Inter-annual comparison of diet and daily ration of larval jack mackerel Trachurus japonicus in the southern East China Sea

We describe and compare the feeding habits of Trachurus japonicus larvae from 2.4 to,8-mm body length (BL) sampled during February to March of 4 consecutive years (2002-2005) in the primary spawning ground along the shelf-break region of the southern East China Sea. Dietary overlap based on Schoener's index revealed high similarities (58-81%) among the 4 years. In larvae 5-8-mm BL, the diet was strongly dominated by calanoid copepodites and Oncaea spp. Larvae <5-mm BL fed occasionally on copepod nauplii and gastropod larvae, as well as the above two main prey items. Feeding was mainly restricted to 0700-2100 h and feeding incidence was 78-90 and 100% in larvae <5 and 5-8-mm BL, respectively, without inter-annual variation. The mean number of prey items per gut was positively correlated with larval size, and in each size class it did not significantly differ among the 4 years. Based on information on the feeding period (14 h), the mean number of prey items, and gut evacuation time (2 h), we estimated that larvae <3-mm BL would consume similar to 4-8 prey items day(-1), and this increased greatly with larval growth, and larvae 7-8-mm BL consume similar to 56-60 prey items larvae(-1) day(-1). We estimated that similar to 0.5% of the available prey items were consumed by larvae day(-1) in 2005 when both prey and larval abundance data were available, implying that the larvae have only limited effects on prey abundance.