YELLOW AND CHINA SEAS - MAR AMARILLO Y MAR DE LA CHINA

ON THE FISHERIES OF SPANISH MACKEREL (Scomberomorus niphonius)

IN THE YELLOW SEA AND POHAI SEA

by

Chan Xin Liu

Research Institute of Marine Fisheries
Liaoning Province, Dalian
The People's Republic of China

SUMMARY

The Spanish mackerel, Scomberomorus niphonius in the Yellow Sea and Pohai Sea is a pelagic fish having long distant migrations. Its fisheries have a long history and chiefly operated with drift nets. This article dealt with the migration, age, growth, mortalities, population dynamics and fisheries management of S. niphonius and reviewed works by other authors.

Age composition of the spawning population in the spring fishing season comprised 6 ages. This observation is somewhat different from the works of other authors, but it makes no difference to the study of population dynamics because of the small numbers of the older fish. Sexual maturity in male fish begins at age 1, and the females mature at age 2. These fish are recruited very young and the majority of fish in the spawning population are young recruits. Their abundance plays an important role in population dynamics.

The survival rate of S. niphonius (0.09) is much smaller than that of S. cavalla (0.52). The average total mortality coefficient is 2.4123, natural mortality coefficient being 0.2566, fishing mortality coefficient being 2.1557. Therefore, even if the fishing effort increases, the catch will not increase correspondingly. The catch from the spawning population in spring fishing season varies in the range of 10,000-30,000 tons.

Some measures for fisheries management are suggested on the basis of the above information, which are chiefly the strict prohibition of trawling the fish below 450 mm in length and limitation of fishing effort.

The present production is situated at a growth over-fishing phase. Hence, it seems that these measures could only be put into practice with some difficulty.

YELLOW AND CHINA SEAS - MAR AMARILLO Y MAR DE LA CHINA

THE HERRING (Clupea harengus pallasi) AND ITS FISHERIES IN THE YELLOW SEA

by

Chang Cheng Ye

Marine Fisheries Research Institute of Liaoning Dalian, The People's Republic of China

Resumen

En este trabajo se discute la distribución, crecimiento, mortalidad y relación stock-reclutamiento del arenque del Mar Amarillo, y se trata de establecer una estrategia para la ordenación de sus pesquerías. Los resultados obtenidos se resumen a continuación:

Se cree que el arenque que se encuentra durante todo el año en las áreas al norte de los 34°N del Mar Amarillo pertenece a una raza del arenque del Pacífico tradicionalmente conocido como arenque del Mar Amarillo.

El patrón de crecimiento del arenque del Mar Amarillo puede ser descrito tanto por la ecuación de von Bertalanffy, como por la ecuación de Brody Robertson. La abundancia de individuos de edad l de las clases anuales de los años 1966 a 1974 fue estimada por el método de Pope, el coeficiente de mortalidad total para cada clase anual a sus distintas edades fue cálculado por el método iterativo.

El reclutamiento del arenque del Mar Amarillo ha mostrado fluctuaciones considerables desde 1966. De la abundancia de individuos de edad l de nueve clases anuales sucesivas entre 1966 y 1974 obtenidas por el análisis de cohortes se observa que la abundancia en la clase anual de 1970 es la más abundante, y que la clase anual de 1970 fue 53 veces más grande que la de 1969. No se ha observado ninguna correlación significativa entre los adultos y el reclutamiento, y se asume que el reclutamiento del arenque del Mar Amarillo es afectado fuertemente por los factores ambientales. En 1972 un fuerte viento del sur "Typhoon N^o 3" impactó seriamente la sobrevivencia de juveniles.

De los resultados obtenidos de la aplicación de un modelo de rendimiento por recluta (Y/R), y considerando los beneficios económicos (los ovarios de arenque maduros tienen un valor bastante alto) se sugiere una estrategia de manejo donde la longitud estándar mínima en las capturas sea de 230 mm., lo que equivale a individuos de 2 años de edad. Además se sugiere que se prohiba a los arrastreros operar desde el invierno hasta la época de desove siguiente. Cuando estas estrategias sean adoptadas, la mortalidad por pesca debe disminuir ligeramente y el stock adulto debe aumentar, pero posiblemente no haya un aumento significativo del tamaño total del stock. Los rendimientos de arenque por parte de los arrastreros disminuirán, pero esto será compensado por un aumento de rendimiento por parte de la pesca que se realiza más cerca a la costa, esto reducirá el consumo de energía. La producción de ovarios de arenque maduro y los beneficios económicos de la pesquería de arenque en general deben aumentar en forma apreciable.

