14 November 2011

KILLER WHALES V. SALMON

Credit: Robert Pittman, NOAA.

What happens when we 'manage' two species in the wild with different—and conflicting—objectives? 

And what happens when one eats the other—and so do we?

That's the question raised in an interesting new paper in PLoS ONE. The authors investigated how many endangered chinook salmon are needed by endangered killer whales to recover their numbers in the northeastern Pacific.

Salish Sea, comprising the Strait of Georgia, Strait of Juan de Fuca, and Puget Sound, surrounding Vancouver Island and Washington state. Credit: SeaWiFS Project, NASA/Goddard Space Flight Center, and ORBIMAGE.

The question gets even more intriguing when you have two countries—Canada and the US—managing the fate of the two species that blithely cross international boundaries as if, you know, they weren't there.

The killer whales in the middle of the conflicted question are known as the southern resident killer whales (SRKW), who summer in the Salish Sea. They eat only fish, and are so dependent on chinook salmon that when they can't get them more adult whales die and fewer calves are born. 

  • Current population of southern resident killer whales: 87 individuals
  • Current chinook salmon stock: 36% of historical run in Canada, 8% in US

Chinook salmon. Credit: Josh Larios via Wikimedia Commons.

The stated objective of US management is to grow the dwindling killer whale population by 2.3% per year over 28 years. 

The authors assessed what the minimum basic caloric requirements were likely to be to make that come true—based on food requirements of captive killer whales, and body lengths of wild whales.

Estimated prey requirements of wild killer whales, based on two plausible values for calorie content of a typical, 4-year-old Chinook salmon. Credit: Rob Williems, et al. PLoS ONE. DOI:10.1371/journal.pone.0026738.

What they found suggests that the chinook salmon can't support both a growing killer whale population and human fisheries at current levels.

What's a fish-eating primate to do? The authors' suggest:

When one protected species relies almost exclusively on another protected species, it can be difficult to develop management frameworks that meet the needs of both species. This can lead to a perception that the needs of the more charismatic species will unfairly trump those of the prey species. In our experience, genuine conservation conflicts often result in management inaction in the absence of a framework in which to assess likely impacts... It is faster to reduce takes of salmon than to increase salmon production, and it is faster to increase salmon production than promote population growth in killer whales. The efficacy of salmon habitat restoration actions can often be measured within a decade, whereas similar measurements will take decades in studies of long-lived species like killer whales.



In other words, maybe we should let the whales get the fish for a while.

There's a lot more interesting stuff going on in this forward-looking paper and luckily it's open access. So you can freely read deeper.

The paper:
  • Williams R, Krkošek M, Ashe E, Branch TA, Clark S, et al. 2011 Competing Conservation Objectives for Predators and Prey: Estimating Killer Whale Prey Requirements for Chinook Salmon. PLoS ONE 6(11): e26738. DOI:10.1371/journal.pone.0026738
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