Habitat-mediated survivorship of juvenile Atlantic cod (Gadus morhua): Fish population responses to fishing-induced alteration of the seafloor in the northwest Atlantic and implications for the design of marine protected areas

Much scientific research has focused on large-scale planktonic egg and larval mortality as a factor contributing to annual fluctuations in year-class strength of fishes. Significantly less attention has focused on small-scale, localized processes affecting survivorship of early benthic-phase juvenile fish, although mortality during this life history stage is sufficient to significantly modify population size. The sustainability of Atlantic cod (Gadus morhua) populations, a species with ecological, economic and cultural significance for New England, is dependent on the continued productivity of off-shore nursery grounds currently at risk from alteration by fishing activity. Key questions in this regard are: (1) the role of seafloor habitat in mediating the survivorship of early benthic-phase cod; and (2) the implications of predator-prey-habitat interactions for the design and allocation of marine protected areas (MPAs) for fish conservation and management in the northwest Atlantic.;Laboratory experiments quantify the effect of fishing impacts to seafloor habitat on mortality rates for juvenile cod. Results indicate that the presence of emergent epifauna (habitat undisturbed by fishing) resulted in a significant decrease in juvenile cod mortality when compared to flat sand (habitat disturbed by fishing) and that density of emergent epifauna is more significant than epifaunal height in reducing juvenile cod mortality. A computer model, parameterized with lab-derived mortality rates and spatial variation in habitat type from seafloor mapping, captures non-linearities in the responses of fish populations to seafloor habitat alteration given variations in fish movement rates, fish densities and MPA size. Comparison of the existing National Marine Sanctuaries to the National Parks, Forests and Wildlife Refuges illuminates a significant disparity in the designation of protected areas between marine and terrestrial systems with respect to the size of protected areas, their number, and the total area and the proportion of U.S. land and waters currently under protection. Primary conclusions include: (1) fishing alteration of the seafloor has a significant deleterious impact on associated Atlantic cod populations; (2) there is a need to incorporate seafloor habitat protection in the designation of MPAs for fish management; and (3) habitat-specific MPAs should be designated in the northwest Atlantic to buffer against environmental and managerial uncertainty.