Population trends of northern fur seals (Callorhinus ursinus) from a metapopulation perspective

Abstract

A metapopulation model was created to explore source-sink population dynamics in northern fur seals (Callorhinus ursinus), for which the largest source population is declining, but smaller, newly established populations are either stable or growing. A declining source population could therefore slow down the growth of newly established populations. We also investigated changes in mortality and birth rates that could stabilize the declining population trend on the Pribilof Islands. The six main breeding populations—in the Pribilof Islands, Commander Islands, Kuril Islands, Robben Island, San Miguel Island, and Bogoslof Island—were modeled as individual age-structured populations, that were linked together using a migration model. The migration model component allowed movement of subadults among the six populations, with rules based on distances between populations, average travel speeds, habitat preferences, and El Nino events. The metapopulation model revealed the importance of immigration to newly established populations during the first 8years of population growth. After the initial growth period, additional immigration had no noticeable effect on population growth in sink populations. The population trend on the Pribilof Islands was difficult to stabilize, and immigration from smaller populations could not slow the population decline. Consequently, conservation of the Pribilof Islands population likely requires a long recovery period following reductions in population-specific mortality rates for adult females, juveniles and pups.