Trophic Shift in the Japanese Anchovy <i>Engraulis japonicus</i> in its Early Life History Stages as Detected by Stable Isotope Ratios in Sagami Bay, Central Japan

Abstract

Using the stable isotope technique and assuming a δ15N enrichment of 3-4‰ per trophic level, Japanese anchovy Engraulisjaponicus were found to increase in trophic position by a half of one trophic level as they grew from 14mm to 30mm in standard length (SL) and by a further third to one half from 30mm to 70-80mm SL, ranging over a total δ15N of 3.5‰ and δ13C of 1.6‰ from recruitment to the fisheries ground to adulthood, during October 1993 in Sagami Bay, Central Japan. Based on the δ15N shift, a similar increase in trophic position with growth was observed in January, March, August and September. Light N, derived from river runoff during the rainy season in August, is thought to have been rapidly incorporated into lower levels of the food chain since a sudden drop in δ15N was observed in anchovy larvae of two size ranges, 19-21mm and 29-31mm SL. Japanese sardine Sardinops melanosticta were found to increase in trophic position more rapidly than Japanese anchovy in the larval life history stage. The isotopic signatures of the larger fish taken from the shirasu (anchovy larvae) fishing nets in August 1994 were suggestive of heavy reliance on anchovy larvae as a food source. The stable isotope technique was found to be effective in estimating ontogenetic change in trophic position in the Japanese anchovy.