PART I (Continued)

CONCLUSIONS

It is widely accepted that the pelagic species are very vulnerable to the intensive exploitation in various parts of the world (Saville 1980). At the same time the environmental anomaly has been known inserting remarkable influence upon these stocks. In the northwestern Pacific Ocean there occurred considerable changes of the abundance and distribution of major pelagic stocks due to environmental anomalies. In such cases it is required to regulate the fishing activities for assisting the recovery of the stocks. Because many fisheries in the area depend on various neritic-pelagic resources of more than one species, it is essential to identify key areas for assuring the effective implementation of fishery regulation for the problem species without hampering exploitation of the other fishes. The identification of key areas requires a thorough knowledge of ecology of the species populations in question.

Unfortunately the existing knowledge is quite insufficient for any species in the northwestern Pacific Ocean. Only the drastic decrease of sardines in the 1940's is fairly well documented, and the recent recovery appears to support the previously proposed hypothesis about fluctuation among the species. The intensive investigations having covered both the commercial catch and distribution of the fish in the sea, especially at the egg and larva stages, since the rise of the sardine stock in the 1930's have efficiently contributed to the advance of biological understanding. Furthermore, the ecological features of sardines make the investigation for this species easier than for the other species. The sardines live and spawn in the limited coastal waters delineated by two distinct warm currents, the Kuroshio Current in the Pacific waters and the Tsushima Current in the East China Sea and Japan Sea, both acting as natural barriers of many neritic-pelagic fishes. The strong demands for these prolific stocks stimulated development of various types of fishing gears which exploit the fish at different stages of life.

As mentioned, the sardine population showed obvious ecological variation in response to changes of the environment and also of the population itself. The increase of population size caused shortage of food organisms and some other necessities as shown by retardation of growth and maturation. In order to adapt such shortage the sardine population expanded their distribution range, and eventually the widely migrating sub-population appeared, the distribution range of which was delineated in the Satsunan Area, which is located near the dividing point of the Tsushima Current from the major axis of the Kuroshio Current. Once the fish spawn in the Satsunan Area, the eggs and larvae are transported to the Pacific waters along Shikoku and Honshu Islands. Off the north-eastern Honshu and southeastern Hokkaido the cold Oyashio Current forms a highly productive mixing zone with the Kuroshio Current, which is characterized by high standing crop of zooplankton (Odate 1980).

It is not well understood yet why the adults moved from the Pacific waters to the Japan Sea through the Tsugaru Strait in the past prosperous years. Although the adults originated in the Japan Sea emmigrated into the Pacific waters in 1956 and 1957 of the adverse period (Hayasi 1960), remarkable migration of adults toward the Japan Sea did not occur yet in the recent years of increasing stock size. It is still uncertain whether this indicates that the present stock size has yet to recover to the previous level or that the exceedingly large amount of catch was achieved by the recent intensified fishing activities. The wide migrants in the past prosperous years might have supported the high production of the species as far as the environmental conditions assure the shift of habitats during the life history. The stock can utilize the warm spawning grounds which accelerate development of the fish at the very beginning stages of life and the highly productive nursery grounds in the mixing zones which provide sufficient food stuff for the enlarged stocks. But the separation of spawning and nursery grounds makes the population vulnerable to environmental changes, especially the significant meandering of the Kuroshio Current in the spawning seasons.

Occurrence of wide migrants was also known in the Pacific herring in the prosperous years. Such ecological changes could exist in common mackerels which have also shown remarkable year-to-year changes of the major fishing and spawning grounds together with variation in amounts of catches. In order to clarify the mechanisms underlying such ecological changes, it is necessary to conduct intensive, direct observations of the fish populations and environmental factors in addition to collection of the relevant data from the commercial fisheries.

Fairly regular cyclic changes were noted in the amount of catch of Pacific saury and common flying squid. They are short-lived species with life span of at most two years. The aforementioned biological data by the direct observations are required for understanding of the periodicity of stock size beyond statistical description. At present, however, there exist gaps between the data requirement and the execution of surveys. The wide expansion of spawning grounds beyond the Kuroshio Current makes it difficult to identify and evaluate quantitatively the spawning activities of the Pacific saury. It is expected that technical development could provide useful means to overcome such difficulties soon in the biological surveys. In the intensively fished areas, it is essential to predict the change of stock size due to environmental causes for the actual benefit to fishing operations. Moreover, such prediction is necessary for achieving rational utilization because similar fishing intensity can deplete seriously "naturally" reduced stocks, even if very practical for "healthy" stocks.

There are two general patterns of fluctuation in major pelagic resources. One is represented by the sardine that showed a wide variation in amount of catch for long-term ranging between 9,215 tons in 1965 and 2,197,744 tons in 1980 and the ratio of the maximum catch to the minimum reached 238:1, but the figures in the successive years do not differ much and do show monotonous decrease or increase. The Pacific saury is an example of the other. The catch ranged within 63,288 tons in 1969 and 575,087 tons in 1958 with the maximum/minimum ratio of 9:1. But the catch often showed fairly large changes year after year. The most typical case is a sudden rise from 196,615 tons in 1972 to 406,445 tons in 1973, and a fall to 135,462 tons in 1974. Sardines enter into fisheries during at least four years from the last half of their first year of life to the fourth year, while sauries live only one or two years. The total stock size depends upon the strength of a single year-class in the saury fisheries, but variations of individual year-classes are "smoothed" away in the sardine fisheries. Instead, the extensive variation ecology occurs together with drastic change of abundance in the sardine population, while a more stable life pattern is retained in the saury population. Most of long-lived species such as herring and common mackerels showed extensive change of fishing grounds as well as amount of catch for long periods of years, but year-to-year change is rather small and predictable on the basis of age composition of catch. Anchovies and flying squids are other examples of short-lived species which vary remarkably on short-term but stays at fairly stable levels over the long-term.

