Previous Page Table of Contents Next Page


PART I - REGIONAL REVIEWS DESCRIBING ENVIRONMENTAL RESOURCES CHANGES AND RELATED FISHERY RESPONSES

PARTE I - ANALISIS REGIONALES DE LOS CAMBIOS EN EL MEDIO AMBIENTE Y EN LOS RECURSOS Y DE LAS CORRESPONDIENTES REACCIONES EN EL CAMPO DE LA PESCA

NORTH WEST PACIFIC AND SEA OF JAPAN - PACIFICO NORDOCCIDENTAL Y MAR DEL JAPON

VARIATION OF PELAGIC FISH STOCKS IN WATERS AROUND JAPAN

by

Syoiti Tanaka

Ocean Research Institute
University of Tokyo
1-15-1 Minamidai
Nakano-ku
Tokyo 164
Japan

Resumen

La plataforma continental alrededor de Japón es angosta y los de peces de fondo son relativamente poco abundantes. Por el contrario, los peces pelágicos son abundantes gracias a las zonas de frentes donde las corrientes frías entran en contacto con las corrientes cálidas, y las pesquerías de peces pelágicos estan bien desarrolladas. En Japón la captura total de peces pelágicos que se alimentan de plancton de aguas cálidas ha aumentado contínuamente desde 2 millones de toneladas hasta 3 millones de toneladas en el período de 1952 a 1975. Las capturas de sardinas aumentaron dramáticamente y la captura total alcanzó 4.7 millones de toneladas en 1981. La captura de estas especies ocupa 30 a 50% de los desembarques totales de peces marinos de Japón. Las capturas de especies individuales muestran una gran variabilidad, las principales especies dominantes cambian con frecuencia. El coeficiente de variación de las capturas es menor del 30% para calamar y anchoveta, alrededor del 50% para saury, jurel y caballa, y es mucho más alto, alrededor de 150%, para sardina. Estos peces pelágicos generalmente desovan en aguas costeras en la corriente cálida de Kuroshio frente al sudeste de Japón en invierno y primavera. Estos se alimentan en las zonas de los frentes del norte en verano y otoño. Las áreas de pesca se forman tanto en las áreas de desove como en las áreas de alimentación. Se cree que el tipo de ruta que sigue la corriente de Kuroshio y sus cambios bruscos tienen una relación estrecha con los cambios en el tamaño de los stocks.

Los índices de abundancia relativa de los stocks de saury en el Océano Pacífico Noroeste fueron cálculados a partir de datos de captura y esfuerzo, y se encontró que la abundancia no fluctúa tan ampliamente como fluctúa la captura. La captura disminuyó a fines de los años 1960s mientras que la intensidad de pesca disminuyó continuamente desde fines de los años 1950s. La relación entre la abundancia del stock desovante y el reclutamiento no está clara. El tamaño de los stocks es determinado por el suceso o fracaso de la reproducción anual. La captura del arenque de Hokkaido fue mayormente hecha en base a las clases anuales dominantes que aparecen cada cierto número de años. El análisis de la relación entre el stock desovante y el reclutamiento revela que en los años 1930s y 1940s no apareció ninguna clase dominante, a excepción de la clase anual de 1939, aun cuando el número de individuos adultos no era pequeño. La comparación de la abundancia de huevos y reclutamiento para el stock de caballa del Pacífico indican que las clases anuales relativamente grandes fueron producidas a partir de abundancias pequeñas de desovantes entre 1954 y 1957 y el stock comenzó a expandirse. La sardina disminuyó dramáticamente a fines de los años 1930s, probablemente debido a una mortalidad masiva de larvas causada por las aguas frías que ocurrieron al sur de Honshu. Análisis de las estadísticas de capturas sugieren que la falta de reclutamiento fue la causa del colapso. Ha habido algunos intentos para explicar la alternancia en la dominancia de especies en base a la interacción entre especies, pero la evidencia directa es pobre. Se piensa que los cambios de las condiciones oceanográficas tienen algún efecto en este fenómeno. Algunas interacciones no lineares entre las especies pueden explicar bien este fenómeno.

Muchos aspectos indican que los cambios de los stocks de peces pelágicos fueron causados por variación del reclutamiento. No se ha presentado ninguna evidencia de sobrepesca. La pesquería de peces pelágicos debe tener una constitución bien adaptada a la variabilidad de los stocks. Un sistema para utilizar en forma integral toda la comunidad de peces debe ser desarrollada. La predicción de las condiciones futuras de los stocks es necesaria. Los sistemas de licencias como medida para limitar el esfuerzo de pesca pueden ser efectivos. La protección de peces inmaduros es deseable para asegurar la abundancia de huevos.

INTRODUCTION

The continental shelf around Japan is narrow except in the West China Sea and the abundance of ground fishes is rather small. On the contrary, pelagic fishes are abundant due to complicated oceanographical conditions, and there are various kinds of fisheries for pelagic fishes. The Kuroshio (warm current) approaches to Japan from the south and flows eastward along the southern coast of Japan and meets with the Oyashio (cold current) which comes down from the north along the Kurile, Hokkaido and northern Honshu. A branch of the Kuroshio enters into the Sea of Japan (Tsushima Current) and meets with cold water masses in the northern part of the sea. Frontal zones and mixing zones are formed in the areas where the cold currents and the warm currents contact.

Pelagic fishes are characterized by a large variability of their stocks. The saury catch showed a large year to year fluctuation depending on oceanographical conditions of the year as will be explained in this paper. Sardine stocks repeated very large variations with a long period of several decades or longer. This periodical change has been found by Uda (1952) when he examined historical documents. According to the data summarized by Ito (1961), the first record on a good catch of sardine appeared in the period of 1558-1569 and again 1573-1591 in the Sea of Japan. In 1624-1643 period, however, a poor catch was reported in the western Sea of Japan. After these records, nationwide trend of good and poor catches were recorded repeatedly; good catch in 1690-1720, poor catch in 1750-1760, good catch in 1790-1830, and poor catch in 1840-1910. In recent years, the catch was very high from 1930 to 1942 reaching a maximum of 1.6 million tons, very low from 1960 to 1972 recording a minimum of only 9,000 tons in 1965. After 1975, a very high catch is being recorded every year. The Hokkaido herring also showed a large long term variation. At the end of the 19th century the highest catch of almost one million tons was recorded, by 1960 this stock virtually disappeared.

As the catches fluctuate in short term and also in long term, prediction of future catch of pelagic fishes is an important subject. Studies on fisheries biology in Japan in the 1930's and 1940's were concentrated on short term fluctuations of catches particularly on the relation between the distribution and movement of fish and ocean-ographical conditions. Many papers have been published on formation of fishing grounds in relation to distribution of frontal zones (shiome) and water masses (e.g. Uda, 1936a; b; Kimura, 1956).

The core of the Japanese fishery management has depended on a licence system since long ago. As the majority of the catch comprises pelagic fishes which show considerable variations, not much importance was placed on other measures than regulation of the number of boats by issuing licences such as catch limit, size limit and mesh size regulation. This circumstance is quite different from that prevailing in Europe and North America where fisheries aim mostly at ground fishes. Although individual species fluctuate greatly, it seldom happens, as will be explained, that many species maintain very high levels simultaneously or all of species are kept at low levels at the same time. Collapse of one species is usually compensated by building up of some other species. The diversity in demand on fisheries products easily accommodates changes in the dominant species, and together with subsidies from the government and a feeling of stability caused by the licence system, the Japanese fisheries have enjoyed their prosperity regardless of changes of species.

In this paper firstly fluctuations of the catch of pelagic fishes in waters around Japan are described and general patterns of life history of fishes are described. Then some results of studies on the dynamics of several species of fishes are introduced. Finally some discussions on the basic principle of the management of pelagic species with high variability are presented.

PELAGIC FISHES IN WATERS ADJACENT TO JAPAN

Changes of catches

Important neritic-pelagic species in waters around Japan include warm water species such as sardine Sardinops melanosticta, anchovy Engraulis japonica, round herring Etrumeus teres, mackerel Scomber japonicus and S. tapeinocephalus, jack mackerel Trachurus japonicus, saury Cololabis saira, yellowtail Seriola quinqueradiata, and flying squid Todarodes pacificus, and cold water species of herring Clupea pallasi. Besides these, oceanic species such as tuna, skipjack and salmon occur in quantities. Here, these oceanic fishes will not be dealt with further. Although yellowtail is a neritic species this is a large sized fish eater. The catch is fairly constant being 27,000-55,000 tons with the average of 44,300 tons since 1960. Recently aquaculture of this species by raising juvenile fish up to a good size has been developed and the production has reached 150,000 ton level and far exceeds the catch from the sea.

All of above mentioned warm-water neritic species except yellowtail have characteristics in common that they are plankton feeders and inhabit in waters under the influence of the Kuroshio. They are important as their catches are very large. The total catch of these warm-water plankton-feeding fishes since 1930 are illustrated in Fig. 1. In the 1930's the sardine stock flourished and the total catch approached to 2 million ton level but it suddenly declined in the early 1940's, and the war further hindered the fisheries and the catch dropped to a minimum in 1945. Immediately after the war, the catch began to increase and recorded 2 million tons in 1952. Then it grew gradually and reached to 3 million ton level around 1975.

In the late 1970's, sardine catch started a dramatic revival while mackerel catch was still kept at a high level of more than 1 million tons. The total catch in 1981 was surprisingly high at 4.7 million tons. The total catch of these species occupied 30% of more of the total catch of the marine fisheries and are always very important for the Japanese fisheries (Fig. 1). The proportions of these species dropped a little in the late 1960's and the early 1970's. This is due to a large catch of Alaska pollock in Alaskan and Siberian waters. Within the waters adjacent to Japan, these species continued to occupy more than 30%. It should be noted that the total catch was relatively stable being between 2 to 3 million tons since 1930 except the war time and after 1977. This stability was realized while the catches of individual species varied considerably.

