UBC Reports | Vol. 50 | No. 4| Apr.
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
“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