REFERENCES

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Hayasi, S. 1962. Fishery biology of the Japanese anchovy, Engraulis japonica (Houttuyn). Bull.Tokai Reg.Fish.Res.Lab. (31):145-268.

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Kondo, K. 1980. The recovery of the Japanese sardine - the biological basis of stock size fluctuation. Rapp.Proc.-Verb.Reun.Cons.int.Explor.Mer, 177: 332-354.

Konishi, Y. 1983. Distribution of eggs and larvae of sardine in the Satsunan area, 1976-1981. Bull.Nansei Reg.Fish.Res.Lab.(15): (In Japanese with English summary).

Marr, J.C. 1968. Report of the Symposium on the Cooperative Study of the Kuroshio and Adjacent Regions (CSK), FAO Fish.Rep.63:57 p.

Morita, S. 1982. Fishery biological study on the herring stocks in the northwest Pacific Ocean - (1). Korfo-Karaginsk herring. Bull.Hokkaido Reg.Fish.Res.Lab. (47):57-92 (In Japanese with English summary).

Motoda, S. and Y. Hirano. 1963. Review of Japanese herring investigations. Rapp.Proc.-Verb. Reun.Cons.int.Explor. Mer. 154:249-261.

Murata, M. and Y. Shimadzu. 1982. On some population parameters of flying squid, Ommastrephes bartrami (Lesueur), in the northwest Pacific. Bull.Hokkaido Reg.Fish.Res.Lab. (47):1-10 (In Japanese with English summary).

Nagaski, F. 1973. Long-term and short-term fluctuations in the catches of coastal pelagic fisheries around Japan. J.Fish.Res.Bd.Canada 30(12):2361-2367.

Nakai, Z. 1962. Studies relevant to mechanisms underlying the fluctuation in the catch of the Japanese sardine, Sardinops melanosticta (Temminck and Schlegel). Japan J.Ichth.9(1-6):1-115.

Nakai, Z., S. Hattori, K. Honjo, T. Watanabe, T. Kidachi, T. Okutani, H. Suzuki, S. Hayasi, M. Hayaishi, K. Kondo and S. Usami. 1964. Preliminary report on marine biological anomalies on the Pacific coast of Japan in early months of 1963, with reference to oceanographic conditions. Bull. Tokai Reg.Fish.Res.Lab. (38): 57-75.

Nakai, Z., S. Usami, S. Hattori, K. Honjo and S. Hayasi. 1955. Progress report of the cooperative iwashi resources investigation, April 1949 - December 1951: 116 p. Tokai Regional Fisheries Research Laboratory, Tokyo.

Odate, K. 1980. Kaiyo plankton/Tohoku-kaiku no kongo sui'iki (Marine plankton in the mixing waters off the Tohoku Region). Kaiyokagaku (Marine Sciences)/Symposium 131:634-645. Kaiyo Shuppan KK, Tokyo (In Japanese without English summary. The Japanese title is underlined with the English translation in parentheses).

Odate, S. and K. Hayashi. 1979. Possible role of the Kuroshio Current in determining distribution of saury in the northwestern Pacific. The Kuroshio IV:849-863. Saikon Publ., Tokyo.

Okutani, T. 1977. Stock assessment of cephalopod resources fished by Japan. FAO Fish.Tech.Paper (173): 7 p. 62 p.

Research Division of Fisheries Agency. 1980. Gyogyo ni kansuru Nisso Kagakugijutsu Kyuryoku Kyotei ni motozuku Dai 13-kai Sanma, Saba oyobi Maiwashi Kyodo Kenkyu Kaigi Keika Hokoku (Progress report of the 13th cooperative research conference on saury, mackerels and sardine based upon the Japan-USSR Agreement on scientific and technical cooperation in fisheries). 215 p. Fisheries Agency, Tokyo (In Japanese without English summary. The Japanese title is underlined with the English translation in parentheses).

Sablin, V.V. and V.P. Pavlychev. 1981. Dependence of migration and catch of Pacific saury upon thermal conditions. Bull.Tohoku Reg.Fish.Res.Lab. (44):109-117.

Saville, A. 1980. Discussion and conclusions of the symposium. Rapp. P.-v.Reun.Cons.int.Explor. Mer. 177:513-517.

Tanaka, S. (This volume). Variation of pelagic fish stocks in waters around Japan. Watanabe, T. 1981. Survival of Japanese sardine at early stages of life. Izvestija TINRO (105):92-107 i (In Russian with English summary).

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

STOCK ASSESSMENT OF COMMON MACKEREL AND JAPANESE SARDINE
ALONG THE PACIFIC COAST OF JAPAN BY SPAWNING SURVEY

by

Taisuke Watanabe

Tokai Regional Fisheries Research Laboratory
5-5-1 Kachidoki, Chou-ku
Tokyo, Japan

Resumen

En relación con las amplias fluctuaciones de la abundancia de recursos pelágicos-neríticos alrededor de Japón, cabe mencionar que desde 1949 se vienen realizando a nivel nacional reconocimientos de huevos y larvas. Se presenta el método de evaluación del recurso por reconocimientos de huevos y larvas que se aplica actualmente en Japón, así como los resultados de obtenidos para dos principales stocks de peces pelágicos-neríticos de la costa del Pacífico de Japón, la caballa común y la sardina Japonesa.

