The decline in wild Pacific salmon populations is not likely caused by sea lice acquired from farmed salmon, a study released Monday suggests.
The findings of the study headed by Gary Marty, a professor at the University of California, suggest that the number of wild salmon that return to spawn in the fall can predict the number of sea lice that will be found on farmed salmon the following spring, which, in turn, predicts the extent of sea lice infestations in young wild salmon.
However, the survival of wild salmon populations appears unrelated to the number of lice found on farmed fish or to farm fish production.
Some experts have argued that separating farmed and wild salmon would help wild populations rebound, but this latest study suggests that is not the case.
"Separating farm salmon from wild salmon — proposed through co-ordinated fallowing or closed containment — will not increase wild salmon productivity and that medical analysis can improve our understanding of complex issues related to aquaculture sustainability," study researchers wrote in their report.
They concluded that other factors, including environmental stress or bacterial and viral infections, might have contributed to the alarming decrease in salmon populations in 2002.
"Productivity of wild salmon is not negatively associated with either farm lice numbers or farm fish production, and all published field and laboratory data support the conclusion that something other than sea lice caused the population decline in 2002," they wrote in the report.
Exposure to sea lice from farmed Atlantic salmon was thought to be the cause of the decline among the spawning fish. Record high numbers of pink salmon returned to spawn in rivers of the Broughton Archipelago in 2000 and 2001, but the returns were followed by a population decline of 97 per cent in 2002 and 88 per cent in 2003.
Fluctuating salmon populations has been a source of concern among politicians and researchers for years.
Earlier this year, British Columbia launched a probe into the disappearance of almost 10 million fish from the 2009 Fraser River sockeye run. That year only about one million fish returned to spawn, prompting the federal government to order an investigation led by B.C. Supreme Court Justice Bruce Cohen.
As part of their study, Marty, along with associates Sonja Saksida, from the British Columbia Centre for Aquatic Health Sciences in Campbell River, B.C., and Terrance Quinn, from the University of Alaska, analyzed 10 to 20 years of fish farm data and 60 years of pink salmon data.
Data showed that the number of pink salmon returning to spawn in the fall predicts the number of female sea lice on farm fish the next spring. This, the researchers said, accounts for 98 per cent of the annual variability in the prevalence of sea lice on outmigrating wild juvenile salmon.
The researchers suggested that "determination of the causes of salmon population decline requires investigation of other variables."
They noted that in 2001, sick juvenile pink salmon frequently had "bleeding at the base of the fins," but, the lesions did not occur in pink salmon exposed to sea lice under controlled laboratory conditions.
Instead, the reddening of the fins was "commonly associated with stressful environmental conditions or bacterial and viral infections." However, none of these differentials were studied in 2001 and "their potential role in fish mortality that year remains unknown."
"Adding medical analysis to multidisciplinary investigations of fish population decline can increase our understanding of the cause and help government agencies develop cost-effective regulations to sustain healthy wild salmon populations," the researchers said in their report.