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Warming Coastal Ocean Temperatures May Lead To Negative Effect For Salmon ‘Recruitment’
Posted on Friday, October 09, 2015 (PST)

While the Pacific Decadal Oscillation is the most important factor in determining how many salmon return from the ocean to streams in the Northwest, a series of other more regional environmental factors also influence that return, according to a recent study.

 

The study found that multiple, inter-related ecological pathways (environmental factors) can strongly influence Oregon coastal coho salmon abundance, with the most important factor being the PDO, an index of large-scale climate variability in the North Pacific Ocean.

 

Among those pathways (17 in all, including two biophysical pathways) coastal ocean temperature (sea surface temperature -- SST) and juvenile salmon prey biomass (the biophysical pathway) had the strongest effects on recruitment of the salmon.

 

Having less impact on recruitment were the North Pacific Gyre Oscillation (NPGO) and the Oceanic Nino Index (ONI).

 

“We found that in years when the coastal ocean was warmer than average there also tended to be reduced salmon prey biomass and lower salmon recruitment levels compared to cool ocean years,” said researcher Michael Malick, PhD Candidate, School of Resource and Environmental Management, at Simon Fraser University. “From a climate change perspective, this result suggests that warming coastal ocean temperatures may have a negative effect on coho salmon recruitment by reducing available prey resources for salmon in the coastal ocean.”

 

There is a degree of uncertainty associated with the environmental relationships, Malick added. “For example, our results indicated that when coastal ocean temperatures were above average there is still a 1 in 4 chance that recruitment would be above average,” he said.

 

The study, “Accounting for multiple pathways in the connections among climate variability, ocean processes, and coho salmon recruitment in the Northern California Current,” was published online August 28, 2015 in the Canadian Journal of Fisheries and Aquatic Science (http://www.nrcresearchpress.com/doi/abs/10.1139/cjfas-2014-0509#.VhaOsPlViko)

 

Malick’s co-authors are Dr. Sean Cox (Associate Professor, Simon Fraser University) and Dr. Randall Peterman (Professor Emeritus, Simon Fraser University), both in the School of Resource and Environmental Management; Dr. Thomas Wainwright (Research Fishery Biologist, NOAA Fisheries, Newport, OR); and Dr. Bill Peterson (Oceanographer, NOAA Fisheries, Newport, OR).

 

The physical network of ecological factors includes such factors as PDO, SST, ONI, NPGO, spring transition and upwelling.

 

“The spring transition variable is meant to capture the timing of the change in ocean conditions that occurs between the winter months when there is little to no upwelling off the coast of Oregon (in fact there can be strong downwelling during the winter caused by southerly winds) and the spring and summer months when there can be strong upwelling off the coast caused by northerly winds,” Malick said.

 

The biophysical networks include copepod and icthyoplankton biomass.

 

According to the study, in the physical network of environmental factors, the two with the most impact on salmon recruitment are cooler surface temperatures and an earlier spring transition date.

 

In the biophysical network, higher prey biomass is associated with higher recruitment.

 

In the course of the analysis, the researchers quantified the uncertainty among the environmental pathway relationships.  For example, “there was a 71 percent chance that recruitment would be 150,000 salmon or less when the PDO was in a warm phase for the physical network and a 62 percent chance for the biophysical network,” the study says.

 

“When the PDO was cool, the probability of recruitment being equal to or below 150,000 was considerably less, with a 54 percent chance in the physical network and a 45 percent chance in the biophysical network,” the study says.

 

Temperature may directly influence salmon in the ocean, but it’s the increase in icthyoplankton biomass caused by lower temperatures that links SST (the physical factor) and biomass (the biophysical factor) to salmon abundance, according to the report.

 

The cooler water is associated with a northern community of copepods with low species diversity, but rich with lipids, good for fish growth. However, the warmer water is associated with a southern community of copepods with high species diversity, but it is poor with lipids.

 

This probabilistic approach, where uncertainty is expressed, is an “important factor in using ecological models to guide decision-making,” the study says.

 

“From a management perspective, our research indicates that environmental factors can be useful indicators of coho salmon recruitment,” Malick said. “However, our results also suggest that focusing on only a single environmental factor can be misleading due to inherent uncertainties in the relationships between recruitment and environmental indicators.

 

“From a longer-term perspective, our findings indicate that future data collection and research efforts should focus on regional-scale oceanographic processes, for example, by obtaining more precise estimates of coho salmon prey resources,” he concluded.

 

An overview and summary graphic of the research can be found at: http://michaelmalick.com/research/bayes-network/index.html

 

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