Estimating Populations of Marine River Otters in Prince William Sound, Alaska, Using Radiotracer Implants

One year after the spillage of oil from the Exxon Valdez in spring 1989, we used radiotransmitters and radiotracer labels to mark river otters (Lutra canadensis) at Knight Island (oiled) and Esther Passage (unoiled) in Prince William Sound, Alaska. Feces recovered from latrines of animals were used as "recaptures" in mark-recapture experiments to estimate the density of river otters in the two coastal study areas. Our methods were evaluated for sources of bias in estimating and comparing population sizes. There was no evidence that densities of river otters at Knight Island (oiled) were less than those at Esther Passage (unoiled) in summer 1990 or that density of river otters at Knight Island was declining. Estimates of river otters in Prince William Sound ranged from 0.28 to 0.80 animals per km of coastline. Our estimates are similar to those for marine river otters in southeastern Alaska, but bias in the different methods should be considered.