UBC Reports | Vol. 50 | No. 4| Apr. 1, 2004

The Mystery of the Disappearing Sockeye Salmon

Something fishy is going on in the Fraser River and UBC scientists are on the case

By Michelle Cook

Fraser River sockeye salmon have been acting strangely of late. Their odd behavior has experts puzzled -- and worried -- because millions of the fish are dying before they reach their spawning grounds. It’s a mystery that could spell the end of the river’s salmon fishery, but UBC and SFU scientists studying the phenomenon think they are close to solving the case of the disappearing fish.

In a project funded by the Natural Sciences and Engineering Research Council of Canada with support from the Pacific Salmon Commission and Fisheries and Oceans Canada, researchers have been examining why some late-run sockeye are migrating up the Fraser more than a month ahead of schedule. Normally, the salmon migrate down the coast of British Columbia and mill about in the Strait of Georgia for several weeks before heading up river in late September.

“In 1995, we started noticing a large group had started to change their behaviour in terms of when they returned,” says Scott Hinch, the forestry sciences professor leading the investigation. “This wouldn’t have been a big problem except that they seemed to be dying in really high numbers. We didn’t realize how high until the phenomenon continued for the next few years.”

“What we saw was fish entering the river four to six weeks earlier than usual and those early migrants died before spawning -- up to 95 per cent of the total run in some years. The mortality rate for sockeye arriving on schedule is about 10 per cent.”

This has had catastrophic effects on the fishery and fish conservation, Hinch adds. Since 1996, close to four million late-run sockeye in total have died during their upstream migration. In 2002, the fishery lost more than $70 million. Some small stocks, like the Cultus Lake sockeye which were emergency listed endangered more than a year ago, have been pushed to the point of collapse.

At first, the researchers were at a loss to explain either the early migration or the high mortality. They formulated several hypotheses before identifying a prime suspect in the high mortality rates: Parvicapsula, a natural kidney parasite that latches onto fish at the mouth of the Fraser.

Since Parvicapsula hitches a ride on all salmon heading up river -- early or otherwise -- the researchers weren’t sure why it only seemed to be having an adverse affect on the fish migrating ahead of schedule. What they did know was that the water temperature in the river is several degrees higher in mid-August than in late September when late-run sockeye normally come in from the ocean. Hinch’s team suspected that the higher river temperatures were affecting the speed of the parasite’s infection and the early migrants’ kidney functions, which play a vital role in helping fish to adjust from salt to fresh water.

Then, by accident, the team discovered a key piece of evidence. While surgically inserting small radio transmitters into a test group of late-run sockeye, researchers noticed that the early arrivals bled profusely during the short operation.

“We’d not seen anything like it before when we’ve inserted transmitters, nor had we anticipated it,” says Hinch. “Blood samples from these fish also showed that the early migrants had poor clotting ability and abnormally high levels of ions. There was clear physiological evidence that the kidneys were malfunctioning and this may be the cause of the high mortality. What it also means is that fish could be bleeding to death during their migration if they get any small nick or cut, which can be common.”

This still didn’t explain why sockeye were starting their long journey up the Fraser so early. Although Parvicapsula was doing a lot of damage to the early migrants in the river, it didn’t appear to be responsible for driving them into the river ahead of schedule.

The UBC team had several theories but it was colleagues from the Institute of Ocean Sciences on Vancouver Island who revealed the clues which may crack the case. Researchers from the Institute told them that recent CTD (conductivity temperature depth) surveys of the Strait along with some historical data they dug up, shows low salinity pockets in some coastal areas that weren’t there before.

Piecing the evidence together, the UBC team determined that some late-run sockeye might be hitting these low salinity pockets of water in the Strait. Thinking that they are in fresh water, the salmon prematurely activate their kidneys and ozmoregulatory systems -- the function they use to adjust to fresh water -- and head straight into the Fraser River instead of milling.

Once in the river, they pick up the parasite earlier than normal. The higher water temperatures limit their ability to fight off the infection. The longer they are in higher temperature water, the more damage the parasite does until, eventually, the sockeyes’ kidneys malfunction.

“Any one of these factors alone probably doesn’t kill them outright,” says Hinch. “It’s the combination of all these things that we think causes them to die. Early migrants probably get two thirds of the way to their spawning grounds, but many just don’t make it. The early fish that do reach the spawning grounds seem to die before spawning.”

Hinch says the team is still a long way from closing the investigation. They still have several hypotheses to examine over the next few summers but the theory, if proven correct, leaves Hinch cautiously hopeful for the future of the Fraser River sockeye.

“Over the last couple of years, there seems to be a segment of the late-run sockeye that are acting normal and now that we understand a bit more of the problem, I’m somewhat relieved and optimistic for these fish,” Hinch says.