The Bristol Bay Salmon "Portfolio"

A new study from the University of Washington, published in the current issue of Nature, uses five decades' worth of data to analyze the population dynamics of Bristol Bay salmon. What they found is a that the massive sockeye salmon run--an average of 40 million fish return each summer--is made up of a few hundred discrete populations, a diversification of risk akin to an investment portfolio--hence, the "portfolio effect." That biological diversity helps stabilize an ecosystem has long been recognized, but this study goes further, pointing out the importance of diversity within one species. As the Seattle Times summed it up:

Though they're all the same species, Bristol Bay sockeye comprise hundreds of populations, each adapted to its own river, stream or tributary. Some of the populations return from the sea after one year. Others spend two years foraging in the ocean before heading back to spawn. Some sockeye flourish when it's cold and wet. Others do better in hot, dry years. That variety means the species as a whole survives and thrives, even when bad weather or a shortage of food in the ocean hammers individual populations.

Or, as the study's lead author, Professor Daniel Schindler, told the paper, "There are enough winners to make up for the losers every year."

I had the good fortune to interview Schindler last year for my Outside feature on the Bristol Bay fishery and the Pebble Mine controversy, and I remember him making the point then that the consistency of the salmon run is dependent on the diversity of its component populations. A more homogenous run would mean more variability and more lean years, and so, for the sake of the fish and the fishery, maintaining this variety of populations is crucial.

As with most news coming out of Bristol Bay, the subtext is Pebble, and the implicit argument Schindler and his colleagues are making is that messing with even the smallest salmon-bearing tributary could damage the Bristol Bay resource. "Our results demonstrate the critical importance of maintaining population diversity," they write in the synopsis, "for stabilizing ecosystem services and securing the economies and livelihoods that depend on them."