Larval dispersal from an energetic tidal channel and implications for blue mussel (Mytilus edulis) shellfisheries

Complex coastal currents control early-stage larval dispersal from intertidal populations, and late-stage settlement patterns, but are often poorly resolved in larval dispersal models. Generally, there is high uncertainty in the timing of larval spawning, which markedly affects larval dispersal. In this study, we describe the physical parameters that induce spawning events in the blue mussel, Mytilus edulis, using a variation of the Condition Index (which relates the mass of meat to the mass of the shell) as a proxy. We developed a high-resolution Eulerian coastal hydrodynamic model, coupled with a Lagrangian particle tracking model, to quantify the potential dispersal of early-stage mussel larvae based on differing spawning dates obtained from field data. Our results showed that (1) the timings of larval spawning cannot be explained solely by ‘thermal shocks’ in the sea or air temperatures (i.e. fluctuations in temperature causing stress); (2) larger spawning events generally occurred during neap tides; (3) the simulated larval dispersal was largely but not always predicted by averaged current pathways (calculated over two weeks period); and (4) simulated self-recruitment was low at sites associated with strong tidal currents. These results have important implications for shellfisheries stock management and sustainability. Specific to this study, simulated mussels from shellfishery beds off North Wales dispersed more than 25 km in one week and so could feasibly contribute to the wider population throughout the northern part of the Irish Sea.