Kokanee kill still poses mystery

They may be dying at a slower rate, but what’s killing kokanee in Okanagan Lake remains a mystery.

An estimated 25,000 freshwater salmon have perished in the north end of the lake since May 13. Most of the dead fish were found in the lake’s Vernon arm last week, but in the last few days bodies have been washing up further south between Okanagan Centre and Whiskey Island.

“My best guess is we’re over the peak, but I wouldn’t bet on it,” said biologist Steve Matthews, who has spent two days on the lake monitoring the carnage.

“It’s still going on, but we’re not seeing the number of stressed, dying fish there were last week.”

Scientists suspect the kill was caused by a disease, virus or parasite, dropping kokanee numbers to their lowest level on record.

The victims are all two- and three-year-olds, each about 15 to 22 centimetres long.

Matthews, of B.C.’s environment ministry, has collected four nearly-dead kokanee which he sent to the ministry’s fish-health lab in Nanaimo for analysis.

“In previous kills, we’ve never been able to collect moribund fish,” said Matthews. “But no one should hold their breath we’ll find the answer.”

Similar die-offs of kokanee have been documented in Okanagan Lake, Skaha Lake, Kootenay Lake, and Canim Lake in the Cariboo district. All of them lasted just a few days in the warm summer months, the affected kokanee were of a narrower age and size range, and their deaths coincided with a storm and/or lightning.

“This one is more mysterious,” said Matthews. “Lightning could have been a factor in previous kills, but not at all for this one.”

Scientists have measured the water’s temperature and oxygen levels and found them normal for this time of year.

The City of Vernon pumped treated sewage into Okanagan Lake from Feb. 23 until last Thursday, but repeated tests show the phosphorus content of the discharge is less than .5 part per million, said Eric Jackson, the city’s director of water reclamation.

Staff have also collected water samples to test for toxicity.

“We don’t believe there is,” said Jackson. “We’re awaiting results from a lab on toxicity tests … We expect to hear from the lab by Thursday.”

Jackson believes fish are dying as a result of the annual turnover of the lake, caused by warm weather heating the surface.

Because this spring has been warmer than usual, toxic compounds may be rising from the bottom layer of the lake and targeting a specific age of kokanee, he said.

Meanwhile, staff at the fish health unit in Nanaimo are preparing for the four moribund fish, now preserved in formalin. Biologist Sally Goldes will analyse the tissue, look for lesions and check for viruses and bacteria.

“We’ve never been able to collect fresh-enough specimens,” she said. “We’ve had these events before, but the samples were dead … Hopefully we’ll get new information which may rule out diseases.”

Results of the tests will take up to three weeks.

“No one has identified exactly what it is that kills them,” said conservation officer Grey Hoyer. “So you can’t rule anything out.”