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NW Fishletter #297, December 14, 2011

[5] Puget Sound Orcas Take A Big Bite Out Of NW Chinook Runs

Researchers from the University of Washington and other Northwest universities have estimated the three resident Puget Sound orca pods may be chewing through up to 90,000 Fraser River chinook every summer--that's nearly one-third of all the spring, summer and fall chinook that return to the Fraser or its tributaries. The results of the study were published Nov. 9 in the online journal PloS ONE ("Competing Conservation Objectives for Predators and Prey: Estimating Killer Whale Prey Requirements for Chinook Salmon," Williams et al., 2011).

Building a model that estimated the caloric requirements of the orca pods, which now total 88 individuals, the authors said recovery efforts for the ESA-listed killer whales may be in direct conflict with other efforts to improve salmon numbers. "For instance," they wrote, "a U.S. recovery goal (2.3-percent annual population growth of killer whales over 28 years) implies a 75-percent increase in energetic requirements." Unfortunately, the orcas' favorite food is fresh chinook salmon.

The scientists said cutting salmon fisheries might help out temporarily so that actions designed to boost fish numbers have time to take effect, but "trade-offs between conservation objectives for predators and prey will become increasingly necessary."

The orca pods don't hang out in the San Juans once the chinook migration is over. Currently, many of them are far down in Puget Sound eating chum salmon that are returning to South Sound hatcheries and rivers. In the spring, some have been sited off the coast of California and near the mouth of the Columbia, where there has been some concern, but there is little evidence that the lack of Snake River spring chinook has aided their decline.

Other research has shown that chinook salmon is their favorite prey and the most nutritious. Unlike non-resident ocean-going orca pods, these southern residents do not consume marine mammals. Luckily, for the U.S., the orcas take a much larger share of Canadian chinook than of those bound for Puget Sound.

The researchers estimated the three pods in Puget Sound would need the energy equivalent of more than 241,000 chinook annually, if that was all they ate. Clearly, said the scientists, that is not the case, but their estimates of how many chinook the killer whales consume during winter months are fairly uncertain.

NMFS considered the needs of orcas before it approved the latest harvest plan for Puget Sound chinook, and the researchers said the issue will clearly affect fisheries in the future. They said a multi-species or ecosystem approach to fisheries' stock assessments and management may be needed to balance the needs of the fishery (First Nations, North American Tribes, commercial and recreational) and the orcas.

"An intriguing policy solution would be to give killer whales a salmon catch allocation under the [Pacific Salmon] Treaty," the researchers said. "This would be consistent with the spirit of Canada's Wild Salmon Policy, which places conservation needs ahead of fishery allocations."

In 1995, the southern resident population was 98 individuals, but dropped to 81 by 2001. It rose again to about 90 whales by 2006, dipped to 83 in 2008 and rose to 88 earlier this year. -B. R.

The following links were mentioned in this story:

"Competing Conservation Objectives for Predators and Prey: Estimating Killer Whale Prey Requirements for Chinook Salmon," Williams et al., 2011

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