Selective mortality of larval Japanese seabass in Ariake Bay, Japan

Abstract

This article describes the growth, mortality, and selection patterns in early larval stage of Japanese seabass Lateolabrax japonicus in Ariake Bay, Kyushu, Japan. Japanese seabass larvae were collected from the spawning ground in December 2007, and juveniles were collected from the nursery ground in March 2008. Otoliths were analyzed to produce back-calculated daily records of size-at-age and growth rate. Back-calculated growth rates and sizes were compared between larvae and juveniles to determine whether selective mortality occurred. A weight-specific growth coefficient (G) and instantaneous mortality coefficient (M) were computed, and the recruitment potential was evaluated from the ratio of M:G. Selection for fast-growing and bigger larvae was evident during the 5–14 days after hatching (DAH). Selective mortality acted to preferentially remove fish that were slow growing and/or relatively small members of the cohort at least during the period examined. Trends in the growth rate differences between larvae and juveniles suggested that the selection process continued beyond 14 DAH although the exact duration over which selective mortality occurred was unknown. Survivors of Japanese seabass exhibited traits consistent with all aspects of the ‘growth–mortality’ hypothesis: faster growth, larger size-at-age, and shorter larval stage duration (LSD), i.e., larvae with faster growth, bigger size-at-age and a shorter LSD selectively survived the larval period. Larvae had higher M (0.323) than G (0.159), resulting in the M:G ratio of >1.0 (M:G = 2.031), suggesting that the larval cohort was rapidly losing biomass. Future studies should look to determine the duration over which selective mortality occurs and the timing of transition between M and G (M = G) and accumulation of cohort biomass.