A numerical study of marine larval dispersal in the presence of an axial convergent front

Estuarine axial convergent fronts generate strong secondary (cross-estuary) flows which can be important for larval dispersal and settlement, and have been shown in this research to aid estuarine retention of natal populations which will promote production. This paper explores several larval migratory strategies, synchronised by sensory cues such as pressure, velocity, salinity and solar radiation, in relation to an estuarine axial convergent front – an important circulatory mechanism that forms in many coastal regions where larvae are concentrated; hence these results have implications for fisheries management. A three-dimensional hydrodynamic model is applied to an idealised channel, parameterised from observations of a well documented axial convergent front in the Conwy Estuary, UK. The model simulates the bilateral cross-sectional circulation of the front, attributed to the interaction of lateral shear of the longitudinal currents with the axial salinity gradient. Axial surface convergence develops during the flood phase of the tide and (weaker) surface divergence during the ebb phase. Lagrangian Particle Tracking Models subsequently use velocities from a range of simulated tidal and climatic scenarios to track larvae in the estuary. The results show that axial convergent fronts aid estuarine retention of larvae. Specifically retention is enhanced for larvae that experience tidal stream transport and diel migration. Tidally-synchronised larvae exhibit the strongest landward dispersal while the modelled copepod (a combination of tidal and salinity cues) exhibits ‘full retention’ in the mid-estuary release location. Finally, the vertical migration speed of the larvae must exceed the background vertical velocities in order to get the greatest enhancement of retention.