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Fish-stocks mystery untangled

Scientist can predict survival rate of salmon
3/11/06 by MARK HUME

VANCOUVER -- It has taken him 15 years, but Ron Tanasichuk, a research biologist with the Department of Fisheries and Oceans, may finally have unravelled one of the great mysteries of the West Coast.

Why do salmon and herring populations vary so wildly from year to year in an ocean where fishing pressures are stable or declining?

Dr. Tanasichuk's answer: It all depends on the krill.

Krill, or euphausiids, are tiny, shrimp-like creatures that swarm through the ocean, providing the bulk of the diet for everything from herring to whales and from salmon to sea birds.


By looking at the stomach contents of thousands of young salmon and herring to find out what the ate and when, Dr. Tanasichuk has been able to determine that one krill species in particular, Thysanoessa spinifera, is a dominant food source.

And by zeroing in on T. spinifera, Dr. Tanasichuk has been able to correlate population trends in herring and in coho, chum and sockeye salmon with the abundance of krill at the time the various fish species first show up in Barkley Sound, south of Ucluelet on the west coast of Vancouver Island.

If there is an abundance of krill when the young salmon emerge from spawning streams and begin to feed in salt water, it gives the the fish a boost that sets them up for survival when they move offshore in their long, open-ocean migrations.

But if there is a krill shortage, the fish grow more slowly and are smaller when they move offshore, and their survival rates plummet.

For decades, fisheries scientists have referred to the open Pacific Ocean as a "black box" because no one could explain what was happening to migrating salmon once they vanished offshore.

Some came back to spawn, some did not, and theories have pointed to everything from illegal drift-net fisheries, to global warming, El Nino and upwellings of deep, nutrient-rich waters.

In some years, there would be large numbers of young salmon emerging from salmon streams, but four years later, low numbers of adults would return from the ocean to spawn.

Dr. Tanasichuk's research is allowing him to get a glimpse into the black box, and a look ahead at what has previously been unpredictable: the survival rate of salmon at sea.

"I'm amazed at how well this works," said the 52-year-old biologist, who is based at the Pacific Biological Station in Nanaimo.

"I can forecast returns really accurately. For five-year-old sockeye, I can forecast returns three years in advance based on euphausiid abundance when they first go out to sea."

Dr. Tanasichuk has made more than 100 research cruises on Barkley Sound, netting krill, juvenile salmon and other fish. From this "gumboot science" has emerged a correlation between diet, krill abundance, growth rates and spawning returns.

"I can forecast wild coho returns to the West Coast, based on the biomass of a certain fraction of the euphausiid biomass. I'm able to forecast coho returns based on euphausiid abundance when the coho are moving out of Barkley Sound," he said, sounding a little surprised by the logic of his food-chain observations.

"I'm able to forecast Barkley Sound sockeye returns a couple of years in advance, because it's got to do with euphausiid biomasses when these animals are first moving out as smolts," he said, referring to young salmon making their first migration to sea.

(Some sockeye come back to spawn as four-year-olds, but others stay out at sea an extra year before returning. But on their outward journeys, both enter Barkley Sound at the same time as smolts.)

Dr. Tanasichuk is also unravelling the relationship between Pacific hake, salmon and krill.

Hake are voracious predators that in certain years move north from California to feed off B.C.'s coast. When hake numbers are high, salmon stocks usually decline.

But Dr. Tanasichuk said only hake eat young salmon incidentally. "They migrate up, I'm convinced, to feed on euphausiids."

That puts them in direct competition with salmon and herring, which are also feeding on krill.

Hake are doing more than stealing the lunch of B.C. fish, however, because they arrive in Barkley Sound just as chum emerge from local rivers. Once in the ocean, the young chum quickly zero in on the schools of krill -- making themselves unavoidable targets of the hake that are feeding on the same schools of krill.

Dr. Tanasichuk said he can forecast spawning returns of chum three years in advance based on both the euphausiid and the hake biomass.

What he initially set out to discover, in 1991, was what was happening to herring stocks in Barkley Sound.

He found that herring are so dependent on krill that when the euphausiid stocks crashed, the size of the adult herring dropped. That led to not only lower survival rates, but also reduced spawning success because small female herring lay fewer eggs.

That led to a downward spiral in herring abundance that he fears may be irreversible. He likens the change to the breaking of a natural machine.

"When a stock is small, the animals will grow more quickly and it's likely they'll survive better and then the females will have more eggs in them because they are bigger fish. But it seems with the drop in the euphausiid biomass that the machine is broken. . . .

"I've done a calculation where this suppression of herring growth by low euphausiid biomass has resulted in probably about an 80-per-cent to 90-per-cent reduction in the number of eggs that should have been produced by this population. What's really significant about that is if the population is at a fairly low level, it's lost its ability to recover, to bounce back."

Dr. Tanasichuk's research has focused exclusively on the waters in Barkley Sound, but he believes what is happening there is an accurate indicator of what is happening elsewhere on the West Coast.

Fish futures

Here are biologist Ron Tanasichuk's predictions, based on his most recent observations:

Herring: All major stocks in B.C. waters should begin declining in 2007; growth rates will be suppressed and adult mortality rates will increase on the west coast of Vancouver Island and in the Strait of Georgia.

Coho: Marine survival rate on the west coast of Vancouver Island will decline to about 3 per cent for the 2006 return year because of reduced food availability in the 2005 smolt year.

Sockeye: In Barkley Sound and on the central coast, returns this year will be similar to 2005, but returns of four-year-old fish in 2007 and five-year-old sockeye in 2005 will be poor.

Chum: Returns to the Nitinat River Hatchery 70 kilometres southeast of Port Alberni will decline markedly in 2007 because of hake predation and competition




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