Collaborative Research: Cobble landscape structure, spatial gradients in predation, and variable demographic bottlenecks in the American lobster

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Understanding how pre- and post-settlement processes interact to govern the dynamics of marine open populations remains a central issue in marine ecology. In this study the American lobster Homarus americanus is used as a model system to determine how nursery habitat structure influences post-settlement mortality and dispersal. The central theme of the study is to find if cobble landscape structure (i.e., patchiness) and gradients in lobster population density, predator abundance and predator diversity along the New England coastline interact to cause regional variation in the strength of a hypothesized juvenile lobster demographic bottleneck. As juvenile lobsters reach the emergent phase they must forage outside of cobble shelter and may become shelter limited even though they have not reached a size refuge from predators (e.g., groundfish). The underlying hypothesis is that cobble patchiness and shelter limitation will cause a demographic bottleneck that limits lobster abundance in regions of high predator density (e.g., southern New England) but not in regions of low predator density (e.g., Maine). Supplementing more than a decade of state-supported postlarval settlement surveys along the New England coast, and NOAA-supported sidescan sonar, remote vehicle and scuba surveys to evaluate the regional linkage between lobster abundance and habitat use with indices of cobble habitat structure and settlement strength this NSF-sponsored research will complement the surveys with manipulative experiments. These are aimed at determining if regional differences exist in: (1) lobster movements among habitat patches; (2) predation risk; (3) local shelter limitation; and 4) shelter fidelity (propensity to switch shelters). In each of two sites in four regions (Rhode Island, Massachusetts, mid-coast and eastern Maine) the investigators will construct experimental landscapes consisting of one fragmented and one non-fragmented cobble patch array. The proportion of juvenile lobsters moving from a source patch where they are released to surrounding destination patches within these landscapes will be correlated with array type, predator abundance, predation risk, and lobster population density to infer the relative contributions of dispersal and mortality in lobster losses from settlement patches. In a separate experiment, size arrays of artificial shelters will be deployed to quantify the degree of size-specific shelter limitation by region. Corresponding predation experiments will be conducted to determine how vulnerability to predation varies regionally. The lobster size that is both locally shelter-limited and still vulnerable to predation will indicate the size at which juveniles are subject to a demographic bottleneck. Finally, they will tag lobsters to measure shelter fidelity to determine how movements between shelters correlates with lobster density, predator indices, or habitat patchiness among sites. Results of these integrated projects will be broadly disseminated. The American lobster is the most economically important single-species fishery in New England. By addressing the black box of lobster post-settlement processes, these scientists will contribute to a long-standing scientific vision of developing a large-scale spatially explicit model that couples pelagic and benthic processes to predict harvests in space and time. Their results will aid resource managers and the fishing industry in the continued development of predictive tools for the fishery. Student training is also integral to this work: the proposed research will form the basis for a thesis or dissertation for an SDSU graduate student, and at least four undergraduates per year will serve as interns earning academic credit. Another important educational component of this project is the continued development of an interactive CD and web-based activity called Hatch-to-Catch already recognized by the National Science Teachers Association as a useful educational tool for K-12 learning (see www.scilinks.org).