Population boundaries are often difficult to define for marine mammals but are critical to conservationists and scientists who study natural
populations. In the North Pacific Ocean, there are two distinct groups of
killer whale (Orcinus orca), one that only eats other marine mammals, such as sea otter and seals, (“transient”)
and another that only eats fish species (“residents”). For decades marine
mammal scientists have been doing surveys of these populations to map their
geographical range but not much is known about the gene flow between the two populations that exhibit very different eating habits. The National Oceanographic and Atmospheric Administration
(NOAA) recently funded a study for genetic testing of these populations to
better understand the similarity and relatedness coded in the whales’ DNA.
From 2001 and 2010, 462 skin biopsy samples were taken from
both residents and transients from the northern Gulf of Alaska to the Sea of
Okhotsk. Scientists used both mitochondrial DNA and nuclear microsatellites to
analyze the samples. One would think that in the vast open ocean with no true
boundaries that gene flow would be common between killer whale populations. However,
this study showed that is not the case in the north Pacific Ocean. Data revealed significant levels of
population genetic subdivision within the two predominant ecotypes. The fish
eating residents are now classified as a single population that ranges from
southeast Alaska through the Aleutian Islands and Bering Sea. The marine mammal
eating transients showed two genetically distinct stocks that have a slightly
overlapping range that begins in the Bering Sea and spreads down to the Prince
William Sound. This study helped further map the geographical range and
understand the dynamics of the unique populations of killer whales in the
northern North Pacific so that NOAA can better manage their habitat.
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