Extremely high water temperatures and depleted oxygen levels have killed large numbers of rainbow trout and fathead minnows in Monastery Lake over the past two weeks. The fish kill prompted concern among residents and an investigation by New Mexico wildlife officials.
According to the New Mexico Department of Game and Fish, staff investigating the lake on July 27 found more than 100 dead fish along the shoreline and said the die-off was associated with elevated water temperatures coupled with low dissolved oxygen.
The fish kill spotlights an increasing problem that affects fisheries as prolonged summer heat threatens lakes and ponds. Wildlife officials say such die-offs are more common during periods of extreme heat.
Teresa Bustillos, a San Miguel County resident, said she and her two sisters have removed 176 dead fish from the shoreline since the fish kill began. “It broke my heart,” Bustillos said.
Many of the fish had already decomposed into skin and skeletons, she said. Others showed red sores, white lesions, missing eyes and other signs of disease, Bustillos added.
According to her, she had never seen the lake’s water temperature reach 80 degrees. A foul odor from the decomposing dead fish hangs over the area, and she believes bacteria associated with the die-off may endanger surviving fish.
She said state wildlife officials are planning to meet in the coming days to consider a possible temporary closure. Until then, she said she will continue removing dead fish and warning visitors about the die-off.
Monastery Lake, a 6-acre pond on Benedictine monastery grounds north of Pecos at about 7,000 feet in elevation, is a popular trout-fishing destination. The state regularly maintains the lake through the Department of Game and Fish’s Open Gate program, allowing public access for bank fishing during daylight hours. Wildlife officials prohibited ice fishing.
Wildlife biologists said the event is consistent with a summer fish kill, which usually happens when water temperatures exceed the tolerance of cold-water species and dissolved oxygen levels drop.
Warm water holds less oxygen while increasing fish metabolism, which raises the demand for oxygen from animals. Hot weather drives algae blooms, which will worsen conditions when they die and decompose, a process that further depletes oxygen. Calm weather also limits surface mixing that is necessary in replenishing oxygen from the air.
Similar heat-related fish kills have been documented elsewhere during periods of extreme weather.
In Minnesota, for instance, the Department of Natural Resources had received more than 200 reports of die-off involving about 100 lakes and rivers by late July. Officials said prolonged heat pushed surface water temperatures beyond the tolerance of cold-water species, such as cisco and trout.
Hundreds of fish also died this year in Delhi’s Sanjay Lake following a drop in oxygen levels amid extreme heat and low water levels. In Liverpool, England, anglers blamed heat-driven algae blooms for the deaths of hundreds of fish in Calderstones Park lake. California’s Lake Elizabeth had also documented a similar die-off in 2024 after high temperatures and low oxygen levels killed up to 1,000 fish.
Wildlife officials said small, shallow waters are vulnerable during prolonged hot weather because rising temperatures and limited water volume can create conditions unsuitable for stocked trout.
The Department of Game and Fish has yet to determine whether Monastery Lake will be closed temporarily or restocked.
With unusually high summer temperatures to persist across the Southwest, wildlife managers warn that other small lakes and ponds are vulnerable to similar heat-related fish kills unless cooler weather improves water conditions.
