Effects of macroalgal habitats on the community and populatión structure of temperate reef fishes

Abstract

Two families of brown macroalgae that occur in sympatry domínate temperate subtidal rocky coasts: the Laminareales, and the Fucales. Both ofthese families are habitat-forming species for a wide variety of invertebrates and fishes. Variation in the presence, density, and composition of brown macroalgae can have large influences on the evolution and ecology of associated organisms. Here, using a series of observational and experimental studies, I evaluated the effects of heterogeneity in the composition of brown macroalgal stands at the population and community levels for reef fishes. A central ecological challenge is the description of pattems that occur at local scales, and how these are manifested at larger ones. I conducted further sampling across a set of sites nested within locations over three regions, Juan Fernández Islands (Chile), Northem New Zealand, and Tasmania (Australia), to evaluate pattems of variation in the diversity and composition of fish assemblages. Specifically, I explored spatial variation in fish assemblages as a function of rocky reef habitats (dominated by brown-macroalgae) and other sources of variation ( abiotic and biotic factors) that potentially mediate the relationship between fishes and reef habitats. Analyses suggest that spatial variation in diversity (e.g., species and trophic) may be explained by spatial variation in depth, temperature, and composition of macroalgal habitats. At each location, only 2-3 families dominated the composition of fish assemblages, but species identities varied among locations. In a subsequent study, I assessed the fish-habitat associations from sites within the Juan Fernández Islands, an isolated eastem Pacific Island that lack large brown macroalgae. I found that, despite the close proximity of these Islands to the South American continent, fish assemblages were mostly composed of endemíc representatives from families that domínate the fish assemblages in New Zealand and Australia. Spatial variation in depth and temperature did not contribute to the observed variation in fish abundance. Instead, I found that benthic habitat-formíng species (particularly foliose brown macroalgae) appeared to limit the abundance ofsome reef fishes. These results suggest that a mixture of large-scale ( e.g., stochastic recruitment) and small-scale processes (i.e., relating to habitat heterogeneity) influence the diversity, compositíon and abundance offish assemblages. Subsequently, I evaluated relationships between reef fishes and macroalgae composition across multiple sites, surveyed repeatedly over four seasons. I found that fishes were associated with different components of heterogeneity in macroalgal habitats, potentially indicating interspecific partitioning of resources that may arise from differential feeding habits and sizesusceptibility to predation. Seasonal variation in the fish-habitat associations was detected, and site differences in macroalgal composítion explained significant variation in the local diversity of fishes. Using a series of small-scale lab and field-based manipulative experiments, I determined the demographic and behavioural responses of reefassociated fishes to heterogeneity in the composition of brown macroalgal habitats. I found that (i) different fish species distinguished between monospecific macroalgae stands (macroalgal identity affected the abundance of 7 ofl5 reeffish species); (ii) there is within-species variation in the response of fishes to macroalgal composition (suggesting ontogenetic habitat shifts); and (íii) the abundance of 5 of 7 reef fish species, and the overall structure of the local fish assemblage, varied with the composition ofmixed-species macroalgal stands. Lastly, I evaluated the potential for fishes to prov1s1on demographic feedbacks to macroalgae. Specifically, I conducted a mesocosm experiment to evaluate the effects of fishes on grazing amphipods, and therefore, the potential indirect-effects offishes on large-brown macroalgae. I found that only one ofthe two fish species studied reduced grazer abundance. Although the second fish species did not consume grazing amphipods, its presence altered amphipod behaviour to significantly reduce grazing efficiency on the macroalgal-host. This study illustrates how density and trait-mediated indirect interactions can have similar effects on primary producers. Overall, my observational and experimental components of this thesis emphasize the influence ofheterogeneity in macroalgal structures on the breadth of habitat use for reef fishes at multiple locations. I found strong behaviourally mediated linkages between the abundance of reef fishes and composition of macroalgal stand. I also provide sorne evidence that mutualistic relationships may exist between kelp and associated fishes.