For years scientists have struggled to understand what is responsible for the declines in abundance of populations of Fraser River sockeye salmon that began in the early 1990s and prompted a Judicial Inquiry by the Government of Canada, the Cohen Commission, in the fall of 2009.
This week an international team of scientists has shed light on this mystery. In the first study to simultaneously consider evidence related to multiple possible explanations for the declines in Fraser sockeye populations, the team gathered and analyzed data on the main factors identified by an expert panel in 2010.
Their findings, published in the journal Conservation Letters, attribute the declines to three factors acting in concert in the Pacific Ocean: increasing numbers of pink salmon, salmon farming along migration routes for juvenile Fraser River sockeye, and warming sea temperatures.
The paper can be found at http://onlinelibrary.wiley.com/doi/10.1111/j.1755-263X.2012.00244.x/abstract
“Although none of these three factors can explain much of the declines in sockeye salmon by themselves, when considered in combination, they appear to play a very important role”, says the lead author of the paper Brendan Connors, a post-doctoral fellow in the School of Resource and Environmental Management, Simon Fraser University.
The team discovered that increasing numbers of pink salmon across the North Pacific Ocean appear to be leading - directly or indirectly - to increasing competition for food with Fraser sockeye salmon, especially after the juvenile sockeye salmon first migrate past large numbers of farmed salmon.
“It is possible that exposure to salmon farms early in life may weaken the ability of sockeye to compete for food with pink salmon, ” said Douglas Braun, a doctoral student in the Earth to Ocean Research Group at Simon Fraser University and co-author on the study.
The study also found that increasing ocean temperature early in life reduces survival of juvenile sockeye, but the effect of warming oceans is weaker than increasing numbers of pink salmon.
The abstract for the study, “Migration links ocean-scale competition and local ocean conditions with exposure to farmed salmon to shape wild salmon dynamics,” says:
“Climate, competition, and disease are well-recognized drivers of population dynamics. These stressors can be intertwined by animal migrations, leading to uncertainty about the roles of natural and anthropogenic factors in conservation and resource management. We quantitatively assessed the four leading hypotheses for an enigmatic long-term decline in productivity of Canada’s iconic Fraser River sockeye salmon: (1) delayed density-dependence, (2) local oceanographic conditions, (3) pathogen transmission from farmed salmon, and (4) ocean-basin scale competition with pink salmon.
“Our findings suggest that the long-term decline is primarily explained by competition with pink salmon, which can be amplified by exposure to farmed salmon early in sockeye marine life, and by a compensatory interaction between coastal ocean temperature and farmed-salmon exposure. These correlative relationships suggest oceanic-scale processes, which are beyond the reach of current regulatory agencies, may exacerbate local ecological processes that challenge the coexistence of fisheries and aquaculture-based economies in coastal seas.”