A mass die-off of an invasive fish species in North Cowichan’s Somenos Lake is believed to have been caused by a lack of oxygen, stewards of the lake say.
Thousands of dead pumpkinseed fish were found belly-up on the lake on Sunday, said Paul Gowland, president of the Somenos Marsh Wildlife Society.
Gowland was out Monday morning conducting his weekly tests on the water and found very low levels of oxygen in the lake.
“These pumpkinseed, they’re quite a tough fish. They’re not native to the area, but they’re very hardy, and they can actually take quite a low level of oxygen in the water and survive, but it was too low for a lot of them,” he said.
He believes recent wind or a drop in temperature churned up the lake, mixing deeper layers of water that were extremely low in oxygen with upper layers, resulting in a sudden drop in oxygen for pumpkinseed fish.
Somenos Lake is a shallow lake that suffers from too many nutrients flowing into it from fertilizer, septic fields and storm runoff, he said. The high nutrient levels enrich the water, creating a fertile environment for algae to grow. When the algae die, they sink to the bottom of the lake and decompose, absorbing oxygen and leaving a water layer at the lake bottom that is nearly devoid of oxygen, he said.
The die-off, while not uncommon, is a sign that the lake is in poor condition, Gowland said. He would like to see long-term solutions from the province and municipality to prevent nutrient run-off from farming and septic fields.
Pumpkinseed fish are native to eastern North America but they have become present in southern ÎÚÑ»´«Ã½ by spreading from introduced populations in the United States and intentional introductions, according to Fisheries and Oceans ÎÚÑ»´«Ã½.
They can be considered a nuisance species and compete with native fish. Pumpkinseed fish are usually found in quiet, slower-moving streams, in small lakes or ponds and in shallow, weedy bays of larger lakes.
The Somenos Marsh Wildlife Society would like to see the invasive species gone, said executive director Paul Fletcher, but it’s unclear what effect a mass die-off will have, because other species may use them for food. “I don’t know whether it’s a good thing or not with that loss of a species.”
There are few other inhabitants in the lake when it’s in an unhealthy state —a small number of three-spined sticklebacks have been seen in the water, he said.
About 50 coho salmon fry seem to have died along with the pumpkinseed fish, while some of the invasive species has survived, Gowland said.
The province was notified and provincial environmental response officers have gone to the lake to take samples, the Ministry of Environment and Climate Change Strategy said in a statement. The province is working with the Somenos Marsh Wildlife Society to find out more about the situation, it said.
People are encouraged to report problems to the province’s 24/7 Report All Poachers and Polluters tip line at 1-877-952-7277.
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