Everything that lives eventually dies. I will, and so will you. Chances are, the reasons why we die will not be a mystery - there will be some known factor, symptom, diagnosis or accident.

When animals die, it can be difficult if not impossible to figure out what happened to cause the death. Veterinarians can sometimes diagnose the cause when domestic pets are involved. But if you have a pond on your property and wake up one morning to dozens of dead fish, your response is more likely to be...

What the heck is happening here?

There may have been clues to a problem with your pond days before any bodies actually floated to the surface. One or two fish may have been found dead. This is not abnormal and will be no real cause for concern. Fish get sick and old and die - just like humans. But then, maybe the following day, you observe fish swimming sideways close to the pond surface. You may notice a fish or two at the edge of the pond, treading water in a shallow area where they are not often seen. Dead fish may have signs of hemorrhage along their fins or in their eyes. What you can't know is that your fish are beginning to suffocate and time is of the essence. Oxygen depletion has occurred.

Is my pond at risk?

Dissolved oxygen levels are too low.

If you have a pond and you have fish, you have a chance of experiencing a major fisk kill due entirely to oxygen depletion. Oxygen depletion is exactly what it sounds like. The fish can't breathe and they die.

In more technical terms, your pond's dissolved oxygen levels have fallen too low to adequately support fish life. Or, the amount of gaseous oxygen (O2) dissolved in an aqueous solution (your pond), is too high in relation to factors supplying oxygen such as aeration. Pond water may be crystal clear, temperatures may not seem severe, but conditions have changed below the surface. Environmental and human-induced stresses in your pond have contributed to oxygen depletion and subsequent fish kills.

Oxygen depletion - typical causes

Overstocking

This is simply the law of supply and demand. Too many fish are trying to live in a pond without enough oxygen to support them.

Overfeeding
Overfeeding (of fish)

Overfeeding fish is not only bad for the fish...the nutrients added to the water by the excess food and the extra waste quickly create a cycle of overload in your pond.

Waste runoff

Have a pasture near your pond? If it isn't your pasture and water runoff after a storm contributes to your pond's water supply, you may end up with nutrient overload and fertilizer issues. Your pond will be subject to whatever your neighbor applies to his or her pasture.

"Acid" rain

Although you'd like to think a nice rain will be good for your pond, a pond on the edge of an oxygen deficit can be pushed into the danger zone by the acid added to the water from dripping leaves and trees. This is an unfortunate fact of life. No matter the environment, acid rain can rapidly change the pH of your water and exacerbate an existing imbalance.

Dying water gunk

Dying leaves and foliage in pond water Debris piles in or near the pond, overhanging trees that regularly drop leaves into the water, invasive water plants (some of which are so detrimental to waterways that they've been declared "illegal"!) add nutrients - more you don't need.

"Dead" water
Rapid turnover of "dead" water from the pond bottom

Bottom water is stagnant water with little or no oxygen. Fish rarely go there and when they go they don't stay long! This water does not mix with upper levels during warm weather. However, during very windy days or after thunderstorms, this bottom water is churned upward, mixes with and then dilutes/decreases whatever oxygen was available in the upper levels.

These primary causes of oxygen depletion identify things a pond owner may be able to consider and control before a fish kill happens. Every pond is so different in construction, size, local environment, amount and type of stock, etc., careful research will usually be required to discover pressure points for your pond. Your county Cooperative Extension Office or state Department of Natural Resources can be extremely knowledgeable about conditions particular to your area.

Oxygen depletion and weather - Summer and Winter kills

Most information seems to focus on Summer fish kills but Winter kills can happen as well - and in fact are more common. Cold, cloudy days, frozen ponds...conditions such as this contribute to Winter fish kills. Aquatic plants get little light and provide less and less oxygen. Summer kills happen for less mysterious reasons...water at the upper level gets too warm, fish head to lower, cooler water which contains less oxygen. Sometimes these lower levels are so oxygen depleted, there is not enough left for the fish to survive. They become distressed, weaken, and begin floating against their will back towards the top - where they often die.

Fish are, by type, more prone to oxygen depletion deaths - Summer or Winter. Pike, perch, bass and bluegill are likely to be less tolerant to low oxygen in Summer. Trout, walleye, bass and bluegill - less tolerant during Winter.

What do I do if I suspect oxygen depletion?

Prevention and Aeration

Prevention is best - if you have time. A water test kit is inexpensive (about $7 - $40.00, I've linked one or two to your left), easy to use, and usually comes with a gradient chart for pH, ammonia, nitrates and water hardness. Are you already seeing dead fish or fish in distress? A dissolved oxygen meter may be what you need. These handheld units usually cost between $300 and $1,300. Higher end models will measure dissolved oxygen and temperature - providing you with even more clues as to water quality. Then aerate. FAST! There are many, many options available for pond aeration - some costing tens of thousands of dollars. This is beyond the budget of most pond owners. A less costly but totally viable solution may be a pond fountain.

There is some controversy amongst "purists" that surface-type fountains do not move enough bottom water to provide any oxygen assistance to fish. Paddle-bottom windmills will keep that bottom water from becoming completely stagnant - the water is always turning over so sudden changes like heavy storms will not "shock" the fish. These are great ideas and great products - if you have $12,000 - $15,000 to spend.

If a pond is in a severe enough state of depletion, no fountain alone may be enough...no matter how new-fangled or complex. In that case, you probably have other circumstances contributing to your oxygen depletion problem that the addition of a fountain won't correct.

However, for those of us without access to municipal funding for this type project or emergency marine biologists on retainer, adding a small, surface fountain may give the fish the extra oxygen they need to get through difficult times.

Buy a fountain - quick!!!

Fountains have different levels of voltage/amps, GPM (gallons per minute), spray height, width and pattern, and cord length. A little research is required but do not discount a smaller fountain even if you appear to need more GPM (according to lake quantity), than you can afford. Remember - you are mainly aiding the oxygen levels of the lake - not trying to be the primary oxygen provider. An added bonus of the fountain is in sheer ambiance! If you've ever sat by a fountain and just listened you'll know what I mean. Lights can be added to further enhance the evening viewing experience.

You may want to read my review of the FountainTech fountain I purchased in response to my first Summer fish kill experience. I have since had one Winter kill, but was able to quickly recover by turning the fountain on 24/7 for the next week. And yes, the fountain worked like a champ during a North Carolina snow storm - temperatures in the upper 20s and low 30s. There is a little photo of it doing it's thing on the Rose Cottage weather cam page. There has been no discussion here of chemical additives. Unless you are that marine biologist, chemicals in and around your pond and your pond's water - should never mix. There are so many unknown factors, the risk of over-correction is high with the addition of any chemical solution without professional guidance. Contacting your local officials is a good first step towards finding assistance.

Having a lake or pond on your property is a joy - but they do take some maintenance. Experiencing a fish kill, Winter or Summer, can be disheartening to say the least. A little knowledge about what helps or hinders available oxygen, and a little preventive planning, can ensure years of enjoyment for both owners and visitors (and even the fish!)

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