INTRODUCTION

According to the memories of the old fishermen and historical background information, Yellow Sea herring have had two recorded peaks of catch in the years of 1900 and 1938. Catches appeared again in small amounts in the landings of trawlers in 1966 and 1967. In 1969 they began to increase gradually with a catch of 4,200 tons, 14,000 tons in 1970 and catches reached the highest level at 174,000 tons in 1972. Then the following catches decreased. Now the catch fluctuates at a low level around 20,000 tons. Papers on Yellow Sea herring published in China are listed in the References at the end of the paper. The information and results of investigation relating to the biology, dynamics of the population, etc. of Yellow Sea herring during the peak at the third time are reported in this paper.

Aspects of the fisheries

At the beginning of the 1970's, Yellow Sea herring were one of the more important fisheries in the Yellow Sea. Then the fisheries were composed of two parts, one off-shore segment and an inshore segment. The former included trawlers, which operated nearly all year round, and purse seines, which operated in winter fishing season to catch wintering populations. The latter included various settled-nets and small purse seiners, which operated in a small area and chiefly caught pre-spawning herring. The ratio of production between these two components was about 1:1.

The variation of the fishery production is shown in Figure 1. The catch has been fluctuating at a low level of around 20,000 tons since 1976. As a result of the decrease of stock size and limitation on trawlers fishing Yellow Sea herring, this fishery has as yet nearly only operated settled-nets during the spring fishing season. The fishery exhibited large variations in a very short period and the accumulation of data, standard. ization of fishing effort, etc., has been difficult.

Fig. 1. Yellow Sea herring, the curve of variation in yield

POPULATION AND DISTRIBUTION

Distribution

During March each year herring are distributed in the shallow waters along the coasts of Shandong peninsula and southeastern part of Liaodong peninsula for their spawning activities. The main spawning grounds are situated in the inshore areas from Shidao to Waihai, Shangdong Province. The area of the spawning grounds is correlated with the population abundance. The water temperature of spawning varies over 1° to 5°C. The first maturation is at age two. Eggs are demersal, and adhere to the sea weeds and other fixed objects. After spawning, the adults move to the deeps and scatter to search for food. In summer they are chiefly distributed in the middle and north parts of the Yellow Sea (34°-39°N, 123°-125°E), and feed in the 60-80 meters in depth areas. In autumn and winter, the area where they are distributed is smaller, and chiefly include the middle part of the Yellow Sea (35°-37°N, 123°30'-125°E), where the water is 70-80 meters in depth. In late winter and early spring (February) the individuals begin to move northwards in groups (by lengths) in spawning migrations. The distribution of immature juveniles is different from that of adults, generally they (at age 1) are distributed a little more northwards, and the adults (at age 2 and above) a little more southwards.

Population

Ye, et al. (1981) have compared Yellow Sea herring to the races of Hokkaido, Japan and Korea employing six main countable features. By statistical tests it is shown that the Yellow Sea herring differ significantly from the other two races in these main countable features. Further, according to exploratory fishing results of twenty-eight surveys from 1971-1978 and more than one thousand fishing records from the industrial fisheries, we have never discovered Yellow Sea herring south of 34°N. Hence we suggested that Yellow Sea herring inhabiting the Yellow Sea north of 34°N all year round is a unique population of Pacific herring, called Yellow Sea herring.

GROWTH PATTERN AND MORTALITY

Growth

The growth curves of Yellow Sea herring are drawn from observed data in Figures 2 and 3. The common growth pattern can be described by a von Bertalanffy growth equation, in which the parameters are: W¿=314g., L¿=308mm, K=0.59, t =-0.54. The inflection of the growth curve in weight is situated at t=1.3 years and W_1.3=91.2g., which is equivalent to 0.290W. Equations (1) or (2) are used to show growth rates. The differences between equations (1) and (2) are that, the growth rate is defined by t in equation (1) as:

Fig. 2. Yellow Sea herring, the curve of growth in weight

while in equation (2) it is defined from observed weights as:

Figure 4 shows the growth rate in weight.

In addition, another growth equation also used to fit the data of Yellow Sea herring is shown in Table 1. The fitting of Robertson's equation is better than that of Brody or von Bertalanffy's equation. The sum of square of residual error of Robertson's equation is smaller than that of von Bertalanffy's, the main reason is that von Bertalanffy's equation does not reflect well the growth of juveniles.

