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Habitat use by a species is a vital component in explaining the dynamics of natural populations. For mobile marine species such as fishes, describing habitat heterogeneity at a seascape scale is essential because it quantifies the spatial extent to which fishes are interacting with their environment. Here, we explored the relationships between habitat metrics and the density and size of kelp forest fishes across a seascape that is naturally fragmented. Multibeam sonar and GIS analysis were employed to create a seascape map that explicitly defined bathymetry and spatial configuration of rocky reefs in southern California (USA). Georeferenced subtidal transects were conducted across this seascape to describe habitat attributes, including the density of macroalgae, and record the number and size of fishes. Multiple regression analyses were conducted to identify which variables of habitat structure were most important in describing numerical density, biomass density, average size, and maximum size for fishes. Responses to different habitat components were dependent on particular species, choice of spatial scale, and the inherent characteristics of the seascape itself. Notably, the relative influence of seascape components was dependent on the configuration of the seascape, where fishes in a more isolated and less connected seascape were more influenced by spatial configuration than fishes in a seascape with greater habitat connectedness. This study demonstrates that explicit habitat maps allow for a more comprehensive understanding of population structure when describing fishes across large spatial scales.

Understanding whether assemblages of species respond more strongly to bottom-up (availability of trophic resources or habitats) or top-down (predation pressure) processes is important for effective management of resources and ecosystems. We determined the relative influence of environmental factors and predation by humans in shaping the density, biomass, and species richness of 4 medium-bodied (10-40 cm total length [TL]) coral reef fish groups targeted by fishers (mesopredators, planktivores, grazer and detritivores, and scrapers) and the density of 2 groups not targeted by fishers (invertivores, small fish ≤10 cm TL) in the central Philippines. Boosted regression trees were used to model the response of each fish group to 21 predictor variables: 13 habitat variables, 5 island variables, and 3 fishing variables (no-take marine reserve [NTMR] presence or absence, NTMR size, and NTMR age). Targeted and nontargeted fish groups responded most strongly to habitat variables, then island variables. Fishing (NTMR) variables generally had less influence on fish groups. Of the habitat variables, live hard coral cover, structural complexity or habitat complexity index, and depth had the greatest effects on density, biomass, and species richness of targeted fish groups and on the density of nontargeted fishes. Of the island variables, proximity to the nearest river and island elevation had the most influence on fish groups. The NTMRs affected only fishes targeted by fishers; NTMR size positively correlated with density, biomass, and species richness of targeted fishes, particularly mesopredatory, and grazing and detritivorous fishes. Importantly, NTMRs as small as 15 ha positively affected medium-bodied fishes. This finding provides reassurance for regions that have invested in small-scale community-managed NTMRs. However, management strategies that integrate sound coastal land-use practices to conserve adjacent reef fish habitat, strategic NTMR placement, and establishment of larger NTMRs will be crucial for maintaining biodiversity and fisheries.