September 30, 2012
For White Sharks, Diet Is Different For Each Individual
April Flowers for redOrbit.com - Your Universe Online
White sharks are the largest predatory sharks in the ocean. They are thought of as apex predators, animals who as adults have no natural predators of their own in their natural ecosystem. Marine scientists have long known that white sharks feed on seals and sea lions, but a new study from the University of California, Santa Cruz demonstrates that there is surprising variability in the diets of individual sharks.
The research team traced variations in diet over a sharks' entire life by analyzing composition of growth bands in shark vertebrae. Carbon and nitrogen isotopes in the shark's tissues serve as natural tracers of dietary inputs.
"We did find that white shark diets changed with age, as expected, but we were surprised that the patterns and extent of change differed among individuals," said Sora Kim, who led the study as a UCSC graduate student and is now at the University of Wyoming.
The study looked at the vertebrae of 15 adult white sharks caught along the west coast; fourteen were caught off the coast of California and one off Baja California. White sharks in this population eat a wide variety of animals, including dolphins, fish and squid in addition to the seals and sea lions. According to Paul Koch, professor of Earth and planetary sciences at UCSC, not every shark eats the same mix of prey animals.
"We confirmed that the diets of many individuals observed at seal and sea lion rookeries shift from fish to marine mammals as the sharks mature," he said. "In addition, we discovered that different individual sharks may specialize on different types of prey. These two types of flexibility in feeding behavior are difficult to document using traditional methods, but may be very important for understanding how the population is supported by the eastern Pacific ecosystem and how it may respond to changes in that ecosystem."
White sharks are found in temperate waters around the globe. They are considered an important, though not terribly common, predator in California's coastal habitats. The National Oceanic and Atmospheric Administration (NOAA) announced that the agency is considering whether to list the west coast population on the Endangered Species Act to protect them.
The Endangered Species Act of 1973 is a law that is designed to provide for the conservation of species that are endangered or threatened. It is also geared towards the conservation of the ecosystems of those threatened species. NOAA's National Marine Fisheries Service (NMFS) shares responsibility for implementing the ESA with the U.S. Fish and Wildlife Service.
White sharks found along the California coast cruise the coast from late summer to early winter before moving offshore in a regular migratory pattern that has been tracked by tagging studies. While their movements may be predictable, the results of this new study illuminate important dietary and behavioral differences between individual sharks.
The shark vertebrae were obtained from various collections, caught at different times and in different places along the coast from 1957 to 2000.
"Interestingly, we do see a small shift in diet as marine mammal populations increased after the passage of the Marine Mammal Protection Act in 1972," Kim said.
The results of this new study are published online in the journal PLoS One.