Fig. 3. Yellow Sea herring, the curve of growth in length

Mortality

The coefficient of natural mortality for Yellow Sea herring estimated from conventional methods by Ye et al. (1981) is about 0.11, but appears not to be reliable. Gulland pointed out that the value is somewhat small.^* In 1980, it was estimated to be about 0.2 by K. Brander during FAO/UNDP Training Course on Marine Resources Evaluation in Bhanghai. Zhang^** (1981) used 0.2 also. So, I also use the value 0.2 as the natural mortality coefficient. The abundances of 1966-1974 year-classes at age 1 from estimates using Pope's method (1972) and also the fishing mortality coefficients for each year-class of various ages are tabulated in Table 3. Table 2 is the yield of year-classes of Yellow Sea herring in successive years at various ages. In addition, catch per unit of effort of 1970 year-class in 1972 and 1973 was used to estimate the total mortality coefficient, which is about 1.05 (Ye, et al. 1981). When the natural mortality coefficient is subtracted from total mortality, the fishing mortality coefficient is found to be about 0.85, which approximates the value of 0.817 in Table 3.

Fig. 4. Yellow Sea herring, the curve of growth rate

*    Personal communication with John A. Gulland in 1981.

**  Zhang Yu-Xi, 1981. The present situation of Yellow Sea herring population and its rational exploitation. (unpublished, China.)

*    absent
**   year-classes
*** fishing years

Fig. 5. Yellow Sea herring, the curve of variation in abundance of year-classes

Fig. 6. Yellow Sea herring, stock-recruitment relation

POPULATION DYNAMICS

The recruitment of Yellow Sea herring shows considerable variations from year to year. Figure 5 is drawn with stock size of nine year-classes (1966-1974) at age 1 estimated with cohort analysis. The abundance of the 1970 year-class is the greatest with about 2940×10⁶ individuals, and 2074×10⁶ individuals caught in total during the fishable life-span (6 years in 1971-1976), whereas the 1969 year-class is the smallest with about 55×10⁶ individuals and 39×10⁶ individuals caught over 6 years (1970-1975). These two year-classes differ by 53 times in stock size and yield. What we are concerned about is how to decide which factor makes the greatest impact on the stock size of Yellow Sea herring, the impact of adult stock on recruitment, or the impact of fishing on recruitment. Figure 6 is drawn to indicate the relation between adult stock and recruitment, in which the recruitment is represented by the number of each year-class at age 1 estimated by cohort analysis, and the yield of the mature individuals was taken as the index of abundance of the adult stock. As a result of short time series, when the data in the Figure 6 are tested statistically the result shows no correlation. It appears that the adult stock of Yellow Sea herring has little impact on its recruitment, or in other words, that fishing has little impact on the recruitment, which is therefore chiefly effected by environment. We have seen for the Yellow Sea herring that strong southward wind has a serious impact on the survival of the juveniles. In 1972, there was a great number of adults, eggs and larvae, but in the last ten-day period of July in that year, there was a typhoon (No. 3) over the northern Yellow Sea along Shandong and Liaoning Province. The wind was southeastward with wind forces of 8-9 on the Beaufort wind scale. As a consequence, few of the 1972 year-class individuals survived.

The main pelagic fish populations exploited in the Yellow Sea and Pohai Sea are Yellow Sea herring, spanish mackerel, common mackerel and pomfret (Stromateoides argenteus), in which the pomfret population is more stable, the catches of Yellow Sea herring has fluctuated around a low level of 10,000-20,000 tons per year in recent years. The common mackerel which migrates into the Yellow Sea also has decreased somewhat, but interspecific changes have not appeared yet, while this phenomenon is more obvious in demersal fish.

Yield per recruit

Equation 3 is used to examine the impacts of fishing pattern on the yield of Yellow Sea herring,

where t and t indicate the maximum and recruited ages, respectively. According to age compositions in 1970-1979, because fish above age 6 contribute about only 0.2%, and age 6 0.4%, age 6 is taken as the maximum age. Other parameters were introduced in above sections. Figure 7 is a curve of constant yield per recruit with recruitment at age 2, being a flattopped yield curve and having a maximum value of y/R at about F=1.0. Figure 8 describes yield per recruit as a function of mean selection age i.e. recruited age, with F=1.0. The maximum of the curve occurs at about t=3. The adjustment of mesh size has more potential for increasing production than that of adjusting F.

