Reproductive territories of northern fur seals on Adams Cove beach, San Miguel Island, California Channel Islands, July 2006.

Hookworm disease has been recognized in northern fur seals since 1896 when F. A. Lucas described the worm from the intestine of a 3-month-old dead fur seal pup in the Pribilof Islands, Alaska. The fur seal hookworm was subsequently described as Uncinaria lucasi by C.W. Stiles in 1901. From the time of their description until the early 1980s, hookworms were highly prevalent in fur seal pups and were responsible for a substantial amount of pup mortality in the Pribilof Islands. Currently, however, there is virtually no mortality associated with hookworms in Pribilof Island fur seal pups, but on San Miguel Island in the California Channel Islands, hookworm disease has become a major source of pup mortality.

This article provides an overview of the biology of hookworms, discusses the change in hookworm disease prevalence in the Pribilof Islands, and reviews a recent experiment conducted by the National Marine Mammal Laboratory's California Current Ecosystems Program in which fur seal pups were treated with drugs to control the disease on San Miguel Island.