DIFFERENCES IN FOOD AVAILABILITY FOR JAPANESE SARDINE LARVAE BETWEEN THE FRONTAL REGION AND THE WATERS ON THE OFFSHORE SIDE OF KUROSHIO

The availability of food for larvae of the Japanese sardine, Sardinops melanostictus, was investigated in the Kuroshio frontal region and th e waters on the offshore side of the Kuroshio, the Pacific coast of ce ntral Japan, in March 1990 and 1991, respectively. Food availability w as assessed by changes in biomass and production of nauplii and small copepods, and RNA/DNA ratios of the larvae during about 2.5 days (the frontal region) or 3 days (the offshore waters) of tracking a drifter released in a patch of the larvae. The biomass of the nauplii tended t o increase with time in the frontal region and to decrease in the wate rs on the offshore side of the Kuroshio during the drifter tracking pe riods. The production of small copepods including nauplii in the water s on the offshore side of the Kuroshio was 14% of that in the frontal region. The sum of the mean food requirements of the carnivorous macro zooplankters and sardine larvae was 11% of the production of small cop epods including nauplii in the frontal region, compared with 136% in t he waters offshore of the Kuroshio. The RNA/DNA ratios of postlarvae s maller than 8 mm in the frontal region were significantly higher than those in the waters on the offshore side of the Kuroshio (P < 0.001). It is considered that the food availability for sardine larvae was rel atively high in the frontal region and low in the waters on the offsho re side of the Kuroshio. The food availability for the larvae probably deteriorated with the offshore shift of the main spawning ground from the frontal region to the waters on the offshore side of the Kuroshio , in the latter half of the 1980s.