Growth-dependent survival mechanisms during the early life of a temperate seabass (Lateolabrax japonicus): field test of the ‘growth–mortality’ hypothesis
Three concepts based on size, time and growth rate are contained in the functional mechanisms of the ‘growth–mortality’ hypothesis, and are known as the ‘bigger is better’, ‘stage duration’ and ‘growth-selective predation’ hypotheses, respectively. Although it is sufficiently documented in fishes that faster growing and bigger larvae selectively survive, synergistic operation of the three components of the ‘growth–mortality’ hypothesis within a species have received little attention. In this study, we have tested the components of the ‘growth–mortality’ hypothesis based on the growth characteristics of original population (OP) and survivors (SV) of Japanese seabass (JSB) Lateolabrax japonicus. Larval and juvenile JSB were repeatedly sampled from the Tango Sea in 2007 and 2008. Otoliths from larvae and juveniles were analyzed to produce back-calculated daily records of size-at-age and growth rate, and were compared between OP and SV. Selective survival for fast-growing and bigger larvae was evident, and our results strongly support the ‘bigger is better’ hypothesis as well as the ‘growth-selective predation’ hypothesis. Growth rates of the individuals that metamorphosed into juveniles had significantly faster growth rates than non-metamorphosing larvae older than 48 days, the minimum age for metamorphosis, suggesting a clear relationship between growth rates and the timing of metamorphosis. On average, SV completed the larval stage ∼13 days earlier than those of OP, supporting the ‘stage duration’ hypothesis. Thus, SV of JSB exhibited traits consistent with all aspects of the ‘growth–mortality’ hypothesis: faster growth, bigger 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. Although maternal influence on growth and survival was evident, factors that regulate growth–mortality mechanisms remain to be explored for JSB.