Abstract
Understanding drivers behind patterns of functionally important groups of fishes is crucial for successful management and conservation of tropical seascapes. Herbivorous fishes are the most prominent consumers of marine primary production which can have profound effects on reef resilience. We explored environmental variables affecting distribution and foraging patterns of herbivorous and detritivorous fish assemblages (siganids, acanthurids and parrotfish) across distinct shallow-water habitats (coral reefs, macroalgae beds and seagrass meadows) during September–November 2016 at Mafia Island, Tanzania (8°00′S, 39°41′E). We performed underwater visual census to quantify fish assemblages, measured habitat features, deployed macroalgal assays and conducted inventories of grazing scars. Multi-dimensional scaling and mixed-effects linear models were used to evaluate differences in fish assemblages and environmental variables influencing abundance and foraging patterns of fishes. Fish communities of focal functional groups differed among habitats. Abundance of herbivores and detritivores as well as relative browsing and scraping was highest on coral reefs compared to macroalgae and seagrass meadows. Adult fish were more abundant on coral reefs while juveniles were abundant in macroalgal beds. Coral cover and crustose coralline algal cover had a positive effect on the abundance of fish in coral reef areas, while macroalgal cover had a negative effect. Contrastingly, in macroalgae habitats, macroalgal cover had a positive effect on the abundance of parrotfish. These results highlight the importance of considering connectivity between macroalgal beds and coral reefs through ontogenetic shifts in habitat use by primarily microphagous parrotfish and of incorporating a range of habitats within coastal management plans.
Highlights
Disentangling which habitat characteristics are critical in shaping abundance, distribution, and ecological processes within the available range of coastal habitats will improve our understanding of marine community patterns and connectivity in shallow coastal seascapes
The algivorous acanthurid Naso brevirostris was one of the most influential species in dissimilarities among habitats as it was abundant in coral reef sites but absent from macroalgal and seagrass habitats (SIMPER, Table 2)
Detritivores were most abundant on coral reef sites and absent from seagrass habitats, while seagrass and algivores and microphages were more evenly distributed across the seascape (Fig. 5)
Summary
Disentangling which habitat characteristics are critical in shaping abundance, distribution, and ecological processes within the available range of coastal habitats will improve our understanding of marine community patterns and connectivity in shallow coastal seascapes. Spatial heterogeneity is a strong driver of distribution and diversity patterns of reef fish (Chabanet et al 1997; Sabater and Tofaeono 2007; Messmer et al 2011; Samoilys et al 2018), where characteristics such as structural complexity and rugosity are important predictors of fish assemblage structure and abundance (Bell and Galzin 1984; Gratwicke and Speight 2005; Graham and Nash 2013). Environmental factors as such may impact coral fish communities on multiple spatial and temporal scales as some fish species might have shifting habitat preferences during different ontogenetic stages and utilize alternative habitats like seagrass meadows, mangroves and/or macroalgal beds during their juvenile life stage (Nagelkerken et al 2000; Wilson et al 2010; Berkström et al 2012). Microhabitat preferences might not be consistent across habitats, and environmental predictors might change depending on habitat identity
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