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Fish die-off ends, cause uncertain

Thursday, July 17, 1986

The mysterious die-off of thousands of two-year-old Kokanee in Okanagan Lake appears to be over with, the cause still to be determined, says the head of the fisheries section with the Environment Ministry in Penticton.
Chris Bull said no new dead fish have been found during the past two days.

At least 10,000 fish, and possibly many more, were killed over the past 1 1/2 weeks.

Officials will never know how many because many fish have already sunk to the lake bottom.

They are also divided over the possible cause.

Lab tests are being conducted on fish samples sent to the Pacific Biological Station in Nanaimo, but Bull said it is difficult to determine the cause of fish kills from lab tests.

Bull feels it was a viral disease, and suggests infectious hematopoietic necrosis, the same one that forced the destruction of 1.5 million rainbow trout at a hatchery in Summerland in January. In past cases, two-year-old fish have proven susceptible to parasites and disease.

Others have suggested an algae bloom particularly toxic to young Kokanee or changes in the lake temperature or oxygen content at certain levels in the lake.

However, a plankton survey by the ministry appears to have ruled out the algae bloom theory.

Pollution was also ruled out because it would have affected other ages and species of fish.

The sickness will significantly affect fishing success next year, said Bull, because a typical annual spawning run is 250,000 fish.

There is nothing the ministry can do to prevent a similar “quirk of nature.” said Bull.