A pattern predicting which sockeye salmon will likely die on their way to spawn in B.C.'s Fraser River basin — possibly linked to a viral infection — has been uncovered by researchers.

Sockeye salmon that showed a certain pattern of proteins being turned on by genes in their tissues were far less likely to survive their migration from the Pacific Ocean to their spawning grounds up the Fraser River, said a study published Thursday in Science.

"It's a pattern that resembles a response to viral-like activity," said Tony Farrell, a zoologist at the University of British Columbia who co-authored the study.

The data collected by a team led by Kristina Miller, a researcher with Fisheries and Oceans Canada, suggest a virus may infect the fish while they are at sea, before they enter the river, and persists as they migrate to their spawning areas.

"We tend to think that it's the conditions in the river whether that determine whether the fish makes it up there or not," Farrell said.

This study suggests that in fact, conditions that existed before they reach the river play a main role.

However, Farrell cautioned that more work must be done to confirm whether a virus is actually a contributor to the higher mortality.

Previous studies had shown that in recent years, 40 to 95 per cent of adult sockeye salmon have been dying before they can spawn, both at their spawning grounds and in the Fraser River on the way there.

Some scientists had suggested that the decline of several species of Pacific salmon, including sockeye, might be related to sea lice from farmed Atlantic salmon in open-net fish farms along the B.C. coast. A University of California study on pink salmon published in December found no evidence the support that, but did notice lesions that might be due to stressful environmental conditions or bacterial or viral infections.

A radiotransmitter tag was implanted in the bellies of the fish when they were in the Pacific Ocean or the lower Fraser River. (Jennifer Burt/Science)

Warming water temperatures in the Fraser River have also been suggested as a possible factor. Miller's paper noted that warmer temperatures reduce delivery of oxygen to the salmons' tissues and may allow infections to develop more quickly.

Miller and her colleagues conducted their study by capturing salmon in both the ocean and the lower part of the Fraser River in 2006. They took biopsies of the fish and implanted radio-transmitter tags in their bellies, each about the size of half a small cigar. About 150 fish were studied.

Tagged fish were detected and identified as they passed receivers at certain locations along the river.

Farrell said the researchers could tell which fish didn't make it past a certain point, but it wasn't possible to tell the cause of death.

"We just know that they just disappeared .… They might have run out of energy, they might have been eaten by a bear or eagle," he said.

Researchers conducted genomic tests on the biopsies — tests that check what genes were turned on at higher or lower levels, producing different levels of certain proteins. One particular signature that has been linked to viral infections was found in many of the fish that died before spawning and could be detected one week to one month before they died.

The researchers found than an ocean-tagged fish with that signature was 13.5 times more likely to die before reaching spawning grounds, while a river-tagged fish was 50 per cent more likely to die before reaching spawning grounds. At the spawning grounds, fish with the signature were nearly four times more likely to die without spawning.

The 2010 Fraser River sockeye salmon run was the biggest since 1913. It was an unusual year, as sockeye salmon runs have been declining since the 1990s. The audio slideshow below shows the 2010 Adams River run near Kamloops, B.C., which is part of the Fraser River system.