Published online 29 October 2010 | Nature | doi:10.1038/news.2010.572

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Sparks fly over theory that volcano caused salmon boom

Could volcanic ash feed ailing fish populations?

Kasatochi volcano as seen from 17,000 feet ASL. Picture Date: October 23, 2008 15:00:00 AKDTThe eruption of the Kasatochi volcano in 2008 has been linked to an unexpected boom in the numbers of salmon in Canadian rivers this year.J. Morris/AVO/USGS

Speculation has been flying this week that a 2008 volcanic eruption on an Alaskan island was responsible for this year's glut of salmon in rivers in British Columbia, Canada. If confirmed, the idea will improve biologist's understanding of the notoriously unpredictable size of salmon runs, and add fuel to the controversial idea of intentionally seeding the ocean with iron to boost diminishing fish stocks. But some researchers contacted by Nature warn that the theory is "far fetched".

After dismally low numbers in 2009, sockeye salmon mysteriously returned in record numbers to British Columbia's Fraser River this year (see 'Canada sees shock salmon glut'). Tim Parsons, one of Canada's most eminent fisheries researchers, has suggested that iron in the ash from the volcanic eruption on Kasatochi island, which spurred a phytoplankton bloom, could have indirectly provided a feast for the salmon. Parsons, an honorary scientist at the Institute of Ocean Sciences in Sidney, Canada, has a government-awarded medal named after him for ocean sciences. So fisheries experts are keen to hear Parsons out and look forward to studies that might confirm the theory. "It's as good as any other theory we have at this time," says Carl Walters at the University of British Columbia's Fisheries Centre in Vancouver.

“It's as good as any other theory we have at this time.”

Carl Walters
University of British Columbia

One way to check the idea, says David Welch of Kintama Research Corporation, a marine science consultancy in Nanaimo, British Columbia, would be to check the scales of salmon that returned in 2010 to see if they experienced an unusual growth burst in the autumn of 2008. "Salmon [scales] have growth rings just like tree rings," says Welch. "That would be a very useful way to test this quickly." Walters says that will happen, but he's not convinced it will be very revealing.

Ashes to food

Parsons' suggestion relies on a study in Geophysical Research Letters by Roberta Hamme of the University of Victoria, British Columbia1. The paper links the 7-8 August 2008 eruption of the Kasatochi volcano in the Aleutian Islands to a huge phyotoplankton bloom later that month. The eruption wasn't particularly large, but a storm spread its ash over a wide area. The resulting bloom was the biggest in 12 years of records, covering 1.5-2 million square kilometres of ocean. "We'd never seen anything like that," says Hamme.

It has long been known that the growth of phytoplankton in the North Pacific is limited by the amount of iron in the water. Dust storms from Asian deserts add doses of iron to the North Pacific, and volcanoes have recently been considered to be another important source2,3. The Hamme paper hammers home that connection. The question is whether such eruptions can have an impact on salmon.

Satellite pictures/Click for a larger version of this image.Roberta Hamme, University of Victoria

Salmon don't eat phytoplankton: they eat zooplankton and small fish, which in turn feed on phytoplankton. Zooplankton take months to a year to reproduce, so a single big burst of food for them over 3-4 weeks doesn't necessarily boost their numbers much, says Welch. Hamme says there were high levels of zooplankton in surface waters in August and September of 2008, but not as high as in early summer, before the eruption occurred.

The salmon that returned to British Columbia this summer would have been in the Alaskan Gulf in the autumn or early winter of 2008, in time to benefit from the food boom. But when Randall Peterman of Simon Fraser University in Burnaby, Canada looked at 18 other populations of salmon that wintered in the Gulf of Alaska in 2008, only three had unusually high return rates. "Whatever effects there were, they would have had to be right where the Fraser River sockeye were, but not where the other fish were," says Peterman. That's possible, but "kind of far fetched", he says. "It's more likely that the large returns this year in the Fraser are due to something closer to the British Columbia coast," he says.

But Walters notes that the last big salmon run in part of the Fraser River, in 1958, came two years after a huge eruption on the Kamchatka Peninsula. "The story makes sense," he says.

Global glut?

If the Alaskan volcano did cause this year's salmon boon, such a glut could happen elsewhere too. But for the hypothesis to work, a series of things have to line up. The volcano has to have iron-rich ash, and has to dump it in those parts of the oceans that are iron-limited: the northern and equatorial Pacific or the Southern Ocean. The eruption has to happen in the spring or summer, when phytoplankton growth isn't limited by low light, and it has to spur the growth of zooplankton rather than algae. And the fish have to stumble on that patch during their critical growth period.

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All this could spur some to think of intentionally seeding the ocean with iron to boost fish numbers. Some companies formed with the controversial intent of dumping iron into the sea in order to combat climate change have also advertised the positive side-effects on fish food. But is that a good idea? "Good god no," says Walters. "Our experience with fertilizing things is it's way too easy to fertilize the wrong thing. In general, it's a pretty dangerous thing to do."

The Canadian Prime Minister ordered an inquiry into what is happening with salmon numbers, and why predictions of the British Columbia salmon runs have been so wrong in recent years — in particular in 2009. They are now considering whether the 2010 boom is a sign of improvement, or just a fluke event — whether caused by the volcano or by something else. 

  • References

    1. Hamme, R. C. et al. Geophys. Res. Lett. 37, L19604 (2010). | Article
    2. Duggen, S., Croot, P., Schacht, U. & Hoffmann, L. Geophys. Res. Lett. 34, L01612 (2007).
    3. Langmann, B., Zakšek, K., Hort, M. & Duggen, S. Atmos. Chem. Phys. 10, 3891-3899 (2010). | Article

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    I hate it when things are more appreciative than people.

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