Post-settlement growth and mortality of brackishwater clam Corbicula japonica in the Kiso estuaries, central Japan

Different cohorts of Corbicula japonica benthic individuals (from new settlers just after larval settlement up to recruits with 12-mm shell length and more) in the Kiso estuaries, Japan were separated based on temporal changes in shell length histograms. The post-settlement growth and mortality were examined for cohorts that were successful in fishery recruitment. Three growth models (Logistic, Gompertz and ALOG) were fitted to the growth data on average shell lengths of different cohorts, the best-fitting growth model varied depending on cohort and further on cohorts that were grouped according to estuary and to either overwintering or non-overwintering. Mortality rates were estimated based on regression models fitted separately to the data on temporal changes in the log-transformed density against sampling dates of individuals before and after fishery recruitment (i.e. attaining 12-mm shell length). For most cohorts, however, there was no significant difference in mortality rates between individuals before and after fishery recruitment, although significant differences were detected in initial densities between individuals of these two groups. It may be suggested that daily mortality rate after fishery recruitment was compensated with fishery mortality, so that total mortality rate was not different between individuals of these two groups.