The world's largest fish is shrinking in both size and numbers, according to a study published in a recent issue of the journal Biological Conservation.
The study by Australian scientists Corey Bradshaw, Ben Fitzpatrick, Craig Steinberg, Barry Brook and Mark Meekan analysed data collected from tourist sightings and estimates of total lengths of whale sharks (Rhincodon typus) on Ningaloo Reef.
Ningaloo Reef in Western Australia is a prime area to see whale sharks and a considerable tourist industry geared for it exists there. From 1995 to 2004 a total of 1333 unique sightings of whale sharks with estimates of length and sex were made.
The authors found that during this period, the mean total lengths of whale sharks sighted decreased by 1.6 metres (from 7.0 to 5.4 metres).
Further analysis of the data "...implies that the decline is being driven by the faster disappearance of the remaining largest individuals " a result consistent with hypothesis that an anthropogenic source of mortality is driving the decline.
The authors also found the population numbers of whale sharks to be decreasing, with tourist operators seeing approximately 40% fewer sharks per hour of searching in 2004 than in 1995.
The authors hypothesize that over-harvesting throughout the species' range within Southeast Asia and the Indian Ocean largely drives this decline, a fact exacerbated by the ability of this species to migrate over large distances (whale sharks are protected by law in Australia but not throughout most of Southeast Asia and the Indian Ocean).
For more information, see the paper: Bradshaw, CJA, BM Fitzpatrick, CC Steinberg, BW Brook and MG Meekan (2008) Decline in whale shark size and abundance at Ningaloo Reef over the past decade: The world s largest fish is getting smaller. Biological Conservation 141, pp. 1894"1905.