Population dynamics and management implications of larval dispersal

Abstract

Strong advective fields in coastal shelf ecosystems, particularly in eastern boundary currents, lead to significant longshore dispersal of the larval phase of meroplanktonic benthic invertebrates. Field observations of larvae and settlement in the California Current by recent workers are an example of the increasing research on the biological/physical mechanisms underlying larval transport. In particular, these studies identify physical/biological interactions that control the spatial distribution of larvae settling successfully. The current study demonstrates how the results of such studies contribute to (1) better understanding of population dynamics, and (2) better population management, using the cyclic covariability in the Dungeness crab Cancer magister population as an example of the former and management of the red sea urchin Strongylocentrotus franciscanus population as an example of the latter. Earlier modelling studies have shown that dispersal influences the stability and synchrony of variability in metapopulations such as the Dungeness crab, and that the spatial scale of covariability is roughly the scale of dispersal. The mechanism identified by recent research implies that the spatial scale of dispersal should be the distance between promontories in the California Current, roughly 100-200 km. Analysis of covariability between time-series of recruitment at different locations along the coast confirm that this is the spatial scale of longshore variability in the Dungeness crab. The spatial pattern in red sea urchin settlement caused by the identified mechanism provides: (1) the basis for spatially explicit management, and (2) an explanation for the observed spatial variability in the degree of overfishing. Research on larval dispersal is also providing the information necessary to design spatially explicit management strategies involving either permanent or temporary fishery closures. Both population dynamics and management require further research to describe the origins of larvae and the early larval phase, in addition to the transport just before settlement.