Primero se estimó el número total de huevos desovados en el mar a partir de los datos de abundancia de huevos colectados por muestreo. A partir del número total de huevos se estima el número total de peces desovantes. lo cual se hace con la ayuda de información biológica y datos de la captura. Si existe información de la composición por edades (y de la captura comercial) se puede estimar tanto la abundancia del stock desovante como de los juveniles. Del examen de la relación entre el stock desovante y el reclutamiento se puede estimar la magnitud de desovantes que tienden a mantener la abundancia del stock a un nivel adecuado.

Caballa Común

Hay un sólo stock de caballa común en la región, la sub-población del Pacífico. La magnitud del stock desovante tuvo un nivel extremadamente bajo hasta mediados de los años 1950s (alrededor de 30 × 10¹² huevos desovados) y de ahí comenzó a aumentar marcadamente, con dos máximos entre 1962-64 (650- 780 × 10¹²) y en 1974-76 (1 × 10¹³). Luego mostró una tendencia declinante con valores alrededor de 270 × 10¹² en sus años recientes.

Las nuevas clases anuales que fueron sumamente débiles en 1948-52, también mostraron un continuo incremento en los años sucesivos, alcanzando dos máximos en 1961 y 1972. Estos declinaron a mediados de los años 60 y mediados de los años 70. Se asume que la disminución en la abundancia del stock fue mayormente el resultado de la marcada reducción del reclutamiento. Sin embargo, es probable que la pesca haya acelerado esta disminución, especialmente por la captura de peces juveniles (edad 2) de clases anuales débiles mediante el uso de artes de pesca no selectiva como la red de cerco.

Durante los dos períodos en que se ha observado un incremento de la abundancia del stock, se ha observado una función dependiente de la densidad en la curva stock-reclutamiento a la cual se ajustan dos diferentes curvas del tipo de Ricker. Esto sugiere que el marcado incremento de la abundancia de las nuevas generaciones, que eran mucho más abundantes que las del stock paterno, haya resultado en un rápido incremento del stock total durante el período de expansión. Sin embargo, la abundancia de reclutas disminuyó marcadamente cuando la población alcanzó un nivel extraordinariamente alto. Después de una serie de reclutamientos bastante bajos el stock total disminuyó drasticamente.

De las curvas de reproducción obtenidas se estima que la magnitud de desovantes (expresada en número de huevos producidos) es de 400 × 10¹², y que el número mínimo requerido para preservar el stock es 200 × 10¹². A juzgar por la información disponible se espera que el stock comience a aumentar en un futuro cercano. Sin embargo, si la pesca comercial causa una reducción masiva de los peces juveniles es probable que la abundancia del stock decline más todavía.

Sardina Japonesa

Hay dos stocks de sardina Japonesa a lo largo de las costas del Pacífico de Japón. La sub-población en la región norte y centro y la sub-población de Ashizuri más al sur.

La magnitud del desove en la sub-población del Pacífico se ha mantenido a un nivel bajo (10-20 × 10¹² huevos producidos) hasta inicios de la década de los años 70. A inicios de los años 60 se produjo un aumento temporáneo (hasta 200 × 10¹²). El desove aumentó desde 1972 con un pico máximo de 580 × 10¹² en 1978, de donde se generó el explosivo aumento de la población en la segunda mitad de la década de los años 70. El stock disminuyó nuevamente, y en años recientes la magnitud del desove se encuentra alrededor de los 150 × 10¹² huevos producidos.

En este caso también se observa una función dependiente de la densidad durante el período de incremento, a la cual una curva del tipo de Ricker se ajusta bien. Se estimó que el nivel de desove deseable para mantener el stock es de 150-250 × 10¹².

El área de desove de la sub-población de Ashizori se encuenta más al sur, aguas arriba en la corriente de Kuroshio. La magnitud del desove de este stock también aumentó marcadamente desde 1978 y sobrepasó en abundancia a la sub-población del Pacífico en años recientes. La sub-población de Ashizuri alcanzó su máximo en 1982, con un desove de alrededor 360 × 10¹² huevos. Aparentemente una cantidad significativa de larvas de esta sub-población habrían sido transportados por la corriente de Kuroshio y se habrían reclutado a la sub-población del Pacífico.

Se asume que el stock de sardina a lo largo de la costa del Pacífico extiende su área de reproducción hacia el sur durante períodos de próspero crecimiento. La magnitud combinada de estas dos sub-poblaciones parece haberse mantenido a niveles bastantes altos en años recientes.

Relaciones Caballa - Sardina

Los procesos que han resultado en cambios en la abundancia parecen ser bastante análogos en ambas especies, lo que puede ser debido a las semejanzas en la forma de vida y el hábitat de estas dos especies. Esto sugiere que ambos stocks han mantenido un potencial intrínseco para aumentar explosivamente aún en los períodos de depresión. Con esto los stocks pueden fácilmente recuperarse cuando el ambiente cambia y se dan las condiciones favorables para la sobrevivencia de los primeros estadios de la vida.

Un aspecto característico observado en el stock de sardina fue la expansión del área de desove corriente arriba de la corriente de Kuroshio. Esto puede haber permitido que el stock realice una biomasa extraordinariamente alta, excediendo el factor espacial limitante que habría sido impuesto en una sóla sub-población. Con el stock de caballa no se ha observado esta deformación estructural. Sin embargo, ambas especies han ampliado su hábitat, ampliando su distribución al máximo posible, durante los períodos de prósperidad.

Por otro lado, a niveles extremamente altos de la abundancia se presenta el efecto negativo dependiente de la densidad, lo que puede dar por resultado una menor variabilidad de huevos y larvas y la eventual disminución del reclutamiento. El stock puede disminuir y disminuirá nuevamente si se presentan condiciones ambientales desfavorables.

Se concluye que los stocks de caballa y sardina en la costa del Pacífico de Japón tienen requerimientos ambientales similares, y que aún cuando las condiciones favorables y desfavorables se dan alternativamente a intervalos de varios años en la misma región, cada especie tiene su propia dinámica.

