One of the most common problems fisheries consultants have to deal with in the summer is unwanted aquatic vegetation. In dealing with private pond owners for over 30 years, I have heard some interesting descriptions of these plants. Pond moss, slime, seaweed, green gunk, and occasionally someone will call and say their pond has an "allergy"(algae). Regardless of the name, unwanted aquatic plants present a major problem to the lake owner.

Aquatic plants fulfill many natural functions in ponds and lakes such as providing cover and food for aquatic and semi-aquatic animals. However, the plants become undesirable when they interfere with human needs such as fishing, swimming, boating, and irrigation.

Unwanted aquatic plants are usually controlled by spraying aquatic herbicides or biologically by introducing a plant-eating animal. Chemical control has its limitations. Chemicals are usually very expensive, require some expertise to apply, and they do not provide prolonged weed control. Even after the vegetation has been treated, the conditions conducive to aquatic weed growth may still exist and reoccurrence of another weed problem is likely. Also, the pond owner's worst nightmare is the possibility of killing too much vegetation too quickly resulting in a major fish kill due to low oxygen from the decaying vegetation.

Biological control requires less work, is less expensive and provides long-term results.

The most common and effective biological control for aquatic weeds is the grass carp. The grass carp, also known as the white amur (Ctenopharyngodon idella) was first introduced into the United States from Taiwan in 1963 by Auburn University and the U.S. Fish and Wildlife Service for aquatic plant control research.

The fish is native to the rivers of eastern Russia and China that flow into the Pacific Ocean. It has been introduced into more than 50 countries throughout the world for weed control and for culture as a food fish.

The grass carp is a member of the Cyprinid Family, which includes goldfish, common carp, and many of our native minnow species. As Cyprinids, grass carp have no jaw teeth. Instead, they have highly specialized pharyngeal teeth and a horny pad in the roof of their throat, which together act as effective grinders allowing digestion of 60 to 70 per cent of the nutrients of the vegetation eaten.

Grass Carp have a very short gut, which allows them to process and eliminate plant material quickly. They are primarily "grazers" and tend to feed in shallow water and on the surface. At times they can be seen feeding with their back and tail extending above the surface.

Grass Carp prefer soft, low fiber, aquatic weed and underwater plants. In general, small grass carp select small, soft plants like duckweeds, filamentous algae, or softer pondweeds. As the fish grow in size, filamentous algae becomes less preferred; duckweed and pondweeds are still preferred, but fibrous plants are more readily eaten. If the more desired species of plants are not available, the fish will feed on terrestrial vegetation hanging over the water. In fact the name "grass carp" comes from its unique ability to consume terrestrial grasses.

The amount of vegetation they will consume depends upon several environmental factors such as water temperature, water chemistry, and the kinds of plants available. Consumption rates also vary with fish size. For example, until they weigh about 6 pounds, grass carp may eat 100 per cent of their body weight per day in vegetation. Larger grass carp consume less in relative terms but do consume larger absolute quantities. Fish weighing up to about 13 pounds will eat 75 per cent of their body weight each day and above 13 pounds they slow down to about 25 per cent of their body weight each day.

I very often hear pond owners say "Well, I think I'll just try putting 3 grass carp in my pond and see how they do. I can always add more." My response is, "Yes, and you can put 3 cows on 20 acres of waist high Johnson grass. They will graze all day and all night but you can't tell where they have been." My point is that you have to stock the fish at a high enough density so that their consumption exceeds the growth rate of the plants.

The number of grass carp required to control weed problems vary depending on the degree of weed infestation, kind of weed, size of the lake, depth of the lake, and region of the country. A number of different approaches have been used to determine the appropriate number of carp to stock. It is recommended that pond owners contact a fisheries professional and have them determine the type of plants present, the degree of infestation, and recommend a stocking rate.

It takes time for grass carp to bring a weed problem under control. Weeds may or may not be controlled at the end of the first growing season. The general rule of thumb for ponds is to stock enough grass carp to control the weeds in one or two seasons, but not so many that they completely eliminate all vegetation in a short period of time.

