Understanding organism-habitat relationships and critically evaluating reference areas is key to marine protected area assessment

Marine protected areas (MPAs) are specified ocean areas designated by management agencies in which extractive human activities (i.e. fishing) are either prohibited (‘no-take’) or limited in some way. MPAs have been implemented globally as a conservation tool to improve the health and function of marine ecosystems. Research has focused on assessing MPA effectiveness; however, certain habitats and communities are often avoided because they are difficult or expensive to monitor. Mesophotic (30-100 m) rocky reef fish communities are valuable commercial and recreational resources that are highly targeted but often overlooked in monitoring due to depth-restricted sampling. We used 2 MPAs in the statewide protection network of California (USA) that prohibit fishing for these species, along with paired reference sites, to test how protection status along with environmental conditions influenced the abundance and biomass of 3 highly targeted species groups with varying life histories and habitat preferences. Depth and habitat were strong predictors for all groups: ocean whitefish Caulolatilus princeps , California sheephead Semicossyphus pulcher , and targeted rockfish ( Sebastes spp.). However, the pattern of these effects differed between the species groups, and the influence of protection was mixed. This work highlights how species with high habitat affinities benefit differently from protection, as a function of depth and habitat representation within the MPA/reference pair. To accurately evaluate MPAs, and the network as a whole, researchers must recognize organism-habitat relationships and incorporate them when assessing conservation efforts.