Reef Studies (CoECRS) in Australia have been assessing the role of fish "grazing" in the ability of reefs to successfully recover from potentially devastating coral-bleaching events related to rises in ocean temperatures.
The research, published in Current Biology, focused on experimentally testing the ability of reefs to recover under two different conditions: the presence of abundant predatory and herbivorous (plant- and algae-eating) fish, and the absence of significant numbers of these fish. The latter condition mimicked the conditions under which coral reefs would recover from a bleaching event in areas also experiencing chronic overfishing.
The experiment assessed coral recovery in a region of the Great Barrier Reef Marine Park that is strictly protected from fishing, such that only coral stands under experimental cages experienced the depletion of large predatory and herbivorous fish. The researchers found that the two groups of coral underwent very different courses of recovery from the bleaching event: Whereas reefs subjected to grazing by large herbivorous fish species exhibited resilience in recovery, recruiting new corals to the reef and keeping algal growth in check, the reef areas from which large fish were excluded showed a distinct erosion in reef quality, with assemblages of algae and plant life overgrowing the reef and preventing the recruitment of new coral.
The study's findings indicate that grazing by large herbivorous fishes – such as parrot fish and surgeon fish - plays a key role in the ability of coral reef ecosystems to recover from bleaching events and maintain resilience in the face of thermal stress due to rises in ocean temperatures, and were also important in keeping recovering corals free from weedy overgrowth.
"The result was dramatic. The coral cover virtually doubled where the fish had access, while the fenced-off areas became overgrown with slimy weed and the corals failed to recover,” team leader Professor Terry Hughes said.
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