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Study Indicates Substantial Salmon Mortality May Occur Later In Migration, Rather Than Near-Shore
Posted on Friday, May 13, 2011 (PST)

The long-held adage that the majority of juvenile salmon mortality takes place during their first days and weeks in saltwater may not strictly be true.


“For at least some populations, substantial mortality occurred much later in the migration and more distant from the river of origin than generally expected,” according to a research report published this week in the scientific journal, Proceedings of the National Academy of Sciences of the United States.


The report, “In situ measurement of coastal ocean movements and survival of juvenile Pacific salmon,” can be found at:


It provides a detailed comparison of the survival of smolts as they migrate down to the mouth of their British Columbia rivers and migrate out of the Strait of Georgia, then charts their return as adults.


Among the findings is that the fate of the calamitously low 2009 Fraser River sockeye was determined in the deep sea, not in the near shore areas. One-eighth of the total mortality occurred in the Strait of Georgia, the findings show, and that the remaining seven-eighths of the mortality occurred after passing northern Vancouver Island.


“We documented the survival of the 2007 tagged Cultus Lake sockeye smolts to their return as adults as part of the failed 2009 adult run to the Fraser -- and with a smolt-to-adult survival rate that closely matched the survival of the untagged run as a whole,” said Kintama Research’s David Welch, lead author. “As a result, we can now demonstrate that it is possible to ‘close the loop’ and come up with the first real estimates of relative mortality in the early and later marine phase of the life history of salmon.”


The study was authored by scientists from Kintama Research Services Ltd., UBC, the Vancouver Aquarium Marine Science Centre, DFO, the BC Ministry of the Environment, and the Seymour Salmon Society. Kintama is an environmental consultancy that developed the technical design for the array and operates substantial elements of the overall POST telemetry array and is based in Nanaimo, British Columbia.


The researchers report direct in situ (on-site) survival and movement estimates for more than 3,500 juvenile Pacific salmon (“smolts”) that were tagged and released in British Columbia rivers during their migration downriver and a substantial portion of their journey north along the continental shelf for the years 2004 through 2007. The fish represent four of the six North American species of Pacific salmon: coho, chinook, sockeye and steelhead.


Each fish was surgically implanted with an individually identifiable acoustic transmitter and the tags were detected with a large-scale acoustic telemetry system, the Pacific Ocean Shelf Tracking array that extends more than 1,500 kilometers along the North American continental shelf, which is centered in coastal waters surrounding Vancouver Island.


As part of the study the researchers used specially programmed tags that transmitted for two months during the outbound migration in 2007 and then turned off to conserve battery power before turning back on again two years later, when the adults returned as part of the disastrous 2009 sockeye run.


The researchers tracked the round trip migration of the two surviving adults (1 percent of the 200 smolts released) returning in 2009, successfully recording the smolts’ movements out of the Strait of Georgia as they migrated north about two weeks apart in 2007, and then recording their migration back in as adults two years later, when both migrated through Juan de Fuca Strait and then up the Fraser River less than 12 hours apart. That 1 percent SAR matched that of the run as a whole.


Another finding was that the surgical tagging process did not appear to hurt the smolts after their release. The size at tagging of fish surviving to reach the northern end of Vancouver Island, some 4 to 6 weeks after release in freshwater, was the same as for all the fish released.


“This finding is important because one of the initial concerns of many scientists was that smaller fish have poorer survival, either because the tag was a greater burden in little fish or because larger smolts simply had a better chance to survive. Neither theory was supported by the results,” according to a summary of the research paper.


For more information see:

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