Assessing the impact of modern environmental stresses on California sea lions (Zalophus californianus) using fluctuating asymmetry

Fluctuating asymmetry, small random differences between the left and right side of a bilateral trait, has been shown to be a successful index of reproductive fitness in many species. Observed asymmetries are thought to reflect an inability of individuals to compensate for developmental instability. I investigated the degree of FA exhibited by California sea lions ( Zalophus californianus) off the coast of California and Mexico using eight cranial characters. I hypothesized that recent environmental stresses, including El Nino events, toxic algal blooms, and fishery interactions, have created a more stressful environment for marine mammals compared with the early part of the century. As predicted, sea lion skulls collected prior to 1940 (1878--1940; n = 34) exhibited significantly less FA than more recently collected animals (1985--2001; n = 141). These results suggest that California sea lions are being more severely affected by recent environmental stresses and that FA may be a reliable bio-indicator of current population health.