Feeding and growth of early juvenile Japanese sardines in the Pacific waters off central Japan

Abstract

Larval and early juvenile growth was backcalculated for individual Japanese sardines Sardinops melanostictus using the biological intercept method based on the allometric relationship between otolith radii and fish lengths. Sardines grew at 0·81 mm day−1 during the larval stage. In the early juvenile stage, they grew from 32·3 to 45·4 mm fork length (L) over a 20-day period (0·64mm day−1). Using the observed relationship between L and wet body weight (W), W = 0·00942 L2.99, W of the sardine juveniles was calculated to increase from 306 to 832 mg during the 20-day period. The carbon (C) requirement to achieve this growth in weight was estimated to increase from 5·7 to 9·6 mg day−1. Stomach contents of the sardines were composed mostly of copepods (73%) and larvaceans (25%). Wet stomach content weight (Ws) was expressed by a power function of the W, Ws=0·731 W0·658. Carbon and nitrogen constituted 41·7 ± 1·5 and 10·0 ± 0·4% of the dry Ws, respectively. Stomach C content increased from 2·0 to 3·9 mg during the 20-day period. Three to four cycles of the daily turnover of stomach contents during the 16 h of daytime, corresponding to a gastric evacuation rate of 0·2–0·3 h−1 under continuous feeding, met the C requirement to achieve the backcalculated growth in early juvenile sardines. The Kuroshio frontal waters seem to provide Japanese sardine juveniles with favourable growth conditions.