Kasatochi’s Ash and the Fraser River Sockeye

Mt. Kasatochi erupting, 23 August 2008. Chris Waythomas, Alaska Volcano Observatory/USGS.

Did a perfectly-timed volcanic eruption temporarily raise a crashing salmon run from the dead? That is the question posed in a short opinion paper by Timothy Parsons and Frank Whitney, both of Fisheries and Oceans Canada, in the current issue of Fisheries Oceanography. The salmon in question are the sockeye (Oncorhynchus nerka) of the Fraser River, which flows out of the mountains into the Salish Sea near Vancouver, BC. Fewer and fewer sockeye have been returning to the Fraser over the past two decades, reaching an all-time low of 1.7 million fish in 2009. The reasons for this decline aren’t totally clear, but probably involve a variety of stressors, from development and agriculture in the watershed to diseases and parasites from salmon aquaculture pens.

Then, in 2010, the fish came back, with a vengeance. Some 34 million of them. More than ever before in the 60 years we’ve been keeping count. And no one had a clue why. It was a topic of bewildered conversation that fall in the UW Fisheries department—and if the salmon nerds at the UW were bewildered, you’d be hard-pressed to find anyone anywhere who wasn’t. The next year, the number of returning salmon dropped back down to around 6 million. This year’s forecast [pdf] isn’t great either.

So what gives? To understand Parson and Whitney’s volcanic hypothesis, you have to understand a few important features of the salmon life cycle, and by extension, facts about salmon research. Salmon, of course, swim up rivers to spawn in fresh water. Large fish swimming up a relatively narrow, shallow passage are, as far as fish go, pretty easy to count. In the case of sockeye, they are even bright red when they do it.

Going the other way is a different story. Young salmon are small, inconspicuous, and numerous. After a short period rearing in their natal river, they swim downstream and enter the ocean where, for all practical purposes, they disappear. Most will die quickly and unceremoniously, eaten by predators or starved by the absence of food in an unpredictable ocean. But a tiny fraction will grow rapidly into adults, and the size of that fraction is incredibly important. For example, survival rates of 2% and 0.5% are both tiny, but they differ by a factor of four—that is, the difference between 6 million and 24 million adults. That fraction is also very difficult to predict. Salmon scientists often refer to the ocean as the “black box:” smolts go in, and adults come out, but what happens inside is largely inscrutable.

Food availability is almost surely a major factor influencing juvenile survival, and in the ocean, all food depends on phytoplankton, which in turn depend on dissolved nutrients in the water. Nitrogen and Phosphorus (N and P) are the most important, but are not always sufficient to fuel blooms. Many areas in the ocean have abundant N and P, but not much green growth. These are known as high-nitrogen, low-chlorophyll (HNLC) areas. The explanation is a lack of “micronutrients” like iron, which, though not needed in quantities as large as N and P, are critical for some cell processes and can thus limit phytoplankton growth. The Gulf of Alaska is one such area.

This is where the 2008 eruption of Mt. Kasatochi, a small volcanic island in the Aleutian chain, comes in. The eruption spewed a plume of ash out over the Gulf of Alaska, which supplied the necessary iron to the surface ocean and triggered a huge phytoplankton bloom. Check out these satellite pictures, which show chlorophyll the Gulf in 2007 and then 2008, along with Kasatochi’s ash plume:

Volcanic ash from Kasatochi triggered a huge phytoplankton bloom in late summer 2008. From Hamme et al. 2010.

In addition, that bloom was dominated by large phytoplankton called diatoms, instead of the small dinoflagellates that are more common in the Gulf. Large algae meant that the primary production didn’t have to go through as long a food chain to get to the zooplankton that juvenile salmon prey on. Large zooplankton could eat those fat, juicy diatoms directly, instead of having to eat smaller zooplankton that had eaten protists that had eaten dinoflagellates. Since only 10% (as a rough rule of thumb) of the original energy makes it through each link up the food chain, the presence of diatoms potentially made the trophic transfer much more efficient by shortening the chain.

That big bloom of diatoms is the heart of Parsons and Whitney’s speculation–that more food was potentially made available to juvenile salmon at a critical moment in their life history. Many more from this year class survived, and returned triumphantly to the Fraser two years later. It’s important to note that this paper is opinion, not definitive research. While the volcano’s effect in triggering the bloom is well-documented, the subsequent effects on salmon are not at all. There’s anecdotal evidence that a similar eruption-salmon explosion happened in Kamchatka once in the 1950’s, but these kinds of events are rare and (for obvious reasons) difficult to study. Still, a really interesting hypothesis…I wonder how it might be tested with some kind of rigor the next time there’s a volcanic eruption in the North Pacific? And more immediately, how might we best incorporate the possibility of large, rare, unpredictable anomalies like this into our fishery management?

References:

T. Parsons, & F. Whitney (2012). Did volcanic ash from Mt. Kasatoshi in 2008 contribute to a phenomenal increase in Fraser River sockeye salmon (Oncorhynchus nerka) in 2010? Fisheries Oceanography DOI: 10.1111/j.1365-2419.2012.00630.x

R. Hamme, P. Webley, W. Crawford, & et al. (2010). Volcanic ash fuels anomalous plankton bloom in subarctic northeast Pacific Geophysical Research Letters (37) DOI: 10.1029/2010GL044629