INTRODUCTION

The major neritic migratory pelagic fish resources around Japan such as common mackerel, Japanese sardine, anchovy, jack mackerel, herring and saury have repeatedly shown long-term large changes in abundance. Dramatic changes in dominance have also occurred among these species. Clear explanations of these phenomena have not been made so far. However, it has been generally accepted that changes in the magnitudes of recruitment in connection with changes in environmental conditions have played an important role though reductions by commercial fishing might have affected them in some cases, especially during the declining period of the stocks. Spawning surveys are among the most effective ways of clarifying phenomena occurring among the reproductive segments of fish life histories.

These species spawn floating pelagic eggs when the adults aggregate on spawning grounds during their spawning seasons. The eggs spawned and the hatched larvae drift on the surface or in the near-surface layer of the sea for a certain period. They are gradually dispersed over wide areas of the sea, dependent on the movement of the water masses, and grow during the drifting period. Japanese flying squid though has a different life history form from those of neritic pelagic fish, but also experiences the same patterns during the egg and larval stages. Taking advantage of the distribution patterns of eggs and larvae, as mentioned above, a quantitative survey of their abundance can be made by a sampling method at suitably designed survey stations using standardized methods and plankton/larva nets.

A nation-wide spawning survey network for the above-mentioned purpose was established in 1949, and intensive observations have been carried out since then on the distribution and abundance of eggs and larvae, physical and biological environmental conditions at each of the survey stations. The major outcomes expected from the survey are: (1) details on the life history form and the abundances of stocks before commercial fishing, (2) information on the major factors involved in the annual change in recruitment and, accordingly, predictions of the magnitudes of recruitment to exploitable stocks in the forthcoming fishing season, (3) the assessment of the stocks concerned through qualitative analysis of data collected by the survey and by the existing fisheries, (4) the estimates of the total amount of allowable catches to be imposed on the commercial fisheries with the aim to maintain the stock abundances at a favourable level in terms of future reproduction and (5) the prediction of the long-term trends on changes in stock abundance.

The author presents, in this paper, the techniques which are currently employed in the spawning survey in Japan and the results of the application to the stock assessment of the two major neritic pelagic fish stocks along the Pacific coast of Japan - common mackerel and Japanese sardine.

The author is grateful to Drs. S. Ito and T. Kawakami, Tokai Regional Fisheries Research Laboratory for their participation in the discussion and for their critical comments. The author also wishes to thank Dr. S. Chikuni, Fisheries Department, FAO and Dr. T. Kidachi, Tokai Regional Fisheries Research Laboratory for their assistance in preparing this paper and for their comments.

TECHNIQUES EMPLOYED

The principles applied in the spawning survey are rather simple and clear - sampling methods and back-calculations - and those techniques have already been well established (Tanaka 1955a, 1955b, Nakai 1962, Nakai et al., 1955, 1962, Watanabe 1970, 1972).

Abundance of eggs spawned

Spawning surveys in Japan are being carried out by a standardized method (a perpendicular tow from 150 m deep to the surface of the sea) with a standardized plankton net (60 cm or 45 cm opening diameter with 0.33 mm mesh size) to maintain a consistency of the data basis. The designation of the survey stations is carefully made in order to fully cover the areas concerned and has become fixed over the years. The timing and the duration of the survey in each year are also designed to fully cover the spawning season of each species while the most intensive observations are usually concentrated in the spawning grounds during the spawning season of each species. An example of the data collected by the spawning survey for common mackerel along the Pacific coast of Japan is shown in Figure 1.

The total number of eggs spawned in the area concerned during the entire spawning season is firstly estimated from the observed egg abundance (density) of the sampling survey. If the sea-area is stratified into two strata (medium: i, smallest : j), which is usually subdivided by geographic and oceanographic conditions of the area in conjunction with the location and extent of the spawning ground in the area, and if the egg abundance (E_j : average number of eggs observed per 1 m² by month) is obtained in an A_j m² area in the j-stratum, the apparent total number of eggs to be observed in a unit area in the j-stratum in the month is given by

The total number of eggs spawned in an i-stratum area in a month (E_i) is then estimated by adjusting the above estimate with the escapement from egg by hatching and the loss by natural mortality during the egg stage as

where      S: survival rate during the egg stage
D: number of days in a given month
d_i: average number of days required for hatching in a given i-stratum area

The total number of eggs spawned in the entire area concerned in a month (E) is accordingly estimated to be

and the variance of the estimate (V(E)) to be

where v_j : variance of E_j in a j-stratum area.

Parental stock abundance and parameters

If the age-composition of the commercial catch is available to represent the age-structure of the stock, the total number of eggs spawned in a given year (E) can be divided into the numbers to have been spawned by the parental (adult) fish by age (i) with the aid of the biological information as

where     NE_i : number of eggs spawned by i-age fish
c_i : ratio of the number of i-age fish in the total adult fish catch
n_i : number of eggs to be spawned by a single female of i-age fish.

Figure 1

An example of the data on the spawing survey of the common mackerel stock along the Pacific coast of Japan. The numerals in each square indicate the number of eggs collected in 10¹² (upper) and the number of survey stations allocated in the square (lower). The survey was carried out in April 1980.

The number of adult (parental) fish by age (NA_i) is then estimated taking the sex ratio of female (r) into account as

and the total adult stock number (NA) as

Similarly, the adult stock biomass by age (PA_i) and the total biomass (PA) are

where w_i : bodyweight of an i-age fish.

