June 2008
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2007 Drought Still Affecting Fish Ponds

By Patrick Cook

The drought of 2007 caused major problems for many recreational fish pond owners across the state. Less water in ponds crowded fish resulting in reduced growth and stress, and sometimes fish kills. In many cases, ponds dried up completely. Welcomed rains this winter have refilled many ponds. However, the 2007 drought will continue to adversely impact many ponds this year.

One of the problems pond owners can expect to have this spring and summer is increased growth of filamentous algae. The reduced water levels last summer allowed weeds and grasses to colonize pond banks and bottoms. As ponds refill and these plants are killed, the physical structure and nutrients provided by the dead plants can result in tremendous growth of filamentous algae.

Often called "pond moss" or "pond scum," this type of algae forms dense mats of threadlike strands. As it grows from submerged objects on the pond bottom, it produces oxygen that is trapped in the mat of strands causing it to rise to the pond surface. The mats can cover large areas of the pond creating an eyesore and interfering with fishing.
Fish kills can also result from heavy infestations. In the late summer and fall, the algae will die. As it decomposes, oxygen is consumed by the decomposing microorganisms so much so that oxygen can be depleted from the pond to the point fish don’t have enough to live.

Filamentous algae are among the most common weed problems in Alabama ponds and, unfortunately, are also among the most difficult to control. Control methods are limited compared to those for other aquatic weeds. Fertilizing the pond early, before there is a problem may prevent the spread of the algae into deep water.

As many pond owners know, fertilization of ponds is necessary for maximum fish production. Fertilization increases phytoplankton, plant-like organisms that are the foundation of the food chain in ponds sustaining all higher organisms (i.e. fish). Fertilization can increase fish poundage (both numbers and size of fish) by as much as threefold. The extra phytoplankton can also shade out and help to limit the growth of filamentous algae and other aquatic weeds. Pond owners should aim to have enough phytoplankton to create 18-24 inches of water clarity. Water clarity can be determined by attaching a coffee can lid, pie plate or some other light-colored shiny object to a pole or line, lowering it into the water and measuring the depth at which the object disappears.

Note that not all ponds need fertilization and fertilizing a pond already rich in nutrients can be harmful and may even cause a fish kill. Also, if you already have a problem with filamentous algae or some other aquatic weed, do not fertilize the pond because it will only make your weed problem worse. If you do not yet have a weed problem and plan to fertilize your pond, you should do it now.

What if you already have a problem with filamentous algae? If you’re willing to put up with the nuisance for a month or so, the best strategy may be to wait it out. Many species of filamentous algae only grow in spring and will begin to disappear in late May and June.

If you’re not lucky enough to have one of these types, herbicides will most likely be required for effective control. Unfortunately, the herbicides most widely used to control algae contain copper as an active ingredient, which can be toxic to fish especially when the pond water has low alkalinity. Before using these herbicides, you should have your water alkalinity tested and consult with a professional.

Grass carp may also help to limit the algae. However, filamentous algae are among the least preferred foods of grass carp. Only at very high stocking rates and when the carp have eliminated virtually all other foods, will they begin to control the algae. Again, herbicides will likely be required.

Along with weeds and grasses, fire ants also took advantage of dry pond banks and bottoms last year and can cause problems in refilling fish ponds. As the colonies are flooded, the ants escape to the surface and form floating rafts on the pond surface and may build a new nest if they drift to shore. Fish in the pond, especially bluegill (bream), will eat the floating ants which can kill the fish if they eat enough of them. This also occurs when ant colonies swarm during spring reproduction and spent winged-males or drones fall onto pond surfaces.

Controlling fire ants on pond edges will not only reduce the likelihood of swarms falling into the water, but also make fishing the pond a more pleasant experience. Although countless home remedies exist, insecticides are the best bet when it comes to controlling fire ants. However, care must be taken when controlling fire ants around fish ponds because insecticides can be extremely toxic to fish. Absolutely no insecticides should be applied to an area of the pond that will flood upon refilling.

On the banks, baits should be used instead of other insecticides because baits contain far less active ingredients. To minimize runoff, apply the bait when the ants are actively foraging so they will collect the bait particles quickly. To determine if ants are foraging, sprinkle a teaspoon of bait approximately five feet from a mound. If you do not see ants collecting the bait within ten minutes, it is not a good time to apply the bait. It is also a good idea not to apply the bait if foliage is wet or rain is likely to occur soon after treatment.

Because of the reduced water levels and crowding created by the 2007 drought, many ponds will have too few bluegills relative to bass. In other words, these ponds are out of balance. If you are catching large, healthy bluegills that are few in number and large numbers of small, skinny bass, your pond is most likely "bass crowded."

Why did the drought reduce bluegill numbers so much more than bass numbers? There are two reasons why and both are related to the positions of the two species in the food chain. Bluegill can be thought of as being in the middle and adult bass can be thought of as being at the top of the food chain. Because bluegills are lower on the food chain, their production is affected by the reduction in microorganisms forming the basis of the food chain sooner. Also, the bluegills were crowded in with the top predator on the food chain, adult bass feeding almost exclusively on small bluegill, with nowhere to hide.

To remedy the situation, you will need to remove 20-25 lbs. of bass per acre in fertilized ponds, and ten lbs. per acre in unfertilized ponds. Although you can, and should, harvest bass throughout the year, it is important to start the process now so bluegill can begin to build up their populations. Target bass less than 14 inches for harvest, but larger bass can be taken. Bluegill production and growth can be enhanced by fertilizing your pond and supplemental feeding with floating catfish food.

If you have any questions about managing your pond, contact your county extension office.

Patrick Cook is a Regional Extension Agent with Alabama Cooperative Extension System’s Forestry, Wildlife & Natural Resources division.