In and after 1952 when sardine and anchovy catches are recorded separately in the statistics, the catches of each species are presented in Fig. 2. In this figure, mackerel includes both S. japonicus and S. tapeinocephalus, but the former predominates in the catch. After 1945, the last year of the war, the catch of squid increased sharply from 100,000 ton level to 500,000 ton level. Then saury showed a good catch in the late 1950's, jack mackerel did the same in the early 1960's, and the catch of each species was maintained at a level of a half million tons for several years. Decreasing trend became obvious for saury since 1964 and for jack mackerel since 1967. Mackerel catch was in an increasing trend since 1950 and the trend was accelerated in the 1960's substituting poor catches of saury and jack mackerel. The catch has been maintained at a high level of more than 1 million tons since 1968. Squid began to decrease in the early 1970's. At that time, offshore fishing grounds of squid were developed in the Sea of Japan but poor catch became notable in the late 1970's including the offshore areas. Anchovy catch was kept fairly constant between about 300,000 and 400,000 tons since 1952. But decreasing trend became apparent since about 1973 and it is at a low level now.

After the collapse in the 1940's the sardine catch recovered to about 300,000 tons in around 1950 mostly in western Sea of Japan and in the East China Sea but this stock declined in the late 1950's. Around 1960 some increase of sardine catch was observed on the Pacific side of Honshu and it was hoped as a sign of overall recovery. But in 1964 unusually cold water prevailed around Japan and the Pacific stock of sardine was crushed. The catch of sardine declined to an extremely low level, recording only 9,000 tons in 1965. However, the 1972 year-class of the Pacific stock was unexpectedly large, and the catch began to increase markedly since 1973. This year-class laid abundant eggs and produced strong year-classes in succession. In 1976 the distribution was expanded as far north as off Hokkaido and the catch increased sharply. In parallel with the expansion of the Pacific stock, other stocks in the East China Sea and the Sea of Japan also increased considerably and the total catch of sardine reached 3 million tons which far exceeds the recorded maximum in the 1930's. Besides the Japanese catch, the Pacific stock is being caught by USSR fleets, yielding a few 10 thousand tons. Recently it is reported that sardine is occurring in the Maritime Province of Siberia.

Fig. 1.

Fig. 1. Total of catches of warm-water plankton-feeding pelagic fishes and its percentage in the total landing of marine fisheries in Japan.

Among changes of the catches of these species, it is noted that increases in mackerel catch took place concurrently with decline in catches of saury and jack mackerel, and again increase in sardine catch simultaneously with decline in catches of squid and anchovy. There are a number of arguments on the interrelation among these fish species (e.g. Mitani, 1970; Kawasaki, 1971). This subject will be dealt with in a later section.

Coefficients of variation (C.V.) of the catch of each species is given in Fig. 2. Squid and anchovy showed only a small variability with C.V. values of about 30% though their catches have declined recently. The values of C.V. are fairly large at about 50% for saury, jack mackerel and mackerel. Among these three, jack mackerel and mackerel showed considerable long term changes while saury showed both short term and long term variations. The value of C.V. for sardine is extremely high, i.e. 156%. Most of this large variability is due to long term changes and short term fluctuation is rather small. The ratio of the maximum catch in 1981 to the minimum in 1965 is as high as 340.

Fig. 2.

Fig.2 Catches of warm-water plankton-feeding pelagic fishes in Japan and the diversity index of their compositions. Sx: standard deviation, x: mean catch, C.V.: coefficient of variations.

Also shown in Fig. 2 is the diversity index of MacArthur (1955), that is H = (Yi/Y) log2(Yi/Y), where Yi and Y are catch of i species and the total of them, respectively. In the 1950's when there was no particularly dominant species, the value of H was larger than 2.5. When sardine collapsed and mackerel formed a single dominant species around 1970, the value decreased below 2.0. The recovery of sardine catch temporarily made the diversity a little larger, but as the sardine catch becomes superlatively dominant the diversity is decreasing rapidly. The diversity is considered to be a measure of community stability and the total catch tends to be low when there is no particular dominant species and hence the diversity is high, and tends to be high when a particular species dominates and the diversity is low.

Fig. 3.

Fig. 3. Schema of current system around Japan. Real line: warm current, Broken line: cold current, W: warm water mass, C: cold water mass, Chain line: front, Shaded area: main spawning ground. (Data from Fisheries Agency).

Fig. 4.

Fig. 4. Fisheing grounds of warm-water plankton-feeding pelagic fishes around Japan. Sd: sardine, An: anchovy, Ja: jack mackerel, Mk: mackerel, Sr: saury, Sq: squid. (Data from Fisheries Agency).

Life history of warm-water plankton-feeding pelagic fishes and their fisheries

Warm-water neritic-pelagic fishes generally spawn in coastal waters inside the Kuroshio off southwestern Japan in winter and spring as shown in Fig. 3. Larval fish are transported eastward or northward by the Kuroshio and feed on abundant food and grow rapidly in the coastal waters of northern Japan or in frontal zone or mixing zone in off-shore waters in summer and autumn. They winter in waters under the influence of the Kuroshio. The adult fish repeat the annual life cycle of northward feeding migration in summer and autumn, and southward spawning migration in winter and spring. Saury are distributed also in far offshore waters and spawn in outside waters of the Kuroshio or in areas of the Oyashio.

These pelagic species are caught in spawning grounds in winter and spring and in feeding grounds in summer and autumn (Fig. 4). Most of the catches of sardine, anchovy, jack mackerel and mackerel are obtained by purse seine fisheries. Saury are caught mostly by the stick held dip net fishery with light, and squid are taken by squid angling. Fishing grounds are generally developed in coastal waters but are extended to offshore areas as well for mackerel and jack mackerel in the East China Sea, for squid in the Sea of Japan, and for saury in the Pacific.

For each species it is considered that subpopulations are different between fish in the Kuroshio waters in the Pacific Ocean and those in Tsushima Current waters in the Sea of Japan. Furthermore, for sardine and mackerel S. japonicus, more than one subpopulation is recognized in each of the current systems. It is thought, however, that these sub-populations could be altered at times depending on such factors as the stock sizes.

The Pacific stocks of mackerel S. japonicus and sardine spawn in the Pacific Ocean off central Honshu and migrate as far north as Hokkaido in summer season. Each of them are the largest stock of the species and is supporting the majority of the total catch of each species. Purse seine boats are large and powerful and 400 tons of catch by one haul is not a rare example for sardine in summer and autumn season.

The Kuroshio often changes its route. Presence or absence of the cold water mass in the Pacific off central or western Japan greatly modifies the route of the Kuroshio. As shown in Fig. 5, 5 types, A, B, C, D, and N, are observed. Each type has different effects on distribution and movement of fish shoals, and mortality in early life stages, and the change of types sometimes causes a turning point of stock size (Watanabe, 1982).

Fig. 5.

Fig. 5. Types of routes of the Kuroshio, south of Japan. (Data from Fisheries Agency).

THE NORTHWEST PACIFIC STOCK OF SAURY

Life history of the saury

The northwest Pacific Stock of saury has a wide spawning ground and a long spawning season. Shoals of the saury migrate southward in the Pacific Ocean off northern Japan in autumn season and start spawning while they are in waters north of the Kuroshio. The majority of the fish, however, cross the Kuroshio and continue to spawn in the vast area south of the Kuroshio till next spring. Adhesive eggs are laid on floating seaweeds. Egg diameter is relatively large ranging from 1.3 to 2.1 mm.

Larvae are drifted eastward or northward by sea currents in waters south or east of Japan and reach as far north as the central Kurile Islands in summer. They feed on abundant plankton in northern waters and grow rapidly. They start southward migrations in late August and are exploited intensively by the stick held dip net fishery. The fishing ground gradually shifts to south as season goes on, from water east of Hokkaido in September, and off northern Honshu in October to water off central Honshu in November and December, and then fishing ground disappears. The catch consists mainly of large sized fish (larger than about 28 cm in body length), medium sized fish (about 25-28 cm) and small sized fish (smaller than about 25 cm). Composition of these three size categories differs depending on year. Small sized fish tend to be abundant in years of poor catch. The age or growth problem is still to be clarified but it is considered that in fishing season in autumn, medium sized fish are just one-year-old and will mature in the season while small sized fish are younger than one year and will mature in the next season as large sized fish. Thus the large sized fish are less than 2-years-old. Sometimes extra large fish larger than 31 cm are caught but they are very small in number. Most saury are supposed to die after spawning.

The fish shoals are concentrated in the first branch (coastal branch) and second branch (offshore branch) of the Oyashio and fishing grounds are formed in these areas. Southward shifts of the fishing grounds coincide with the shift of the Oyashio front very well and it is apparently due to this that the southward movement of the fish is obstructed by the front (Matsumiya and Tanaka, 1976b). Southern limits of the fishing ground well correspond with the isothermal lines of surface water temperature of 16°C or 18°C (Tanaka, 1971a; b). When the front stays in the water off northern Japan for a long period of time, fish shoals do the same and good catches are expected. On the contrary, if the Oyashio current penetrates sharply into the mixing zone, fish disappear quickly from the fishing ground and the catch is generally poor.

Population dynamics of the saury

The catches of saury in the Pacific off northern Japan in the period of 1954-1963 were high, ranging from 227,000-528,000 tons (average 389,000 tons). Since 1964 they dropped down to a level of 200,000 tons or less, and recorded very poor catches of 46,000 tons in 1969 and 73,000 tons in 1970. Kurita el al., (1973) analysed the dynamics of the northwest Pacific stock of saury and the effects of fishing on it. For the saury fishery, catch yij and the numbers of fishing operations xij are reported from sample boats for each 10-day-period. Relative index of the abundance of i 10-day-periods can be calculated using these data. The abundance index Pi is usually calculated by the formula

However, a simpler formula

is applicable in case of the saury fishery. Effective fishing intensity of i 10-day-period fiis

fi = Yi/Pi
3

An index of the abundance of available stock for each season may be given by the maximum Pi in the season (Pmax).

The catch and index of the abundance are shown in Fig. 6. The trend of the index is different from that of the catch. The catch fluctuated considerably in 1954-1963 period and dropped down since 1964, while Pmax showed much smaller fluctuation and did not show any decreasing trend until 1967. The distribution and movement of the saury are greatly affected by oceanographical conditions of the year and hence the catch is quite variable. If, for instance, fish are concentrated in a small area and stay there for a long time catch per unit effort would become high and a good catch would be obtained. Contrarily, if fish are distributed dispersedly or move quickly to the south and leave the fishing round, a bad catch would be expected. Pmax suggests that the abundance of available stock was fairly constant though the catch fluctuated considerably.