If the total number of the adult fish in the total catch (CA) taken by commercial fisheries is known, the catch rate of adult stock (CRA) is given by

where     CA : total number of adult fish caught by commercial fisheries
FA : fishing mortality coefficient of adult stock
MA : natural mortality coefficient of adult stock
SA : survival rate of adult stock.

If the age-composition of the adult stock (NA_i) is obtained for the age i to j for the successive two years (years t and t + 1), the survival rate of adult stock (SA) can be estimated by the following equation

SA = (NA_{i+1, t+1} + NA_{i+2, t+1}+.NA_{j, t+1})/(NA_{i, t} + .NA_{j-1, t})

The total mortality coefficient (ZA = FA + MA) is then estimated by

SA = e^-ZA

and accordingly, the fishing and natural mortality coefficients (FA and MA) are estimated by equation (6).

Juvenile stock abundance

If the age-composition of the total catch of juvenile fish taken by the commercial fisheries (CA_i-1 CA_i-2 .) is available, the abundance of juvenile fish at given ages (i-1, i-2, ) can be estimated by a series of back-calculations. That is, if the natural mortality coefficient (M) during the juvenile stage is assumed to be the same as that in the adult stage (MA) which has been given by equation (6), the fishing mortality coefficient during the oldest juvenile age (F_i-1) is estimated by the equation below as

where NA_i : number of the youngest adult fish in the successive year,

and subsequently, the total mortality coefficient and the survival rate during the same age (Z_i-1 = F_i-1 + M and S_i-1 = e^-Zi-1). The stock number of the juvenile fish, which is one age younger than the youngest adult fish (NJ_i-1) is then estimated as

In the case that the survival rate (S_i-1) is obtainable from the age-composition data of the catch, the stock number could be estimated directly by equation (8). The stock number for the retroactive ages (i-2, i-3, ) could be estimated through similar back-calculations.

Total allowable catch

The favourable magnitude of spawning is first estimated, with the aim to sustain the stock abundance at a favourable level in terms of future reproduction. It has been ascertained for the major neritic pelagic stocks around Japan that a density-dependent relationship is observed between the number of eggs spawned by a parental stock and that of the brood, which fit the Ricker-type reproductive curve well. The cumulative number of eggs spawned in a brood during its adult stage (P) is therefore expected to reach a peak at a certain level of the magnitude of parental spawning (NE_t : number of eggs spawned by the parental stock in year-t). That is

where, A and B are constants. The abscissa value of the peak of the brood against the parental spawning would give the most favourable level of spawning by the parental stock. The abscissa where the brood turns to a sharp decrease in accordance with the decrease in parental spawning may give the minimum or safe spawning level required to prevent deterioration of the stock.

If the desirable or required level of spawning in the next year (t + 1) is provisionally given as NE_t+1 (number of eggs to be spawned) the desirable/required survival rate in the year-t would be given in terms of the number of eggs, namely, the survival of anticipated eggs over the potential spawning by the provisional adult stock (one-age younger fishes in year-t), by the following equation as

where

NA_i-1,t = NJ_i-1,t: The number of the provisionsal youngest adult fish in the next year (t + 1) at age-i

n_i : the number of eggs to be spawned by a single adult female of age-i

r : sex ratio of female

The expected total number of adult fish in the next year (NA_t+1) which should correspond with the anticipated spawning (NE_t+1) is therefore

Assuming here that natural mortality coefficient (M) is constant regardless of the age of fish and is known, the fishing mortality for the year-t (F_t) is estimated by the survival rate obtained (S_t), and accordingly the catch rate of the adult fish in the same year (CRA_t), as

The total allowable catch of adult fish in the year-t (TAC_t), which corresponds with the desirable/required spawning in the next year (NE_t+1) is therefore to be

Figure 2

Annual change in the estimated total number of eggs spawned and the nominal catch of the common mackerel stock along the Pacific coast of Japan during 1951-81 (Watanabe, 1977, and supplement).

STOCK ASSESSMENT

The assessment of the two major neritic pelagic stocks along the Pacific coast of Japan, common mackerel and Japanese sardine, has been made being based on the data collected by the spawning survey.

Common mackerel

Common mackerel in the region comprises a single stock, the Pacific sub-population. The catch from the stock began to increase in the early 1950's from a low level at about 70-80,000 tons and reached a peak at about 0.95-0.98 million tons during the early 1970's (Figure 2 and Table 1). After a sharp decline down to about 0.6 million tons in the mid-1970's, it increased markedly again reaching the highest with about 1.3 million tons in 1978. The catch shows a declining trend thereafter and was about 0.5 million tons in 1981.

The magnitude of the spawninig of the stock increased markedly around the late 1950's from an extremely low level at about 30 × 10¹² in the total number of eggs spawned and reached a peak with 650-780 × 10¹² during 1962-66 (Figure 2 and Table 1). After a temporary but sharp decline down to 160-190 × 10¹² around 1970, it increased again quite remarkably and reached its maximum of 960-1,020 × 10¹² during the mid-1970's. It shows a declining trend thereafter and was about the 270 × 10¹² level in 1981.

NA: Data not available

The year-class strength of spawners (ages 3-6), which had been extremely weak during the 1948- 55 year-classes, also showed a steady gain in successive year-classes and reached a peak in 1961 (Figures 3 and Table 2). It declined once during the later half of the 1960's but recovered soon after with a further rapid increase reaching the highest in 1972. The abundance of spawners in recent years, however, shows a sharp declining trend.

Figure 3

Change in the number of adult fish by age in each of the year-classes of the common mackerel stock along the Pacific coast of Japan for the year-classes 1948-78. The number of fish was estimated from the observed egg abundance. The numbers at age IV and above are shown with the accumulated numbers including those of all the younger ages.

