By KIM FUNDINGSLAND
A number of North Dakota lakes and rivers warmed up much more quickly than normal this summer. While warm water is welcomed by swimmers and such, it can be devastating for fish populations. Some bodies of water have already become so warm that fish have died, literally suffocating due to the heat. Without any relief from high temps, the possibility of additional fish kills throughout the state remains high.
"Normal surface temps might get into the 80s, even upper 80s in some years," said Scott Gangl, North Dakota Game and Fish Department fisheries management section leader. "We've had 90s in some places and it is only July. The nights are short and really haven't allowed water to cool down much. That means our average temperature for the day is so much higher."
According to Game and Fish, no less than a dozen fish kills have been reported in North Dakota recently. While the number of affected water bodies is not yet considered unusual or unexpected, the kills have occurred quite early in the season. In addition, additional lakes and rivers are reaching temperatures high enough to stress fish. Stressed fish struggle to survive.
"We're watching it pretty close. For the most part, conditions set up this summer are already in motion," said Gangl. "We always preach good water quality and good farming practices, but this year it has been just plain heat and calm conditions. It hasn't been really windy."
Wind creates waves that cools lakes and adds oxygen. Under calm conditions, lakes retain heat, algae blooms die and dissolved oxygen is consumed. Because of the early warm-up this year though, many lakes are now in a Catch-22 condition. Wind may actually turn deadly.
"There's a lot of dead water out there now," explained Gangl. "To mix it up may do more harm than help if the dead water mixes with the good water. There's a lot of ammonia and gases contained in those dead areas."
A summer kill, explained Gangl, usually starts at the bottom of a lake, below where stratification occurs. Stratification is the level of lake where warm water above is separated from cooler water below. That level is known as the thermocline.
In most years fish utilize the cool water below the thermocline to escape summer heat, but that is not necessarily possible in all lakes this year. Due to the extreme conditions experienced throughout much of the state this summer, the cool water in many locations has become a death zone for fish. It is devoid of dissolved oxygen because of decaying algae that has settled and consumed the available dissolved oxygen.
For example, a 50-foot-deep lake with a thermocline at 25 feet may be devoid of oxygen from 25 to 50 feet. Above 25 feet the water is susceptible to becoming too warm for fish to survive. That means fish are literally being caught between water with no oxygen below them and water too hot to live in above them. That scenario, says Gangl, is thought to be responsible for a recent kill of rainbow trout at Northgate Dam in Burke County.
"Trout are the most susceptible to warm water. Pike are a cool-water fish. They can survive up to about 80 degrees, but anything much warmer gets too stressful. Pike start tipping over in the mid-80s," said Gangl.
At Lake Skjermo, located near Fortuna in the northwest corner of the state, a fish kill occurred due to water becoming too warm for perch and pike. In the James River south of LaMoure, temperatures became so warm that pike were killed.
"There was just nowhere for them to go," said Gangl. "The surface temperatures were hot and the flows were low."
None of the lakes on which summer kills have occurred to date have been completely unexpected. Some of the lakes had experienced kills in the past. However, a few of the kills have grabbed the attention of the public and biologists alike.
One such kill occurred recently on Devils Lake, generally thought to be too big of a body of water to worry about regarding oxygen content during the summer months. A few northern pike recently got caught in shallow bays during adverse conditions and were found floating along the shoreline of Stump Lake, an extension of Devils Lake.
"Big water is not so much of a concern," said Gangl. "At Devils Lake there were some localized fish kills in small areas, but the system is so large that a few dead fish isn't going to affect the lake so much."
Gangl emphasizes that the fish kills reported so far have not generally been total fish kills. For instance, at Northgate it was the trout that died. There was no evidence of a loss of walleye or bluegill, two other species in Northgate. The minimal Stump Lake kill did not include walleyes, perch or white bass. Nevertheless, what lies ahead remains impossible to predict.
"If there are more problems, a lot of the lakes will probably be lakes that haven't killed in the past," warned Gangl.