A landmark migration study on the West Coast that tracked thousands
of young salmon as they swam down rivers and then went out to sea has
upended one of the longest-held tenets of fisheries science.

Until
David Welch and his colleagues surgically implanted more than 3,500
young salmon with electronic tags, it had been believed the high
mortality afflicting salmon happened mostly in river estuaries as fish
made the transition from fresh to salt water.

But Dr. Welch,
president of Nanaimo-based Kintama Research Services Ltd., said an array
of listening posts strung for more than 1,500 kilometres along the
coast allowed researchers to follow the fish as they migrated out of
B.C. rivers and headed north, swimming an average of 20 kilometres a
day.

“The scientific body for a century has said the marine
survival problems are happening very early in the life history. Now we
are measuring that and saying, ‘Sorry, it doesn’t look like that.’ Most
of the mortality is happening more than a month after entering the
ocean,” said Dr. Welch, who published new research on the subject this
week after gathering data for several years.

The study made use of
a marine telemetry array called POST, for Pacific Ocean Shelf Tracking,
which picks up signals from electronic tags surgically placed inside
the body cavities of young salmon, most of which are about 150
millimetres in length when released.

Once in the ocean, the main
body of fish headed up Georgia Strait, on the east coast of Vancouver
Island, while a smaller number went out Juan de Fuca Strait and up the
west coast of the island.

The fish – sockeye, steelhead, coho and
chinook – mostly survived the early stages of their migration and were
tracked for four to six weeks until they were lost after passing the
last POST array.

“Most of the mortality happened beyond the north
end of Vancouver Island. Now, whether they dropped dead from sea lice
one day past where we [last] measured them or some other disease
problem, or whether it was some place two years out in the ocean, we
can’t resolve that – it’s just that we know most of the mortality
happened beyond the Strait of Georgia, in the Queen Charlotte Sound
area,” Dr. Welch said.

The study estimates one-eighth of the
mortality occurred in Georgia Strait and seven-eighths occurred after
passing northern Vancouver Island.

There has been speculation that
fish farms, which are concentrated in ocean channels near the northern
end of Vancouver Island, might be exposing migrating wild fish to sea
lice and disease.

But Dr. Welch said his study doesn’t shed any light on that controversy.

“I
do want to emphasize that our results do not say the fish farms did
play a role, it’s just that the fish passed the salmon farms and at some
point after that, died,” he said.

Dr. Welch said he is currently
helping to design a study that will use the electronic-tag technology to
directly examine the issue of whether migrating salmon are impacted by
fish farms.

A small number of the tagged salmon carried extra
batteries, which were turned off after about a month and then turned on
again two years later. Among that group, two fish were picked up again
by the POST array when they returned as adults.

Dr. Welch said it
was an exciting development, because it revealed the remarkable
synchronicity of migrating salmon. Heading north, the two fish left
Georgia Strait one week apart – two years later they returned to
Vancouver Island only 12 hours apart, and within two hours of each other
re-entered the Fraser River.

Dr. Welch gave a partial preview of
his research to the Cohen Commission earlier this year, but the detailed
paper was published this week in the Proceedings of the National
Academy of Sciences of the United States.

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