Environmental fluctuations in the Bering Sea and North Pacific Ocean are reflected in the tissues of one of its apex predators, the northern fur seal (Callorhinus ursinus). The seals migrate and forage throughout this region most of the year and return each summer to their rookeries off Alaska. Their body tissues reflect the incorporation of prey throughout the region and can provide a record of ecosystem data where human instrumentation does not exist. Vibrissae (whiskers) and canine teeth from the seals have been collected from 1993 to present, while additional archived samples dating to the mid-20th century are available at the National Marine Mammal Laboratory. Stable isotope ratios within these tissues are incorporated from their diet derived from organisms found within the Bering Sea and North Pacific Ocean. Stable carbon and nitrogen isotope ratios (δ13C and δ15N) are found in vibrissae and are used to identify trophic relationships between the seals and their prey, while stable isotopes of oxygen and carbon (δ18O and δ13C) in canine teeth are used to detect environmental fluctuations in the marine ecosystem. Vibrissae are continuously growing tissues that reflect the animal's diet for at least one year. Fine resolution sampling provides trophic information on weekly to monthly time scales and reveals dietary and geographic changes within a year. Specific years can be identified in the tooth's growth layers. Each layer will be analyzed for δ18O and δ13C and these data can be correlated to inter-annual environmental shifts (temperature and salinity) in this region. Determining how environmental changes are reflected in northern fur seal tissues is an important step in discovering how ecosystem changes affect the animals and their prey, and why these predators are in dramatic decline.