Growth-selective predation hypothesis revisited for larval anchovy in offshore waters: cannibalism by juveniles versus predation by skipjack tunas

Abstract

The 'growth-selective predation' hypothesis was revisited for larval Japanese anchovy Engraulis japonicus, focusing on larval cannibalism by juveniles and larval predation by skipjack tunas Katsuwonus pelamis, in offshore waters. Larval anchovy and predators were captured simulta- neously in June 1997 and in May 2000 in the western North Pacific. Growth rates estimated through otolith microstructure analysis, as well as somatic sizes, were compared between the ingested larvae from the stomach contents of the predators and the surviving larvae from the original population. Size-selective mortality was directed negatively for cannibalism by juveniles and positively for pre- dation by skipjack tunas. The cannibalised larvae had lower growth rates than the larvae from the original population in the same larval size range. On the other hand, a similar comparison showed no differences in larval growth rates for predation by skipjack tunas. Larval cannibalism by juveniles would potentially regulate growth-selective survival as well as survival rate itself during early life history stages of Japanese anchovy, while predation by skipjack tunas would influence survival rate itself but not growth-selective survival.