Evidence Of Intermittent Residency In The Northern Fur Seal Callorhinus Ursinus

This study found evidence of intermittent, multi-year residency periods in northern fur seals (Callorhinus ursinus) using stable isotope ratios in vibrissae and canine teeth. Northern fur seals migrate from the Bering Sea during summer months to lower latitudes and slightly warmer waters of the northeastern Pacific Ocean and California Current in the winter. To determine the length of time spent away from the Bering Sea, growth rate was estimated using the covarying oscillations ?13C and ?15N, estimated to be 0.09 mm/day. The ?13C and ?15N in vibrissae from 30 male fur seals showed a minimum of 13 separate periods of stable covariance covering 3.25+ cm, indicating at least 1 year in warmer, less productive waters of the northeastern Pacific Ocean. The vibrissae isotope ratios were used in conjunction with ?18O from tooth dentin growth layer groups of 20 male northern fur seals; they showed significant enrichment in ?18O in 50% of the animals at age 1-2 years, which indicates extended periods of time spent in lower latitudes in the North Pacific Ocean as ?18O is typically enriched in warmer, less productive waters. Significant changes in ?18O were found to be ̃0.2‰ enrichment per 10º south latitude, while longitude was found to have 0.2‰ enrichment per 50˚ East longitude. These data show that latitudinal changes, those related to the southerly migration from the Bering Sea to the northeastern Pacific Ocean, are a stronger factor in the shifts in dentinal ?18O than longitudinal shifts. These intermittent periods of occupation are important when estimating population abundance of northern fur seals, especially pups and juveniles.