Fig. 7. Yellow Sea herring, yield against fishing mortality with t=2.0

Fig. 8. Yellow Sea herring, yield against mesh with F=1.0

MANAGEMENT STRATEGY

The management strategy should be related to management objectives and this presents complex problems. Gulland (1977) discussed the problems. These problems were also discussed in detail at the Penaeid Shrimp Workshop at Key West (1981). In discussing the strategy for fisheries management of Yellow Sea herring, we must consider:

(1)  The economic benefits: The mature ovaries of herring are extremely valuable and an important export item, earning foreign exchange.

(2)  According to the actual situation of Yellow Sea herring fisheries, any growth over-fishing can be accepted. Nevertheless, recruitment over-fishing must be prevented. For the Yellow Sea herring, there is no reliable evidence that there is any relation between adult abundance and recruitment. Under this circumstance it is still important to retain abundant spawners to prevent recruitment collapse.

As mentioned above, for the strategy of management of Yellow Sea fisheries a lower fork length of 230 mm. Limit for catches is equivalent to 2-years of age; trawlers are prohibited from operation from winter to the next spawning period. After these strategies are adopted, fishing mortality coefficient should decrease slightly, adult size should increase somewhat, but it will not have significant effects on stock size; the catch of trawlers will decrease seriously, their decreased catch will be compensated by the increased number of settled-nets near the shore, energy consumption will be reduced, the production of herring mature ovaries and the economic benefits of the whole herring fisheries will increase appreciably. These strategies have already been adopted by the fisheries management office.

REFERENCES

Gulland, J.A. 1977. The Management of Marine Fisheries. University of Washington Press. Seattle.

Jiang, T.W. 1981. Preliminary observations on the artifical hatching and embryonic development of Hunag Hai herring. Acta Oceanologica Sinica. Vol. 3. No. 3.

Lu, Xiaosheng, et al. 1962. The herring inhabiting in Yellow Sea. The Fifth Proceedings of the Western Pacific Fisheries Committee. Science Press, Beijing.

NOAA Technical Memorandum NMFS-SEFC-98. Interim Report of the Workshop on the Scientific Basis for the Management of Penaeid Shrimp. Key West, Florida, U.S.A. Nov. 1981.

Pope, J.G. 1972. An investigation of the accuracy of virtual population analysis using cohort analysis. Int. Comm.Northwest Atl.Fish.Res.Bull. (from FAO Fish.Circ.No. 701).

Tang, Q.S. 1980. The sexual maturity, fecundity and biology of herring in the Yellow Sea. Marine Fisheries Research No. 1.

Ye, C.C. 1980. An estimate of the abundance of Yellow Sea herring. Chinese J.Zoo. No. 2.

Ye, C.C., Q.S. Tang and Y.J. Qin. 1981. The Huang Hai (Yellow Sea) herring and their fisheries. J.Fish.China. Vol. 4. No. 4.

YELLOW AND CHINA SEAS - MAR AMARILLO Y MAR DE LA CHINA

ON THE BIOLOGY AND FISHERY OF THE GOLDEN SARDINE
(Sardinella aurita Val.) ALONG SOUTHERN FUJIAN AND EASTERN GUANGDONG COASTAL WATERS

by

Qiu Shuyuan¹ (S.Y. Chiu)¹ and Huang Tsongchion²

¹ Department of Oceanography Xiamen University Fujian, The People's Republic of China

² Xiamen Fisheries School Fujian, The People's Republic of China

Resumen

Los autores y sus colegas investigaron la biología y pesquería de la sardina dorada (Sardinella aurita) entre 1971 y 1977 y en el presente trabajo se resumen los principales resultados obtenidos. Para los estudios biológicos se colectaron y examinaron 7 022 especímenes en base a los cuales se llegó a determinar los siguientes aspectos de la estructura de la población:

La longitud del cuerpo varió entre 159 y 258 mm, la longitud promedio fue 185.2 mm, y el grupo dominante de tamaño se encontró entre los 160 y 210 mm. El peso del cuerpo varió entre 46 y 240 gr. Los grupos dominantes en cada año tuvieron pesos entre 50 y 70 gr. en 1971; 70 y 90 gr. en 1972; 80 y 100 gr. en 1973: 90 y 110 gr. en 1974; 70 y 90 gr. en 1975; y 110 y 130 gr. en 1976.