The stock number of the juvenile fish at age-2 was estimated from the catch data (number of fish caught at age-2) and the estimated stock number of adult fish at age-3 for the year-classes 1961 -78. A comparison among the estimated stock number, catch in number and fishing mortality coefficient at age-2 was made (Figure 4 and Table 3). It is interesting to see that the fishing mortality coefficient tends to increase when the stock number decreases. Another notable thing found in the comparison is that the catch and fishing mortality coefficient of the age-2 fish decreased drastically during the early 1970's in contrast to the sharp increase in the stock number. This might have been caused mostly by the low availability of the fish in the northern coastal fishing grounds where a substantial amount of age-2 fish had usually been taken by purse-seiners during the feeding migration of the fish to the north. It is supposed that the fish might have migrated to further off-shore areas during the above-mentioned period probably in connection with the burst of the sardine stock in the coastal areas where intensive fishing on sardine by purse-seiners was carried out.

The relationship between the estimated total number of eggs spawned by the parental stock in each year and that spawned by each of the successive broods during their adult stage (ages 3-5) shows a density-dependent function which fits the Ricker-type reproduction curve well (Figure 5 and Table 4). It is noted that the two different curves are fitted to the two different growing periods of spawning magnitude, i.e. 1951-64 and 1969-74 year-classes respectively. The spawning of broods declined far below the reproductive curves during the imtermediate year-classes (1965-68) of the above-mentioned two year-class groups. Both the curves obtained show that the highest spawning by a brood is expected at about the 400 × 10¹² egg level of parental spawning. The change in the parental spawning during 1969-71 and the reproductive relationship afterwards suggest that even if the spawning of parental stock declined substantially (down to 160-190 × 10¹² eggs), the spawning by broods could soon recover if the environmental conditions remain favourable for a successive few years. It is assumed, however, that a further decline in the parental spawning might be deteriorative as brood spawning decreases precipitously. It is estimated, in this connection, that the 200 × 10¹² level of parental spawning would be the minimum level which would ensure that stock abundance would be kept at a safe level.

The commercial catches during 1977-78 (0.95-1.27 million tons) were well balanced with the estimated annual allowable catch for the period (1.0-1.3 million tons) to maintain the spawning magnitude at a favourable level (400 × 10¹² eggs). However, this level was exceeded in 1979 with 1.1 million tons catch over the allowable catch of about 0.65-0.85 million tons for the year. The allowable catch has become even smaller since 1980 in conjunction with the decline in the spawning magnitude of parental stock. The catch though has also been declining substantially since 1980; catches in 1981 (0.45 million tons) again exceeded the estimated allowable catch (0.27-0.36 million tons) required to maintain spawning at a safe level (200 × 10¹²). However, the parental spawning during 1980-81 was still higher than the above-mentioned level with 390-270 × 10¹² eggs though it showed a declining trend (Figure 2 and Table 1) and juvenile fish of the 1980 year-class appeared to be abundantly distributed in many nursery grounds in the region. Provisional information on the result of the spawning survey carried out in 1982 also indicated a fairly high abundance of larvae in the region. It is expected, therefore, that the stock will shift from the current low level to another expanding phase in the near future, being supported by the strong year-classes of 1980 and 82. However, if massive reductions in juvenile fish stocks are made by commercial fishing with non-selective gears such as purse-seine, stock abundance could decline still further.

Figure 4

Change in the estimated stock number (N), commercial catch in number (C) and fishing mortality coefficient (F) at age-II of the common mackerel stock along the Pacific coast of Japan for the year-classes 1961-78. The natural mortality coefficient (M) was assumed to be 0.5 throughout the period (Watanabe, 1977, and supplement).

Figure 5

Reproductive relationship between the estimated total number of eggs spawned by the parental stock in each year (E) and that spawned by each of the broods during their adult stage, ages 3-5 (P) in the common mackerel stock along the Pacific coast of Japan. Numerals in the figure denote the year-classes (Watanabe 1977, and supplement).

Japanese sardine

Japanese sardine along the Pacific coast comprises two stocks, the Pacific sub-population in the central and northern areas and the Ashizuri sub-population further south (Figure 6). It has been well known that the sardine stocks around Japan deforms their distribution pattern flexibly in accordance with the change in stock abundance and environmental conditions (Ito, 1961, Nakai, 1960, 1962, Watanabe, 1982).

The commercial catch from the Pacific sub-population, which had remained at an extremely low level during the 1950's with 10-20,000 tons of annual catch, further declined drastically in the mid-1960's after a temporary increase during 1961-62 up to 100-200,000 tons and reached its lowest levels in 1966 with only 3,000 tons catch (Figure 7 and Table 5). The catch, which showed a gradual recovery afterwards, increased again remarkably since 1973 and in 1977 reached 1 million tons. It has sustained a continuous increase thereafter and exceeded 2 million tons in 1981.

Figure 6

Spawning ground of the two major sub-populations, the "Pacific" and "Ashizuri", of Japanese sardine along the Pacific coast of Japan. Shadowed areas indicate the concentration of eggs and larvae collected by the 60 cm opening diameter plankton net by the R/V SOYO MARU during January to March 1980.

NA: Data not available

Figure 7

Annual change in the estimated total number of eggs spawned and the nominal catch of the Pacific sub-population of Japanese sardine during 1949-81. The change in the former in the Ashizuri sub-population is also shown for 1977-81 (Nakai 1962, Watanabe et al., 1979 and supplement).

