Destratification of ponds means more fish for anglers to catch

Using aeration to prevent ponds from stratifying in the summer can increase the biomass of largemouth bass a pond can support.

Every body of water, from large reservoirs to small ponds, supports only a certain weight of bass. The fisheries biologists’ term is biomass, the weight of all bass in a pond or per acre. Dr. Wes Neal, Fisheries Extension Specialist at Mississippi State University, has found a way to increase bass biomass in a typical Mississippi farm pond managed for largemouth bass and sunfish — a three-fold increase in bass biomass. No fertilizer. No feed. Just aeration of the pond to prevent stratification.


In Mississippi, most water bodies without water flowing through them develop a layer of warm water in the upper water column and a layer of cooler water deeper in the water column in the summer. The zone of rapid temperature change between these two layers is the thermocline. A water body in this condition is said to be “stratified,” and even shallow ponds can stratify.

Two aspects of stratification are important to the pond owner. First, the upper water layer, where oxygen-producing photosynthesis occurs, does not mix with the lower water.

Second, oxygen is consumed by bottom-dwelling invertebrates and bacteria and fungi decomposing organic matter that sinks to the bottom. That happens year-round. But in the summer when the pond is stratified and the upper-water layer does not mix with the bottom-water layer, oxygen is depleted in the lower-water layer.

Fish can’t live in these low-oxygen conditions. Neither can large bottom-dwelling invertebrates like mayfly and dragonfly larva that are important sunfish food, leaving only small invertebrates tolerant of the low-oxygen conditions that are of lesser food value to the sunfish. Lacking food, the sunfish biomass is reduced. And less sunfish biomass, less bass biomass.

Stratification prevention

The best way to keep the entire pond — top to bottom — habitable year-round for fish and their forage is to prevent stratification. This can be done several ways. Neal chose the most economical approach: aeration. The aerators add oxygen to the water, but more importantly they create an up-flowing water current. Proper placement of one or more aerators produces sufficient water flow to prevent stratification.

The study was conducted on a six-acre pond with well-established bass and bream populations on the Mississippi State campus. The pond had and average depth of about 6.5 feet and maximum depth of 13 feet. The pond was largely devoid of aquatic vegetation and received limited fishing effort.

Neal used Vertex Water Features (Pompano Beach, Florida) aerators, but other aerators designed for this purpose would also work. The aerators were operated continuously from 2012 to the end of the study in 2018.

The bass response

During 2010 to 2012 and prior to aeration, bass biomass ranged from 32 to 38 pounds per acre. During 2013 to 2015, a time period Neal termed a transitional period, bass biomass ranged from 38 to 39 pounds per acre.

During 2016 to 2018 — three years after aeration began — bass biomass jumped to 89 to 116 pounds per acre.

The bass population size structure shifted from a wide distribution of bass sizes up to 22 inches during the pre-aeration and transitional years to a large increase in the numbers of bass 6 to 13 inches, but few bass over 14 inches. The increased bass biomass and numbers were the result of increased recruitment.

In the foreground is the air pump that supplies air to diffusers. The buoy in the pond marks the diffuser. A ring of bubbles around the buoy is air bubbles rising to the surface. (Photo courtesy J.W. Neal)

The more abundant small bass declined in condition and suggested bass overcrowding. In 2017 and 2018, Neal removed 21 to 28 percent of the bass biomass. The bass biomass remained unchanged.

Neal’s conclusion: destratification increased both the productivity and habitable volume of the pond.

The next step

Tripling bass biomass is significant, but the fish were small and skinny, likely with slow growth. These are typical conditions in a bass crowded pond. The management recommendation for bass crowded ponds is to harvest bass. With bass biomass tripled, Neal suggested bass harvest should at least triple. That is not easily accomplished by fishing.

Neal did not measure sunfish biomass nor the abundance of large bottom-dwelling invertebrates, but the fundamentals of aquatic ecology allow working backwards, down the food chain from the bass. The young bass likely benefited from the greater habitable space and also the expected increase in large invertebrates. But these bass need abundant forage fish for rapid growth to larger sizes.

Sunfish — the primary forage for bass in the pond — would benefit from the greater abundance of invertebrates in the unstratified pond, increasing their productivity and ability to support the bass population. But the high density of smaller largemouth bass needs to be greatly reduced to allow sunfish survival and achieve a greater sunfish biomass.

Neal also suggested, in addition to reducing the bass population, the use of threadfin shad as an additional forage fish. The unstratified conditions and altered nutrient dynamics should, based on basic aquatic ecology, increase production of phytoplankton and zooplankton to support the threadfin shad.

Ponds are simple ecosystems that follow basic ecological principles. Although simple and well-studied, there are still new things to learn about managing farm ponds.

About Hal Schramm 182 Articles
Hal Schramm is an avid angler and veteran fisheries biologist.

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