Se identificaron cinco grupos de edad (edad 0 a IV). Los grupos de edad predominantes entre los desovantes fueron variables: en los años 1971-1972 y 1975-1977 predominaron las edades I y II, y en los años 1973-1974 predominaron las edades II y III. Entre las edades I y II se observó un incremento en la longitud del cuerpo de 22.6 mm; 20.7 mm. entre las edades II y III, y 24.0 mm. entre las edades III y IV.

La proporción sexual hembras:machos fue 1:0.96 en 1972; 1:0.91 en 1975; 1:0.81 en 1976; y 1:0.94 en 1977.

La fecundidad promedio en 1972 (de 9 especímenes) fue 52 185 y en 1973 (12 especímenes) fue 70 720. En 1971 la mayor parte de los desovantes eran del grupo de edad I (160-180 mm), en 1972-74 la mayor parte de los desovantes fueron de edad mayor (180-200 mm).

El desove ocurre entre Marzo y Julio. Las áreas de desove se encuentran distribuidas en las aguas costeras (40-50 metros de profundidad) de la provincia del sur de Fujian y este de Guangdong, los límites del área de desove no están bien definidos.

Del análisis de 56 contenidos estomacales en 1972 y 144 contenidos estomacales en 1976 se observan resultados diferentes en cuanto al alimento dominante. En general Sardinella aurita es un pez zooplanctónico y su actividad alimentaria es baja.

Se han examinado los factores hidrológicos sobre pesca y cambios en los aspectos biológicos con la finalidad de explicar la causa del aumento y disminución de las capturas anuales, pero no se ha encontrado una explicación satisfactoria. Considerando la complejidad del problema, se propone un estudio más completo e intenso.

Estudios del comportamiento de los peces, la productividad del plancton, la extensión del afloramiento y mezcla de las corrientes, así como la relación con otros peces pelágicos, pueden ayudar a resolver este intrincado problema.

INTRODUCTION

The golden sardine (Sardinella aurita Val.) is a pelagic warm water fish widely distributed in the warm part of the three great oceans. It also inhabits in coastal waters of the South China Sea and the southern part of East China Sea. Since the development of a purse-seine fishery using electric lights in the southeastern China coast in the middle sixty's, the annual catches of the sardine have greatly increased. According to and recognizing the incomplete statistics, the annual yield of the peak year (1971) amounted to 100,000 metric tons. The most important fishing ground lies in the coastal area of southern Fujian and eastern Guangdong province.

Since the early 1970's, an investigation into the biology and fishery of the golden sardine has been carried out by the author and colleagues. The following is a summary of the result of the investigations from 1971 to 1977.

BIOLOGY OF THE GOLDEN SARDINE

Specimens of the golden sardine have been collected and measured since 1971 to study: the body length; weight and age composition; growth rate; spawning season and spawning ground; age at first sex maturation; sex ratio; fecundity; food composition and feeding intensity; etc. A total number of 7,022 specimens were examined within seven years of investigation.

Body length

The body length (fork length) of the spring spawners ranges from 159 to 258 mm. The average recorded body length was 185.2 mm. The dominant body length group varies from year-to-year, ranging from 160 to 210 mm. Within the years of 1971-1977, there is a general increasing trend.

Body weight

Body weight of the spring spawning population ranges from 46 to 240 g. (1971-1977). The dominant body weight groups are: 50-70 g. (1971), 70-90 g. (1972), 80-100 g. (1973), 90-110 g. (1974), 70-90 g. (1975), 110-130 g. (1976). The average body weights of the age groups I, II, III, and IV in the years 1971-1977 are given in Table 1.

Age Composition

Ages of the golden sardine are determined by the scale method. There are 5 age groups (0-IV) among the young and mature populations. The predominant age groups in spring spawners are I and II (1971-1972; 1975-1977), II and III (1973-1974) age groups. The summer populations are mostly 0 group, next is I group, the percentage of II and III age groups are very small.

Growth rate

Based on the back calculation from aged specimens of the spawning population in 1972- 1974. The average fork lengths of I, II, III and IV age groups is 171.1 mm, 193.7 mm, 214.4 mm, 238.4 mm, respectively. Hence, the body length increment is 22.6 mm from age I to age II, 20.7 mm from age II to age III, 24.0 mm from age III to age IV.

Sex ratio

From data collected in 1972, 1973 and 1975 to 1977, the female spring spawners are usually slightly more numerous than the male. For example, the average sex ratio (♂ : ♀) is 1:0.96 in 1972, 1:0.88 in 1973, 1:0.91 in 1975, 1:0.81 in 1976, 1:0.94 in 1977.