In most situations where weeds have already become a problem, 5 to 10 grass carp per surface acre will achieve weed control. In severely weed-choked cases, higher rates of 15 to 20 grass carp per acre may be necessary to attain control. In some cases, it is sometimes more effective to treat the pond with a herbicide first, and then stock moderate numbers of grass carp for weed maintenance. In ponds with largemouth bass present, grass carp for stocking should be 9 to 12 inches long to avoid being eaten by the bass. Approximately 5 to 6 years after initially stocking grass carp, restocking may be necessary to maintain control since some will die of natural causes or be lost to bird or predation by otters. Also, carp are less effective when they are older.

The older carp that remain in the lake can reach weights of 10 to 20 lbs and will consume fish feed if an automatic feeder is being used to feed bluegill. I have seen many methods used to remove these larger fish such as bow and arrow, 22 rifle or shotgun. However one of the best ways is by angling.

Russell Thornberry, editor of BUCKMASTER MAGAZINE, likes to catch large grass carp on a fly rod. Like any good fly fisherman Russell tries to “match the hatch.” To create a fly that looks like a fish pellet, he takes a piece of cork from a wine bottle and trims it to the size of a fish food pellet. He then burns a hole through the cork with a hot needle and forces a small hook through the cork. He also ties a fly the size of a pellet out of spun deer hair. I have also seen pond owners take a pellet and use a drop of superglue to attach a pellet to the hook.

Throw out a few malt cups of feed and wait for the amur to surface. As they swim at the surface and begin to suck in the feed, toss your pellet fly a couple of feet in front of the amur and hang on. Don’t try to horse them since these are strong swimmers. Take your time, enjoy the fight and wear them down.

Once you land the fish don’t discard them. Grass carp are excellent table fare. They have firm white-textured flesh and are delicious to eat. Dr. Tom Lovell, Professor Emeritus at Auburn University’s Department of Fisheries conducted a number of taste test comparing grass carp to largemouth bass, catfish and rainbow trout. All fish were prepared in the same manner and presented to a panel of evaluators. “In almost every test grass carp ranked in the top two. This was a big surprise to a lot of people, according to Lovell. Don’t throw those grass carp away; they may be the best fish you never ate.

Click for Larger View During the 1950s Dr. H.S.Swingle of Auburn University, after years of research, determined that stocking largemouth bass and bluegill was the best set of species to provide long-term fishing in ponds and small impoundments. This combination of bluegill and largemouth bass stocked at a 10:1 ratio provided stability and a “balanced” population.

Balance refers to a situation where both the bluegill and largemouth bass are reproducing and providing catches of harvestable fish of both species. Keep in mind that to this early generation of pond owners a harvestable bluegill was 4 ounces and a harvestable largemouth bass was less than a pound. This combination of stocking largemouth bass and bluegill at a 10:1 ratio was adopted by most game and fish agencies in the southeast during the early 1950s.

Today’s landowners have different objectives. They want quality largemouth bass fishing that includes catching high numbers of bass in the 2- to 4-pound range, with a reasonable possibility of catching a largemouth bass 8 pounds or larger. To obtain this type of fishery requires more than fertilizing and feeding.

Fighting Mother Nature

Dr. Rich Noble, retired fisheries professor at North Carolina State University says that largemouth bass management is a fight against Mother Nature. Noble stated, “In nature, largemouth bass populations tend to be dominated by small fish. For viability of the population it is an advantage to have large numbers of small reproducing bass rather than moderate numbers of large fish. Lake fertility doesn’t change the size distribution much. If we are going to have big bass, we have to get the food to them, and the small bass are a big problem preventing that.”

Carrying capacity of largemouth bass

Click for Larger ViewTo understand why these small bass are such a problem we need to look at the total pounds of fish a pond can produce and the changing relationships among largemouth bass, bluegill and other prey species. At a given fertility level, a pond supports predictable poundage of fish. This total poundage or carrying capacity is typically comprised of 25 percent bass. Biologists speak of a pond’s carrying capacity of largemouth bass in terms of pounds per acre. The total pounds of largemouth bass per acre is pretty consistent over time, even if the size composition of the bass population changes. Therefore, you can have a high number of small bass or a smaller number of larger bass.

