Some thoughts on fertilization and salmon numbers

  • Oct. 7, 2013 5:00 a.m.

The following are some thoughts from Dr. Skip McKinnell, Deputy Executive Secretary of PICES, The North Pacific Marine Science Organization. In July, Dr McKinnell wrote a paper challenging the theory that a volcano led to a large Sockeye run in 2010. Here, he offers some thoughts on whether an ocean fertilization experiment, like that carried out last year by the Haida Salmon Restoration Project, could be behind high pink salmon returns, as has been implied by those close to the experiment.Do you believe there is anything credible in this theory?Dr. McKinnell:There is no question that pink salmon eat animals that grow in the Gulf of Alaska. They don’t eat chlorophyll directly (which is what was produced by the iron) but chlorophyll, especially that created in a diatom bloom, is at the base of a foodweb that can lead to salmon as well as many other competitors. Nevertheless, there are some challenges for the…theory. These challenges are interspersed…below.How long do BC pink salmon remain at sea and where?Dr. McKinnell: Pink salmon hatch in the gravel during the winter after their eggs are laid. They stay in the gravel until the yolk sac has been used up then emigrate into the river currents and head downstream to the ocean (Strait of Georgia for Fraser River pink salmon). The earliest arrivals of pink salmon in the Strait of Georgia are probably late March and early April. They can be found feeding in the Strait of Georgia through summer before migrating out the Strait of Juan de Fuca (south) or Johnstone Strait (north) to the continental shelf. Most salmon biologists would not object to the statement that they follow a northward migration along the continental shelf during that first year at sea, but the stock-specific details of who appears when are not well known. Pink salmon eventually move offshore where they remain in the Gulf of Alaska feeding and growing until they return the next summer to spawn. 99.999999% exhibit this two year long life-history pattern.Could a cohort of pink salmon passing through the Haida Eddy in July 2012 be already returning to the Fraser?Dr. McKinnell:If the Haida Eddy was beyond the 200 mile limit in July 2012, there won’t be any juvenile Fraser River pink salmon swimming through it. …They are known from decades of sampling to be on the continental shelf or perhaps slightly beyond it on the continental slope. Haida eddies tend to travel southwestward after they are formed in winter at Cape St. James (i.e. not in the direction of migrating juvenile Fraser River pink salmon).Could a plankton bloom at that time in their life cycle have swelled their numbers for return a year later?Dr. McKinnell:If the bloom and the pink salmon don’t intersect in time and space, how could it? As for swelling numbers, I ask why 26 million is remarkable when 18 million in 2011 was unremarkable? In my view they are both remarkable. As with the Cohen Commission-stimulating runs of sockeye in 2009/2010, the pink salmon return this year is remarkable mostly because it was compared to the DFO 50:50 forecast of 8 million. A 50:50 forecast has equal probability of getting more or less than that number. Given that 18 million parents returned in 2011, the more remarkable thing is why the DFO forecast was so low so I looked up the original DFO document. It says: “for Fraser Pink Salmon, the forecast ranges from 4,794,000 to 17,111,000 fish at the 10 percent to 90 percent probability levels. The median (50 percent probability) forecast of 8,926,000 Pink Salmon is below the long-term (1959-2011) average return (12,580,000). The Fraser Pink forecast originally presented to CSAS was generated by the power model without the sea-surface salinity environmental covariate, due to the unavailability of salinity data at the time of the review. The original forecast was higher than the forecast presented in the current paper, and ranged from 6,881,000 to 27,687,000 returns at the 10 percent to 90 percent probability levels, with a median (50 percent probability) forecast of 14,010,000 returns. Fraser Pink forecasts are highly uncertain given the changes to the return estimation methods through time.” The text that follows in quotations was taken from the Pacific Salmon Commission website (www.psc.org) “A return this season of 26,000,000 Fraser pinks would imply a marine survival rate of approximately 5 percent, which exceeds the long-term average marine survival rate of 3.4 percent.” Is a 1.6 percent increase remarkable? That would involve historical comparisons to understand, but for pink salmon 5% seems pretty good.Could a single bloom (that disappeared within a month) be responsible for such increased runs of returning salmon?Dr. McKinnell: It is imperative, in my view, that any abundance of salmon needs to be compared with the abundance of the parents that produced it, and better yet, with the abundance of juvenile pink salmon that they produced. Representative sampling of salmon at sea has its own problems but a colleague in Alaska has been doing that for over a decade and produces remarkably accurate forecasts of pink salmon harvests in SE Alaska (the most abundant pink salmon in North America). As of yesterday, just the catch alone of pink salmon in SE Alaska was 89 million (many become spawners for the next generation). This was also quite a bit over the forecast (54 million) and note that SE Alaska pink salmon began their northward migration in 2012 beyond the Haida Eddy. To the north, pink salmon catches in Prince William Sound were about 89 million. If I was seeking a potential cause for these widespread high catches I would consider a much larger geographic scale than a point-source iron supplement that was located outside of the range of what is known of the juvenile pink salmon migration.

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