Fig. 6.

Fig. 6. Relative abundance index (thick line) and the catch (thin line) of saury in the north-west Pacific Ocean off northern Japan. (Kurita et al., 1973)

A relative index of fishing intensity, that is the ratio of the total catch to the index of abundance (Yi/Pmax), is given in Fig. 7. The ratio indicates a relative value of the rate of exploitation. In this figure, the number of boats of the saury fishery is also presented. The relative index showed a cosiderable fluctuation according to the fluctuation of the catch, but still it indicated a clear decreasing trend. This trend well agreed with the trend in the number of the boats. It can be said that poor catch after 1964 occured while fishing intensity has been reduced considerably.

Fig. 7.

Fig. 7. Relative index of the rate of exploitation (thick line) of saury in the northwest Pacific Ocean and the number of boats of stick held dip net fishery (thin line). (Kurita et al., 1973).

Seasonal change of Pi within a season is generally that at the beginning of fishing season in August immigration of fish into fishing ground is not completed yet and the value of Pi is low. It increases rapidly and reaches a peak usually in September and then turns to decrease gradually toward the end of the season in November or December. The rate of decrease after hitting the peak is different year by year and in some years a high level of Pi continues for a fairly long period but in some other years Pi decreases quickly. Trends in 1961 and 1962 when more than 400,000 tons of catches were obtained are compared with those in 1966 and 1967 when the catches were about 200,000 tons in Fig. 8 (Tanaka, 1971a; b). Between these two groups of years, there was almost no difference of Pmax. However, decreasing rate was much different after mid October and caused a large difference in the catch.

Fig. 8.

Fig. 8. Seasonal change of the available stock abundance of saury in the northwest Pacific Ocean. (Tanaka, 1971b).

Isothermal lines of the surface water temperature in mid October are shown in Fig. 9 taking 1962 and 1966 for examples. In 1962, isothermal lines ran in an east-west direction and prevented southward migration of the fish and hence the fish stayed long in the fishing ground. On the other hand in 1966, the Oyashio projected to the south and the fish disappeared quickly from the fishing ground.

Fig. 9.

Fig. 9. Isotherms of surface water temperature and fishing ground of saury in mid 10 days of October. (Tanaka, 1971b).

Matsumiya and Tanaka (1976a) analysed length frequency data together with the catch and effort data and estimated annual rate of exploitation and the abundance of stock for the size categories of large and medium fish. The escapement post fishing season number was calculated and compared with the abundances of stock or recruitment. Matsumiya and Tanaka (1978) drew the reproduction curves separately for the large sized fish and medium sized fish assuming a two year lag from spawning to recruitment. After this paper was published, some doubt was thrown on the ageing of saury.

Here only the large sized fish is considered because age of them is assumed to be two-years-old. A stock and recruitment relation is presented in Fig. 10. Assuming two years as the maturation age, two independent cycles, an even-year-cycle and an odd-year-cycle, are defined like for pink salmon. Before 1960, failure in reproduction in the 1966 year-class caused a serious decline of the cycle. The odd-year-cycle also failed in reproduction in 1963 and again in 1967 and dropped down to an extremely low level. During these years no signs of a shortage of parent fish could be detected.

Because of its peculiar life history the saury is fished intensively in a short time period immediately before spawning, and effects of fishing do not appear as growth over-fishing but appear in reproduction through insufficient number of spawning fish. In the case of the northwest Pacific stock of saury, there is no evidence that the spawning stock was depleted by fishing. Furthermore, the poor catch happened after the intensity of fishing decreased substantially. Analysis of the stock and recruitment relation indicated that the decline of the stock was brought about by failure of reproduction. In 1971, the spawning stock was very small. Nevertheless this year-class produced in 1973 the largest catch since 1963. In summery, the saury catch is influenced: in short term by distribution and movement of fish affected by oceanographical conditions of the year and; in long term, by the stock level determined by success or failure of sequential reproduction.

Fig. 10.

Fig. 10. Stock and recruitment relation of the large sized saury in the northwest Pacific Ocean. Numerals: year-classes. (Matsumiya and Tanaka, 1978).

SOME EXAMPLES OF STOCK AND RECRUITMENT RELATION

Hokkaido herring

The Hokkaido herring, like as other stocks of herring, showed remarkable year-class fluctuations. The catch was supported by dominant year-classes which appeared once every few years. Fishing was conducted by set nets and gill nets in spring for adult fish which concentrated in the coastal area for spawning. The maximum catch of 970,000 tons occurred in 1897, but a decline of catches started in the early 20th century from southern parts of the spawning grounds, and extended northward. In the 1930's frequency of occurrence and also the strength of dominant year-classes decreased markedly. A sign of serious decline of the catches appeared in late 1930's. Suddenly in 1939 a large year-class came up. This year-class provided temporary good catches around 1945 and dominated the catch for 20 years. Disappearance of this year-class ended the fisheries of the Hokkaido herring.

Age composition data on the catches were collected for a long time. Only the number of gear is available as fishing effort data, but main gear was set net and fishing intensity is considered not to vary considerably. Thus the number in the catch by age is assumed to represent the survival process in the stock. Fish first appear in the catch as 3-year-olds, reach a peak in number at about age 5, and then turn to decrease in geometrical fashion with constant survival rate peculiar to each year-class. Reasons for difference in survival rate between year-classes are not known. The above mentioned 1939 year-class had unusually large value of survival rate at 0.73.

Stock and recruitment relations of the Hokkaido herring have been analysed (Tanaka, 1960). As the survival rate is not the same among year-classes, year-class strength is represented by the catch at age 5 when recruitment is completed. Fecundity is roughly proportional to the body weight of fish and hence total catch in weight is taken as an index of spawning. The result is presented in Fig. 11. It is rather difficult to find any relations as the points scatter very broadly. Nevertheless it is clear that up to the 1920's abundance of spawning was large and recruitment was also large on the average though it showed considerable variations. In the 1930's recruitment was kept very low and then the abundance of spawning began to decrease. The abundance of spawning decreased to low levels by 1939, but the large year-class appeared and the abundance of spawning was restored considerably in 1940's. Yet, no dominant year-class appeared again. Occurrence frequency of dominant year-classes is quite different between adult catch levels of larger than and smaller than 400,000 tons. If this trend was a real one inherent to biology of the herring, then it could be said that the Hokkaido herring stock collapsed by insufficient spawning due to overfishing. However, if the reproduction curve inherent to the stock had a shape usually observed, and that seems to be more reasonable, reason of the collapse was not deficiency in the spawning but a high mortality of fish before recruitment caused by some natural factors. The factor of high water temperature has been suggested (Hatanaka, 1949) but the factors are still not well understood.

Fig. 11.

Fig. 11. Stock and recruitment relation of the Hokkaido herring. Numerals: year-classes. (Tanaka, 1960).

Pacific stock of mackerel

The Pacific stock of mackerel (S. japonicus) inhabit the coastal waters inside of the Kuroshio and spawn in water around the Izu Island in the Pacific off central Honshu. Spawned eggs and larvae are transported by the Kuroshio and extend their distribution eastward and northward. Young fish come back and congregate in waters off central Honshu and winter there. One-year-old fish extend their distribution as far north as northern Honshu in summer and come back again to water adjacent to central Honshu in winter. In third year they join the adult fish stock as two-year-old fish, and migrate as far north as Hokkaido and southern Kurile and feed there in summer. When autumn comes, they move to the waters off northern Honshu, and begin to migrate southward along the coast and to return to the wintering area. They spawn for the first time at age 3 around the Izu Island. The fish in feeding seasons are caught by the purse-seine fishery off Hokkaido and northern Honshu in summer and autumn season. In wintering and spawning seasons they are caught by the angling and scoop nets in waters off central Honshu and the Izu Islands.

The abundance of spawning is estimated every year since 1951 (Watanabe, 1972; 1982). The abundance was less than 50×1012 until 1956 but started to increase since 1957 and in 1961-1968 it repeated large fluctuations around 500×1012 level. During 1969-1971, it dropped below 200×1012 temporarily but increased again since 1972 and reached to 1,000×101 level in 1974-1976. After this time it declined sharply. Spawning level in 1982 was reported to be very poor.

The age composition of the spawning stock was estimated every year and Watanabe allotted the abundance of spawning to each year-class and calculated the total abundance of eggs spawned by each year-class throughout its life. Comparison of spawnings by year-classes with the spawnings of the year of birth gives a stock and recruitment relation. The result is shown in Fig. 12. Points scattered near the origin in 1951-1953, started to move upward due to good reproductions continued from 1954 to 1957. Up to 1963, points moved toward the right forming a Ricker type curve, but the reproduction turned poor in 1964 and the stock showed a temporary fall. It started to increase again since 1969 and produced another Ricker type curve at a higher level. These two reproduction curves suggest excessive spawnings in 1964 and 1974.

The stock repeated large long term variations, with a peak in the mid 1960's originating from good reproduction in 1956 and 1957 year-classes, a decline around 1970 caused by a poor reproduction from 1964 to 1968. Another peak in the mid 1970's was brought about by a high reproduction since 1969. The catch was maintained at levels of one million tons or higher from 1968 to 1980. In 1978, the maximum of 1.3 million tons was recorded. The catch in 1980 was 1.0 million tons but it dropped drastically in 1981 down to only 420,000 tons. Fishing in 1982 was again poor. Although there is some possibility of overfishing and shortage of spawning around 1978 to explain the latest decline, long-term variations before these recent years indicated that the primary cause of the stock variation was the success or failure of reproduction and the meandering of the Kuroshio. Reproduction of year-classes from 1958 to 1962 and from 1970 to 1974 fit well with Ricker type curves, but it is noted that the levels of the curves are different between these two periods.

Fig. 12.

Fig. 12. Stock and recruitment relation of the Hokkaido herring. Numerals:year-classes. (Tanaka, 1960).

Causes of collapse of sardine in the 1940's

A peak of catch in the 1930's seen in Fig. 1 was due to a good catch of sardine, and more than 80% of the total catch shown in the figure was accounted for by sardine. The catch of sardine hit a peak of 1,586,000 tons in 1936 and then turned to decline sharply (Kurita and Tanaka, 1956). Various explanations were presented for this decline. Nakai (1949) explained that at that time a large cold water mass appeared south of Honshu and the route of the Kuroshio was type A (see Fig. 5), and hence sardine larvae transported by the Kuroshio from the main spawning ground south of Kyushu in those years drifted away to far offshore waters and died from starvation, causing extinction of recruitment. Nakai's explanation was accepted to be most plausible. In waters off California, the sardine stock collapsed almost simultaneously with the Japanese sardine and it has been found that extinction of recruitment was the cause of the collapse (anon., 1953).

