Whale shark at Ningaloo reef. Photo: David Ross of Ningaloo Reef Dreaming.

The iconic whale sharks that congregate at Ningaloo Reef off Western Australia are declining in number, according to a recent study published in the Journal of Animal Ecology.

Researchers from AIMS and Charles Darwin University have used 12 years of whale shark photographs from Ningaloo Reef to monitor and predict trends in population size. The scientists' models show a steady decline in numbers of the giant fish.

The study follows increased efforts over the past decade to learn more about the Ningaloo whale shark aggregation due to fears that over-harvesting outside of Australian jurisdiction could pose a threat to the gentle giants.

"Because these animals migrate several thousand kilometres, Australia's whale shark population is shared with many other countries in Southeast Asia and around the Indian Ocean," AIMS fish biologist Dr Mark Meekan said.

"Although many countries including India and Taiwan have recently halted or reduced their commercial take of whale sharks, continued harvesting throughout Southeast Asia is probably still occurring."

Because whale sharks grow slowly and reproduce infrequently they are particularly vulnerable to over-exploitation.

Whale sharks are now recognised as a rare species and in decline worldwide. In 2001 the species was listed as nationally threatened under the Environment Protection and Biodiversity and Conservation Act 1999 .

Dr Corey Bradshaw from Charles Darwin University said that understanding more about the movement patterns and reproductive biology of whale sharks may hold the key to their protection.

"A big gap in our understanding of whale sharks is how often they breed, and how many offspring they produce. We also know very little about how populations are connected and this information will tell us a lot about the probability of future decline and potential extinction," Dr Bradshaw said.

In Western Australia, whale sharks are a huge drawcard for the tourism industry, bringing in over $6 million each year.

Although it's too early to tell what a population decline could mean for this industry, scientists say that improved conservation and further research will be critical for the survival of the species.

"Piece by piece, scientific research is beginning to unravel the biology and ecology of what were previously very elusive creatures. Rigorous scientific studies on whale sharks are helping to protect the species and the tourism industries that rely on them," Dr Meekan said.

While Drs Bradshaw and Meekan said their findings were only preliminary given the long generation time of whale sharks, they predicted that evidence for declines will increase as science uncovers more information about the activities of whale sharks during their long migrations.

The team has recently returned from another tagging expedition at Ningaloo Reef where five additional sharks were tagged. Because few whale sharks have been fitted with the expensive tags, each tagged shark contributes valuable information about the population's migration patterns.

Scientists hope that understanding more about the mysterious whale sharks will lead to increased protection for the species and the long-term sustainability of the associated eco-tourism industry.

17 June 2011