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Editor's note: This article originally appeared in the July 2006 issue of FLW Outdoors Magazine. Learn more about FLW Outdoors Magazine and how to subscribe by clicking here.
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Fishery managers often fertilize ponds to produce more pounds of fish. With good habitat and harvest management, the greater biomass of fish can quickly be turned into big bass. Why not get a bigger bag of fertilizer and produce more and bigger bass in lakes and reservoirs?
It makes good ecological sense. The fertilizer stimulates the growth of planktonic algae. The energy in these primary producers passes through the food web to zooplankton and bottom-dwelling invertebrates, then to forage fish, and finally to top predators, such as bass. Positive relationships between phytoplankton and sport-fish abundance have been repeatedly demonstrated in lakes and reservoirs. Good ecological sense or not, fertilization in warm-water reservoirs hasn’t worked.
Researchers conducted fertilization experiments in Lake Mead in Nevada and Grayson Lake in Kentucky. They were able to stimulate algae blooms in the coves where fertilizer was applied, and even boost shad production, but the researchers found no significant effects on predator fish, including largemouth bass.
The reasons are not totally clear, but one obvious factor is water flowing through the reservoir that flushes and dilutes the fertilizer. Lake Mead is used for hydropower generation, so some flow is inescapable. Because Grayson Lake had a moderate flushing rate, the forward-thinking researchers applied the fertilizer to a small cove largely isolated from the rest of the reservoir by a sandbar. Despite their reasoning, little effect of the fertilizer was measured.
Phosphorus is usually the limiting nutrient in aquatic systems, so pond managers add high-phosphorus fertilizer to low-fertility ponds. Phytoplankton quickly take up the added phosphorus. What the plants don’t quickly absorb is bound by other chemicals or sediment particles and settles to the bottom of the lake where it is unavailable to the phytoplankton in the light-rich upper waters.
To compensate for this, pond managers apply phosphorus frequently throughout the entire growing season. The reservoir experiments applied fertilizer for only a portion of the growing season. Applying fertilizer throughout the growing season may have produced better results, but fertilizer is expensive. In Grayson Lake, seven fertilizer applications from April to June cost about $100 per acre. Applying fertilizer through the end of the growing season in October would have tripled the cost. Therefore, while fertilizing may be feasible and effective for ponds and small lakes, it simply is not affordable or effective for larger bodies of water.
Many lakes, however, receive ample nutrients from the watershed. These eutrophic (nutrient-rich) systems produce more fish. Nutrients from fertile watersheds enter the water with every runoff event, an effect equivalent to frequent fertilization.
A little fertilization is good, but too much is called pollution. Excessive nutrients, especially phosphorus, cause dense algae blooms. Besides being aesthetically displeasing, the algae blooms cause oxygen depletions, odor problems and declines of sport fish. Extensive efforts have been made to reduce phosphorus inputs to our surface waters. Most of these efforts were successful, and our waters and sport fisheries have benefited. Ironically, from a fisheries perspective, advanced wastewater treatment efforts have been too effective in a few systems, and fish abundance and growth have suffered.
Given a choice, I’ll take the cleaner water. There may be fewer pounds of fish per acre in these lower-nutrient lakes, but good management and cooperative anglers can ensure that catch rates and fish quality remain high.
