Responses of temperate reef fishes to alterations in algal structure and species composition

Reef habitats in the South Atlantic Bight comprise only a small proportion of the available benthic habitat, but support enormous densities of fishes, invertebrates, and algae. Because these reefs are distant from shore and in deeper waters (usually >20 m), we know very little about the factors affecting the distribution of fishes or the structure of fish assemblages on these reefs. Our goal in this study was to determine how variance in macroalgal structure and species composition on reefs off the coast of North Carolina affects reef fishes. Manipulations of algal density and species composition in artificially created 1.5 m2 plots indicated that: (1) patches with Sargassum filipendula had greater numbers and higher species richness of reef fishes than patches without algae; (2) fishes responded to variability in Sargassum density with high fish abundance in patches with higher algal density, but this response varied among fish species; (3) greater numbers of individuals and species of fish used Sargassum patches compared to patches of 2 other brown algae common in this region (Zonaria and Dictyopteris); (4) fish densities were higher in patches with taller algae and greater percent cover of algae. Many of the fishes using experimental patches were small, young-of-the-year individuals. Our results indicate that variability in the structure of seaweed beds on reefs in the South Atlantic Bight can influence the distribution and abundance of reef fishes. Although the algal beds in the South Atlantic Bight are more diminutive and less extensive than kelp beds of cold temperate waters, they may be a critical habitat for juvenile reef fishes in this region, and the algal species dominating the beds will have a large effect on their value as juvenile habitat.