It’s been a stressful season on the Colorado River. The unusually low water level and warm weather has caused a strain on not only local raft guides, but on the river’s ecosystem as well. The indigenous and invasive species of fish on the Colorado River are feeling the effects.
“Low flows and elevated temperatures can be extremely stressful for aquatic life,” explained Katherine Creighton, the native aquatics project leader for the Utah Division of Wildlife Resources. It can cause the fish to suffocate due to the depleted oxygen amount in the river. The U.S. Geological Survey temperature gauge has measured river temperature fluctuations between the high 70 to low 80 degree marks in July. “Additionally, seasonal monsoons can flush sediments and contaminants into the mainstream which, when not diluted by higher flows, can exacerbate the existing stressors of a drought year,” Creighton added.
Local guides on the Colorado River have begun to notice small numbers of belly-up fish in the river. “Almost every day I see what looks like bleached catfish floating on the surface of the river,” said Jonah Boyer, a guide at Navtec Expeditions. ”I’ve seen catfish halfway dead swimming up to the boat, almost like it’s asking for help. It’s really sad to see all the fish dying in the river.” The Colorado River is home to invasive catfish and other sportfish that escaped from reservoirs, such as smallmouth bass and walleye. “It is not surprising that non-native fish species are less tolerant to the extreme fluctuations in temperature, flow and turbidity that can occur naturally this time of year,” explained Creighton. “It is difficult to speculate on the exact cause of the catfish mortalities that have been recently observed by boaters on the Colorado River. However, it is not surprising given the current environmental conditions of the river.”
It is interesting to note that no native fish deaths have been reported this year.
Multiple stressors on the fish, especially during a low water year, can result in a large number of fish deaths on the river. On Desolation Canyon in 2012, monsoon weather created a large flood down a side canyon which had previously caught fire; it caused thousands of fish to die. According to Creighton, researchers speculated that the low volume river didn’t have enough water to dilute the debris, ash and fire retardant that washed into the river.
That same year on the Shoshone section of the Colorado, a heavy monsoon swept in a significant amount of debris and sediment into the waters. The next morning thousands of dead fish appeared along the riverside, both native and invasive species. Biologists at Colorado Parks and Wildlife in Glenwood Springs suspect the fish died from the depleted oxygen from massive amounts of silt since the water could not dilute itself.
In other years with moderate to high river flows, significant environmental damage has not always resulted in large numbers of dead fish. Fish biologists from the Utah Division of Wildlife and the Moab Field Station received reports from canoers about an oil slick on the Green River in 2014, but no large numbers of fish deaths were witnessed, according to the project report from the Division of Wildlife Resources. They did find oil stains on rocks and vegetation in the wash, indicating that oil had flowed atop the water.
In 2015, the Gold King Mine Spill in Colorado was an environmental disaster which resulted in a significant amount of wastewater pouring into the Animas River, turning the water color to a bright orange. A sizable fish kill was anticipated in the San Juan River, the drainage to the Animas, but interestingly, no belly-up fish were found by the researchers at the Division of Wildlife Resources. It’s speculated that there were not enough multiple stressors to cause the fish to die.
The Colorado River Basin has four species of endangered fish: humpback chub, bonytail, Colorado pikeminnow and razorback sucker. They are endangered, in part, due to water development and habitat fragmentation, officials say. Despite the major stressors on the fish this year, there are benefits to a low water season for the native fish. “Researchers suggest moderate flows are most beneficial for humpback chub reproduction and recruitment,” said T.A. Francis of U.S. Fish and Wildlife Service. According to the 2017 Field Report, in 2016 and 2017 the number of adult humpback chubs detected by PIT (passive integrated transponder) tag antennas in the Colorado River was comparable to the numbers in 1998 and 1999, which means the fish is becoming more abundant. “I theorize that in recent years, moderate peak spring runoffs coupled with early large fall rain events may have created improved conditions for humpback chub reproduction and recruitment,” said Francis.
There are positive effects for the spawning patterns of the other native fish as well. Colorado pikeminnow spawn at the end of high water season and razorback sucker spawn at the beginning – therefore the native fish spawn earlier and have a longer growing season. In 2012, another low water season, the fish grew larger than average and had a higher survival rate going into their first winter. “We are hopeful that the 2018 cohort will benefit from an earlier spawn, higher growth rates and increased over-winter survival similar to what we saw in 2012,” said Creighton. Utah Division of Wildlife Resources researcher Chelsea Gibson documented spawnings of both razorback sucker and Colorado pikeminnow while conducting a monitoring trip on the Colorado River earlier this month, indicating a successful spawn for both of these endangered species.
Individuals are encouraged to report any native fish deaths to the Division of Wildlife Resources at (435) 259-3780.