Macroalgal height is more important than species identity in driving differences in the distribution and behavior of fishes

Macroalgae are key foundation species in temperate reef systems in part because their structure can transform the physical composition of the reef landscape. Previous assessments of the effects of algae on fishes have mostly focused on effects of changes in the density and identity of algae and often ignore how the unique structure provided by each alga contributes to such patterns. We performed a series of experiments and surveys to test the relative importance of algal size versus identity on the abundance, species richness, and behavior of fishes. In experiments, fish abundance and species richness did not differ among algal species but varied among sizes of algae. Fishes engaged in resting/refuging behavior were more abundant near tall than short algae, but the abundance of fishes engaged in feeding did not differ among sizes or species of algae. Field surveys of macroalgae occurring naturally on reefs detected similar patterns: fish abundance and richness were positively related to algal size, though there was considerable variation that was not explained by algal size. Differences in the fish community among algal species indicated that certain algae (e.g. Macrocystis pyrifera) were more important than others in providing habitat for fishes due to their size. Moreover, the influence of an invasive alga, Sargassum horneri, on the fish community appeared to be predictable based on its size. These results reveal that the importance of macroalgae as structural habitat for fishes may be more related to size than to species.