Although the total landings of coastal pelagic fishes around Japan have been fairly stable for 10 years, the catch of individual species has shown violent fluctuations. The catch of Hokkaido herring and the sardine is decreasing and shows no signs of recovery. The catch of saury has also decreased sharply since 1960, but that of mackerel has increased substantially. There are big yearly fluctuations in the landings of squid, horse-mackerel, and anchovy, but no consistent trend either way has been shown. Apparently the great variations in stock abundance relate to the hydrographic complex in the waters around Japan. As most of the important coastal pelagic fishes spawn and spend their early life in the area of the continental shelf of the southern half of the Japanese islands, the location and the strength of the Kuroshio current play an important role in the ever-changing environmental conditions. Although the causes of these fluctuations are largely unknown, they can be classified as long, intermediate and short-term. The large-scale fluctuation over a long term, as in the case of the Hokkaido herring and the sardine, may possibly be due to evacuation of the main spawning grounds. The intermediate period of fluctuation is caused by the degree of survival of the young. A variety of causes must be responsible for the short-term fluctuation, including survival, for which there is no consistent trend, and the effect of the Kuroshio current on the distribution of the young fish.
Length–length and length–weight relationships for four small pelagic fishes in the Kuroshio–Oyashio current system
