World's largest fish is shrinking

The world's largest fish, the gentle and solitary whale shark, is getting smaller, an international conference heard this week.

This has led to concerns that the future of this highly migratory fish may be threatened.

Whale sharks live in tropical waters around the world and are sometimes spotted in protected waters at Ningaloo Reef in Western Australia.

Dr Mark Meekan and colleagues from the Australian Institute of Marine Science studied continuous records from log books filled out by ecotourist operators operating at Ningaloo Reef.

Meekan told the International Whale Shark Conference in Perth this week that the average size of the fish has declined, from just over 7 metres in 1995 to around 5.5 metres today.

Researchers don't know exactly why the fish is shrinking. But they speculate that over-fishing in unprotected international waters, injuries caused by collisions with sea vessels and a drop in the average age of the fish could be reasons.

"Any fish population that is undergoing unsustainable mortality usually shows a drop in average size of individual fish, and a drop in abundance. So what we're seeing at Ningaloo is particularly worrying, because these waters are protected," says Meekan.

"If we're losing the adults in the population, leaving only juvenile whale sharks, then we'll have no population there to reproduce. That's a real concern."

The whale shark is an elusive, slow growing, plankton-eating, oceanic fish.

It only occasionally ventures to a handful of coastlines around the world, including those along India, the Seychelles, Kenya and Somalia.

Because of this very little is known about them. Only one pregnant female has ever been found and she had a litter of 300 pups.

Meekan says 'top order' animals such as large sharks are a good barometer of the ocean health.

"They're like the canary in the coal mine, so we do need to pay attention to the signals they are giving us."

Protecting the whale shark
Conference delegates called for countries to try harder to protect the whale shark and its habitat.

They called for a move away from harvesting the sharks to sustainable alternatives, like carefully managed ecotourism.

"The evidence points to serious declines in the abundance of whale sharks in some parts of the world following even short periods of exloitation," the delegates say in a communiqué released at the end of the conference.

Source: www.abc.net.au