Size- and density-dependent overwinter mortality of anchovy juveniles in the Bay of Biscay

Abstract

For small pelagic fish, recruitment success is mainly determined by the degree of survival over the early stages of life. In the case of European anchovy in the Bay of Biscay, major mortality events take place during spring and summer, and thus, the juvenile population in autumn is considered as an indicator of the next year recruitment. However, overwinter mortality could be an additional process regulating anchovy recruitment success in this area. Anchovy juveniles winter mortality was estimated over eleven years, between 2003 and 2013, using acoustic estimates of juvenile abundance (in autumn) and age-1 adults (the following spring) from two independent surveys. The results show that mortality is highly variable among years, with values ranging between 0.004 and 0.016 d(-1). Size- and density-dependent mortality were also evaluated during the analysis. Observed size distributions of juveniles in autumn were projected to the following spring assuming different size specific growth rate distributions in the population. Thereafter, the projected size frequency distributions were compared with the observed ones. The results indicate that mortality is higher for small individuals in most of the years (7 of 11). The mortality rate is positively correlated with juvenile abundance in autumn but not with mean juvenile length. The results suggest that although a density-dependent effect could be one of the main factors regulating the survival of juveniles during winter, size-dependent mortality could also impact recruitment. Understanding the factors that trigger size-dependent mortality during winter could contribute to the management of this fishery.