Environmental fluctuations in the eastern Pacific Ocean are reflected in the tissues of one of its most vulnerable apex predators, the South American fur seal (Arctocephalus australis). These seals live along the Atlantic and Pacific coastlines of South America and forage in the waters over the continental shelf. The coastal waters off Peru is a region of great environmental fluctuations due to periodic (every 2-7 years) El Niño-Southern Oscillation (ENSO) events, and the recently identified ENSO Modoki, which result in ecosystem-wide food web changes. Fur seal body tissues reflect the incorporation of prey from the region; continuously growing tissues can provide a timeline of environmental data in an ecosystem where changes regularly occur and human instrumentation to measure such changes is sparse. Vibrissae are continuously growing tissues and fine resolution sampling along their length provides trophic information on weekly to monthly time scales over several years. Teeth are composed of annual growth layers representing the seal's entire life and provide data on annual temperature and salinity changes where they and their prey reside. 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 found along Peru's coastal margin. Stable isotopes of oxygen and carbon (δ18O and δ13C) in canine teeth are used to detect changes in sea surface temperatures and related primary production, both direct measures of ENSO events. Vibrissae (whiskers) have been collected and analyzed from adult female seals immediately following the 2009/2010 ENSO event. Preliminary data indicate distinct periods of decreased and increased primary production; these periods likely correspond to regional ENSO events impacting the food web. We propose to continue a more comprehensive analysis of vibrissae and teeth from mother-pup pairs collected from 2011-2015 to identify productivity and trophic changes within the region and correlate those changes to climatic anomalies. The combined isotopic data from both tissues reveal how abiotic, ecosystem-wide changes influence the trophic dynamics and resultant survivability of the vulnerable South American fur seal.