Kurita (1957) compared annual natural increase or production with the stock size assuming various levels of rate of exploitation and found that the relation was not like a parabola of the logistic curve but the natural increases in 1941-1943 were extremely low against the stock sizes of the years. He concluded from this analysis that recruitments of 1938-1941 year-classes were very poor due to some natural causes. He did not specify any natural factors but coincidence of timing of recruitment failure may endorse the Nakai's hypothesis.

Assuming a logistic type model for the sardine stock, and giving various values of the parameters of the logistic curve, r and K, trajectories of stock abundance from 1915 to 1950 are calculated applying the actual catch records. When r is large and K is small (e.g. r = 0.68, K = 107 tons) the stock declines once due to overfishing but it returns, increasing rapidly after 1942 due to reduced catches and cannot simulate the actual situation in which stock level was kept very low for a long time. If r is small but K is large (e.g. r = 0.04, K = 30×106 tons), stock size is reduced considerably by fishing and stays at a low level similarly to the actual situation. But here r value is unreasonably small. This analysis leads to a conclusion that unless parameter values change, it is not likely that the sardine stock was reduced by overfishing and kept at a low level under a pressure of fishing. Even if overfishing was a real one, it is considered to be true that the productivity of the sardine stock was reduced drastically in the 1940's due to some causes other than overfishing.

SOME DISCUSSION ON INTERACTION BETWEEN SPECIES

Alternation of dominancy between saury and mackerel

As mentioned previously the catches of individual species of the warm-water plankton-feeding pelagic fishes in watres adjacent to Japan showed large fluctuations while the total catches have been fairly stable. There, alternations of dominant species were observed. As seen in Fig. 2, saury catch decreased while mackerel catch increased rapidly. Both the Pacific stocks of saury and mackerel feed in the frontal zone where the Ovashio and Kuroshio meet and spawn in waters under the influence of the Kuroshio. Both of them feed mainly on zooplankton. Hence the saury and mackerel are undoubtedly in competition. Mitani (1970) speculated on the alternation of the dominant species that i) the saury stock was depleted by overfishing, ii) the mackerel stock was in an upward trend at that time, and hence iii) the mackerel stock obtained advantages over the saury stock in competition for food and this resulted in the alternation of dominant species from saury to mackerel. This speculation is questionable because firstly there is no evidence of overfishing of the saury and secondly there is no explanation why the mackerel was in an upward trend. Further, any change caused by simple competitive interactions is reversible and so when fishing intensity of the saury was reduced the situation would likely have returned to the former state and dominancy of mackerel could not be maintained.

Kawasaki (1971) criticised Mitani's argument and stated that the alternation of species was caused by a change of the oceanographical environment. He denied possibility of overfishing by a reason that predation on plankton feeders by skipjack, tuna, porpoise and other predacious animals in the mixing zone is estimated to be much larger than the catch. Special features of oceanographical conditions in the Pacific off northern Japan at the time when the alternation occurred were that the Kuroshio shifted south and the Oyashio retreated north enlarging the mixing zone between these two currents. The mackerel mostly inhabit the Kuroshio and feed in the mixing zone. On the other hand, saury are mainly distributed north of the Oyashio and the retreat of the Oyashio pushed them further offshore and they could not use abundant plankton in the coastal waters. Kawasaki concluded that this situation caused the change of position from the the saury to mackerel. Among three species, (sardine, mackerel and saury) sardine is distributed in most offshore waters. Although it may be possible that some types of oceanographical conditions are favourable to coastal species and unfavourable to offshore species, it seems implausible that the same conditions last for more than 10 years. The problem of alternation of species should be discussed from two aspects; one is a trigger factor such as change of oceanographical conditions or effect of fishing, which destroys a stable state and causes alternation of species, and the other is a mechanism such as interaction between species, which stabilizes the new situation after change.

Nonlinear interactions between species

Shirakihara and Tanaka (1978) confirmed Vandermeer's (1973) suggestion that nonlinear interaction between two species could produce alternations of dominant species. Generally, two species interaction can be expressed in a form of simultaneous differential equations:

where Ni is the stock size of i species, Gi is a function of N1 and N2. The condition for equilibrium of N1 or N2 is:

Gi(N1,N2) = 0 or G2(N1,N2) = 0 (5)

Taking N1 on the abscissa and N2 on the ordinate, these equations present curves which are called isoclines. Intersections of two curves are equilibrium points of the system. In case of the linear interaction, isoclines are straight lines and there is only one intersection. In ordinary circumstances this point is a stable equilibrium. When the equilibrium is broken by some disturbance, stocks always respond to restore equilibrium at the same point. Changes are reversible (Larkin, 1963). Under nonlinear interactions, however, isoclines from curved lines and may have more than one intersection. Therefore, it is possible a system has two stable equilibrium points. In such a system, there is a possibility of irreversible change that stocks at one equilibrium point are disturbed and pushed out of the point and drawn to the other equilibrium point. If original equilibrium point corresponds to point A in Fig. 13 where species I is dominant and new equilibrium point to point B where species II is dominant, then such irreversible change means an alternation of dominant species.

There is a possibility that nonlinear interactions really exist between the sardine and mackerel stocks. The mackerel prey upon the sardine. Therefore, a large stock of the sardine may be profitable for the mackerel. However, once the sardine stock grow up to such a high level as in recent years, the sardine would become a strong competitor occupying a part of the mackerel's habitat and exercizing unfavourable influences over the mackerel. Isoclines for the mackerel could have a form like curve I in Fig. 13. For the sardine, the mackerel are predators and also competitors on one hand but they prey upon anchovy and jack mackerel which are competitors of the sardine on the other. A small mackerel stock may allow anchovy and jack mackerel stocks to grow larger and may cause a low growth of the sardine stock. Hence the isocline for the sardine could be a curved line as shown by curve II in Fig. 13. This argument is nothing more than speculation with an over simplified model but still suggests an importance of studies from such a view point.

Fig. 13.

Fig. 13. Isoclines of two interrelated species with nonlinear interactions. Arrows indicate directions of movement.

MANAGEMENT OF NERITIC-PELAGIC FISHES

It has been made clear from above discussions that neritic-pelagic fish stocks are very variable depending on natural environmental changes. Particularly, successes or failures of recruitment determines variation of the entire stock. There is almost no evidence that fishing is playing an important role in this large variability. Cessation of recruitment occurs even when spawning is fairly abundant. At the same time, there are quite a few examples of building up of stocks triggered by a relatively strong year-class produced from a very small egg production. This does not mean however, that fishing has no effect on fish stock at all. The effect of fishing must be measured in relation to the productivity of stock. High productivity can keep stock growing under an intensive fishing. On the contrary, even a weak fishing may accelerate collapse of stock under a low productivity due to unfavourable natural conditions.

In management of pelagic fish stocks, high variability of productivity depending on changes in environment should be kept in mind. It could not be possible even by complete closure of fishing in the 1940's to maintain the sardine stock at the high level of the 1930's. Under such a situation, fishing which is compatible with low productivity of stocks should be adopted. The fishery should maintain resilience under unfavourable productivity, to build up quickly when the environment allows more intense fishing.

It is certainly not easy to expand or reduce a fishery according to variation of productivity because a fishery is an economical activity. However, the fact observed in Japanese fisheries should be noted. That is, variability in the total catch of warm-water plankton-feeding pelagic fishes is much smaller than that in each species. Productivity of the sea must be fairly constant, not changing like catches of individual species. In order to maintain a stability of a fishery, the entire community of neritic-pelagic fishes should be exploited as a whole. Fortunately, most of neritic-pelagic fishes are caught by the same type of fishing gear. Japanese purse seine boats select freely their target fish which is most profitable to them among sardine, anchovy, jack mackerel and mackerel. The fishery can be continued regardless of large variabilities of individual stocks if it is exploiting the community of neritic-pelagic fishes as a whole.

One of the serious problems in doing so is the differences in prices and demands among various species of fishes. In Japan each of the above-mentioned species has a basic demand as human food. When sufficient amounts of fish cannot be supplied for human food, due to poor catch, then the price goes up and income of fishermen is kept relatively high in spite of a small catch. When the catch becomes very large and far exceeds the demand as human food, the price drops down and a large demand is created as material for reduction and feed. By such a mechanism, Japanese fisheries for neritic-pelagic fishes can survive under very unstable conditions of resources. It seems to be necessary to develop such a flexibility in demand and organization for circulation.

The proper management of neritic-pelagic fishes which show large fluctuations depends much upon the prediction of productivity. To this end, predictions of recruitment as well as of abundance of available stock are needed. Such research as stock and recruitment relations, relations between the distribution and movement of fish and oceanographical conditions and the abundance of prerecruited fish are required.

Notwithstanding extensive researches, it is very difficult to predict precisely the productivity of stocks. Therefore, except long living species such as herring, a regulation by catch limit system seems not to be effective. Regulation of fishing effort by, say limiting the number of boats, may be effective.

Tanaka (in press) calculated yield per recruit and egg production per recruit for various values of natural mortality M, life span T and K of Bertalanffy's growth curve. He found that provided that only adult fish are exploited, effect of fishing would not be serious for short-life species such as anchovy and relatively long-life but large K value species such as sardine. Assuming the age of first capture is at the age of first maturation, yield per recruit increases with the value of fishing coefficient of F. Egg production per recruit at F = 1.0 (roughly 40 to 50% in the rate of exploitation) is reduced to about 1/3 for sardine and 1/2 for anchovy relative to that at F = 0. A decrease in egg production by this extent seems not likely to reduce recruitment considerably. This situation is altered substantially if immature fish are exploited. With F = 1.0, the egg production would not be reduced less than 1/3 for anchovy, but may be reduced less than 1/10 for sardine.

Adults and immatures of pelagic fish often form different shoals and selective fishing only for adult fish is relatively easy. It is desirable to try to reduce the catch of immature fish as much as practicable. In any case, reduction of egg production to low levels should be avoided. Optimum egg production may vary depending on the environmental conditions and hence efforts to keep egg production at a fixed level are not practical.

