Quantification of key biological processes determining the dynamics and the assessment of the anchovy population in the Bay of Biscay: growth, reproduction, demography and natural mortality.

Abstract

Growth, reproduction, demography and natural mortality determine the dynamics and assessment of fish populations. Studies on these biological characteristics of the anchovy in the Bay of Biscay are presented, along with a review of their estimation procedures: Regarding growth, the validation of the age determination procedure is presented with an assessment of its performance since 1987. Age determination, given a date of capture, requires knowledge of the typical annual growth pattern of otoliths, their seasonal edge formation by ages and of the most typical checks. These features are described, along with a summary of the annual growth in length. The study of reproduction focuses on the spawning fraction (S), i.e. the fraction of mature females spawning per day. The mean proportion of day_0 and day_1 was selected as the best estimator of S due to its statistical properties. S was about 0.4 (CV=18%). For the study of the demography, the Daily Egg Production estimator of Biomass (DEPM) has been extended to produce population at age estimates and variances through cluster sampling of ages (or using age-length keys). The application to the DEPM series revealed the dominant role of the 1-year-old recruits in the population. Finally, the closure of the anchovy fishery between 2005 and 2010 provided an opportunity to estimate natural mortality (M) using data from DEPM and acoustic surveys since 1987. M estimates were around 0.9 if common for all ages. However, we found firm evidence that natural mortality at ages 2 and older is markedly higher than at age 1, which suggests senescent mortality. These results implied changes in the perception of the dynamics and assessment of this population.