Published online 29 April 2008 | Nature | doi:10.1038/news.2008.783

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Where have all the seals gone?

Researchers clash over killer whales' role in 'megafaunal collapse'.

Has the lack of whales to munch on caused killer whales to switch to sea lions instead?GETTY

The latest salvo has been fired in a rancorous battle over the cause of steep declines in the north Pacific’s marine mammals.

Back in 2003, Alan Springer of the University of Alaska Fairbanks and his colleagues proposed that commercial whaling had forced whale-eating killer whales to switch to other prey1. This, they argue, triggered a ‘sequential megafaunal collapse’ as the killers first ate the harbour seals, then switched to fur seals, Steller sea lions and finally sea otters.

This theory proved controversial, in part because it ran strongly against the previous assumption that declines in fish stocks were largely responsible, which had led to some restrictions on fishing takes in the region.

“It has been over four years since [this theory] was published and we are both surprised and dismayed by how it has polarized a segment of the research and management communities,” say Springer and his colleagues in a new paper that takes on some of their critics2. “Much of what has been written to date has been accusatory, with little searching for common ground or a way forward toward further understanding.”

Sharp declines

Marine mammals have declined steeply in the North Pacific. From 1970–2000, the number of Steller sea lions in the region fell by 80%, according to the US National Research Council. Similar declines have been recorded in harbour seals, fur seals and sea otters.

While many pinned the blame on multiple factors, including declining fish stocks and hunting, Springer pointed out some evidence that did not fit this theory. Sea-lion prey was abundant in many areas where sea-lion populations were in decline, for example. And sea birds that dine on the same species as these mammals seemed to be doing well. Springer’s analysis also showed that the mammal populations had declined sequentially — with the number of harbour seals dropping off in the late 1970s, sea lions in the late 1980s and sea otters in the early 1990s. A simultaneous decline would have been expected if all these mammals were affected by the same lack of food.

To Springer, predation seemed a more likely culprit. He pointed out that between 1949 and 1969 whalers took more than 62,000 whales from the North Pacific. For the roughly 10% of 'transient' killer whales, which eat mammals rather than fish, the larder was a bit bare; Springer proposed that this had made them switch from eating whales to a tasty seal-based diet.

Others say that there's little evidence that killer whales ever relied on great whales as a main part of their diet. “I personally don’t believe the data support the hypothesis that large whales were key prey for transient killer whales,” says Douglas DeMaster, a marine mammals expert at the National Oceanic and Atmospheric Administration (NOAA) in Seattle, Washington.

Out of order

DeMaster and others also say that the declines in sea mammals weren't sequential3. "I’m not saying they’re exactly simultaneous; but to me it’s clear they’re concurrent," says NOAA researcher Paul Wade. So that wouldn't rule out lack of fish as a cause of the decline.

Earlier this year DeMaster published a paper outlining evidence of anthropogenic causes — mainly by-catch from commercial fisheries, commercial hunting and both legal and illegal shooting — of declines in the number of Steller sea lions4.

Full historical data for the decline of these species simply do not exist, both sides admit, forcing researchers to use incomplete information. Different statistical approaches seem to yield different answers about whether sea mammals fell one after another or generally at the same time.

Moving on

"I don’t sit on any side of this debate," says Ian Boyd, a marine mammal expert at the University of St Andrews, UK. "What both groups need to do is sit down and come to a consensus about what key data need to be collected [to decide between the hypotheses]. Because of the acrimony that now exists it’s going to be more difficult to do that."

Springer's latest work does provide an idea of what to do next. “The last part of that paper talks about a way forwards,” says DeMaster. “That’s helpful.” It notes that as populations of great whales should recover now that whaling has been vastly curtailed, so killer whales might switch back to eating them in preference to seals. This would offer some support to Springer's hypothesis.

But how long it will take for this to become apparent, if it does, is unclear. “I doubt we’re nearing a resolution,” says Springer. “The poles are still fairly far apart.”

In the meantime, the US Endangered Species Act means that the NOAA must adopt a precautionary approach, acting to restrict fishing even if this isn't definitely the culprit behind sea mammal declines. "It would take more than just a preponderence of evidence for the agency to change its current management strategy," says DeMaster. 

  • References

    1. Springer, A. M. et al. Proc. Natl Acad. Sci. USA 100, 12223-12228 (2003). | Article | PubMed | ChemPort |
    2. Springer, A. M. et al. Mar. Mamm. Sci. 24, 414-442 (2008). | Article |
    3. Demaster, D. P. et al. Prog. Oceanogr. 68, 329-342 (2006).
    4. Atkinson, S., DeMaster, D. P. & Calkins, D. G. Mamm. Rev. 38, 1-18 (2008). | Article |
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