REFERENCES

Anon. 1953. California Cooperative Oceanic Fisheries Investigations, Progress Report, 1 July 1952 to 30 June 1953, pp.44.

Hatanaka, M. 1949. Effect of the variation of water temperature of the spawning ground on the breeding of spring herring in Hokkaido (preliminary report). Bull.Inst.Agr.Res. Tohoku Univ. 1:105-108.

Ito, S. 1961. Fishery biology of the sardine, Sardinops melanosticta (T. & S.), in the waters around Japan, Bull.Japan Sea Reg.Fish.Res.Lab., 9:1-227.

Kawasaki, T. 1971. Recent discussion of the fluctuations of mackerel and saury resources. Bull.Jap.Soc.Fish.Oceanogr. 18:16-24.

Kimura, K. 1956. A theory of congregation and separation of fish by oceanographic condition - I. Referring to Pacific saury fishing of Tohoku regional sea in fall, Bull.Tohoku Reg.Fish.Res.Lab., 7:103-145.

Kurita, S. 1957. Causes affecting the size of sardine stock in the waters off Japan and adjacent regions, with particular reference to the catch declining since 1941, Bull.Tokai Reg.Fish.Res. Lab. 18:1-14.

Kurita, S. and C. Tanaka. 1956. Estimation of annual catches of sardine and anchovy in Japan, 1926-50, using the amounts of processed Iwashi products, Bull.Jap.Soc.Sci.Fish., 22:338-347.

Kurita, S., S. Tanaka and M. Mogi. 1973. Abundance index and dynamics of the saury population in the Pacific Ocean off northern Japan, Bull.Jap.Soc.Sci. Fish. 39:7-16.

Larkin, P.A. 1963. Interspecific competion and exploitation J.Fish.Res.Bd.Can., 20:647-678.

MacArthur, R. 1955. Fluctuations of animal populations, and a measure of community stability, Ecology, 36:533-536.

Matsumiya, Y. and S. Tanaka. 1976a. Dynamics of saury population in the Pacific Ocean off northern Japan - II. Estimation of the catchability coefficient q with the shift of fishing ground, Bull.Jap.Soc.Sci.Fish. 42:943-952.

Matsumiya, Y. 1976b. Numerical implication between the distribution of surface temperature and the saury fishing ground in the Pacific Ocean off northern Japan, Bull.Jap.Soc.Fish.Oceanogr. 29:30-40.

Matsumiya, Y. 1978. Dynamics of the saury population in the Pacific Ocean off northern Japan - III. Reproductive relations of large and medium sized fish, Bull.Jap.Soc.Sci.Fish. 44:451-455.

Mitani, F. 1970. On the long-term fluctuation of pelagic fish resources, Bull.Jap.Soc.Fish. Oceanogra. 16:187-191.

Nakai, Z. 1949. Iwashi wa naze torenai? (Why cannot the sardine be caught?). Suisan Kikan, 2:92-101.

Shirakihara, K. and S. Tanaka. 1978. Two fish species competition model with nonlinear interactions and equilibrium catches, Res.Popul.Ecol. 20:123-140.

Tanaka, S. 1960. Studies on the dynamics and management of fish populations, Bull.Tokai Reg.Fish.Res.Lab. 28:1-200.

Tanaka, S. 1971a. A proposal on the studies on the relation between the fisheries resources and their circumstances, Bull.Jap.Soc.Fish.Oceanogr. 18:12-16.

Tanaka, S. 1971b. Tohoku, Hokkaido oki no sanma shigen nitsuite (On the saury stock off Tohoku and Hokkaido regions), Zen-sanma, 5(1):4-7.

Tanaka, S. in press. A mathematicl consideration of the adaptation strategy of marine fishes, Res.Popul.Ecol., Suppl.3.

Uda, M. 1936a. Locality of fishing centre and shoals of "katsuwo", Euthynnus vagans (Lesson),. correlated with the contact zone of cold and warm currents, Bull.Jap.Soc.Sci.Fish. 4:385-390.,/p>

Uda, M. 1936b. Fishing centre of "sanma" Cololabis saira (Brevoort), correlated with the head of Oyashio cold current, Bull.Jap.Soc.Sci.Fish. 5:236-238.

Uda, M. 1952. On the relation between the variation of the important fisheries conditions and the oceanographical conditions in the adjacent waters of Japan 1. J.Tokyo Univ.Fish. 38:363-389.

Vandermeer, J.H. 1973. Generalized models of two species interactions: A graphical analysis, Ecology. 54:809-818.

Watanabe, T. 1972. The recent trend in the stock size of the Pacific population of the common mackerel off Honshu, Japan, as viewed from egg abundance, Bull.Jap.Soc.Sci.Fish. 38:439-444.

Watanabe, T. 1982. Distribution of eggs and larvae of neritic migratory fish in relation to the Kuroshio, Bull.Coast.Oceanogr. 19:149-162.

NORTH WEST PACIFIC AND SEA OF JAPAN - PACIFICO NORDOCCIDENTAL Y MAR DEL JAPON

SOME EXPLANATION FOR CHANGES IN ABUNDANCES OF MAJOR NERITIC-PELAGIC STOCKS IN THE NORTHWESTERN PACIFIC OCEAN

by

Sigeiti Hayasi

Far Seas Fisheries Research Laboratory Shimizu, Japan

Resumen

Hay once productos que se extraen de los recursos neríticos-pelágicos que son importantes para la pesquería Japonesa. Estos son sardina, anchoveta, postlarvas de estas dos especies, arenque redondo, arenque del Pacífico, caballa, jurel, sábalo, saury del Pacífico, calamar común de aleta y calamares miscellaneos. La captura total se ha mantenido alrededor de dos millones de toneladas de 1953 a 1970, aumentó desde entonces, y ha pasado de 4 millones de toneladas por año entre 1977 y 1980. Los estados vecinos también han extendido sus actividades de pesca, pero entre 1970 y 1980 la flota Japonesa todavía producía entre el 74 y 80 por ciento de la captura total. Aun cuando todavía es posible explicar las fluctuaciones de los recursos pelágicos-neríticos en base a los datos Japoneses, se hace cada vez más necesario extender la cooperación internacional en el campo de biología pesquera, especificamente en el caso de arenques, sardina, caballa y saury. Las estadísticas Japonesas de 1953 a 1980 indican que de estos once productos, nueve de ellos asi como las poblaciones de peces relacionadas experimentaron aumentos o disminuciones más o menos notables durante los últimos 28 años. El arenque del Pacífico se ha mantenido en una disminución continua, mientras que las capturas de calamares miscellaneos muestran un incremento constante debido a la explotación de nuevos stocks.

Investigaciones anteriores revelan que hay algunos factores ambientales que han afectado seriamente la abundancia, distribución y otras características biológicas a las especies costeras. La caída y recuperación de la sardina está bastante bien documentada sobre la base de los datos tomados de la pesquería comercial y de los censos de huevos y larvas y las observaciones oceanográficas acumuladas a partir de los años 1930. Las capturas aumentaron en los años 1920s junto con una rápida expansión de las áreas de pesca, no solamente en las aguas alrededor de las Islas Japonesas pero también a lo largo de la costa oeste del Mar del Japón desde la Península de Corea hasta Sakhaline. La pesquería que se fue expandiéndo rápidamente dependió de la extensa migración de las subpoblaciones. El área principal de desove está localizada en el área de Satsunan, extremo sur de la Isla de Kyushu. Los huevos y larvas son arrastrados a lo largo de la costa del Pacífico de las Islas de Shikoku y Honshu por la corriente de Kuroshio. Los peces pasan el segundo y tercer año de sus vidas frente al noroeste de Honshu, donde las aguas de mezcla aseguran una alta productividad del plancton de forraje. Los adultos son principalmente capturados en el Mar del Japón y migran hacia el sur para el desove a fines del otoño e inicios del invierno. Los extensos serpenteos de la corriente de Kuroshio resultan en la acumulación de larvas de sardinas en las poco fértiles aguas mar afuera, y los stocks de larvas sufrieron una alta mortalidad a mediados de los años 1940. Desde entonces las áreas principales de distribución fueron cambiando de año en año y éstas se encontraron en el Mar Oriental de la China frente al este de Kyushu en 1951 a 1953, en el Mar de Japón cerca a la parte central de Honshu alrededor de 1955, y en las aguas del Pacífico a lo largo de la parte central de Honshu desde 1957. Los peces parecen haber ido incrementado gradualmente en los años 1960s en las aguas del Pacifíco a pesar de la interrupción debida a una anomalía ambiental en 1963. La actividad del desove se ha extendido rápidamente desde 1972, y una significativa cantidad de huevos han sido encontrados en el área de Satsunan a partir de 1976, cuando las capturas sobrepasaron un millón de toneladas, tal como ocurrió en el próspero período anterior en los años 1930s.

Se han detectado cambios ecológicos similares en el arenque del Pacífico. La caballa común también ha mostrado cambios significativos en sus áreas de pesca y áreas de desove. Se nota que todas estas poblaciones están compuestas de tres o más grupos de edad.

Las capturas de saury del Pacífico han mostrado una amplia variación en años sucesivos. Pero no hay ninguna tendencia notable a largo tiempo en esta especie. Las áreas de pesca en esta especie varían, siempre en las aguas del Pacífico, de zonas costeras a zonas mar afuera debido a condiciones oceanográficas. Especies de vida corta como anchoveta y calamar, además del saury, muestran una significativa variación en el tamaño de sus stocks y también frecuentemente en las áreas de concentración que varían a corto plazo, lo que hace que los pronósticos previos al inicio de la estación de pesca sean muy útiles para estabilizar las operaciones de pesca. Sin embargo, la extensión geográfica de las áreas de desove y áreas de pesca parecen ser más estables en las especies de vida corta que en la sardina, herenque y otros peces de vida más larga.