The magnitude of spawning in the Pacific sub-population (Figure 7 and Table 5) shows that fluctuations resemble those of the commercial catches, that is, a low level (10-20 × 10¹²) until the mid-1960's with the exception of a temporary recovery during the early 1960's (200 × 10¹² eggs) a drastic depression during the latter half of the 1960's reaching the lowest (1 × 10¹² eggs) in 1970 and a rapid increase since 1973. It reached an extraordinarily high peak in 1978 at about 580 × 10¹² eggs. These increases since 1973 resulted in the burst of the stock abundance during the mid to late 1970's and eventually in the prosperous catch by the commercial fishery as mentioned above. It decreased again thereafter but is still at a high level in recent years with about 150 × 10¹² eggs estimated from the surveys.

The reproductive relationship of the Pacific sub-population shows a density-dependent pattern during the increasing period which fits the Ricker-type reproductive curve well (Figure 8 and Table 6). It shows a rapid increase in the broods' spawning since the 1972 year-class. Spawning by the 1974-76 year-classes increased greatly exceeding that of parental stock and formed a peak of the reproductive curve at about the 450 × 10¹² egg level. The broods' spawning decreased substantially afterwards though the magnitude of parental spawning had remained at a very high level. The desirable spawning magnitude of the parental stock was estimated to be 150-250 × 10¹², and the safety level to be 100 × 10¹² where the spawning by a brood decreases precipitously against the further decline in the parental spawning.

The estimated annual allowable catches for a desirable level of spawning of the Pacific sub-population in recent years were high, at 2.03-2.23 million tons against the 1.01-2.06 million tons of annual commercial catch. The spawning survey carried out in 1982 indicated a lesser abundance of eggs and larvae in the central and northern areas of the region. This was, however, ascertained to be caused mostly by the delay in maturation of the extraordinarily strong year-class of 1980 (Figure 9 and Table 7), which did not mature at age-2 and subsequently did not spawn in 1982. It is quite likely, therefore, that the very large spawning will be made in 1983 when the abundant 1980 year-class joins in the spawning stock at age-3. The same phenomena were commonly observed when the stock was at an extremely high level during the 1930's (Nakai, 1962).

The spawning ground of the Ashizuri sub-population is located further south where the up-stream current of the Kuroshio flows (Figure 6). The spawning magnitude of the stock also increased markedly since 1978 and exceeded that of the Pacific sub-population in recent years reaching the highest in 1981 at about 360 × 10¹² eggs (Figure 7 and Table 5). The combined spawning of the two sub-populations has therefore remained at a very high level even in recent years with 400-500 × 10¹² eggs.

It is assumed that one of the major reasons for the burst in the spawning in the Ashizuri sub-population would be the substantial immigration of parental fish, especially the 1977 and 1978 year-classes from the Pacific sub-population which is presumed to have taken place during 1980-81 in connection with a change in environment, i.e. the disappearance of the meanders of the Kuroshio in the southwestern area of the region. This may explain, on the other hand, the sudden unusual decrease in the broods' spawning in the Pacific sub-population during 1977-78 (Figure 8), namely, due to emigration of spawners to the south. Another notable fact observed in the Ashizuri sub-population is that the spawning ground has been expanding, during the increasing period, further west down to 130°E where up-stream of the Kuroshio influence more on the transfer and dispersion of eggs and larvae to the north and east from its original area (132-135°E) (Figure 6). It is apparent, in these circumstances, that a substantial amount of the larvae of Ashizuri sub-population have been transferred and therefore recruited to the Pacific sub-population since the former had gained large abundance. It is also known that the intermingling of juvenile and adult fish has widely taken place between the two sub-populations after the expansion of the distribution range in each (Watanabe et al., 1979, Watanabe, 1982). This indicates that the sardine stock along the Pacific coast of Japan would have expanded its habitat to nearly its greatest range during the late 1970's.

The abundance of post-larvae in the entire region increased remarkably after 1977 (Figure 9 and Table 7). The 1980 year-class was, inter alia., strongest and mostly supported the massive catches along the Pacific coast of Japan during 1981-82 with more than 2 million tons of annual catch.

In summary, the Japanese sardine stock along the Pacific coast expanded its reproductive area and habitat during the bloom period and realized an extremely large abundance. The entire stock abundance and spawning magnitude are sustaining very high levels in recent years.

Figure 8

Reproductive relationship between the estimated total number of eggs spawned by the parental stock in each year (E) and that spawned by each of the broods during their adult stage, ages 2-4 (P), in the Pacific sub-population of Japanese sardine. Numerals in the figure denote the year-classes.

DISCUSSIONS

Common and species-specific natures in the mackerel and sardine stocks

The processes resulting in the changes in abundance appear to be quite analogous with each other and may have originated from the similarity of life history form and habitat of the two species. This suggests that these stocks have maintained an intrinsic potential burst even during the depressed period. The stocks could, therefore, readily begin to increase when the environment changes to more favourable conditions for early life history stages. For instance, the abundance of both common mackerel and sardine stocks both changed to increasing phases during the nearly same periods (early 1960's and early 1970's) and the spawning magnitude of both stocks showed a substantive decline during the same period (1969-71).

The Ricker-type reproductive relationship was also assumed in both the stocks. That is, the spawning by broods increases markedly exceeding that of parental stock during the expanding period, which results in a rapid increase in the entire stock abundance. However, the broods' spawning reduces its increasing pace in accordance with the increase in the entire stock abundance and finally becomes substantially lower than that of the parental stock once the entire stock abundance has reached an extraordinarily high level. If such a serious decrease in recruitment continues for a few years and if unfavourable environmental conditions surround the stock, the entire stock abundance would drastically decline, which may however enable the stock to shift in a growing phase some time in the future. (See discussion by Sharp, Csirke and Garcia this Volume).

