Can gray whale management units be assessed using mitochondrial DNA?

Résumé

Although most eastern North Pacific gray whales (Eschrichtius robustus) feed in Alaskan waters north of the Aleutian peninsula, some have been reported as long-term feeding residents in more southern waters ranging from northern California to southeast Alaska. The population history of this smaller putative southern feeding population is unknown. Recently, native Americans of the Makah tribe attained permits to harvest up to five whales per year in Washington State waters. Managers need to know whether southern summer residents could be potentially depleted through low-level harvesting. This paper investigates the feasibility of using genetic data to assess the plausibility of two possible population histories for the southern feeding group: panmixia with the northern feeding group and a single colonisation event less that a century ago. We find that a genetic study would most probably result in an unambiguous answer to the question of whether the southern feeding group is a separate population founded by a single colonisation event. Simulations show that a single founding event in the last century would result in genetic differentiation 97.8% of the time (a = 0.05) between the two feeding groups. Further, sensitivity analyses of uncertain parameters used in the model show that the results do not depend on the values of growth rate, mitochondrial allele frequency distribution or population size of the eastern North Pacific gray whale after commercial harvest.