Las especies neríticas-pelagícas parecen formar ecosistemas más o menos interelacionados como se sugiere por el hecho de que la captura total ha sido más o menos estable o ha mantenido un incremento más o menos constante a pesar de la notable variación de las especies individualmente, especialmente aquellas que tienen una gran biomasa. Además, estos recursos sorportan la mayor proporción de la producción de peces en el Océano Pacífico noroeste. Por lo tanto, cualquier regulación de las actividades de pesca tendientes a asistir la recuperación de especies reducidas por causas naturales debe evitar disturbar la producción de las otras especies que sustituyen a las especies problema en estas áreas. Es esencial que se seleccionen las áreas claves donde la población de una especie problema debe ser protegida de la presión de la pesca. Para que la ejecución de las regulaciones pesqueras sean practicables, es indispensable tener un amplio entendimiento o un amplio conocimiento de la ecología de las principales especies.

Las estadísticas de capturas y esfuerzo no siempre proveen información confiable sobre la abundancia de los stocks pelagícos debido a las diversas fuentes de variación en la accesibilidad y vulnerabilidad a las pesquerías. Al evaluar los stocks de sardina y caballa común solo se utilizan los censos de huevos en forma extensa. Sin embargo, los trabajos de campo y de laboratorio, requieren mucho tiempo, especialmente para especies con áreas de desove muy extensas. Las técnicas acústicas no están todavía lo suficientemente maduras como para permitir evaluaciones cuantitativas de los stocks de peces en el noroeste del Océano Pacífico. Se requiere un desarrollo tecnológico para proveer medios útiles que permitan introducir en breve tiempo medios más eficientes para medir los stocks pelagícos.

INTRODUCTION

Variations of fish stocks assert considerable influence upon Japanese society in which the marine products have long been most dominating sources of animal protein. Uda (1952) and Nakai (1962) quoted historical documents which have suggested wide fluctuation in the stocks of sardine and other neritic-pelagic fishes that might have resulted in prosperity and destruction of villages in various parts of the country.

The remarkable decline of sardines in the 1940's was a very embarrassing event for the society which was suffering definitive shortage of the animal protein supply. In 1949 the Cooperative Iwashi Resources Investigation was initiated with the aim of clarifying causes of the drastic decline of sardine resources as the first nation-wide systematic investigation of fisheries resources in this country (Nakai et al., 1955). The decline of sardines drove the fishing fleet to catch substitute species such as anchovy, jack mackerel and common mackerels and the investigations were amended so as to cover all the major plankton feeders in 1955 (Ex. Com., Conf. Invest. Neritic-Pelagic Fisher. Japan, Fisheries Agency 1961). At the present stage biologists have realized that the wide fluctuation in amount of stock of a species was often compensated by that of other species, and total catch of all the plankton feeders have stayed at fairly constant levels (Nagasaki 1973).

In and after 1960, USSR, Republic of Korea and other countries have expanded the fishing activities in the northwestern Pacific. The fishing exploitation encouraged related investigations. In 1963 the Cooperative Study of Kuroshio or CSK was initiated as a joint activity of UNESCO and FAO (Marr 1968). Japan and USSR organized joint consultations of the stocks of Pacific saury each year since 1968. The consultations expanded the topics recently, so as to cover common mackerels since 1976 and sardines since 1980 (Research Division of Fisheries Agency, Japan, 1980). The international cooperations have deepened the understanding of fisheries resources in relation to the fishing activity and oceanographic changes.

Briefed in this report are time series of catch statistics of eleven commodities important to the Japanese neritic-pelagic fisheries for 28 years from 1953 to 1980 compiled from the official statistics (Hayasi unpublished), together with international data since 1970 in the northwestern Pacific (FAO 1975, 1981). Changes of stocks due to environmental causes are included here from major papers explaining fluctuation in the amount of catch. Fishing-induced changes of stocks are described in a separate paper (Tanaka - this volume).

CATCH STATISTICS OF MAJOR NERITIC-PELAGIC SPECIES

In 1955 the Japanese fishery biologists have listed eight important neritic-pelagic species or groups of species including sardine or ma-iwashi in Japanese language, Sardinops melanosticta (Temminck & Schlegel), round herring or urume-iwashi, Etrumeus teres DeKay, anchovy or katakuchi-iwashi, Engraulis japonica (Hottuyn), common mackerels of ma-saba, Scomber japonicus (Houttuyn), and goma-saba, S. tapeinocephalus Bleeker, jack mackerel or ma-aji, Trachurus japonicus (Temminck & Schlegel), scads or muro-aji-rui, Decapterus spp, common flying squid or surume-ika, Todarodes pacificus Steenstrup, and shirasu or mostly postlarvae of sardine or anchovy, together with some miscellaneous coastal fishes. The word "important" means that these eight commodities have appreciable biotic production or such potential. Even though recent changes of life styles resulted in shifts of preference for food stuffs including fishes, and the demand has abated for these species in the fresh fish market, they are still important as the raw materials for aquaculture, poultry, food industries, etc. Furthermore, these species represent major tertiary producers in the neritic-pelagic biota. Thus changes of these species populations determine not only the nation's nominal catch, but also the predacious species which comprise many high priced food items. Ecologically, three species should be added to the list, Pacific herring or nishin, Clupea pallasi Cuvier & Valenciennes, in the borealic waters, Pacific saury or sanma, Cololabis saira (Brevoort), that represents significant biomass in the mixing waters of the warm Kuroshio Current and the cold Oyrshio Current and the miscellaneous squids, of which a kind of flying squid called aka-ika in Japanese language, Omnastrephes bartrani (Lesuer), comprises the most significant portion.

Expansion of non-Japanese fishing activities resulted in the decline of the share of Japanese in the production of the eleven commodities from about 4/5 in 1970 to about 3/4 in 1980 (FAO 1975, 1981). The Japanese share is especially low, 5 to 30 percent, for herring, which has been mostly taken by USSR and Chinese fisheries. Anchovies were reported mostly in Japan, over 77 percent in 1970 to 1973, but Korean catch gradually increased, lowering the Japanese share below 50 percent since 1978. Percentages of Japanese catch of other commodities appear to have been stable, 81 to 100 percent for sardine, 70 to 100 percent for round herring, 70 to 93 percent for mackerels, 90 to 99 percent for jack mackerel, 84 to 100 percent for scads, 57 to 83 percent for Pacific saury, 85 to 92 percent for common flying squid, and 65 to 87 percent for the miscellaneous squids. Thus, Japanese data still appear to represent the changes of most of the stocks under discussion.

Table 1 shows the amount of catch together with means and coefficients of variation of the eleven commodities based on the official statistics. The period of 28 years was chosen because the statistics survey system in this country was re-established in 1951, and the figures became reliable for most of the commodities since 1953. A longer series of statistics since 1926 indicates that the herring catch has been on the continuous decline and the years under discussion remain in the general decreasing period. The miscellaneous squid increased since 1964 due to recent exploitation of some flying squids in the north- and southwestern Pacific Ocean as substitutes to the common flying squid. However, the 28-year period covers rise and fall of the other species, and appears to represent enough duration of the usual fluctuations.

The figures given in Table 1 indicate significant variation exceeding 50 percent for sardine, mackerels, herring, jack mackerel and miscellaneous squids. The large variations of herring and miscellaneous squids must be responsible to the considerable change of stocks and fisheries. Pacific saury and scads follow these unstable species with the coefficient of variations of around 50 percent. The other species are more stable with coefficients of variation of 26 to 34 percent.

EXPLANATION OF FLUCTUATIONS IN FOUR COMMODITIES

Systematic surveys have indicated possible environmental causes responsible for the wide fluctuation in the sardine population. The same factors might have had different effect upon ecologically related species so as to stabilize the amount of total catch of these species. Information was also made available for identifying non-fishing factors on the changes of herring, saury and flying squids. In the past, the environmental factors appeared to have played leading roles in determining the abundance and distribution of the neritic-pelagic populations in the northwestern Pacific Ocean.

Sardine and related species

The sardine, besides Alaska pollock, Theragra chalcogramma (Pallas), represents the most prospering species in the North Pacific, having exceeded two million tons in annual catch. There are many fishes which are ecologically and commercially related to the sardine. A commodity called iwashi in Japanese language refers to either sardine or a group of round herring, and some other clupeids as well. The three species occur over almost the entire coastal waters around the Japanese Islands except northern Hokkaido and also the Ryukyu Islands. The same fishing gears catch all these three species, often concurrently. Furthermore, the most productive purse seine fishery for them also largely depends upon common and jack mackerels.

The wide variation in amount of catch of sardine was accompanied by extensive geographic shift of the fishing and spawning grounds. When the co-operative investigation was commenced in 1949, it was urgent to identify the major causes of decrease of sardine catch among three general possibilities, drop of availability due to change of migratory routes, or decline of stock size due to either overfishing or environmental causes. With the aim of clarifying the mechanisms underlying the drastic changes of sardine population, Nakai (1962) analysed the data taken in the years up to 1960. Later Kondo (1980) and Watanabe (1981) added information obtained in the following years of the recovery.

Table 1.

Yields of major neritic-pelagic fishes in Japan, 1953-1980.

