Whale sharks — giants of the fish world that should strike terror only among tiny creatures like the plankton and krill they eat — are imperiled by over-fishing of the species in parts of its ocean range.
That threat is emphasized in a new study, funded by UIC and the Shark Research Institute in Princeton, N.J., by geneticists led by Jennifer Schmidt, a University of Illinois at Chicago associate professor of biological sciences and reported online in the journal PLoS One. Co-authors of the report are Marie Levine, executive director of the Shark Research Institute; Mary Ashley, professor of biological sciences at UIC; and Kevin Feldheim, director of the Pritzker Laboratory at the Field Museum in Chicago.
Schmidt and her colleagues studied the DNA of 68 whale sharks from 11 locations across the world – the Indian and Pacific Oceans and the Caribbean Sea. Results showed little genetic variation between the populations, their DNA proves they are genetically quite similar, which indicates migration and interbreeding among far-flung populations of these sharks. This means that “whale sharks in protected waters cannot be assumed to stay in those waters, but may move into areas where they may be in danger,” says Schmidt.
The species is believed to have originated about 60 million years ago. Whale sharks are found in tropical and warm temperature waters and are filter feeders – one of only three known filter feeding sharks species (along with the basking shark and the megamouth shark). It feeds on phytoplankton, macro-algae, plankton, krill and small nektonic life, such as small squid or vertebrates. The many rows of teeth play no role in feeding. They reach a length of 18 meters and more (50+ feet) and weigh over 20 tons. Male sharks reach sexual maturity when they are about 25 to 30 years old and measure about 8 meters, so it will take a long time for the species to recover from recent population declines.
Whale sharks are listed as threatened, but not every country protects them. The large animals are especially prized by fishermen for meat and fins used in soup. While these animals are protected in many parts of their range, they are fished legally and illegally in some countries.
Schmidt credits some countries for closing whale shark fisheries and hopes that efforts such as ecotourism programs, which sometimes include swims with the gentle giants, may prove an attractive economic alternative to fishing.
“The only real threat to whale sharks is us,” said Schmidt. “To design proper conservation plans, we need to understand the sharks’ lifestyle. We can only protect their habitat if we know what habitat they use.”
But little is known about the shark’s biology, perhaps because they have been studied primarily near shore, while mature animals may breed and give birth out in the open ocean. Nor is much known about neonatal or juvenile sharks, including where they grow to maturity, or how and when they move between regions. That has made devising effective conservation efforts a problem.
The study presented describes the first identification of microsatellite loci in the whale shark, and the use of these loci to analyze population structure across a panel of whale shark DNAs from three different ocean basins, which showed little genetic differentiation between geographic populations. Rather, the data confirm a history of gene flow between populations, supporting migration and interbreeding between these seemingly disparate groups. Such data are supported by satellite tracking studies that show frequent mid-range and periodic long-range migrations. Though this level of gene flow is sufficient to genetically normalize populations, it is unlikely to be sufficient to reestablish depleted populations. As whale sharks cross geographic and political boundaries in their movements, international protection should be sought to ensure the continued survival of this species.
Gene flow between geographic sampling populations could be mediated directly, by individual animals traversing large distances to interbreed with distant populations, or could be more incremental, as animals breed with near neighbor populations and their offspring subsequently move to yet more distant areas. As data become available about additional whale shark aggregation sites, it appears that a band of whale sharks spanning the mid-latitudes is plausible.
Find Jennifer Schmidt’s and her co-authors’ full analysis and findings in the research article.
Copyright © 2009 by Marine Science Today, a publication of OceanLines LLC