The effects of multiple stressors on female northern elephant seals were examined by conducting an experimental manipulation where individuals were exposed to acoustic disturbance at-sea coupled with experimentally elevated cortisol. Health metrics (energy store, stress hormone, immune status, and contaminant burden) and vital rates were measured before and after the post-breeding foraging trip. Natural variation in mercury burden had additive and synergistic effects with cortisol on the endocrine (T3, T4, and rT4) and immune (IgG) systems. Energy gain varied over a foraging trip (foraging success) and was correlated with circulating blood cortisol level (GLMM, p 0.001). Females with very poor energy gain had cortisol levels 20 times normal levels and exhibited a lower probability of pupping the following season. Differences in energy gain were not associated with spatial distribution or diving depth in our experimental post-breeding data (n = 63) or our long-term data across post-breeding and post-molting trips (n = 463). Females with lower energy gain rates had significantly lower mean dive durations. While acoustic disturbance had a slight effect on the mass gain of females, variation in prey availability associated with environment fluctuations swamped all other effectors. Elephant seals provide an experimental system to study the interaction of multiple stressors on vital rates on a capital breeding marine mammal, providing insight into other more difficult-to-study marine mammals.
Costa et al. (Wed,) studied this question.