Disentangling the effects of macroalgae on the abundance of temperate reef fishes

Habitat heterogeneity may mediate the relationship between organisms and their environment. However, the demographic and behavioural responses of organisms to different sources of habitat heterogeneity (e.g., structural complexity and composition) may vary, and consequently, different sources of heterogeneity may interact to shape the abundance of individual species and composition of a species assemblage. We focused on habitat-forming macroalgae, and conducted a set of laboratory experiments to determine the macroalgal preferences of two species of temperate reef fish. In a subsequent field experiment, we manipulated macroalgal heterogeneity at two sites in Wellington, New Zealand, to determine the relative importance of different sources of habitat heterogeneity to the abundance of locally common reef fishes. Specifically, we manipulated three sources of habitat heterogeneity: (1) macroalgal species identities; (2) combinations of macroalgal species (i.e., mixed stands); and (3) macroalgal density. Our laboratory experiments indicated that two common fishes readily distinguish and exhibit preferences for different forms of macroalgae. Our field experiment indicated that the abundance of reef fishes varied as a function of experimentally induced habitat heterogeneity. We detected within-species variation in responses to macroalgal composition (suggesting ontogenetic habitat shifts), and larger-scale influences on the abundance of reef fishes (effects attributable to location). Macroalgal identities affected the abundance of 7 of 15 reef fish species. Composition of macroalgal stands shaped the abundance of 5 of 7 reef fish species, and the overall structure of the local fish assemblage. Generally, heterogeneity in vegetative structures appeared to increase breadth of habitat use for reef fishes. This work suggests strong behaviourally mediated linkages between the abundance of reef fishes and the composition of vegetative structures in a temperate, macroalgal-dominated ecosystem.