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Fish kill a mystery

Okanagan Lake kokanee stocks at their lowest ebb ever, in wake of puzzling die-off

by Don Plant
The Daily Courier
Thursday, May 21, 1998

A mysterious fish kill in Okanagan Lake could be solved after scientists analyze the organs of two nearly-dead kokanee plucked from the lake Tuesday.

Up to 50,000 kokanee salmon have perished in the north end of the Lake since last week. Most of the dead fish were found in the Vernon arm of the lake, but remains have been discovered as far south as Okanagan Centre.

Biologist Steve Matthews estimates five to 15 per cent of the lake’s kokanee stocks aged two to three years have died. Scientists suspect the kill was likely caused by a disease, virus or parasite, dropping kokanee numbers to their lowest level on record.

“We’re at the lowest ebb we’ve been at with kokanee populations,” he said. “This is the last thing we need.”

The die-off probably wasn’t caused by humans, but residents say a concentration of adolescent kokanee died near the City of Vernon’s outflow of treated sewage by Tronson Road. Matthews, who motored by the area in a boat Tuesday, ruled out sewage as a cause, saying fish are doing well down the lake from the discharge.

“I really think it’s not man-caused,” he said. “I don’t think people should automatically assume that because there’s some dead fish appearing that there’s some toxin in the water.”

It’s hard to determine the cause of death from dead fish, so Matthews and biologist Vic Jensen spent several hours on the lake looking for live ones. They found two moribund kokanee, dissected them and preserved the organs in Formalin.

The samples have been sent to a fish health unit in Nanaimo, where scientists will determine the cause of death. Results will take about two weeks.

Fish kills have happened in Okanagan Lake three or four times in the last 10 years, said Matthews, but usually in the summer when it’s warmer. The worst one-year toll was up to 100,000 kokanee.

“A native lady said this phenomenon has been observed every few years since the turn of the century,” said Ron Taylor, member of the Oceola Fish and Game Club. “There seems to be no rhyme or reason. It could be the lake turning over or a lack of oxygen.”

About 10 kokanee, each six or seven inches long, were visible from Lloyd and Arlene Duggan’s house in Carrs Landing Wednesday morning.

“They’re laying on their sides — one’s on the beach, the rest are in the water,” said Arlene. “I haven’t seen a big die-off like this in the 11 years we’ve been here.”

Witnesses first reported dying fish last Wednesday. They’re widely dispersed, but pose no risk to people swimming in the lake, said Matthews. Whatever’s killing them is targeting only young kokanee, and it’s not clear when the carnage will end.

“We don’t know if it’s getting worse. Fish are still dying. It’s hard to say what’s happening. We don’t know if we’re over the peak yet or not,” he said.

There’s no reason for people to be alarmed, say experts. If you find dead fish on the shoreline, bury it or use it as fertilizer, said Matthews.