Bass have optimum sizes of food fish (prey) that are needed for fast, efficient growth. Eating food smaller than the optimum size is inefficient because more energy is burned in catching small prey than is gained by eating it. Bass growth is dependent on prey growing to the right sizes to become optimum for each of the various sizes of bass in the population. High populations of small bass deplete the bluegill reproduction, leaving few bluegill to grow to the proper size for larger bass growth.

Correcting populations of small largemouth bass

How do we correct an overpopulation of small bass? The most economical and efficient management approach is to decrease the population of small bass by angling. The bass size structure can be shifted upward by aggressively removing the small bass. The total poundage of bass in the lake will not increase, but the remaining fish will grow larger.

What size largemouth bass should you remove? Typically, the bottleneck occurs at bass lengths of 10 to 16 inches. Largemouth bass within this size range are so abundant that they deplete prey needed for growth by larger bass. I usually recommend the pond owner start out removing all bass under 16 inches until an established harvest quota is met. Sometimes it is very difficult for anglers to remove bass that are 8 to 10 inches because it goes against their “catch and release” mentality. Also, if you catch a bass that is 18 inches or larger that is very skinny, take it out. It is also important to remove these bass in as short a time as possible. If the harvest is stretched out too long the bass will replace themselves.

Harvesting male largemouth bass

Click for Larger ViewAlthough bass can be removed anytime of year to improve size structure, an excellent time to remove high numbers of bass is spring. Catch rates are usually highest in the spring and bass can be sexed this time of year. Since the females are the ones that grow larger and reach trophy size, it is to the pond owner’s advantage to remove as many males as possible. Males are highly vulnerable as they move into shallow water to prepare spawning nests and guard the young. Also, harvesting bass early will allow the forage (bluegill) to increase their spawning success, thus more food for the remaining bass.

As a general rule, most fishery professionals recommend the removal of approximately 30 pounds of bass per acre from a well-fertilized pond. This can be accomplished on a good weekend on a 2-acre pond. However, as the lake increases in size it becomes a challenge to harvest the quota of bass. I know of two lakes 75 to 100 acres in size that had 2,000-3,000 pounds of bass removed in one year by angling, so it can be done. It may require you to have a few tournaments with friends to get enough fishing pressure to accomplish your harvest goals.

What size largemouth bass to harvest

Selective harvest the largemouth bass population, just like fertilizing or feeding, should be continued year after year. The bass population, by nature, will continue to produce large numbers of small bass, which will be contrary to your goal of larger quality bass.

Click for Larger ViewAs the population responds to selective harvest the size needing reduction will change, typically requiring larger fish to be removed. If you started by removing fish less than 16 inches, after a few years you may want to move this up to 18 inches and under. Hook and line sampling can give a good indication of the size distribution of bass in the population, especially the overabundant size range.

Quality largemouth bass management is best achieved by the use of multiple management techniques– such as fertilization, feeding, stocking additional prey species, aeration, etc. However, harvesting bass by angling to thin the population is one of the most effective and least expensive management practices that can be initiated to produce quality bass fishing.

largemouth bass

Every year my partner, Barry Smith, and I get hundreds of phone calls from pond owners whose primary management objective is to produce quality largemouth bass fishing. Usually this conversation includes the following statement: “I am catching a lot of largemouth bass that are a pound in size, but they do not seem to be growing.”

Largemouth bass/bluegill stocking:

More than 50 years ago, research by the Fisheries Department at Auburn University, Auburn, Ala., identified the combination of largemouth bass and bluegill as the set of species that would best provide long-term quality fishing in private ponds and lakes. This basic stocking combination holds true today.

Selective largemouth bass harvest:

However, during the 1970s bass tournaments and TV fishing shows began to preach Catch-and-Release and Don’t Kill Your Catch. This is fine for public reservoirs that have heavy fishing pressure, where protecting populations of adult bass may be important. But, it goes against basic management philosophy of private ponds and lakes. Selective bass harvest is an important and necessary management tool for small impoundments.

