Monday, 12 November 2012

Healthy reefs dependant on seaweed eating fish

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KeppelIslandReefGreat Keppel Island Reef in the Great Barrier Reef. The study compared two sites at a similar latitude on opposite sides of Australia. Image: Lee LeFeverSEAWEED eating fish keep coral reefs clean, but a new study has found that only a small number of fish species take the primary role of removing dominant seaweed that can overtake reefs.

Lead author Adriana Verges, who conducted the work while at Edith Cowen University says fish that eat seaweeds are important, and they keep coral reefs 'clean'.

“If seaweeds are not eaten they can often overtake the entire system," she says.

The study compared two sites at a similar latitude on opposite sides of Australia; The Keppel Islands in the Great Barrier Reef and Ningaloo Reef.

Video cameras were used to quantify the rates of consumption on the dominant brown algae (Sargassum myriocystum) by herbivorous fishes at both reef sites.

The study found that four species (Naso unicornis, Kyphosus vaigiensis, Siganus canaliculatus and Platax pinnatus) dominated seaweed feeding in both systems, accounting for 80 per cent of all bites in both sites.

However, the total number of species observed biting on Sargassum differed dramatically between the two reefs, with 23 species feeding in Ningaloo, and just eight in the Keppel islands.

Large differences in biomass and composition of herbviorous fishes were found between the reefs—both being significantly higher in Ningaloo.

Dr Verges says she was surprised to find such a huge difference between the sites.

“Ningaloo Reef is more similar to mid-shelf and outer-shelf reefs of the Great Barrier Reef than to inshore reefs like the Keppels, it's remarkable.”

The reef in Ningaloo occurs only a few meters from the mainland, while mid-shelf and outer-shelf reefs in the Great Barrier Reef are tens of kilometres out.

Dr Verges says this shows Ningaloo has a healthy coral reef system.

The comparison shows that even in highly diverse systems like Ningaloo Reef, only four species of fish have a significant effect on large seaweeds.

“Key fish species performing this role are shared among systems as different as Ningaloo and the Great Barrier Reef.”

In recent decades, roving herbivores have been identified as key elements of coral reef systems.

Over fishing of these consumers is considered a significant factor in reef degradation worldwide.

“The results calls for conservation programs that focus on the protection of ecological processes in coral reefs, such as herbivory, by protecting important fish species,” Dr Verges says.

While these fish species are not generally fished in Australia, unicornfish are severely overfished in other Indo-Pacific coral reefs.

Research was a collaboration between ECU and the Ningaloo Research Program, an initiative from the Western Australia Marine Science Institution.

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