There are a few “cool weather” conditions that affect the health of fish in your pond.

North Florida’s hot humid summer seems a distant memory for now.  The cool temperatures, falling leaves, and shorter days are a welcome relief.  Since activity in and around your pond may be reduced this time of year, there are a few “cool weather” conditions  you should be aware of.

The winter conditions we enjoy also influence the biological and ecological characteristics of your pond.  Cooler temperatures, shorter days, and dying vegetation all affect your pond’s water chemistry, which in turn affects the biology of fish and other plants and animals that live in your pond.

Dissolved Oxygen

One of the driving forces influencing water chemistry is the amount of dissolved oxygen in the water.  This dissolved oxygen is what is absorbed through fish gills to “breath” and is used by other plants, animals, and bacteria in a number of biological processes.  One of these beneficial processes is the decomposition of organic waste materials (uneaten fish food, decaying plant and animal materials, etc.).  Because cold water “holds” more dissolved oxygen than warm water, your pond’s dissolved oxygen levels are likely higher in the winter than in the summer, which is good, but there are other factors that can influence those levels.  Shorter day lengths and cooler temperatures mean less sunlight and energy for plants to use for photosynthesis, therefore reducing the amount of oxygen produced.  The fluctuating temperatures of a typical north Florida winter can speed up or slow down the beneficial bacterial decomposition processes.  So, if your pond has excess organic wastes and a high fish density, it may become stressed due to low dissolved oxygen levels and slower decomposition.  A fish kill can result.

Nitrite

Another chemical that can occur in higher amounts in cooler weather is nitrite.  High nitrite concentrations in heavily stocked ponds occur more frequently when temperatures are fluctuating.  These fluctuating temperatures result in the interruption of the beneficial nitrogen cycle due to decreased plankton and/or bacterial activity.  High nitrite concentrations can result in “brown blood” disease.  In this disease, nitrite is absorbed through fish gills turning the blood a brownish color.  The nitrite interferes with healthy hemoglobin function and can cause suffocation because the fish is unable to absorb enough dissolved oxygen.  Largemouth bass and bluegill are relatively resistant to high nitrite concentrations, whereas catfish and tilapia are less resistant.

Fish Behavior

Fish are cold-blooded animals.  They are unable to maintain a constant body temperature like we do.  Their body temperature is affected by water temperature, and their metabolism and activity level fluctuates with water temperature.  Fish are more active in warmer water and less active in cooler water, therefore, fish will not feed as vigorously in the winter as they do in summer.

Management Considerations

• Because low water temperatures reduce fish activity as well as beneficial bacterial decomposition processes, reduce the amount of feed you provide your fish in the winter.  Use the fish’s feeding behavior as your guide.  Fish will generally slow feeding when water temperatures are at or below 60 degrees.  Uneaten food will add excess organic matter to the pond increasing the chances of low oxygen levels and a fish kill, especially as warmer weather approaches.
• If you have an aerator, keep it operative during the winter, especially during warm spells and cloudy weather.  It is probably not necessary during periods of strong, windy, cold fronts passing through our area, but watch your fish for signs of oxygen stress! (See this UF Publication for signs “Dissolved Oxygen for Fish Production”) If you don’t have an aerator and your pond is productive with a high number of fish, plants, and algae, consider installing one, especially as the weather turns warmer this spring and summer.
• If you remove dead shoreline plant material from your pond banks, don’t throw the plants, or any other organic wastes, into the water.  Doing so will increase the amount of organic material that needs to be decomposed, and increase the chances of oxygen depletion and a possible fish kill.
• Check condition of fish feed, pumps, aerators, and structures around the pond, and maintain or replace as needed.
• If you have had aquatic plant problems in the past, now is a great time to contact your County Extension office for control information and plan ahead.  Some plant species can be managed beginning in late February as they start to grow.  Also, scout the pond and pond banks for non-native invasive plant species (your UF Extension Agent can help you identify them).  These actions are part of an integrated pest management approach that may include the use of herbicides, mechanical control, and/or grass carp (a plant eating fish).

Winter is a great time to perform maintenance checks and plan ahead.

North Florida’s fish and farm ponds play an important role in many aspects of agricultural and rural life, from irrigation to food production.  Maintaining good water quality is critical to the health of these ponds.  Your County Extension agent has much more information on the above topics.  Also, many Extension Agents in the Panhandle have the tools to test your pond’s oxygen level, as well as other water quality indicators.  Please contact them if you have any questions.

Below are links to additional information and references used for this article:

• Florida Crop/Pest Management Profile: Aquatic Weeds
• Managing Florida Ponds for Fishing
• Nitrite in Fish Ponds
• Pond Mixing
• Cold-induced fish kills in Florida waters
• Dissolved Oxygen for Fish Production
• The Role of Aeration in Pond Management

Judy Ludlow