OUTDOOR BRIEF

DISSOLVED OXYGEN

Massive amounts of fish die at reservoir

Nevada Department of Wildlife fisheries biologist Chris Drake discovered a major fish die-off when he traveled to Wildhorse Reservoir after receiving a call from a concerned angler regarding dead fish at the popular fishery.

"We estimate that approximately 8,000 to 11,000 trout and 750 to 1,000 perch died over the last couple of days," says Drake. "We believe it is due to the lack of dissolved oxygen within the water."

While there have been occasional smaller fish kills over the past few years during the heat of the summer, Drake says that this die-off is more significant than is normal. The problem appears to be related to several factors.

The combination of unusually hot weather and more than the usual number of sunny days this summer has produced above normal algae growth in the lake. As algae grows, oxygen is released into the water for aquatic organisms to use. However, when the algae dies, as it did just recently, and settles to the bottom and decomposes, it becomes anaerobic and the oxygen is depleted as the dead plant matter breaks down.

"The dissolved oxygen (DO) level was 1.1 to 1.6 parts per million (ppm) on the surface and 0.7 to 1.4 ppm on the bottom when I tested on Monday," said Drake. "Trout require DO levels of 3.0 ppm or higher for normal respiratory function."

Coupled with the low levels of DO are low lake levels and warm water, which doesn't hold as much dissolved oxygen to begin with. The end result was high stress to the trout without suitable habitat and a significant fish kill occurred. Drake estimates that the majority of the fish washing up on shore were trout (approximately 95 percent), consisting of rainbow, tiger and a few brown trout. The average size of the trout was 16 to 17 inches, with fish up to 22 inches being found dead in deeper water. Some yellow perch were also found to be stressed and dying, with the average size being around 9 inches.

With the large number of fish lining the shorelines, anglers and campers can expect conditions to be extremely smelly because of the decomposition of the fish. NDOW will continue to monitor the extent of the fish kill and will perform population surveys later this fall once the weather cools down.

Drake says that fish may continue to die if current weather conditions don't change and that fishing will be poor over the next two to three weeks until temperatures start to cool down as fish are in survival mode and not eating. He also wants to remind anglers that all fishing regulations are still in effect and that restocking of desired game fish will resume once water quality levels are safe and productive and will be determined by current reservoir storage and anticipated runoff in 2008.

Survival of smaller fish in the reservoir is expected to be fair to good as low DO fish kills generally target the larger fish due to their higher metabolic rate and greater oxygen demands. NDOW records indicate that in an average year approximately 100,000 trout are stocked into Wildhorse Reservoir.