Abstract
Coral reefs have changed radically in the last few decades with reefs in the Caribbean now averaging 13% coral cover and 40% macroalgal cover (mostly Dictyota and Lobophora). So, it is time we re-evaluate which species are key to the process of herbivory in these new conditions. The role herbivorous fishes play in controlling macroalgae is often considered by managers and researchers at a guild or family level, but greater resolution is needed to understand the impact of herbivores more fully. We performed feeding assays and behavioural observations of fish feeding to quantify the removal of the most common macroalgae by different herbivorous fish species. In total, we ran 34 hour-long trials using Dictyota and Lobophora across two sites and conducted over 34 hours of observation of 105 fish from eight species in the Cayman Islands, Caribbean. We show that many nominal herbivores did not consume macroalgae but instead targeted the epibionts on macroalgae and other substrates. In fact, only three fish taxa consumed macroalgae as a significant proportion of their feeding: one species of surgeonfish (Acanthurus coeruleus), one species of parrotfish (Sparisoma aurofrenatum), and the third, the chubs (Kyphosus spp.), is a group of species which is not consistently considered as part of the herbivore community in the Caribbean. From our observations, an individual A. coeruleus can consume ~44 grams of Dictyota per day, while S. aurofrenatum can consume ~50 grams and Kyphosus spp. can consume ~100 grams. These values are significantly more than all other herbivorous fish species and suggest these three taxa are key macroalgal consumers in the Caribbean. These results highlight that disentangling the role of individual herbivore species is necessary for critical species to be identified and protected. Furthermore, as reef conditions change, we need to re-evaluate the key functions and species to be more effective at protecting and managing these important ecosystems. With far higher macroalgal coverage than in the past, the few browsing species that remove macroalgae may be increasingly important in promoting reef health.
Highlights
In the Caribbean, coral cover has declined to a regional average of 13%, while macroalgal cover is ∼ 40% of the forereef
All individuals we observed fed from the benthos during these observations except for three Kyphosus spp. which remained in the water column during the observation period
After multiplying the mean weight (g) of algae removed per bite (Figure 4) by the number of bites counted in each video (Figure 3, upper two panels), we found no significant differences in grams consumed by the three main herbivore species at either site (Site One Dictyota: n = 10, p = 0.553, Kruskal–Wallis; Site One Lobophora: n = 10, p = 0.111, Kruskal–Wallis; Site Two Dictyota: n = 5, p = 0.095, Mann–Whitney U; Site Two Lobophora: n = 9, p = 0.546, Mann–Whitney U; Figure 3, middle two panels)
Summary
In the Caribbean, coral cover has declined to a regional average of 13%, while macroalgal cover is ∼ 40% of the forereef (between 28 and 45% by ecoregion, AGRRA, 2018). Of the ∼40% macroalgal cover, Dictyota and Lobophora are often the most abundant genera (Cardoso et al, 2009; DiazPulido et al, 2011; Suchley and Alvarez-Filip, 2017) This increase in macroalgae can cause problems for corals and the resilience of the reef system by reducing their growth (Tanner, 1995; Box and Mumby, 2007) and fecundity (Kuffner et al, 2006), increasing the prevalence of disease (Birrell et al, 2008) and pre-empting space and inhibiting coral recruitment (McCook et al, 2001; Birrell et al, 2008; Venera-Ponton et al, 2011). With macroalgae covering almost half of the reef, more attention should be paid to the species that consume these algae
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