Behaviour influences larval dispersal in shelf sea gyres: Nephrops norvegicus in the Irish Sea

The western Irish Sea seasonal gyre is widely thought to play an important role in the local retention of resident larvae. This mechanism could be particularly crucial for the larvae of the heavily fished crustacean Nephrops norvegicus (L.), as their sediment requirements highly restrict where they are able to settle. Recent research suggests that the gyre may be becoming less retentive due to changes in atmospheric forcing; thus it is now crucial to understand how the gyre influences larval dispersal. This investigation addresses the hypothesis that shelf sea gyres reinforce larval retention, using a biophysical model with vertical migration, habitat selection and temperature-dependent pelagic larval duration (PLD) configured to match the behaviour of N. norvegicus larvae. The results suggest that the gyre does increase the likelihood that passive larvae remain within the western Irish Sea when the larvae remain fixed at the depth of peak gyral flow. However, retention rates are significantly lower when vertical migration is introduced, and there is no evidence that the gyre promotes larval retention amongst either vertically migrating larvae, or larvae that require muddy sediments for successful settlement. In contrast, vertical migration is shown to be favourable for local retention in the eastern Irish Sea. PLD varies by a factor of 2 according to release date and location. The simulations suggest that whilst some highly limited and almost entirely unidirectional larval exchange may occur, the distinct sites largely rely upon local recruitment.