On the other hand, a negative density-dependent effect on any reproductive relationship may arise when the entire stock has expanded its habitat to its widest possible extent at an extremely high level of abundance. That is, there is an apparent physical/qualitative decline in parental fish, such as delays in growth and maturity, decreases in fecundity, and deterioration in the nutrient condition of eggs etc. (Nakai, 1962, Watanabe, 1970, 1982), may inevitably follow which might result in fewer and lower viability of eggs and larvae, and eventually in serious decreases in recruitment. The stock could and will drastically decline again if unfavourable environmental conditions surround the stock under such conditions. In fact, the delay in growth and qualitative deterioration in maturity were observed in the common mackerel stock during the late 1970's and the delay in growth and maturity in the 1980 year-class of Japanese sardine.

Figure 9

Change in the estimated abundance of the post-larva in index (PL) by body-size-group of the Japanese sardine stock along the Pacific coast of Japan for the year-classes 1971-81. The indices for the 8 mm size-group and above are shown with the accumulated values including those of all the smaller groups. The larvae were collected by perpendicular tows with a 60 cm opening diameter plankton net by the R/V SOYO MARU during January to April every year.

One specific nature observed in the sardine stock was the expansion of spawning and nursery grounds to the up-stream area of the Kuroshio. This may have enabled the stock to greatly expand its habitat and migratory areas to realize eventually an extraordinarily large biomass exceeding the prior spatial limiting factors imposed on the original single population level. Such a structural deformation has not been clearly observed in the mackerel stock. However, it may be consistent for both species that the stocks have expanded their habitat to the widest possible distribution within the limiting factors of each species in the region during the prosperity period.

The three phases are assumed to be employed in the expanding process of the sardine stock, namely, (1) recycling of the stock is being made at sub-population (or race) levels and no intermingling occurs among each unit, (2) the spawning grounds are substantially expanded in accordance with the increase in the stock abundance in each of the sub-populations and the immigration from one to the other or the intermingling takes place to some extent and (3) further expansion of spawning/nursery grounds are made, accompanied by the enlargement of habitat along with the extraordinarily large abundance, and a large migratory area is formed by juvenile and adult fish extending beyond the prior ranges of each sub-population (Nakai, 1961, 1962). The prosperity stage is realized at phase-3 but the stock may have been over-populated at the same time (Watanabe, 1982). The current status of the Japanese sardine along the Pacific coast is considered to be the late-2 or early-3 stage of the above-mentioned phases.

It should also be noted here the important role of dynamic changes in the Kuroshio flow in the change of the sardine stock, especially the role of the large meanders in the southwestern area of the region. In fact, the strongest year-class of sardine appeared in 1980 when a large meandering of the Kuroshio temporarily disappeared. It is also reported that some changes in the meanders would have often boosted the burst of feed organisms for larvae in the area. The Kuroshio has, thus, played a very important role in both physical (transfer and dispersion of eggs and larvae) and nutritional (feed organisms) conditions of the sardine stock.

It is concluded that the common mackerel and Japanese sardine stocks along the Pacific coast of Japan require similar environmental conditions for their prosperity and that although the favourable and unfavourable conditions are formed alternatively, with several years time interval for each sequence, the species-specific dynamics of the several stocks abundances are independent of the physical and quantitative status of each other stock.

Management strategy

It is probable that commercial fishing could nip in the bud potential increases in abundance of a depleted stock if a significant stock reduction is made when the stock begins to recover. However, the magnitude of the potential and the pace of the recovery of mackerel and sardine stocks appear to be larger and fast enough to overcome the ordinary reduction by commercial fishing. It is also unlikely that fishermen would impose intensive fishing on mackerel and sardine when the stocks are at a low level.

It is apparent, from the analyses made in this paper, that once the stocks have started growing, the increases in the abundance greatly exceeds the fishing mortality, even if the reduction is massive. The effect by fishing on these stocks is therefore not so significant so long as the reduction remains within the range which corresponds with the stock size. It becomes seriously significant only when the stocks are in a declining phase, these being accompanied by lesser recruitment. Fishing should be limited on such occasions within the allowable catches which correspond with their minimum or safety level of spawning magnitude (200 × 10¹² eggs for common mackerel and 100 × 10¹² eggs for the Pacific sub-population of Japanese sardine) firstly to prevent any further deterioration of the stock and secondly to ensure the stock to shift soon in the growing phase.

FUTURE STUDIES

Stock assessment by spawning survey, although still involving several problems to be overcome in the future, is one of the most effective ways to assess dynamic changes in stock abundance from the reproductive sector of fish, specifically for neritic pelagic fish resources as the author has described in this paper. In addition to technical improvement of the survey methods, tools, handling of samples and data processing, the following studies have been identified as needing to be carried out intensively in the future, as the improvement of the accuracy on the estimate of annual recruitment is, inter alia, essential for the better assessment of these resources.

(1)  Observation on and evaluation of the physical quality of adult fish as spawners with suitable measures and the analysis of the time-series data on the change in fecundity and the number of eggs really spawned by adult fish in conjunction with the changes in abundance/density of the stock and environment.

(2)  Clarification of the mechanisms and their roles in mortality during the early stage of life.

(3)  Observation on and analysis of the changes in distribution pattern and the structure of the stock in connection with the changes in environment, abundance of the stock and inter-species relationships with the other neritic pelagic fish stocks.

(4)  Examination of the catch data and data collection system to obtain a better estimate (unbiassed) of age-compositon of the stock.

(5)  Comparative studies with the results obtained by the other methods and among the other species in conjunction with the clarification of the common and species-specific natures.

The author believes that the above-mentioned studies would greatly encourage the progress in the studies in each species and would improve also the global understanding on the dynamics of neritic pelagic resources including jack mackerel, saury, herring, anchovy, Japanese flying squid etc. at the community level.

REFERENCES

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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. 38:439-44

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