January 25, 2007
Learning about sharks: Dissection offers chance to gather new information
Fishing for a salmon shark with a rod and reel would be like fishing for a marlin “times three,” according to a UW shark expert who’s witnessed their ferocity.
These sleek predators — which are in the same family as great whites — can weigh as much as 700 pounds and have been clocked swimming faster than 50 knots, or 58 miles per hour.
Scientists have been aware of salmon sharks for more than 50 years in the waters stretching from Oregon to Alaska, but what’s known of the biology of the sharks that frequent the Eastern Pacific has come mainly in the last five years, according to Vince Gallucci, UW professor of aquatic and fishery sciences. So little is known that it’s difficult to assess wildlife management questions when they arise. For example, perhaps the sharks are a contributing factor keeping sea lions on the endangered species list, if the sharks are out-competing sea lions for the salmon, pollock, herring and squid they both eat.
Gallucci and his students had the rare opportunity recently to learn more about the reproductive tract of a pregnant salmon shark. He says he knows of 80 samples of salmon shark that have been collected in recent years and this is the first time anyone has had access to an intact reproductive tract with embryos, or pups, as the young are called.
The female was caught while University of Alaska assistant professor Bob Foy and his students were conducting fish surveys near Kodiak Island. She was an average-sized female, measuring more than 8 feet in length, Gallucci says. Foy sent Gallucci and his students her 4-foot long reproductive tract that included a 17-inch-long ovary and four embryos.
Each embryo was about a foot long, weighed between 1 and 2 pounds and had teeth.
Some sharks that give birth to live young are not like mammals with a placenta that allow nutrients to flow from mother to embryo. In order to grow, the shark embryos need something to eat.
In sandtiger sharks, for instance, the uterus could be aptly termed the war-womb as the embryos attack each other until only one remains.
While salmon shark embryos don’t eat each other, they do grow by eating a stream of unfertilized eggs, each a quarter inch or less, that the mother produces during pregnancy from her extra large ovary.
From the dissection of the reproductive tract Gallucci, Foy and their students hope to be able to eventually determine the age and development stage of the embryos, add to the information on how many pups females have and confirm that the shell gland they found is not used by females to store sperm, as it is in some other species.
Several years ago Gullucci’s group and Alaskan colleagues discovered that female salmon sharks migrate from the Gulf of Alaska and the Bering Sea to an area south of the Columbia River and back, a trip of about 4,000 miles.
The researchers hope the new information will help them further understand the lives of sharks in the North Pacific.
