Intrapopulation Foraging Response of Zalophus californianus to Environmental Stress

Periods of oceanographic thermal variation in the Northeast Pacific between 2013 and 2017, characterized by an El Niño and a Pacific marine heat wave (“The Blob”), diminished primary productivity, which reduced carbon transfer through the food web. Opportunistic consumers compensate for loss of prey through the consumption of other prey, whereas dietary specialists exhibit population level survivorship responses. The California sea lion (Zalophus californianus) does not exhibit a uniform response to reduced prey availability across the population. As was the case during the Unusual Mortality Event in 2014, there were adult females that supported the energetic costs of raising offspring and nursed young until they could be successful foragers themselves, whereas still many young animals died. Distinct individual foraging responses to fluctuations in prey availability persists within the population, such as diving deeper (as indicated by instrumentation) or foraging farther from the rookery, although the extent of which is unknown. While instrumentation provides more accurate global positioning, data on forage variation within the population is limited by cost or life of the instrument. Another approach to understanding foraging ecology is stable isotope analysis of tissues, collected by either opportunistic or strategic means. Isotopes are deposited in tissues after the metabolization of prey and will indicate general forage location and trophic level (carbon and nitrogen, respectively) of prey. Vibrissae (whiskers) of adult Z. californianus grow at a continuous rate and remain chemically inert after growth, thus storing isotopic ratios along vibrissae and generating a timeline of forage habits for an individual. Isotopic ratios of vibrissae were analyzed for vibrissae segments corresponding with pre, during, and post warming between 2013 and 2017, indicating forage location and trophic level of prey consumed and to what extent variation within the population persists through perturbations.