Vertical migration effects on the dispersion and recruitment of European anchovy larvae: From spawning to nursery areas

Recruitment of European anchovy has been traditionally thought to largely depend on the passive transport and dispersion of eggs and larvae from spawning to nursery areas. Knowledge of the factors influencing the vertical distribution of fish early stages, and consequently influencing the transport, is a crucial issue in fisheries science. The aim of this study is to assess the relevance of diel vertical migration (DVM) as a mechanism involved in the transport of European anchovy larvae toward nursery areas taking into account age/stage-dependent vertical migration (i.e., the maximum migration). We developed a simplified vertical migration sub-model for anchovy larvae included in an Individual-based (IBM) hydrodynamic coupled model. Two types of simulation experiments were conducted: (1) ‘Pure’ Lagrangian (passive) transport experiments and (2) biological behavior transport experiments with a realistic scheme for egg-buoyancy, larval growth, and DVM scheme. We detected high variability in the trajectory and final position of larvae with 14mm length between the passive and biological behavior experiments. The particles were less clustered in the passive transport experiment. In the biological experiment the particles were aggregated depending on the mesoscale oceanographic structures, evidencing a transport associated to filaments and meanders. The formation of schools was facilitated by the transport in filaments and larvae transported within filaments generally avoided the nucleus of the eddies and the central part of the North Current. Moreover, our results suggest that there is interconnectivity between all the anchovy sub-populations in the NW Mediterranean and that the Gulf of Lions and the shelf waters surrounding the Ebro River Delta are the most important areas for anchovy reproduction in this region. Additionally, we detected that the pre-recruitment is higher in those years when the larvae retention is favored. We underline that bottlenecks in larval transport modeling are related to the scarcity of knowledge in developmental biology and behavior of anchovy larvae and emergent mechanistic processes.