Oxygen levels fingered in North Pond fish kill

SILVERTHORNE — A sudden drop in oxygen levels is the most likely cause of a recent fish kill in Silverthorne’s North Pond, but human activity may also be a factor, according to town manager Kevin Batchelder.

The park near the elementary school is a fly-fishing hotspot for catch-and-release anglers, but reports of dead trout during the past couple of weeks caused the town to get the water tested for pollutants.

“It came back negative for any type of toxicity,” said Batchelder. It may be that the dead fish were mishandled by anglers, he added. Batchelder said that an angler may have illegally been keeping some fish on a stringer, and then dumped them back into the pond.

Colorado Division of Wildlife (CDOW) officials plan to lab test several dead fish to try and pinpoint the cause, said aquatic biologist Jon Ewart. Several trout have been frozen and stored for transport to a lab in Brush. It may be a few weeks before the results are in, Ewart said.

By analyzing the tissue of the dead fish, biologists should be able to tell if they died of oxygen deprivation, as Ewart suspects. Initial reports of dead fish coincided with an early cold snap. A sudden temperature change can affect oxygen levels by killing algae. As the plants decompose, the oxygen in some parts of the pond can be rapidly depleted, Ewart explained.

Surplus nutrients can also be a factor in similar settings, Ewart said. Too much fertilizer running off into ponds or streams can result in an over-abundance of algae, he said, careful to explain that he doesn’t know if that’s the case at North Pond.

Overall, the fish population in the pond appears to be healthy, Ewart said, explaining that the fish kill probably only affected a small number of trout in some parts of the pond.

“You’d think the fish would just swim away, but it’s not so. They just get lethargic and die,” Ewart said.

It’s not clear how many trout died in the recent incident. Batchelder said it may have only been four or five fish. But local fishing guide Mitch Vogt, of Cutthroat Anglers, said the number may have been higher. Vogt was also concerned because he wasn’t able to spot schools of minnows that have been plentiful in the shallower water near the shore.

“We had concerned customers coming in. They wanted to find out what’s going on,” Vogt said.

Batchelder, also an avid angler, said he walked around the pond late last week and spotted numerous minnows, which are a good food source for the larger trout.
A recent biological survey by the Colorado Division of Wildlife showed that the pond has a healthy, dense population of trout, he concluded.

<i>Bob Berwyn can be reached at (970) 331-5996, or bberwyn@summitdaily.com.</i>