CalCOFI Reports, Vol. 11, 1967 - California Cooperative Oceanic ...

REPORTS VOLUME XI, 1 JULY 1963 TO 30 JUNE 1966 53Assessment of PopulationEven though many estimates have been made of thecharacteristics inherent to the population dynamics(Yokota 1953, Yokota and Asami 1956, Hayasi andNondo 1957, Hatanaka 1960, Tanaka 1960, Hayasi1961, Yoshihara 1962, Asami 1962, Beverton 1963) ,it has been concluded that the anchovy is one of themost typical organisms indicating limitations of thepresent system of the population dynamics (Tanaka1960, Hayasi 1961). The most comprehensively acceptedopinion on the effects of fisheries upon the stockis that the fisheries may not seriously damage theanchovy stocks, because of the short life span andvigorous reproduction of that species, even if increaseof amount of fishing efforts often causes decrease ofthe catch per unit effort (Ex. Com., Conf. Invest.Neritic-Pelagic Fisher. Japan 1962). Techniques offorecasting the anchovy stock size have succeeded inlimited areas, where the life history of the fish isunderstood (Ex. Com., Conf. Invest. Neritic-PelagicFisher. Japan 1963).Hayasi (1961) summarized a general belief amongfishery biologists engaging in the Cooperative Investigationson effect of fisheries upon the anchovy populationas follows :“One of the most important biological features ofthe Japanese anchovy is the short life span, practicallyabout two years in duration. Another important featureis the wide-temporal and areal extent of thespawning activities.Deduced on the basis of these features are thefollowing two general conclusions on fluctuation inthe size and distribution of the population. Firstly,the environmental change in the early stage of life ofa space-time group may not exert any serious effecton the amount of recruitment for the year, becausethe population consists of a number of such groups.The wide-extended spawning indicates that the speciescan propagate under various environmental conditions.Secondly, the natural mortality rate is regardedto be high because the anchovy, being small in size,represents the major prey of diverse fishes, and becausethe life span of two years is much shorter thanthat of other neritic-pelagic species such as thesardine, many of which live for five or more years(Nakai and Hayasi 1962). Therefore, changes of themortality rate by natural causes, especially thosebased on predation and competition, may play moreimportant roles than the changes of the fishing intensityin determining the population size of the anchovy.The consistent regional distribution of catch suggeststhat the well-developed fisheries in the Pacific watersand the Seto Inland Sea have not seriously depletedthe stocks therein. ’ ’In the Pacific waters along Honsyu, it is possibleto estimate sizes of large stocks at five developmentalstages; (1) eggs in offshore, (2) postlarvae on thefishing grounds of sirasu fisheries on the outer coast,(3) immatures in the fishing grounds in the bays,(4) adults at the end of their first year of life and(5) adults at the early half of their third year oflife on the fishing grounds along the Boso Peninsulaand north. The egg stock must be proportionate withthe stock of parents, mainly I-age fish. Hayasi (1961,1962b) compared the stock sizes at these five stagesof the same year classes under a hypotheses thatfluctuations in these stock sizes must be correlated ifthe following conditions are satisfied: (1) these stocksare actually related in the course of ontogeny, (2)the older three stocks are not overexploited, (3) theaccessibility and vulnerability of the stocks do notremarkably vary between year classes, and (4) theestimates of abundances are reliable. Actually highcorrelation coefficients, ranging between 0.90 and0.98, were found for most of 8 to 10 year-classesbetween the four stocks except the immatures inthe bays.Taking the correlations between stock sizes as wellas information on spawning, migration, age and durationof life span, it has been concluded that no fishingactivity exerts any serious damage on the anchovystocks in the major fishing grounds. The correlationsbetween sirasu catch and either one of the older stagesreveal that the fishing did not reduce the youngeststock severely. The fisheries in the bays exploited onlya portion of the immature stock. The two-boat purseseiners aiming at the adults did not exploit 0-agefish so seriously as to affect the stock sizes of I- and11-age fish. The exploitation of 11-age fish did notaffect size of the major local group because the oldestanchovy may die within the year, and may not muchcontribute to the reproduction (Hayasi 1961, 1962b).The level of stock sizes during 1950 through 1959was assessed on the basis of the reproduction curveproposed by Ricker (1954). In constructing the curvefor the Japanese anchovy, the abundance of eggs inany two successive years was used because this measureis free from such a bias as inherent to the catchstatistics, and because the spawners are mainly I-agefish. The egg abundance spawned by two successivegenerations are not well correlated with each other,except those reproducing in 1950 through 1952, whichproduced less than 500 X 10l2 eggs. Therefore, it canbe stated that the classes spawning since 1953 werelarge enough to supply a large amount of landings(Hayasi 1961,1962b).On the basis of the correlations, formulae are givenfor predicting the catches of 0 and I1 age adults inseveral months or two years prior to the fishing seasons.Application of the formulae to the catch for pastdata showed that the predicted amount agreed withthe actual catch for 8 or 9 years out of 10 (Hayasiand Kondo 1962). With the use of those formulae andcatch statistics, the landings have been predicted asabout 26,000 tons for 0-age adults during Octoberthrough December of 1961, and about 50,000 tons forthe 11-age adults in the early half of that year. Theactual landings of 0- and 11-age adults in those periodswere 23,000 tons and 47,000 tons, respectively (Hayasi1962b). This procedure was not applied for thefollowing seasons because of occurrence of the remarkableanomaly of the environmental conditions (Nakaiet al. 1964). Thus, the prediction based on the correlationsbetween catches at different stages are applicableonly in the case when the migratory routeof fish, type of fishery, and environmental conditions