This article written by Kurt Knebusch was originally published on the website of the College of Food, Agricultural and Environmental Sciences.
COLUMBUS, Ohio — Long, cold, snowy winters, like this one, can lead to big fish kills in ponds. But there are steps you can take to help fish survive, said an expert at The Ohio State University.
The key is a pond’s oxygen level, said Eugene Braig, aquatic ecosystems program director in Ohio State’s College of Food, Agricultural, and Environmental Sciences.
“Many game fishes become stressed when dissolved oxygen concentrations fall below 5 parts per million,” said Braig, who works in the college’s School of Environment and Natural Resources. “Very few fish species can tolerate dissolved oxygen concentrations of 2 ppm or less.”
The problem is, without intervention, the oxygen level in ice- and snow-covered ponds can drop from 8 ppm or more at the start of winter to 1 ppm toward the end — curtains for bass and bluegill, for example. Here’s what Braig suggests.
If you’ve never installed a pond aerator, such as a bubbler, then shovel off some of the snow from the ice. Try to clear 25-50 percent of the pond’s surface — provided, as a rule of thumb for safety, that the ice is at least 4 inches thick, he said.
How it helps: Shoveling some of the snow lets in sunlight. Plants and algae still living in the water below can resume their photosynthesis. The process adds oxygen to the water.
Run a bubbler
If you have installed a bubbler, run it whenever ice starts to form, ideally from shallower depths. When the pond is ice-free, turn it off. Another option is running an air stone, which produces clouds of very small bubbles.
How it helps: Bubblers and air stones infuse oxygen into the water, which helps both in winter and summer. They also roil the water surface and so keep holes in the ice. There, oxygen in the air will diffuse into the water, which likewise raises the oxygen level.
Raise your bubbler
In winter, raise your bubbler so it’s not on the bottom, where it normally sits in summer. Elevate it to about 2 feet below the surface of the water. Or move it to shallow water. Set an air stone, too, about 2 feet deep.
How it helps: By raising a bubbler or air stone, you limit circulation of the water column. The deep water stays deep, the shallow water, shallow.
In summer, mixing those waters is a plus. But in winter it can hurt. It brings up and chills down the relatively warmer deep water, where fish often ride out the worst of the cold. Without this refuge, with nowhere to go but into mixed-up, all-frigid water, a fish’s stress goes up. So does its risk of turning belly up.
But don’t both shovel and bubble
Pick one practice or the other, Braig said — either shovel or aerate, but don’t do both. “Aerated ice,” he said, “is not likely safe enough to support a person’s weight to allow snow removal.”
Hope for a thaw, even a short one
Even in wicked winters, a warm spell can thaw a pond for a few days, according to “Winter and Summer Fish Kills in Ponds,” a fact sheet by Ohio State University Extension, the college’s statewide outreach arm.
“Oxygen levels quickly rebound when a pond becomes ice-free,” the fact sheet says. “One timely warm period of two to three days can greatly reduce the possibility of a fish kill.”
Manage aquatic plants and algae
Managing aquatic plants and algae starts when you first dig the pond, Braig said. The shoreline should have a 3:1 slope, or 3 feet of run for 1 foot of drop. This limits the area of shallow water, where summertime plant growth can boom.
In an existing pond, use chemical, mechanical and biological controls, if and as needed, to keep aquatic plant growth in check.
“Given their valuable function in producing oxygen, there’s a value in managing an appropriate coverage of aquatic plants,” he said. “Depending upon your management goals for the pond and its fishery, having up to 25 percent of the surface area in plant cover is probably appropriate.”
How it helps: Having the right coverage of aquatic plants contributes to more stable oxygen levels through photosynthesis, he said.
Also, managing for limited plant coverage serves to limit how many plants will eventually die and decompose. Decomposition uses up some of the oxygen in the water — sometimes lots of it — and can lead to too-low levels in winter. Less rotting of dead plants means more oxygen for fish.
Start with a deep enough pond
Braig said in Ohio the recommendation is for 25 percent of a pond’s area to be more than 8 feet deep, and in the northern part of the state, the same percentage to be 10-12 feet deep.
How it helps: “Ponds with a greater volume of water potentially have a greater reservoir of dissolved oxygen and tend to be less susceptible to winter kill,” he said.
To learn more about preventing winter fish kills, Braig suggests these sources, all from OSU Extension:
• “Winter and Summer Fish Kills in Ponds,” go.osu.edu/FishKills.
• “Controlling Filamentous Algae in Ponds,” go.osu.edu/ControllingAlgae.
• Ohio Pond Management, $12.50 at go.osu.edu/PondManagement.
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Much of the information in this story came from the winter 2014 edition of Braig’s online newsletter, Your Pond Update, senr.osu.edu/YourPondUpdateWI2014.
All photos: iStock.