Decline in Whale Shark size

Many of our marine species are being harvested and the overfishing is affecting the way our aquatic friends are growing and maturing. The biological processes of marine fish have been severely disrupted and the main portion of this study is about the size or more specifically the decrease in size of Whale sharks in the Ningaloo Reef, Australia over the past decade. According to this examination our over harvesting of whale shark has significantly decreased the size of the species by nearly 2 feet in the past 8-10 years. Is over harvesting the main reason for this and if so can we stop it?

This study is important to understand that marine species harvesting may cause the species to be more susceptible to extinction. The whale shark has become much smaller and the main suspect of this is size selective fishing, which is removing particular size classes of a marine species. The larger whale sharks are being harvested over the smaller ones. This may change the population’s age structure, growth and maturation rates. “Changes to demographic rates and the evolutionary patterns induced by size-selective fish- ing can increase extinction risk in harvested species” says Jennings about the selective harvesting.

The whale shark is the world’s largest fish and that is why the decrease in the size of them has been very shocking. The whale shark like most sharks grow slowly, matures late, and has extended life spans. Such traits are likely to limit annual recruitment and increase susceptibility to over-exploitation by humans, adds Smith and Bradshaw. With the whale shark size decreasing the main suspect is the overfishing of larger species, which in turn cause breeding of the whale shark to become between only smaller ones. Ratner and Lande state, there are three main mechanisms that may drive changes in body size of harvested populations: (1) abiotic factors affecting growth and development (e.g., large-scale climate or regime shifts); (2) biotic changes such as density-modified growth rates and (3) changes to demography and genetic composition via harvesting.

The sharks were studied in much detail and many other factors were taken into consideration when tracking them. The study had spotter planes used with boat skippers and in-water shark spotters all with experience in the field of shark tracking. The pilots estimated the lengths but they collaborated with the boat skippers and further validation of the whale shark size was done with the snorkelers. Lengths of sharks were only accepted when all the groups agreed. The presence of the snorkeler does not influence the shark’s behavior and the vessels also used strict rules. This ensures that pa- trons do not approach within 3m of the shark while snorkeling, and vessels must remain a minimum of 30 m from the shark for a maximum of 90 minutes, says Davis. Tracking the same shark was taken into account and so was the weather the shark was found in. They made sure they did not affect the sharks behavior and lifestyle.

Fig. 1

(Credit: http://journals2.scholarsportal.info/tmp/15565618855922104545.pdf)

Many whale sharks were tracked; in fact the study made one of the largest ever databases of whale sharks. The study used a lot of math and used graphs and equations to create important ways to contrast and compare the whale shark size and change. Four linear and non-linear models were made to compare the lengths of the sharks across the years. The whale lengths from 1995-1996 differ greatly to that of 2003-2004 with many of the larger whale sharks decreasing in size. The 1995 sharks averaged a length of 7.0m. In 2004 the sharks average length decreased to only 5.4m in length. These changes go along with the studies idea of over harvesting of fish because the larger fish are disappearing faster then the other sizes.

The team suspects the whale shark fishery of Southeast Asia the reason for most of the harvesting. Chen and Phi-pps add, the commercial harvest of whale sharks principally supplies markets in Taiwan, where fins (preferably from large individuals) are used for soup and the flesh is sold for human consumption. The examiners believe this to be the main reason for the fish decrease. Although Taiwan has recently halted the commercial harvest of the species, with other countries doing the same. They still suspect illegal and legal exploitation of whale sharks in much of Asia.

With many of the whale shark commercial harvesting companies slowing down or coming to a halt the whale shark should have a better chance of survival. The study believes a slow but coming change in the downward trend in whale shark size because of the exploitation coming to an end. They still want to be precautionary and proactive about adopting more conservation efforts for the all-marine species. Whale sharks are protected in all Australian waters, to truly save the marine species we need to become international to have fish mortality decrease. The experimenters hope to potentially see entire ocean basins protected and an increase in tagging to identify and learn more about whale shark living.

By: Tashi Nanglo

Works Cited

Bradshaw, Corey . "Decline in whale shark size and abundance at Ningaloo Reef over the past decade: The world’s largest fish is getting smaller." Science Direct (2008): 12. Web. 13 Oct 2010.

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