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Deep lake fish kills attributed to oxygen, water temperature

Date09/15/2011
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MOUNTAIN HOME – Dead fish in Bull shoals, Norfolk and Beaver lakes in north Arkansas have resulted in questions and concerns from a number of Arkansans.

It is a natural and somewhat seasonal phenomenon, according to Ken Shirley, a veteran district fisheries biologist with the Arkansas Game and Fish Commission. The situation was complicated this year because of unusual and prolonged high levels on the lakes resulting from heavy spring rains.

Shirley said, “High springtime inflows into the large reservoirs are a great benefit to our fisheries. The resulting huge spawns of crappie, walleye and largemouth bass often dominate those lake fisheries for years. Similar spawns of threadfin or gizzard shad, sunfish, and invertebrates result in fast growth of all our game fish,” he explained. “However, this is not without its cost. Access to our waters is often limited or even damaged,” he added.

“Another detriment also is quite common though less well known by the angler. Organic matter like leaves flushed into the lakes decays, taking up oxygen in the cold water below the thermocline,” Shirley says. The thermocline is the point where shallow warm water meets cold deep water.

“While many cool water fish like walleye and striped bass concentrate at the thermocline where they can acclimate to the temperature and water quality conditions, there is also often a deep plume of cold oxygenated water flowing along the bottom which also has fish.”

Shirley explained that fish using that deep plume of oxygenated water have to travel up through a deep or anoxic area to reach more abundant oxygen.

“Many do not make it. If they do, they may find the surface water too hot for them to acclimate quickly or their gas bladder expands so much that they pop to the surface, unable to swim back down, eventually dying from the bends or temperature shock.”

Striped bass, walleye and yellow perch are the most numerous of fish being found dead in recent days on two of the lakes. Beaver Lake doesn't have a population of yellow perch and the kill in Beaver Lake primarily affected only a few walleye and striped bass.

Bull Shoals no longer is stocked with striped bass although it once was and still has some older fish remaining. Beaver Lake has a revitalized population of walleye. Yellow perch, smaller relatives of walleye, are not a significant sport fish in Arkansas but are a food source for stripers, walleye and other predator fish.

Shirley said, “Fish kills like this occur in normal high water years on Norfork, preventing its stripers from reaching the huge size for which they are capable. Larger fish are affected sooner than smaller ones. Bull Shoals and Beaver, with less fertile water, usually suffer these kills only after extreme high water events such as this year. High water temperatures speed up the oxygen depletion, making this year even worse than would have been expected from high inflows alone.

“While these fish kills were anticipated this year, and on Bull Shoals and Norfork may be severe enough that anglers notice the temporary decline in walleye and striper populations, the increase in food and great spawns will be even more apparent in the future. These fish kills usually occur over a period of weeks and will end when the surface water temperatures decline enough that the layers begin to mix.”

After several months of high water, Beaver Lake is back near normal. Lake Norfork is slightly above normal, and Bull Shoals Lake is currently about five feet above normal.