Declining abundance of gentle giants

The iconic whale sharks that congregate at Ningaloo Reef off Western Australia, providing the basis for an important marine tourism opportunity worth up to $20 million per annum for the regional economy around the small township of Exmouth, appear to be declining in number and size, according to a recent analysis of shark sightings provided by the industry.

Whale sharks grow slowly and reproduce infrequently; because of this they are particularly vulnerable to over-exploitation and can sustain few losses above their natural mortality rate. The species was listed as nationally threatened under Australia’s Environment Protection and Biodiversity Conservation Act (1999).

The pattern of spots on these animals provides a unique identifier of individual sharks, much like human fingerprints. Researchers from AIMS and Charles Darwin University have been using a 12-year photographic library of whale sharks from Ningaloo Reef, collected by the local tourism operators and others, to track re-sightings of the same individuals. This led to the conclusion that the number of ‘known’ sharks returning to Ningaloo annually has been declining faster than would be forecast from natural attrition, leading to the reasonable hypothesis that this reflects additional mortality from fishing somewhere within their enormous range.

This information is being used to inform and validate international efforts to monitor and control illegal fishing upon these gentle giants of the sea. The Department of Environment and Conservation in Western Australia is measuring the impact of ecotourism on sharks visiting Ningaloo by recording the number of times individual sharks are seen during the whale shark tourism season. This knowledge supports protection of the whale sharks and sustainability of this valuable regional industry. In addition, the Australian Department of Environment and Water Resources is using this new information to inform funding priorities for research, such as the development of ecotourism industries based on whale sharks in Indonesia.

Unique whale shark spot patterns allow identification and tracking of individuals.

March 10, 2008