Inter-cohort differences in growth, condition and feeding of juvenile anchovy (Engraulis encrasicolus) in the Gulf of Béjaia (Algerian coast, SW Mediterranean): Implications for recruitment success
Résumé
Since the spawning season of European anchovy spread over more than six months we compared the growth, condition and feeding of juvenile anchovy originating from two different cohorts (summer and autumn 2007) in the Gulf of Béjaia (Algerian coast, SW Mediterranean). Juvenile anchovy fed predominately on zooplankton, the main prey items being copepods, and to a lesser extent on molluscs larvae, polychaete larvae and ostracods. Prey composition in the diet of the two cohorts differed from each other, and the vacuity index was significantly lower for the autumn cohort (20%) than for the summer cohort (70%). Juveniles from the summer cohort had higher growth (somatic and otolith growth) and better condition than individuals from the autumn cohort. Growth rates were 0.74 mm d−1 and 0.59 mm d−1 respectively. We also found that otolith daily increments widths were higher for the summer cohort. Sea surface temperature (SST) and sea surface chlorophyll-a concentration derived from satellite imagery (SeaWiFS), were used as the main environmental variables to analyze anchovy growth, condition and feeding. No relationships were observed between chlorophyll-a concentration and juvenile vacuity index, nor growth rates and condition. Our study suggests that growth and condition differences among the two cohorts were mainly due to water temperature differences encountered by the fish during their larval and juvenile periods. High growth rates and condition of individuals from the summer cohort may have conferred them ecological advantages and enhanced their survival. It has been suggested that, in this region, late spawned cohorts contribute less to the recruitment of the adult population