Fisheries restrictions and their cascading effects on herbivore abundance and macroalgae removal at Kenyan coral reefs

Abstract

The increase of macroalgae at degraded reefs impedes several ecosystem services and calls for effective methods to facilitate a return to coral dominance. Removal of macroalgae (browsing) is typically realized by fish, but the role and identity of browsers at the heavily-fished East African coast is still largely unknown. This study investigated how browsing pressure at Kenyan reefs (−4.700, 39.396) related to fisheries management and herbivore community. From October 2018 to January 2019, consumption during 24-h buffet assays using the brown macroalgae Sargassum and Padina was determined and video recorded at six sites: two in fished zones, two in marine reserves (traditional fishing allowed) and two in no-take zones. Herbivorous fish composition, biomass and sea urchin density were also determined. Consumption of Sargassum and Padina was nearly three-fold lower in the fished zones (26% and 28% of macroalgal biomass consumed, respectively) compared to the no-take zones (62% and 82%), with intermediate consumption in the marine reserves (48% and 71%). Herbivore biomass was seven-fold higher in no-take zones and included substantially more browsers (mainly unicornfishes, Naso spp.) and scrapers (scarids), which were associated with the higher browsing pressure. Browsers and scrapers were predominantly responsible for the consumption of macroalgae as determined by video recordings, though key species differed across sites. In contrast, damselfish-dominated fished sites were associated with high sea urchin densities and low browsing pressure. These results indicate that fishing restrictions are likely to support reef resilience by increasing herbivorous fish biomass of key species and thereby promote macroalgae removal.