Fecundity

The average individual absolute fecundity was 52185 in 1972 (9 specimens) and 70720 in 1973 (12 specimens). Generally, the fecundity increases with body length and body weight but there is no definite regularity in our small samples.

Age and length at first sex maturation

From observations on the maturity of gonads, the age at first sex maturation is considered to range from one year or two years old. In 1971, most of the spawners are of I age group (160-180 mm) while in 1972-1974, the majority are older groups II plus (180-200 mm). The smallest specimen observed at first sex maturation is 173 mm (♀), 174 mm (♂) in 1975.

Spawning season and spawning ground

The spawning season lasts for a long period (March to August) in this area, but the peak season is from late March to May. The spawning ground situates in coastal waters (40-50 m, in depth) of southern Fujian and eastern Guangdong province, but the boundary is not exactly delimited.

Food consumption and feeding intensity

An analysis of 56 stomach contents in 1972 showed that the food of adult fish differs considerably from that of young fish. The adult fish feed mainly on diatoms (Nitzschia sp.) and blue algae, next are small zooplankton such as copepods and ostracods. The young fish feed primarily on plantonic crustacea among which copepods and brachyuran larvae are the most important, secondary are diatoms (Coscinodiscus spp.) and fish larvae.

However, analysis of 144 stomach contents of the adult fish (fork length 170-230 mm) in 1976 showed quite different results. The majority of food items were planktonic copepods, decapods, brachyuran larvae, mysids, and chaetognatha, whereas the young fish (41-70 mm) fed mainly on diatoms (mostly Coscinodiscus spp.); next were copepods.

The feeding intensity is relatively low. Examination of the relative stomach fullness among the spawning season revealed that most had half full stomachs (Class II) or had less than 1/3 stomach filled with food (Class I). The summer young fish population had less full stomachs (mostly Class I).

FLUCTUATIONS OF THE ANNUAL CATCHES OF THE SARDINE FISHERY

The annual catches of the sardine fishes of southern Fujian and eastern Guangdong provinces have increased since the development of the purse seine fishery employing electric lights in these coastal areas. The quantity of catches of southern Fujian in the years 1971-1978 are shown in Table 2.

From Table 2 it is clear that among the eight years (1971-1978) there are two peaks (1971, 1978) and two troughs (1973, 1976) of the annual yield.

The average catches per vessel in the years 1969-1978 are:

ANALYSIS OF THE CAUSES OF FLUCTUATION OF ANNUAL CATCHES

The quantity of annual catches of any kind of marine fish may be influenced by three main factors:

(1)  Changes of hydrological (such as temperature, salinity, currents, etc.) and meteorological factors (such as windy days, etc.);

(2)  Changes due to fishing effort (such as over-fishing);

(3)  Changes of biological factors (such as number of spawners, strong or weak generation, etc.).

Concerning the factor (1), there were no great changes overall in water temperature, salinity, or sea currents in the area discussed except that the water temperature in March 1973, was 1-2°C higher than that of the other years. This factor might influence the ripening of gonads hence the date of spawning season and finally the quantity of catch. But the effect is not so serious as to cause a sudden drop of the catch.

Concerning factor (2), the greatly increased fishing effort of the purse seine fishery with electric lights from 1969 to 1972 certainly has had profound effects. The most noted effect is diminishing the number of spawners, thence perhaps the number of larvae and young fish. The decline of catch in 1973 might be due to this factor. But the fact that the annual catch of the golden sardine has gradually recovered and another peak was observed in 1978 cannot be explained relative to fishing alone. Particularly since the fishing intensity from 1971 to 1977 increased rather than decreased.

Concerning factor (3), the weak generation of the year-class of 1973 might have influenced the magnitude of catches in 1974-1976, so the subsequent decline of catch in 1976 may be attributed to this factor. However, why the catch of 1978 reached another peak is not certain. Maybe it was due to the strong year-class of 1975. It should be noted that the catches of other pelagic fishes such as round scad (Decapterus maruadsi), the Japanese mackerel (Scomber japonicus) and the round herring (Etrumeus micropus), etc. were all good in the same year.

Thus, what are the true factors controlling the fluctuation of annual catches? At present it is quite difficult to answer. Considering the complexities of this question, more thorough and intensive studies are needed, including fish behaviour (distribution, migration, feeding and breeding), plankton productivity, the extent of upwelling and mixing of water masses and currents as well as the interrelationship of other pelagic fishes, etc. These must be combined and employed as comprehensive tools to resolve this intricate problem.