Sockeye salmon spawning on the rivers and streams of Washington state, British Columbia and southeastern Alaska have been producing fewer and fewer adults over the last six decades, a new study suggests.
In one dramatic example, the Fraser River's early Stuart sockeye run produced 20 adults for every spawning sockeye during the 1960s, but productivity had dropped to about three adults per spawner by the mid-2000s, said Randall Peterman, co-author of the study and a professor at Simon Fraser University in Burnaby, B.C.
Around Washington state, British Columbia, and eastern Alaska, the story's been much the same, with some populations dropping below the replacement ratio of one adult per spawning salmon, he added.
The topic known as productivity — which is measured by the number of adults produced by each spawning salmon — is addressed in a paper published today in the Canadian Journal of Fisheries and Aquatic Sciences.
"People who rely on salmon for their livelihoods, or their First Nations food and social and ceremonial purposes, really find sockeye populations very valuable, and so it's important to keep them going at a productive level," said Peterman who conducted the research with post-doctoral fellow Brigitte Dorner.
"Furthermore, there are very strong and important concerns about the long-term viability of many sockeye populations as well as other salmon populations, other species."
Since sockeye salmon are adaptable, their declining productivity may suggest that something is going wrong in the ecological system, he added.
Declines on wide scale
Originally produced for the Cohen Commission, the judicial inquiry examining the 2009 collapse of the Fraser River sockeye salmon run, the study crunched data on the productivity of 64 sockeye salmon stocks in Washington state, B.C., and Alaska between 1950 and 2009.
What emerged was a trend showing sockeye-salmon declines on the Fraser River were not unique and were happening on a wider scale and much farther north than originally anticipated.
The study found there have been "rapid and consistent decreases" in sockeye salmon productivity in stocks between Puget Sound in Washington state to Alaska's Yakutat peninsula.
Not experiencing the declines, though, have been areas in central and western Alaska, where productivity has remained stable or even increased, he added.
Peterman said the study didn't look at the potential causes of the declines, but he thinks the large spatial trends suggest some causes are more likely than others.
More research needed
He said some people have argued that perhaps habitat degradation and pollution on a regional level may be affecting sockeye-salmon productivity.
But he said that's unlikely because populations in pristine and heavily disturbed areas have showed similar downward trends.
"So this seems to suggest that the causal agents are more likely either pathogens, predators or reduced food supply. Could be one or all of those."
Research on the causes of the declines must continue if government agencies, which are responsible for managing the species, are to come up with any appropriate action, he added.
"What our research does is help propose to other researchers what types of mechanisms they should be looking for, that is mechanisms that operate in areas where many fish stocks overlap or at least where there is some large multi-regional oceanographic or climatic patterns that could affect many populations at once," said Peterman.