MDC explains winter fish kills are no cause for alarm

JEFFERSON CITY, Mo. – The sun comes out and the temperature climbs bravely into the 40s. You shake off cabin fever with a stroll around a lake or pond and through the neighborhood. Birds are singing and the fish in the pond are… dead!

People all over Missouri are calling Missouri Department of Conservation (MDC) offices with stories like this one. They are sad to see dead fish and concerned about what might be killing them. The Department of Conservation appreciates callers’ vigilance, but officials say the fish die-offs documented so far are not related to serious pollution or disease issues.

MDC Resource Scientist Rebecca O’Hearn says winter fish kills are normal occurrences. What is not normal, she says, is the number and size of this year’s winter kills.

“We are getting dozens of reports of significant winter fish kills from all over the state,” says O’Hearn, “from places with shallow water, such as Eagle Bluffs Conservation Area, and from huge water bodies such as Truman and Table Rock lakes, Lake of the Ozarks, and Pomme de Terre and Stockton reservoirs.”

The abnormal size and number of fish kills is related to abnormally cold weather. Missouri State Climatologist Pat Guinan says the period from Dec. 1 through Feb. 28 was the coldest since the winter of 1978-79 and the ninth-coldest on record.

How cold was it? Guinan says it was cold enough to cause a rare phenomenon known as “frost quakes” in eastern Missouri. These occur when the temperature drops so fast that freezing soil expands rapidly and builds up pressure similar to the pressure in much bigger seismic zones deep in the earth’s crust. Release of this pressure produces popping, cracking noises like the ones people reported hearing on Jan. 26 and Feb. 2.

That same cold dropped the temperature of water and created unusually thick, widespread ice. Conservation Department Fisheries Management Biologist Greg Stoner observed that Lake of the Ozarks was a solid sheet of ice from Bagnell Dam to Truman Dam in January, a rare occurrence.

Then came the snow. Winter storms dumped more than a foot of snow overnight in parts of Missouri. O’Hearn and Stoner say that can be a lethal combination.

“Deep snow can prevent light from reaching aquatic plants,” says O’Hearn. “Without light, plants begin to die, and when they die, they not only are not releasing oxygen into the water, their decomposition actually consumes oxygen. If that goes on for long enough, like it has this year, fish can suffocate.”

O’Hearn says low-oxygen winter kills are fairly common in small, shallow ponds, but this year’s unusual conditions caused them to happen even in large lakes.

Stoner says that temperature alone can cause winter kills. Fish normally go into winter with enough fat reserves to survive. But colder-than-normal water can exhaust those reserves. Small fish are most vulnerable to cold stress, and that is part of the reason Lake of the Ozarks suffered a large-scale die-off of shad.

“We knew going into the fall that we had a very strong year-class of shad produced in the spring of 2013,” says Stoner. “When yearling shad are that abundant, they are generally smaller than average going into the winter, and with less fat reserves. Follow that up with a hard winter, and the result can be widespread die-offs of shad, especially the yearlings.”

As bad as die-offs may seem, Stoner says they seldom have a serious effect on fish populations as a whole.

“Shad can live eight years or longer,” Stoner says, “so losing a significant portion of one year-class still leaves plenty of adults to spawn this spring. If there is any impact on the game fish in the lake, it will likely be a temporary shortage of food this spring and a reduction in the rate of growth for game fish still too small to eat anything bigger than the smallest shad.”

Stoner says the impact of winter kills on fishing prospects are harder to predict on smaller bodies of water. The outcome depends on water depth, duration of snow cover, and fish population size. Ponds with prolonged snow cover sometimes experience total kills and require restocking.

Stoner and O’Hearn urge people who see fish kills to report them to the nearest Conservation Department office. Such reports help MDC evaluate the extent and severity of losses. It is always possible that a particular fish kill resulted from pollution or some problem other than winter conditions. If pollution is suspected to be killing fish or wildlife, call the Missouri Department of Natural Resources Environmental Emergency Response Line at 573-634-2436.

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