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Recent
publicity in the Australian media has highlighted a report
that was published in the Journal of Animal Ecology
earlier this year about estimating population trends in
the world’s largest fish. The media interest stems from
the fact that the original report is based on the study of
photo-identification and recapture studies at Ningaloo,
Australia’s world renown whale shark site, and suggests
that the population of whale sharks there is declining.
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Researchers
in Australia and Thailand warn of a significant
decline in whale shark populations over the
past 12 years. Photo Johannes Schwabe.
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The scientific publication by Bradshaw, Mollet and Meekan
describes how data from the re-sightings of whale sharks
over a 12 year period was used in 16 population model
combinations and that in 10 of these the population was
projected to be in decline. The models used allowed for
different age classes, the age at maturity, the frequency
of reproduction and potential survival rates for each of
these groups. The analysis indicated that there was a
difference in the survival probability of whale sharks
based on their size, with larger sharks of 9 m having an
81% probability of survival compared to only 59% for a 5 m
shark.
Models that ignored the effects of size on the probability
of survival, did not predict a decreasing population
trend. Those models, however, which did allow for this
indicated a decline in the population and that the age at
maturity and the frequency of reproduction (i.e. every
year or every other year) had a significant additional
effect on the rate of decline.
This study echoes a similar situation to a report on the
number of whale sharks found off Thailand, which was
published in Oryx in 2006 by Theberge and Dearden. This
study used sighting data recorded by one of the
live-aboard dive boats that kept a log of search effort
and all whale shark sightings. In this area, there was a
92% decrease in whale shark sightings from 1991-1992
compared to 2000-2001. The report does note that there may
well be a number of factors affecting the numbers of
sharks seen including the effects of El Nĩno, when
the highest numbers were found and La Nĩna when the
fewest were found.
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