Abstract
The conservation and management of site-attached assemblages of coastal reefs are particularly challenging because of the tremendous environmental variation that exists at small spatial scales. In this sense, understanding the primary sources of variation in spatial patterns of the biota is fundamental for designing effective conservation policies. We investigated spatial variation in fish assemblages around the windward and leeward sides of coastal islands situated across a gradient of riverine influence (13 km in length). Specifically, relationships between rocky reef fish assemblages and benthic, topographic and physical predictors were assessed. We hypothesized that river induced disturbances may overcome local habitat features in modeling spatial patterns of fish distribution. Fish assemblages varied primarily due to the strong directional gradient of riverine influence (22.6% of the estimated components of variation), followed by topographic complexity (15%), wave exposure (9.9%), and benthic cover (8%). The trophic structure of fish assemblages changed from having a high abundance of invertebrate feeders in macroalgae-dominated reefs close to river mouths to a high proportion of herbivores, planktivores and invertebrate feeder species in reefs with large boulders covered by epilithic algal matrices, as the distance from rivers increased. This gradient led to an increase of 4.5-fold in fish richness and fish trophic group diversity, 11-fold in fish biomass and 10-fold in fish abundance. Our results have implications for the conservation and monitoring of assemblages patchily distributed at small spatial scales. The major role of distance from river influences on fish assemblages rather than benthic cover and topographic complexity suggest that managing land-based activities should be a conservation priority toward reef restoration.
Highlights
One of the primary challenges of environmental studies is to determine the variables that influence the spatial distribution and structure of assemblages
Benthic cover surveys revealed that the rocky reefs were dominated by epilithic algae matrix (EAM), soft coral and fleshy algae, which together accounted for more than 90% of the total substratum cover
Haemulon steindachneri, Serranus flaviventris were very abundant in the 8 sites from 1 to 4.5 km of distance, whereas only 1 individual of H. steindachneri was observed at the 4 locations from 11 to 13 Km
Summary
One of the primary challenges of environmental studies is to determine the variables that influence the spatial distribution and structure of assemblages. In the case of reef fishes, these variables include benthic composition [6,7,8], topographic complexity [9,10], depth [9,11], wave exposure [12,13,14], and river discharges [15], among several others All of these variables may interact to shape the abundance of individual species and the composition and structure of a fish assemblage
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