Larval Advection and Control Regulating Recruitment and Population Connectivity in an Upwelling System

Connectivity among marine populations is poorly known, because we have little idea of where planktonic larvae go. Spatial variation in currents can cause populations in some areas to receive or export more larvae than other populations, even though habitat quality may be similar among locations. However, our recent work suggests that sweeping generalizations regarding the role of oceanographic processes affecting larval transport and settlement in productive upwelling regions along the western margins of continents require much closer scrutiny to provide reliable information on larval connectivity between source and sink populations. Contrary to the prevailing view, larvae of most invertebrates appear to exert considerable control over their movements, remain very close to shore and recruit onshore in strong upwelling conditions. We are testing our conceptual model regarding the behavioral and physical mechanisms regulating cross-shelf transport, supply and connectivity across an upwelling cell using three approaches. First, we estimated connectivity of a representative species (porcelain crab Petrolisthes cinctipes) using a Bayesian modeling approach in which prior estimates of larval transport and connectivity (a dispersal kernel) were combined with field estimates of habitat quality, larval production, and larval settlement to obtain updated estimates of connectivity patterns. Estimated connectivity patterns reveal considerable spatial heterogeneity in the strength of larval sources, a consequence of both variation in population density and estimated oceanographic dispersal distances. We anticipate that our approach will be broadly applicable worldwide in situations where information on dispersal from larval tracers or circulation models is unavailable. Second, we are developing a natural elemental marking technique for determining population connectivity of species that do not retain calcified structures during larval development. This approach has been used to establish larval connectivity of up to 21 populations for 5 years from northern California to southern Oregon. Third, we are incorporating suites of larval behaviors across the shelf into ROMS models to estimate larval transport and connectivity for crustaceans in northern California. Our results indicate that larvae of many species may settle closer to natal habitats than is commonly believed, even in highly advective upwelling regimes. Taken together, these estimates of population connectivity will be useful in managing fisheries stocks and in designing and evaluating networks of marine protective areas in upwelling regions.