I remember when I was a teenager, my brother and I would go bass fishing in a neighbor’s pond, and our idea of catch-and-release was to drag a bass fillet in corn meal and release it in a skillet of hot grease. We didn’t realize it at the time, but we were actually helping that pond from becoming overcrowded with small bass.

largemouth bassMost fertilized bass/bluegill ponds in the southeast will have a carrying capacity of 400 to 600 pounds of fish per acre. These ponds will usually average 400 pounds of bluegill and 100 pounds of bass per acre. A 1-pound largemouth bass will require 5 to 6 pounds of live bluegill to gain a pound of weight. When ponds are stocked initially with bluegill and bass fingerlings, the young bass have an unlimited food supply. At the end of one year the bass begin to reproduce, adding thousands of little ones to the pond. Soon the bass are eating more bluegill than the pond can produce, and the bass growth stops.

Deer biologists and cattle ranchers will tell you that when a heard reaches carrying capacity, for continued growth you have to increase the food supply to the animals by ‘THINNING THE HERD’ or adding supplemental feed. Better yet, a combination of both works the best. The same is true for good bass management.

In a pond or lake that accumulates too many small bass, the adult bluegill population begins to decline over a period of four to six years because the small bass eat all of the bluegill reproduction before they have an opportunity to grow to 6 inches. Have you ever noticed in new ponds that the big bluegill seem plentiful for the first several years, then around year five or six, it is more difficult to catch those big stringers of slabs? The average life span of a bluegill is only five to six years. If you can’t replace the ones that you catch or that die of natural mortality, the numbers quickly dwindle. With few bluegill to spawn and more bass produced every spring, the condition of the fish population continues to go downhill.

My first recommendation to a pond owner who has a stunted bass population is to remove as many small bass, 14 inches or less, as soon as possible. This is not difficult in a two-acre pond, but can be a challenge in a lake that is 20 acres or larger. It can be done, but it requires a lot of effort.

Stocking intermediate bluegill:

The next step is to replenish the bluegill that have been overgrazed by the many small bass. This is best accomplished by stocking significant numbers of 3- to 5-inch bluegill, a size that is called “intermediate” by fishery biologists. This size bluegill is at or near sexual maturity and is usually large enough that the average-size bass has difficulty eating it.

For best results, intermediate bluegill should be stocked at approximately 500 per surface acre. In small ponds or where budgets are not restricted, stocking rates as high as 1,000 per acre have produced excellent results in a few months.

Bluegill that are a tenth of a pound (1.6 ounces) are sexually mature and can begin spawning when the water temperature approaches 78 degrees F. In the South, this means that bluegill can spawn every 30 days from May through September.

Fertilizing with Biologic’s Perfect Pond Plus:

Also keep in mind that a fertilized pond can produce three to four times the bluegill that can be produced in an unfertilized pond. This translates to three to four times the food available for your bass. Keep your pond with a good green color (plankton bloom) with a visibility of 18 to 24 inches. Use 5 pounds of Biologic’s Perfect Pond Plus per acre beginning in March and fertilizing through September. If the pond does not develop a green color, have the water tested to see if it needs an application of agricultural limestone. Just like your food plots or garden, fertilizer reacts better in ponds that have adequate levels of lime.

Automatic fish feeders:

Use an automatic fish feeder to feed the bluegill multiple times daily. This will increase their growth rate, their condition, and reproduction potential. An automatic fish feeder is one of the best investments a pond owner can make. Not only does it help grow big bluegill, but it also provides an area that concentrates fish for your kids to catch. And, it is just fun to watch the fish feed and observe how fast they grow.

The benefit of this supplemental stocking of intermediate bluegill is often seen in the fall or early winter, as the offspring of this stocking begin to gain size and weight, contributing to the food supply of the remaining bass. In crowded or stunted bass populations, continued bass harvest is necessary each year. Usually, removing a total of 30 to 40 pounds of small bass per acre during the year is sufficient to prevent them from becoming overcrowded again.

There are other supplemental forage species that can be added to the pond, such as threadfin shad, golden shiners or tilapia (where legal). These forage species should often be stocked with the intermediate bluegill, but not instead of the bluegill.

Remember your goal is to produce a bass that has a figure like Rosie O’Donnell’s. She didn’t get that figure without the groceries.

Don Keller is a Certified Fisheries Scientist.

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