The selection process from larval to juvenile stages of anchovy (Engraulis encrasicolus) in the Bay of Biscay investigated by Lagrangian simulations and comparative otolith growth

The relation between growth and survival was investigated using the otolith growth data collected during repeated larval surveys ( May - July) and a juvenile survey ( September) undertaken in 1999 on anchovy spawning and nursery grounds in the Bay of Biscay ( NE Atlantic). The paper describes the methodology for reading the larval and juvenile otoliths, for reconstructing the correspondence in space and time between juveniles and larvae using Lagrangian simulations, and for comparing the otolith growth rates among the reconstructed sub-cohorts. Virtual buoys were released weekly on the grid of a three-dimensional hydrodynamic model and their trajectories were tracked. The origin of an individual was determined by selecting the trajectory beginning on its hatching week and ending at the minimum distance of its sampling location on the sampling date. Larvae and juveniles with the same spatio-temporal origin were selected and supposed to belong to the same sub-cohorts. The surviving juveniles showed faster growth rates during their larval period than the pool of larvae they were estimated to originate from, which supports the idea of growth-selective survival. Alternative interpretations ( transport and gear selectivity) are discussed. Variations in otolith growth pattern also suggest a higher juvenile growth over the shelf break than in oceanic waters.