August 20 2007 Nevada Department of Wildlife
Nevada Department of Wildlife fisheries biologist Chris Drake discovered 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 last 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%), 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 due to 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.
The Nevada Department of Wildlife is the state agency responsible for the
restoration and management of fish and wildlife resources, and the promotion
of boating safety on Nevada’s waters. Wildlife offices are located in Las Vegas,
Henderson, Winnemucca, Fallon, Ely, Elko, and Reno. For more information
on wildlife or wildlife issues, contact the agency web site at www.ndow.org.
Nevada Department of Wildlife fisheries biologist Chris Drake discovered 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 last 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%), 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 due to 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.
The Nevada Department of Wildlife is the state agency responsible for the
restoration and management of fish and wildlife resources, and the promotion
of boating safety on Nevada’s waters. Wildlife offices are located in Las Vegas,
Henderson, Winnemucca, Fallon, Ely, Elko, and Reno. For more information
on wildlife or wildlife issues, contact the agency web site at www.ndow.org.
| WILDHORSE RESERVOIR FISH KILL |