Understanding The Breeding Structure Of Killer Whales
July 13, 2013

Understanding The Breeding Structure Of Killer Whales

April Flowers for redOrbit.com - Your Universe Online

Orcas, or killer whales, are found in every ocean in the world making them a global force to be reckoned with. They have remarkable social bonds and sophisticated hunting techniques that make them the top predators in the oceans.

For a long time, scientists assumed these clever, highly mobile whales bred with each other freely in the seemingly homogeneous ocean. Dedicated researchers have furthered our understanding of the oceans complexity and the world of the killer whales, showing us the truth of killer whale breeding is far more nuanced than previously thought.

The National Oceanic and Atmospheric Administration (NOAA) is the agency responsible for conserving and managing killer whales in US waters, and it faces a major challenge: it must identify orca subpopulations, understand their needs and develop effective and sometimes unique ways to manage them. By determining the patterns of similarity and relatedness coded in the whales' DNA, managers gain important insights into how these subpopulations arose, what factors shaped them and what drove divergence in the species complex. Far beyond simply identifying the boundaries of whale stocks, this study has important implications for evaluating the status of killer whale populations. The results of the study were published via Open Access in the Journal of Heredity.

Recently, other studies have shown that distinct groups of Orcas gather in the same place seasonally. Scientists were unsure if this was because of feeding or mating patterns. Genetic evidence and observations of individual whales have now shown these whales exhibit low levels of "gene flow," or breeding among subpopulations. The new study sheds light on these principles at work among killer whales in Alaska and the northern reaches of the North Pacific Ocean.

A lack of data for the far western reaches of the North Pacific and uncertainty about the population structure have led to very broad stock designations for killer whales in the waters of the western Gulf of Alaska, Aleutian Islands, Bering Sea, and Russia. Fish-eating "resident" killer whales in the far North Pacific are currently considered a single stock that ranges from southeast Alaska through the Aleutian Islands and Bering Sea. Formally known as "transients," Bigg's killer whales are currently managed as two separate stocks with overlapping ranges. The two are the "Aleutian and western" stock (Gulf of Alaska, Aleutian Islands, and Bering Sea) and the much smaller "AT1" stock, which appears to range primarily throughout Prince William Sound and the Kenai Fjords.

The NOAA team collected 462 skin samples from both resident and transient wild killer whales and characterized individual genetic variability using two different genetic markers (mitochondrial DNA and nuclear microsatellites).

Kim Parsons, NOAA scientist from the National Marine Mammal Laboratory, and her colleagues discovered further subdivision within the whale stocks. Because the evidence is so compelling, Parsons and her team believe it's time to revise the killer whale stocks in the region. The genetic testing also revealed a lack of breeding between the two killer whale types, highlighting the distinctness of the sympatric Bigg's and resident killer whales on a local scale.

The findings of this study corroborate a 2010 global genetic study of killer whales and what many marine scientists already suspected, based on years of studying wild orcas by tracking their movements and comparing their vocalizations, preferred prey and social structure. The team says much work remains to be done with the killer whales, but that powerful genetic tools have given us another vital piece of the puzzle.