Overfishing of sharks may have already impacted our coral reefs

A study by marine biologists at The University of Western Australia has found that fishing by Indonesian shark hunters may have resulted in changes to fish communities on a major reef off Western Australia’s North-West coast.

The study, published in the journal Marine Ecology Progress Series, concluded that sharks influence the number, kind and size of fishes on coral reefs sharks and that they should be protected from overfishing.

Shark populations have declined by up to 90% in some regions in the past 50 years, yet their ecological role, particularly on coral reefs, remains contested.

Shanta Barley -web

Dr Shanta Barley

Dr Shanta Barley and colleagues from UWA and the Australian Institute of Marine Science compared fish communities at the Rowley Shoals, a marine protected area in northwestern Australia, to the nearby Scott Reef, where Indonesian shark hunters have legally reduced populations of these predators.

Using video rigs consisting of pairs of cameras that were either placed on the seabed or carried by divers swimming along the reef, the researchers filmed fish communities at both locations. They then used specialist software to identify the species present and their number, length and weight.

The study found that removal of large shark species by fishers may lead to explosions in smaller shark species such as the white tip reef shark (Triaenodon obesus), which was 570 per cent more abundant and had 190 per cent more biomass at Scott Reef relative to the Rowley Shoals.

The researchers also found that small to medium sized fishes (< 30 cm) known to be eaten by reef sharks were 200 per cent more common in the shark-fished area relative to the marine reserve, as were a range of plant-eating fishes that sit at the bottom of the food web.

“These results suggest that removing sharks from coral reefs may not lead to a classic ‘trophic cascade’, which is a series of knock-on effects that start at the top of a food web and percolate down to the bottom, but rather alter fish communities at every level,” Dr Barley said.

“This is worrying, because fishes at the bottom of the food web may maintain reef health by eating algae.”

In addition, the study found that fishes were more diverse where sharks were abundant.

“There is debate around whether predators are the ‘guardians’ of biodiversity or whether they actually reduce diversity by ‘eating’ species to local extinction, but our research suggests the former,” Dr Barley said.

Like all natural experiments, there are limitations to what conclusions can be drawn from the study about the ecological role of sharks on reefs. However such approaches are one of the few techniques available to ecologists to understand these disappearing and elusive animals.

Co-author of the study Professor Jessica Meeuwig said one outstanding question that is not answered by the research is whether sharks primarily influence reef fish communities as predators or competitors.

“Regardless, the evidence is growing that when we lose sharks, we lose coral reefs as we know them,” Professor Meeuwig said.

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