Identification of two hatch date-dependent cohorts and variation in early growth rate of mud carp (Cirrhinus molitorella) in the Pearl River Delta, China

Cirrhinus molitorella is one of the most important commercial fish species in the Pearl River, but there are few published studies on the early growth of this species. In the present study, seasonal recruitment patterns, temporal variations in early growth rates and environmental variables were analysed. In all, 1512 individuals were collected during monthly sampling from July 2015 to January 2016, ranging from 30.8- to 84.9-mm standard length (SL). A subsample of 276 juveniles was aged from 68 to 193 days, with hatch dates between 2 February and 19 September, by determining daily increments in otoliths. Monthly abundance data and SL frequency distributions showed that there were two major temporal cohorts of juvenile recruitment, which hatched primarily in April and July. Average growth rate, estimated by the linear relationship between age and SL, was greater for the April cohort (0.49mmday–1) than the July cohort (0.37mmday–1). Mean increment widths from the 23rd increment onwards were significantly greater for the April than July cohort (P