Recreational fish ponds are very popular here in Walker County. Home ponds vary in size but are generally small impounded bodies of water ranging from one-half to three acres.
I get many calls each year from pondowners who lose part or all of their fish population. While several things can cause this, by far the most common are fish kills due to oxygen depletion.
Many factors can cause such oxygen depletion kills, some of which can be controlled by the pond owner while many others cannot.
Most oxygen depletions occur during warm weather usually from June through September. The warm water holds less oxygen than does cool water. This problem is compounded by the fact that fish increase their metabolic rates during warm weather resulting in their needing higher levels of oxygen to sustain them.
Under these conditions, fish are much more likely to succumb to oxygen depletion problems.
Another cause of oxygen depletion problems is the weather. Sunlight is necessary for the tiny algae and planktons to produce oxygen in your pond. Periods of cloudy weather, especially accompanied by hot temperatures and no wind conditions, make this problem much worse.
In such cases the oxygen consumption by your fish is greater than the amount of oxygen being produced in your pond resulting in a fish kill.
Another extremely common weather related event that can cause oxygen problems is summertime afternoon thundershowers. We have certainly had our share of those lately. Ponds are generally stratified meaning that the deep layers of water are cool while the surface water tends to be warmer.
The warm layer of water near the surface is where most of your oxygen is located while the very deep parts of the pond usually contains very little oxygen. Summer thunderstorms dump another cool layer of water on top resulting in a rolling or mixing affect of the water layers. The result is that what oxygen is in the pond will be dispursed and decaying organic matter stirred from the bottom will be as well. Again, the result is an oxygen depletion fish kill. We refer to this as a pond “turn-over.”
Stocking fish at too high rates can also cause problems with overcrowding, increased stress and oxygen depletion. Many of us pond owners do a fairly good job with our original stocking, but without periodic checking and fish harvesting the population can grow quickly in a home pond.
Poor weed control can also contribute to oxygen depletion fish kills. Most pondowners wait until aquatic weeds hamper fishing until they implement a control strategy. I am guilty of this myself, and I get very frustrated while I try to fish and spend more time cleaning weeds off my lure than I do removing fish from my hook. Many of you know exactly what I’m talking about.
Ponds with two feet deep edges will have less problems with weeds than most other ponds. I also recommend stocking grass carp or white amur fish into the pond to help with weed control. There are also a number of aquatic herbicides that can be used if you use them carefully and only according to the labeled directions.
One additional word of caution. You can actually cause an oxygen related fish kill by applying herbicides on heavily weed infested ponds during the summertime.
Here is how it happens. The herbicide does its job by killing the undesirable weeds. Once the weeds are dead and begin to decay, the decay process takes oxygen right out of the pond. A good practice would be to use spot treatments or treat only a fraction of the pond at a time and space the applications out at one week or so intervals.
Finally, overfeeding or overfertilization can cause oxygen problems. A deep green or blue-green color often results. For those of us that do fertilize ponds, try using a sechi disk, to measure the algae bloom in your pond.
The disk is nothing more than a small rectangle or circle suspended from a measuring stick. If the submerged disk disappears from view between eighteen and twenty inches (give or take), you have about the right amount of bloom. If it disappears from view at eighteen inches or less, you are at increased risk for having a fish kill.
If the fish appear at the surface of your pond and appear to be moving slowly or acting sluggish, this could be the first sign that an oxygen related fish kill is eminent. I have even seen fish congregate very close to the shore and actually appear to be panting for air. It is best to observe fish exhibiting this behavior very early in the morning since the oxygen level in your pond begins to drop around nightfall and hits its lowest level about sunrise.
Unfortunately most pondowners only discover an oxygen depletion when it is too late. Aeration is the only sure way to correct the problem and save your fish. This means the use of supplemental aeration such as PTO driven aerators or pumps to pull water from the pond and spray the water over the surface of the ponds. There are emergency techniques such as running an outboard motor locked in a stationary position and others, but these are often too little too late to do much good.
By implementing good pond construction, fertility, and weed control measures and by learning to recognize the conditions that can cause oxygen problems and the signs that your fish are under stress, you can be prepared to act accordingly and reduce your chances of losing fish to summertime oxygen depletion problems.