Juvenile sockeye salmon that enter the Strait of Georgia in
British Columbia soon after emerging from the gravel of the Harrison River, a
tributary of the Fraser River, are contributing a larger proportion of adults
returning to the Fraser River basin than they did more than ten years ago.
Harrison River sockeye with its sea-type life history,
emerge from the gravel, migrate to the Fraser River estuary to rear and, within
the same year, migrate into salt water and stay longer in the Strait of Georgia
than lake-type juveniles that spend one to two years in freshwater before
migrating to saltwater, according to a recent study.
The increase in Harrison River sockeye adult returns that
occurred from 2005 to 2011 is likely due to a change in the Strait’s ecosystem
caused by warmer ocean temperatures, the study says.
“We found that a rare population type of sockeye salmon in
the Fraser River have recently become abundant because of a change in the
Strait of Georgia ecosystem,” said Richard Beamish, emeritus scientist and
former director with the Department of Fisheries and Oceans in British
“We speculated that the change in the ecosystem was an
increase in the preferred prey at the time these sea-type sockeye salmon enter
The study showed that these sea-type sockeye salmon enter
the Strait of Georgia later in the year when the more abundant lake-type have left
and at a smaller size than the lake-type.
A change in the ecosystem probably produced an abundance of
preferred prey that matched the time of ocean entry and allowed the fish to
grow faster and quicker, a key to improved salmon survival, Beamish said.
Most sockeye salmon have a lake-type life history in which
fry emerge from the gravel in a river and migrate to a lake where they rear for
one to two years before migrating to the ocean.
Sockeye with a sea-type life history are less abundant.
Juveniles of this life history enter the ocean during the year they emerge from
the gravel and, although rare, it is found throughout the distribution of
sockeye salmon, according to the study.
The recent success of this life history in a changing ocean
identifies the importance of managing for biodiversity of the sockeye salmon
populations in the Fraser River basin, the study says.
“Early Ocean Life History of Harrison River Sockeye Salmon
and their Contribution to the Biodiversity of Sockeye Salmon in the Fraser
River, British Columbia, Canada,” was published online March 7, 2016, in
Transactions of the American Fisheries Society, http://www.tandfonline.com/doi/abs/10.1080/00028487.2015.1123182?journalCode=utaf20
Beamish’s co-authors are Chrys Neville, Ruston Sweeting and
Terry Beacham, all biologists at the DFO Pacific Biological Station; Joy Wade,
Fundy Aqua Services, B.C.; and Lingbo Li, a postdoctoral scientist at the
School of Oceanography, University of Washington.
From 1952 to 2005, Harrison River adult sockeye contributed
just 1.1 percent of the total Fraser River sockeye salmon population. However,
from 2005 to 2011 the average per year jumped to 6.9 percent of the total
production, including 27.9 percent in 2009 and 27.6 percent in 2011.
What happens when the fish leave the gravel as juveniles and
enter the Strait of Georgia is complicated. For the study, scientists surveyed
juvenile abundance each year in 1998 – 2010. 2008 – 2010 surveys in the Strait
found that 0 percent to 5 percent of the juvenile sockeye were from the
They found that virtually all lake-type juveniles had left
the Strait by mid-July and that Harrison River fish enter the Strait later than
lake-type fish and their entry extends over a longer period. The sea-type
juveniles migrated into the Fraser River estuary as early as April, with most
appearing between June 6 and July 4, but some appearing as late as October,
“showing that movement into the Strait of Georgia is prolonged and can take
place over about 6 months,” the study says.
In most years the largest catches of Harrison River sockeye
in July were from the east side of the Strait (few were found in the Strait’s
open waters prior to mid-July). Some 70 percent to 99 percent of the juveniles
caught in Howe Sound (east side) were from the Harrison River. The juveniles
migrate there due to a preference for the low salinity of the water and the
high abundance of prey, the authors surmise. That is analogous to the migration
of lake-type fry into a lake.
By September, the percentage of Harrison River sockeye found
throughout the Strait of Georgia was closer to the percentages found in Howe
Sound in July. Harrison River fish enter the open ocean areas of the Strait
roughly eight weeks later than the average lake-type fish and after virtually
all of the lake-type juveniles have left the Strait, the study says.
“We showed that these sea-type sockeye salmon remain in the
strait until fall which is a residence time that is about twice as long as the
lake-type,” Beamish said. “There is a
strong relationship between the abundance of the juvenile sea-type fish in the
fall and the subsequent abundance of adults from the brood year, indicating
that it is the conditions within the Strait of Georgia that were mostly
responsible for the recent increase.”
He added that the increased abundance of a particular
species of plankton was the main reason for the improved survival.
“The management implications are that the Harrison
population should be managed separately, if possible,” Beamish said. “The study
also provides another example of the need to recognize that all populations of
Pacific salmon have evolved a resilience to climate changes and thus all
However, he said, “we have a long way to go if want to
understand, rather than be surprised by, future ecosystem changes. There needs
to be an increased investment in the study of ocean ecosystems and an improved
coordination of effort nationally and internationally.”
Beamish is also co-author of the award-winning book, The Sea
Among Us, about the Strait of Georgia ecosystem. It has been nominated for five
awards and won three awards, including a British Columbia book prize.