Implications of spawning migration patterns of the giant mud crab Scylla serrata (Forskål, 1775) on opportunities for larval dispersal

Connectivity is an essential driver for aquatic species distribution, genetic variability and stock structure. The giant mud crab (Scylla serrata) is a coastal portunid commonly associated with estuaries and mangrove systems. This species has been observed to undertake a seaward spawning migration, as the larval development is known to be more successful under the stable environmental conditions typically found in marine waters. The larvae return to the coastal areas through advection, where they are recruited and enter the estuaries after metamorphosing into the first instar. Here, we used numerical modelling to test hypotheses regarding probabilities of larval settlement of the giant mud crab and the effect of the distance offshore from which females release the eggs. Our scenarios considered the biological characteristics of larvae and oceanographic conditions for six locations for mud crabs along a complex coastline - the Queensland east coast, Australia. The models suggest that all locations tend to self-supply, and to exchange mud crab larvae with other regions, but in different magnitudes. The spawning distance offshore considerably affects larval distribution and settlement. The main drivers for larval advection in areas within the continental shelf are wind patterns and coastal currents, while offshore along the Australian continental slope, the main drivers are ocean currents. Self-recruitment is predominant, although we also observed a significant degree of connectivity between each location and the surrounding coastline. Short spawning migrations benefit self-recruitment in all scenarios, but long offshore migrations favour connectivity among different locations. This source/sink balance seems to depend on the local oceanographic features. Nevertheless, offshore spawning by the giant mud crab has the potential to provide for successful recruitment in a variety of environmental contexts. This study provides novel predictions of the probabilities of larval settlement for mud crab populations considering ocean advection that can be applied to different contexts.