Modeling the effects of fishing and. implications for the design of marineprotected areas: Juvenile fish responses to variations in seafloor habitat

A number of recent studies have linked post-settlement survivorship of Atla ntic cod (Gadus morhua) with the complexity of the seafloor to which fish s ettle. Survivorship is greater in habitats of higher complexity (e.g., pebb le-cobble substratum with emergent epifauna > pebble-cobble > sand), where cover provides shelter from predators. Fishing with mobile gear such as bot tom trawls and dredges reduces the complexity of seafloor habitats. We used a dynamic model to (1) link patterns in habitat-mediated survivorship of p ost-settlement juvenile cod with spatial variations in habitat complexity, (2) simulate habitat change based on fishing activities, and (3) determine the role of marine protected areas in enhancing recruitment success. Densit y-dependent natural mortality was specified as three alternative functional response curves to assess the influence of different predator foraging str ategies on juvenile survivorship during the first 12 months of demersal exi stence. We applied the model to a theoretical patch of hard-bottom substrat a and to a case study based on seafloor habitat distributions at Stellwagen Bank National Marine Sanctuary (Gulf of Maine, Northwest Atlantic). Our re sults demonstrate that patterns in the shape of response surfaces that show the relationship between juvenile cod survivorship and density as well as movement rate were similar regardless of functional response type, that juv enile cod movement rates and post-settlement density were critical for pred icting the effects of marine protected-area size on survivorship, and that habitat change caused by fishing has significant negative effects on juveni le cod survivorship and use of marine protected areas can ameliorate such e ffects.