 HerringSardineRound-
herring
AnchovyShirasuJack
mackerel
ScadsMackerelsSauryCommon
flying
squids
other
squids
Total
1953275 175343 62352 503243 47620 433239 115..234 776253 661420 08647 8382 130 686
1954132 251245 64047 988304 03122 717250 073..297 393292 717398 74530 2442 021 799
195546 837211 34266 363391 52228 237238 372..244 402497 002383 41937 4702 144 966
195635 557206 30259 947346 70628 616246 416..266 197327 813299 48636 1131 853 153
195747 265212 23852 815430 21121 150281 56831 065275 329421 530364 36539 2512 176 787
195838 348136 65456 544417 28123 198282 12742 247268 442575 087354 22538 7702 232 923
195916 925119 58147 067356 23228 602409 61622 640294 543522 566480 66738 3292 336 768
196015 36778 10148 877349 17521 381551 60344 119351 149287 071480 66141 7362 269 240
196123 595127 04926 724366 93424 006510 73231 400337 785473 792383 99340 1942 346 204
196220 749108 00227 169349 45126 313449 01219 228408 725483 160536 47042 3482 520 627
196314 59755 86128 908320 60918 696441 17521 691464 886384 548590 64741 2312 382 849
196456 64316 24332 279295 89738 355496 45123 095495 664210 689238 29067 8681 971 474
196550 1699 21529 073405 90632 824526 88533 602668 574231 377396 90282 0712 466 598
196648 61713 46526 309407 66533 771477 08437 304642 423241 840382 89986 7072 380 084
196763 67216 80124 023365 24034 976327 87895 474687 474220 087477 012104 1002 416 737
196867 92024 37835 187357 66841 597311 37546 7171 015 279140 204668 36490 0652 798 754
196985 24020 56129 156376 80132 378282 81757 7711 011 40663 288478 16095 1732 532 751
197097 37416 76723 895365 47135 479215 56053 7511 301 91893 129412 24091 9372 707 485
1971100 48357 42947 126350 69240 657270 87044 6001 253 892190 288364 349102 7562 823 142
197262 19857 88348 767369 68850 819151 77842 0641 189 910196 615464 365119 9952 754 082
197382 658296 86440 422335 42558 251128 05454 6661 134 503406 445346 672116 2283 000 188
197476 273351 68446 164287 51639 308165 48950 2351 330 625135 462355 005117 7592 955 520
197566 617526 04743 936245 16447 052185 66949 6471 318 210221 573378 344137 0663 219 325
197666 0831 065 69252 476216 66460 150127 70478 963978 826105 419300 963169 9753 222 915
197719 8731 419 82644 829244 93342 00587 45798 3441 355 309253 465264 287227 9634 058 291
19786 7081 637 38050 573152 42841 19457 99295 1391 625 865360 213257 117243 8244 528 433
19796 8191 817 03449 461134 57755 31382 515101 3681 414 183277 960212 841301 8534 453 924
198011 1542 197 74438 416150 60655 19753 66491 3131 301 121187 155331 225344 9774 762 527
MEAN58 399406 76542 034319 21335 810282 10952 768791 100287 648393 636104 7802 766 724
S.D.53 104614 92211 88781 86412 242153 78526 451464 458138 714103 82883 164788 641
C.V.9115128263455505948267929

After Hayasi (unpublished).

Fig. 1.

Fig. 1. Spawning ground and migratory routes of sardine in eight periods between 1925 and 1978, After Watanable (1981).

Nakai (1962) noted expansion and shrinkage of distribution range of the fish according to the change of catch. Amount of Japan's catch of sardine increased from 410,000 tons in 1923 to 1,480,000 tons in 1933. Significant fisheries occurred in Korea in 1925, and Coast Range and Sakhaline in 1930. The total catch reached a peak of 2,700,000 tons in 1936 and 1937, of which Korean catch comprised about one million tons. The expansion of the fishing area indicates enlargement of the whole distribution range accompanied with the rise of stock size. One of the major features of the prosperous period of 1925 to 1940 is appearance of widely migrating group of the fish. In the period about 70 percent of sardine eggs were spawned in the Satsunan Area off southern tip of the Kyushu Island, although the spawning activities occurred widely in the waters south of central Honshu (Fig. 1). The eggs and larvae drifted northeastward along the Kuroshio Current, and spent the first year of life in the coastal waters along Shikoku and Honshu Islands. The I- and II-age fish in the second and third years of life lived in the Pacific waters along the northeastern Honshu. A high correlation coefficient between abundance of the immatures in the Pacific waters and abundance of the adults in the following years in the Japan Sea, as well as seasonal shift of the fishing grounds, indicates that the fish migrated to the Japan Sea through the Tsugaru Straits between Honshu and Hokkaido Islands in the autumn and winter of the third year of life. Thus the major distribution range expanded in the Pacific waters at the young and immature stages before and in II-age, and in the Japan Sea at adults stage of III-age and older. Considerable increase of the population size resulted in the delay of growth and maturation. The first maturity occurred at II-age of body length of about 18 cm in the prosperous years, while at almost the same size but at II-age in the following adverse years of and after 1948.

The Kuroshio Current began meandering in and after 1934, and left the coast, most extensively in the middle 1940's, reaching southward Lat. 30°N, and a large-scale cold water mass appeared in the course of drifting route of sardine eggs and larvae. The larval sardines were accumulated in the infertile Kuroshio waters west of the cold water mass (Fig. 2). Due to starvation, the larval stock suffered a mass mortality and the supply of recruitment was cut from the spawing ground to the nursery ground. Eventually the prolific wide migrating stock collapsed and the amount of total catch over the whole sardine area decreased drastically. After the curtailment of population, the spawning stock shrank to become too small to reach the Satsunan Area, and the spawning grounds shifted year after year (Fig. 1).

Fig. 2.

Fig. 2. Distribution of surface temperature and direction of the Kuroshio Current off western Japan in winter, 1933, 1941 and 1950. After Nakai (1962).

Nakai (1962) believed that the sardine may spawn in the Satsunan Area off the southern Kyushu when the stock size would become large enough to drive the adult fish to wide migrations, pursuing the food plankton, and then the fish may prosper if they could use the nursery grounds in the highly productive waters off the northeastern Honshu. The reduction of stock size and improvement of efficiency of fishing gear in the 1950's raised the fishing mortality of the sardine. Even though there was no evidence indicating any overexploitation of the population as a whole, it was recommended to prevent the general increase of fishing effort upon the species, and to adopt regulations of fishing in key areas. In 1951 to 1953, the purse seiners exploited significant amount of sardines off western Kyushu which might have seriously disturbed the southward migration to the spawning ground. Thus the fishing operations could have effected the stock size not only due to thinning but also by disturbing migration and spawning. In the 1950's, sardine fisheries became gradually more significant in the Pacific waters along Honshu, together with appearance of remarkable spawning grounds therein. An analysis of catch statistics and the egg census converted to the spawners' stock size in the eleven years from 1950 to 1960 indicated it necessary to avoid intensive exploitation of the dominant year-classes. This proposal was based on the fact that the catch of adults in number varied from 1.1 to 235.3 millions, while the egg abundance remained within a narrower range of 7.04 to 46.87 trillions; and then the rate of exploitation appeared to have sharply risen to about 30 percent when operations upon the dominant classes were very profitable, although it remained less than 5 percent in eight years among eleven. The fishing intensity for three dominant classes which occurred in 1951, 1956 and 1957, was too high to permit these stocks being abundant enough at the adult stage for reproduction of sufficient recruitment. The continuing tendency of increase of stock size was suddenly hampered by cooling of the water in the major spawning ground off the Boso Peninsula of central Honshu in January to May 1963. The anomaly of the temperature reached as much as 14°C in February and March. The drastic cooling caused retardation and southward shift of the spawning activities of sardines (Nakai et al. 1964).

In spite of further decline of the catch in 1963 to 1972 the stock appears to have been on the gradual increase in the central and southern waters off Honshu and Shikoku Islands. Kondo (1980) noted that the Kuroshio Current meandered for the eight years from 1963 to 1971, but flowed near Honshu Island in the spring of 1972 to 1974. The gradually increasing sardine stock had an opportunity to bring strong year-classes in the 1972 and 1974 spawning seasons. Watanabe (1981) explains the shrinkage and recovery of the population on the basis of temperature anomaly and egg abundance.

In the early 1950's a significant stock was found in the East China Sea and Japan Sea along the warm Tsushima Current. The amount of catch as well as eggs and larvae previously expanded there until 1951, but only in the waters west of Kyushu. The waters surrounding the Japanese Islands have been warmed since then, and the spawning grounds shifted northeastward along central Honshu, accompanying delay of spawning season. The northern shift of spawning grounds caused further shrinkage of the nursery grounds, and consequently a rapid curtailment of the stock itself (Fig. 3). Since the major distribution range moved to the Pacific waters in and after 1956, sardine stock increased gradually except the sudden interruption in and just after 1963, and spawning grounds have expanded in the Pacific waters off central Honshu in the early 1970's (Fig. 4). The survival rate in the very beginning stages of life before postlarva was found to be high for the 1972, 1974 and 1975 seasons. The 1972 year-class was a large portion of the stock and produce sufficient recruitment in the following years in spite of fairly concentrated fishing activities. The gradual increase of stock size resulted in the expansion of distribution range, and the spawning retrieved in the Satsunan Area since 1976. Along with this recovery of the historical dominant spawning grounds, the increase of catch from the northward migrants in the feeding phase off the southeastern Hokkaido denotes new expansion of the territory of the sardine population. The westward shift precipitated the spawning season, and then occurrence of the postlarval sardine coincided with the high abundance of copepods' nauplii, thus raising the survival rate of the fish at the early stages of life. Up to 1979, however, the stock size has never reattained the maximum level found in the preceding prosperous phase. The reproduction is still limited within localized sub-populations, and the wide migrants have not occurred yet. Konishi (ms) reported rapid expansion of spawning activities in the Satsunan Area from coastal waters to offshore waters even across the Kuroshio Current during six years between 1976 and 1981, implying that the distribution of eggs and larvae in the offshore water is usual in the expansion phase of the sardine population.

Fig. 3.

Fig. 3. The catch of sardine in Japan and neighbouring areas, and fishing and spawing grounds in relation to water temperature (W.T.) at a fixed station off Shioya-saki Cape, 1906-1975. Max, D, Min and I denote the maximum, decreasing minimum and increasing periods of catch. "P" and "J" in the entry of Fishing G denote the Pacific waters and Japan Sea with major fishing grounds given by lines. "Sa", "Ts", "B", "To" and "Iz" in the entry of Spawning G denote Satsunan, Tsushima Current, Boso Peninsula, Tosa Bay and Izu Islands areas, respectively, with major spawning grounds given by lines. "A"s under short lines for W.T. indicate duration of persistency of the A-type cold water mass that occurred at the position given in the index map. After Watanade (1981).

Fig. 4.

Fig. 4. Distribution of eggs of sardine in the Pacific water along Honshu, 1970-1974. After Watanabe (1981).

Sardine catch in Korea was less than 10,000 tons until 1975, but jumped up to 50,000 tons in 1977 and stayed at significant levels since then. USSR fleet landed the sardine in 1978 to 1980 at a high level of around 300,000 tons (FAO 1975, 1981). This includes the catch made in the waters around the Japanese Islands. All the information indicates recovery of the sardine population and expansion of the distribution ranges.

The other two species of iwashi have been quite stable in amount and geographic distribution of catch. Anchovies spawn in the coastal waters to offshore waters throughout the year in the southern waters. Such wide space-time range of reproduction could compensate a failure of recruitment in an area or in a season by that in the other areas or seasons (Hayasi 1962). It could be easily understood that shirasu fisheries aim at postlarvae of either sardine or anchovy whichever the species is abundant, and then that the amount of catch has been fairly stable.

