Exploring the consequences of complex habitat loss for the New Zealand blue cod, Parapercis colias

Complex coastal habitats including seaweed and kelp forests are declining throughout the world due to the impacts of ocean warming, coastal development, overfishing, and pollution. Complex habitats provide shelter, food, nesting, and nursery sites, allowing for high biodiversity and productivity that supports valuable economic, cultural, and recreational services. From the perspective of prey, habitat-forming macroalgae offer physical and visual refuges that reduce their risk of predation. However, for predators, this refuge availability presents a trade-off. Whilst prey might be more abundant in the presence of complex habitats, they are also likely to be more difficult to catch and consume, creating uncertainty around the consequences of habitat loss for predatory species. Here we explore the trade-off between prey availability and predation success for a model coastal predator, the New Zealand blue cod, Parapercis colias, which is a valuable commercial and recreational fisheries target. Field surveys quantified differences in prey abundance inside and outside complex habitats, whilst mesocosm experiments determined the influence of refuges on prey consumption. A simple food web model was developed to explore how prey availability and predation success trade-off in the presence of habitat complexity to determine predator population dynamics and vulnerability to fishing. Results indicate that increased prey abundance compensates for decreased predation success within complex habitats and allows for high densities of predators. However, the effects of habitat loss are dependent on the mechanisms by which complexity increases prey abundance. If complex habitats act only as predation refuges, then a loss of complexity is advantageous for predators, increasing their abundance and reducing their vulnerability to overfishing. However, if complex habitats also enhance the carrying capacity of prey through mechanisms unrelated to predation, then habitat loss reduces predator abundance and increases vulnerability to overfishing. Our study highlights the need to understand specific mechanisms that promote population abundance in complex habitats and shows how this knowledge will help us to better predict the impacts of habitat loss for coastal fisheries.