Relationship between stock levels and the population structure of the Japanese anchovy

Abstract

The anchovies distributed around Japan are only one species of Engraulis japonica Houttuyn although there are eight major species in the genus Engraulis in the world. Japanese anchovy stocks show considerable stability, fluctuating within a narrow range of 3–4 times, 1906–1984. This is in contrast to that of the Japanese sardine which shows fluctuations of up to 400 times. This phenomenon, together with the positive correlation found in abundance between consecutive developmental stages, makes it difficult to explain changes in abundance merely by chance or probability, in other words, by the “critical period”; concept The most important factor concerning mechanisms that stabilise stock levels is the population structure. Several local populations of the Japanese anchovy are dispersed geographically over a wide area, each of which is composed of various year classes and seasonal reproduction groups. The existence of these various groupings adapted to diverse environmental conditions maintains the stability of the overall stock. The second major factor is the existence of a population regulation mechanism that changes the reproduction mode and regulates the number of individuals in response to environmental conditions. In the 1950s to 1960s the stock level was maintained by spring spawnings of the large‐sized group. Since the end of the 1960s, the condition factor and maturity coefficient of the large‐sized group gradually decreased, their spawning activity in spring declined and the abundance of the large‐sized group also decreased. The decrease in abundance of the large‐sized group appeared to trigger or increase the spawning potential of the small‐sized group which spawns in summer‐autumn. Biological characteristics of the small‐sized group include relatively high spawning frequency and spawning batch fecundity compared with the large‐sized group, with shortened longevity. In this way, there are two types of reproduction modes, and the longevity, migration and maturing period of the anchovy change according to the reproduction mode. The Japanese anchovy skilfully adapted to its environment by changing its mode of life and population structure and a population regulation mechanism is working throughout the life of the species.