Modelling dispersal dynamics of the early life stages of a marine flatfish (Solea solea L.)

Connectivity throughout the life cycle of flatfish remains an open question, especially during the early life stages. Their effective management requires understanding of how spawning grounds and nurseries are connected and what processes influence larval retention and dispersal. The case of sole (Solea solea L.) is of particular interest because it is one of the most valuable commercial species in the North Sea, although stocks are chronically overexploited and variability in interannual recruitment is high. The transport of sole larvae from the spawning grounds to the nurseries is driven by hydrodynamic processes, but the final dispersal pattern and larval survival/abundance might be influenced by both behavioral and environmental factors. Therefore it is important to understand the relative impact of hydrodynamics, environment, behavior and ecophysiology on sole larval dispersal. Here we use a particle-tracking transport model coupled to a 3D hydro-dynamic model of the North Sea to investigate interannual variability of the transport of sole larvae over a 12-year period (1995–2006). A sensitivity analysis is performed to assess the relative impact of hydrodynamics, temperature and behavior on the recruitment dynamics to the nurseries. Four scenarios have been tested: (i) constant forcing of sea surface temperature during all years but varying meteorological forcing and river runoff, (ii) constant meteorological forcing during the whole period but varying sea surface temperature and river runoff, (iii) no vertical migration and (iv) an extended drift period (max. 30 days) before settlement if the larvae are not close to a suitable sediment type. Results suggest that year-to-year variability of larval supply to the nurseries is high, both in terms of abundance and larval source (balance between retention and dispersal). Sensitivity analysis shows that larval abundance at the end of the larval stage increases considerably if a settling delay is included. The impact of vertical migration on larval transport and the variations in larval retention at the nurseries due to varying meteorological conditions and sea surface temperature forcing are not spatially consistent