Mackerels, jack mackerel and scads also show appreciable variation in amount of catch. Geographically the jack mackerel and scads were more abundant in the East China Sea and Japan Sea than in the Pacific neritic waters. The major fishing grounds of common mackerels shifted from year to year; most abundant in the East China Sea and western Japan Sea in 1953 to 1955 but in the Pacific waters along Honshu in 1956 to 1968. The catch rose in the East China Sea in 1969. For the 1970's products of mackerels are almost the same in the Pacific waters and in the East China Sea and Japan Sea. The catches started a rapid decline in the Pacific waters since 1980, but stayed high in the Tsushima Current area. The fluctuations in these species are not explained yet in spite of recent intensive national and international scientific effort. Although China, Korea, USSR and other states are producing significant amount of these scombroids, the Japanese share is stabilized at 70 to 80 percent (FAO 1975, 1981).

Pacific herring

Significant fisheries have existed for herring on both western and eastern sides of the northern North Pacific. The fishery had been dominatingly important in the northern Japan, and the Hokkaido Fisheries Experimental Station conducted extensive surveys of the species since its establishment in 1902 (Motoda and Hirano 1963).

The fishery was initiated in the 16th Century, and covered almost the entire coast of Hokkaido Island and northern Honshu Island in the 19th Century. The landing reached a peak of 975,000 tons in 1897, when a stock called "Hokkaido spring herring" had supported the fishery. The spring herring continuously declined in the 20th Century. The recent statistics indicate that the herring has been most abundantly taken in the USSR waters that produced 78 percent of the total catch in the northwestern Pacific Ocean. The USSR catch also decreased from 350,000 tons in 1970 to 57,828 tons in 1978 with slight recovery to 70,000 tons in the following two years (FAO 1975, 1981). The commercial exploitation of herrings by Japanese and Russian fisheries has been abstained in various parts or from various local stocks in the northwestern Pacific Ocean (Morita 1982).

Motoda and Hirano (1963) described ecological changes of the spring herring from 1907 to 1960 when the catch decreased continuously. During the prosperous years, herrings of this stock used to appear in the Japan Sea off Hokkaido in February and March. Then the fish migrated southward, and entered into the coastal waters for spawning. The spawning area shrunk northward, and the growth rate rose together with decline of catch. The age of first maturation decreased from three years to two years, and abnormal eggs were more frequently observed in the spawning beds than before. It is assumed that the spring herring stock had declined due to change of migratory routes of the spawners as a result of drastic and wide environmental variation in the northern Japan Sea and Okhotsk Sea, and even over the North Pacific Ocean. The most probable cause is the rise of water temperature off the entire coast of Kollaido since the years around 1932 to 1938, more evidently in and after 1955. The decline of stock size seemed to have resulted in the spring herring becoming a localized stock occurring year round in the waters along the west coast of Hokkaido.

Pacific saury

Amongst the wide distributions range over the northern Pacific Ocean, the fishing activities of sauries occur in the limited waters around the central Kurile, southeastern Hokkaido and northeastern Honshu. The operations are mostly concentrated in a short period of the year from September to November. Wide fluctuations in amount, geographical distribution and size composition of the available stocks required extensive studies to find out any means useful for obtaining a pre-season forecast.

Recently Fukushima (1979) investigated long series of catch and effort statistics, biological data including distribution and maturity, and oceanographic information obtained from 1950 to 1970, after the saury fishermen introduced the presently used stick-held dip net with attracting lamps, known as bouke ami. This type of fishing technique allowed rapid expansion of the fishing ground toward offshore waters, and rose the amount of catch drastically to a level of 287,648 tons in the annual average between 1953 and 1980, from a lower level of about 30,000 tons in the preceding drift net operations of 1906 to 1949. The fishing grounds of Japanese boats cover a range west of Long. 153°E, and south of Lat. 44°N. Russian fishermen operate the same gear in the northern area above Lat. 45°N (Sablin and Pavlychev 1981).

Sauries of the northwestern Pacific Ocean spawn in the Kuroshio Front and south along the Japanese Islands. The spawning activities last from October off eastern Honshu to the following May off Shikoku. Young and adult fish migrate through the waters off northeastern Honshu in April and May along branches of northwardly expanding Kuroshio Current, and either into the Oyashio Front in June, and then traverse the Oyashio Front and aggregate south of the Kurile Front. In early and middle July large-sized fish of 30 cm in modal length immigrate the northern part of distribution range, reaching as north as Lat. 47°N to Lat. 50°N. Low temperature and rapid change of day length in the northern limit accelerate sexual maturation of the fully grown fish, and drive them to the spawning grounds. Medium-and small-sized fish of 27 to 28 cm or less in modal length are sexually immature in the summer, and continue northward feeding migration until the lowered temperature drives the fish to southward migration in August. In early and middle September, major fishing grounds form off eastern Hokkaido and northern Honshu, north of Lat. 38°N over a projection of the Oyashio Intermediate Front. A warm water mass frequently occurs around southeastern Hokkaido in the month, and often alters position of the Intermediate Front, and then the fishing grounds (Fig. 5). The pronounced warm water mass drives sauries toward offshore waters. When the water mass is weak, good fishing sites occur in the nearshore waters (Fig. 6). In areas without the intermediate water the fish move quickly and are not easily attracted by the fishing lamps. Most fish leave the northeastern Honshu in the late November, and the stick-held dip net fisheries cease their operation. In the winter, the majority of fish enter into the waters around the Kuroshio Current and the Counter Current, north of the sub-tropical convergence.

Because the warm water mass off the southern Kurile Islands near the boundary between Japan and USSR determines the position of the best fishing grounds at the beginning of fishing season, the international proportion does not stay constant year after year, the Japanese catches having comprised 56 to 83 percent of total production (FAO 1975, 1981). Success of fisheries depends on the size of individual fish, abundance and distribution of the available stocks. There are three general size groups, large-sized fish around 30 cm in modal length, medium-sized fish of 26 to 28 cm, and small-sized fish of 24 to 25 cm. The relative abundance of these three groups varies remarkably. Off the northeastern Honshu the fishing grounds form near the coast in the years when the first branch of the Oyashio Current, the nearest flow from the coast, develops intensively, making the operations highly profitable with less time required for cruises of fishing boats. Thus the amount of catch raises in the years when both large-and medium-sized fish are available for fisheries and the nearshore first branch of the Oyashio Current develops (Table 2). Odate and Hayashi (1979) demonstrated correlations between the abundance of larvae and the amount of catch in nine years from 1969 to 1977. The relationship also provides a basis of the pre-season forecast.

Fig. 5.

Fig. 5. Schematic presentation of hydrographic conditions in the vicinity of the Polar Front, with zonal structures.
CN: Cold water mass off Nozima-zaki Cape at Lat. 34°54'N,
Long. 139°52'E.
WH: Warm water mass off Hokkaido Island.
Wl, W2. W'l, C1 and C2: Warm and cold water masses in the transition area.
I.-Cold Zone: Inner cold zone.
O.-Cold Zone: Outer cold zone.
After Kawai (1955).

Fig. 6.

Fig. 6. General longitude position of the major fishing ground of Pacific saury at the beginning of stick-held dip netting season given in bars and rectangles, and of the major flow of the Oyashio Current given in closed circles in the left Panel A, and the extent of area of the coatal first branch of the Oyashio Current given in number of latitude and longitude 20 minute squares indentified as such by temperature of 5°C or below at 100 m depth layer.
After Fukushima (1979).

Table 2. Amount of catch of Pacific saury in Japan related with position of northern tip of the Kuroshio Current, general features of the Oyashio Current, position of major fishing areas and dominant size group of individual fish in the commercial catch.

Northern- most path of the KuroshioTrend of the OyashioMajor fishing areaDominant size in catchAmount of catch (Typical years)
38°00'NThe 1st branch developsCoastal
Long. 142°E-145°E
Medium-sized fish
(27-28cm)
200 000
(1950-1953)
37°30'NThe 1st and 2nd bran- ches developCoastal and nearshore
Long. 142°E-146°E
Bimode of large and medium-sized fish (30cm and 27-28cm)400 000
(1955-1959)
37°00'NThe 1st branch is weak, and the 2nd branch developsOffshore
Long. 144°E-148°E
The same as above but some change in the medium-sized fish (30 cm and 26-27cm)300 000
(1960-1963)
36°30'NThe 1st branch is weak, and the 2nd and 3rd braches developOffshore
Long. 145°E-149°E
Large in even years or medium in odd years for 1964-1967, or large in odd years or medium in even years for 1972- 1975200,000
(1964-1967) = 1972-1975)
36°00'NThe 1st and 2nd bran- ches are weak, and the 3rd branch developsOffshore
Long. 145°E-151°E
Principally small fish
(24-25cm)
100 000
(1968-1971)

After Fukushima (1979).

Flying squids

Okutani (1977) reviewed papers dealing with cephalopods exploited in Japan. The study of stock assessment has been concentrated to the most prolific common flying squid, of which amount of catch showed fluctuations at interval of nine years. The average catch for each interval has shown a gradual decrease. Kitano (1979) noticed that years of low catch, i.e., 1910, 1919, 1928, 1937, 1946, 1955, 1964 and 1973, for 78 years appeared exactly one year after a minimum temperature in 1909, 1918, 1927, 1936, 1945, 1954, 1963, and 1972. He also proposed a minor periodicity with another interval of 4.5 years. The recent statistics indicate the minimum in the 1950's occurred in 1956 but not in 1955. The discrepancy, however, may not seriously affect his conclusion.

After decline of the common flying squid, Japanese fishermen have exploited several other neritic squids. Of these species, Ommastrephes bartrami (Leuseur) or aka-ika in Japanese language, is the most successfully exploited and the amount of production reached 17,000 tons in 1974, and 188,000 tons in 1980. Murata and Shimadzu (1982) tried to evaluate the stocks off the southern Hokkaido and estimated the rate of exploitation exceeding 30 percent. Even though the estimates are not yet fully reliable, the gradual decline of catch per unit of effort and diminishing of size of individual squids appear supporting an inference that the fishery might have already asserted serious effects upon the stocks.


Previous Page Top of Page Next Page