Fluctuating Asymmetry in the Saddle Patch Shape of the Pacific Ocean Killer Whale (Orcinus orca) Populations

The killer whale (Orcinus orca) is a top predator and one of the most contaminated marine mammal species in the world. Due to different prey preferences and life styles, killer whale populations accumulate persistent pollutants differently, and therefore are exposed differently to this stress. Stress may express itself in a population as an increase in the relative number of individuals with asymmetric presentation of a trait that is normally symmetrical. This phenomenon is called fluctuating asymmetry. There are many environmental and genetic factors that can cause fluctuating asymmetry. We have used the symmetry of the killer whale's saddle patch pattern behind the dorsal fin as an indicator of fluctuating asymmetry in six Pacific Ocean populations. The southern resident killer whale population seems to be remarkably more asymmetrical than the other studied populations. Although many possible environmental factors could cause asymmetry, we suggest that small population size, development of reproductively isolated ecotypes and possible inbreeding as genetic factors are causing asymmetry in the southern resident population.