Differential impacts of coral reef herbivores on algal succession in Kenya

Abstract

In shallow-water systems, fisheries management influences herbivory, which mediates ecosystem processes by regulating algal biomass, primary production, and competition between benthic organisms, such as algae and corals. Sea urchins and herbivorous fishes (scrapers, grazers, browsers) are the dominant herbivores in Kenya's fringing coral reef and their grazing influences coral-macroalgal dynamics and dominance. Using experimental substrata and grazer exclusions, we tested the hypothesis that herbivores differentially affect algal composition and succession using 3 levels of fisheries management: fished reefs, community-managed closures (<10 yr old, <0.5 km2), and government-managed closures (20 to 40 yr old, 5 to 10 km2). In fished reefs and government closures, herbivores facilitated maintenance of early successional algal species, such as turfs, associated with sea urchins in the former and scraping fishes in the latter. Crustose coralline algae were only abundant in government closures, and video recordings showed that fish grazing was greatest at these sites, most notably for parrotfishes (scrapers). A combination of sea urchins and small grazing and detritivorous fishes was present in community closures, which allowed macroalgae to quickly develop from turf into early then late successional stages. These reefs may represent an intermediate or transitional system of herbivore dominance characterized by macroalgae. Consequently, reefs in heavily fished seascapes initially protected from fishing may require additional management efforts to facilitate the recovery of larger-bodied scraping fishes, including bans on capturing parrotfishes and restricting gear (e.g. spearguns) that target these species. © Inter-Research 2014.