Study examines ocean effects on chinook

Productivity of wild chinook salmon from the Columbia River to northern Alaska is subject to large-scale atmospheric and ocean circulation trends, especially the North Pacific Gyre Oscillation, according to a recent study.

Other studies of coho, pink and sockeye salmon stocks have found that trends in productivity for these stocks have more to do with localized ocean trends.

The study also found a more alarming trend: the differences in timing and productivity among the various wild chinook stocks are becoming more synchronous, meaning that there are fewer variations and less diversity, all a result of ocean trends that are caused by global climate change.

“Productivity of chinook shows covariation on very large scales, from Oregon to Western Alaska,” said researcher Brigitte Dorner, an independent fisheries scientist who lives on Lasqueti Island in British Columbia. “This is unusual for salmon, and it suggests that there are some factors operating on equally large scales that influence chinook productivity. These factors are likely linked to large-scale atmospheric and ocean circulation phenomena, which affect chinook through mechanisms such as food availability and predator prevalence.”

She said the link between chinook productivity and the large-scale circulation patterns -- besides the NPGO, the study considered the Pacific Decadal Oscillation and the location of the bifurcation in the North Pacific current as it reaches North America -- is concerning for two reasons.

The first is climate change, which is affecting the large-scale circulation patterns in ways difficult to predict in the longer term.

“Consequently, we may expect that chinook productivity patterns may start to diverge from what we have seen historically and therefore also become more difficult to predict,” Dorner said. “One particular development that has been linked to climate change is an intensification of the NPGO, with more intense phases of low and high values. Assuming this trend continues, we can expect to see more of a ‘roller-coaster’ pattern of large swings in chinook productivity.”

The second is that productivity patterns of separate populations have become more synchronized, “meaning there is less ‘balancing out’ of declines by increases elsewhere,” she said.

“It is difficult to infer causality without actually studying the direct mechanisms by which large scale circulation patterns affect chinook productivity in more detail, but just logically speaking, increased synchronicity is consistent with what you would expect if factors related to large scale atmospheric circulation patterns were becoming relatively more important as drivers of productivity,” she said.

The combination of increased synchronicity and intensification has potential for serious impacts on chinook-dependent species as well as chinook fisheries, Dorner concluded.

“Spatial and Temporal patterns of covariation in productivity of Chinook salmon populations of the Northern Pacific” was published online last month in the Canadian Journal of Fisheries and Aquatic Sciences.

Dorner’s co-authors are Matthew Catalano, professor, School of Fisheries, Aquaculture, and Aquatic Sciences, Auburn University, and Randall Peterman, professor emeritus, School of Resource and Environmental Management, British Columbia.

Overall, the recent changes (changes found during the period 1995 to 2009) – increased synchronicity and extreme swings in productivity – may reduce the resilience of chinook salmon to the effects brought on by climate change and the resulting habitat modifications, the study says.

Climate change appears to be driving the intensification in the NPGO, which in turn may be responsible for the increased synchronicity at least to some degree, Dorner said.

“On the other side of the equation, increased synchronicity, especially when combined with more extreme swings in productivity, would be expected to make stocks more vulnerable to a variety of adverse factors, including factors linked to climate change,” she said. “For example, if major flooding or drought affects a stock or group of stocks at a time when populations throughout the area experience a phase of low productivity, that has potentially more serious and long-term consequences than the same event occurring during a time when at least some of the nearby populations are doing well and can compensate for local losses through recolonization, maintenance of genetic diversity, etc.”

The study looked at 24 distinct stocks of wild, not hatchery, chinook salmon, even though many of the stocks have a substantial hatchery component. The focus on wild stocks was to permit a greater understanding of chinook survival dynamics without the confounding influence of hatchery practices, the study says.

Of these stocks, run timing for both the juvenile and adult life history vary widely. Outmigration for chinook differs with ocean-type (subyearlings) which migrate in the first year in fresh water and freshwater type (yearling) which spend their first year in freshwater, according to the study.

Adult run timing also varies, with fish returning in spring, summer or fall.

Increased synchronicity can be “expected to result in more year-to-year variability in harvest levels and more frequent fishery closures because fewer stock abundances will be high when others are low, especially since the predominant trend in recent years has been towards lower productivity,” the study says.

“One of the biggest challenges facing fisheries scientists and managers is the non-stationarity of stock productivity (i.e., where the mean and/or variance changes substantially over time),” the study says. “Regardless of whether such changes occur because of altered ocean conditions or freshwater conditions, the success of efforts to sustainably manage Pacific salmon harvests and rebuild low-abundance populations depends on understanding the causes of changes in salmon productivity.”