Differences in food availability for Japanese sardine larvae between the frontal region and the waters on the offshore side of Kuroshio

Abstract

The availability of food for larvae of the Japanese sardine, Sardinops melanostictus, was investigated in the Kuroshio frontal region and the waters on the offshore side of the Kuroshio, the Pacific coast of central Japan, in March 1990 and 1991, respectively. Food availability was 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 pitch of the larvae. The biomas of the nauplii tended to increase with time in the frontal region and to decrease in the waters on the offshore side of the Kuroshio during the drifter tracking periods. The production of small copepods including nauplii in the waters on the offshore de of the kuroshio was 14% of that in the frontal region. The sum of the mean food requirements of the carnivorous macrozooplankters and sardine larvae was 11% of the production of small copepods including nauplii in the frontal region, compared with 136% in the waters offshore of the Kuroshio. The RNA/DNA ratios of postlarvae smaller than 8 mm in the frontal reqion 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 relatively high in the frontal region and low in the waters on the offshore 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.

Winter mixed layer depth and spring bloom along the Kuroshio front: implications for the Japanese sardine stock

“…Combination of the these previous studies and our results, and the positive correlation between MLD and phytoplankton density give an important implication for sardine stock variation. Since copepod eggs and nauplii are the main food source of sardine larvae (Nakata et al 1995), low spring chl a, corresponding to a shallow winter mixed layer, might worsen the feeding conditions for sardine larvae; this is because the relationship between chl a and nauplius density is not linear (Dr. Tohru Ikeya pers. comm.…”

“…Nutrient, phytoplankton and zooplankton distributions and hydrography are considerably different north and south of the Kuroshio axis (Kawai 1972, Nakata et al 1995, Sukigara et al 2011. For example, satellite data show a high density of chlorophyll a (chl a) along the northern side of the Kuroshio axis (Fig.…”

Wind mixing, food availability and mortality of anchovy larvae Engraulis encrasicolus in the northern Adriatic Sea

“…Economou 1987) and in relation to the structure and persistence of local concentrations of food particles (e.g. Nakata et al 1995, Lochmann et al 1997. The benefits of vertically structured aggregations of food particles for larval survival were highlighted by Lasker (1975), such vertical structuring of the plankton being related to wind mixing and water column stability (e.g.…”

Growth of larval and early juvenile Japanese anchovy, Engraulis japonicus, in the Kuroshio‐Oyashio transition region

“…The larvae have a strong positive taxis to turbidity (Uotani et al, 1993), so the shoals tend to be formed in turbid waters (Funakoshi, 1990). The production of copepods is higher in the coastal waters than in the frontal and offshore waters of the Kuroshio (Nakata et al, 1994(Nakata et al, , 1995. This results in a density-dependent decline in growth of larval S. melanostictus in the ®shing grounds (Watanabe and Kuroki, 1997).…”