Genetic identification of a small and highly isolated population of fin whales (Balaenoptera physalus) in the Sea of Cortez, México

Abstract

For many years, researchers have speculatedthat fin whales are year-round residents in theSea of Cortez (= Gulf of California). Previouswork by Bérubé and co-workers has shownthat the degree of genetic diversity among finwhales in the Sea of Cortez at nuclear andmitochondrial loci is highly reduced. However,the relatively unobstructed connection with theNorth Pacific Ocean argues that Sea of Cortezfin whales are part of a much larger easternNorth Pacific population given the extensivemigratory ranges observed in fin whales andbaleen whales in general. The low degree ofgenetic variation might thus simply be due tohistoric fluctuations in the effectivepopulation size of an eastern North Pacificpopulation. In order to test if the reducedgenetic variation detected among fin whales inthe Sea of Cortez is due to small populationsize or a past bottleneck in an otherwise largeeastern North Pacific population, we analyzedthe geographic distribution of geneticvariation at a single mitochondrial (controlregion) and 16 nuclear loci in samplescollected from fin whales in the eastern NorthPacific (n = 12) as well as the Sea of Cortez(n = 77). Our results showed that fin whalesobserved in the Sea of Cortez constitute ahighly isolated and thus evolutionary uniquepopulation, which warrants special conservationmeasures given the current low estimate ofabundance of approximately 400 individuals.