Brown shrimp on the edge: Linking habitat to survival using an individual-based simulation model

In many coastal areas, natural habitats are being fragmented and lost to encroaching human development. These landscape changes can affect the production of recreationally and commercially important fisheries because many exploited species of fish and shellfish are estuarine dependent and utilize coastal marshes as nursery grounds. Brown shrimp are an example of a commercially exploited species that may be highly affected by changes in the spatial distribution of habitat types. We used a spatially explicit, individual-based simulation model to explore the role of marsh vegetation and edge habitat in brown shrimp survival. The model simulated shrimp movement, mortality, and growth of individual shrimp from arrival as postlarvae to 70-mm body length, when they emigrate offshore. Simulations were performed on 100 X 100 m spatial grid of 1-m(2) cells, with each cell labeled as "water" or "vegetation". Predation mortality was influenced by shrimp size, movement, and habitat. Simulated shrimp growth depended on temperature, habitat, and local shrimp density. We examined the relationships between shrimp survival and marsh attributes (amount of vegetation and edge habitat) by simulating a series of four habitat maps that we created from aerial photographs. Biological parameters were derived from published estimates and from field data. We corroborated the model by comparing the simulated shrimp abundance with summary statistics from long-term monitoring data, by comparing the simulated density with fine-scale patterns observed in field studies, and by comparing simulated and measured stable-isotope values. Surviving shrimp grew faster, moved less, spent more time in vegetation, and experienced slightly higher local densities than shrimp that died during the simulation. Habitat maps with more edge habitat invariably produced higher simulated shrimp survival rates. High-edge habitats increased survival by providing shrimp more direct access to vegetation without additional movement-related mortality and density-dependent growth costs associated with low-edge habitats. Model predictions were robust to higher numbers of initial postlarvae and to alterations to the movement rules. The results of this study suggest that the management of brown shrimp should be extended from protecting the spawning stock through catch regulations to also protecting the estuarine life stages through habitat conservation and restoration.