Many species of marine crustacean larvae are able to migrate vertically in the water column to avoid surface flows that may carry them away from their preferred developmental or settlement habitats. Because of this, maps of surface currents do not explain larval distribution patterns and hence cannot be used to identify adult populations that are most likely to replenish local populations or seed distant ones. This fact has been observed by the researchers leading this project in strong upwelling centers off California for 45 species of crustaceans. From the literature and their own research, they have identified six primary modes of behavior by which larvae exert control over their position offshore—by exploiting vertical shears in horizontal flows. These vertical migrations may be triggered, for example, by a certain developmental stage or by tidal cycles, heat or hydrostatic pressure, among other things. The main objective of this project is to incorporate these six larval behaviors into a Lagrangian particle transport module within a Regional Ocean Modeling System for the California Current to study larval transport and connectivity of populations along the coast and within the new MPAs. For each of the six behaviors, thousands of virtual larvae are being released into the surface layer (depths less than 30 meters) from 1 kilometer to 10 kilometers from shore every other day, transported by modeled ocean currents and tracked for 180 days. Maps of habitat type have been incorporated into the “settlement” component of the model. Settlement is assumed to occur if larvae return to shore at the end of their pelagic larval stage, either 30 days or 60 days depending on the model “run.” Results from the project’s first year further underscore the importance of vertical migrations to settlement success. Along the Central California coast, larvae that were assumed to descend beneath the surface layer were 500 times more likely to be retained at the coast 30 days after release than those larvae that remained with the surface boundary layer. Settlement success rates were 145 times greater for larvae that avoided surface flows. Among the final outputs from this project will be the construction of 1.5 kilometer-resolution circulation model from Baja California, Mexico to the US-Canadian border that will include climatological estimates and interannual variability in connectivity statistics from larval trajectory output.
Realistic Behavioral-Physical Models of Connectivity for a Network of Marine Protected Areas