WWW.WAS.ORG • WORLD AQUACULTURE • MARCH 2014 41 An estimated 2.6 million small water bodies dot the landscape of the conterminous United States (Smith et al. 2002). In the southern region of the country, most of these entities are manmade ponds of which a proportion — possibly 10 to 15 percent — is managed for recreational sportfish production. The majority of sportfish ponds range from about 0.4 to 10 ha in surface area, and they are typically stocked with a combination of sunfish Lepomis spp. and largemouth bass Micropterus salmoides. Sportfish ponds often are fertilized to increase phytoplankton productivity and stimulate growth of natural food organisms for greater fish production. Ponds constructed in acidic soils tend to have low alkalinity water and are often limed to make fertilization more effective. Considerable effort and money are expended annually for maintaining the productivity of sportfish ponds. Much of the research on sportfish pond fertilization has been conducted at Auburn University. Dr. H.S. Swingle and his colleague, E.V. Smith, began this effort in the 1930s (Swingle and Smith 1938, 1939) and investigations have continued. The major findings of the effort will be briefly summarized because they have been and continue to be instrumental in developing recommendations for fertilizing sportfish ponds in other southern states and food fish ponds worldwide. Review Liming. Ponds in areas with highly-leached, acidic soils usually have waters with pH above 6, but total alkalinity concentration typically is between 5 and 15 mg/L (Boyd 1990). Such waters A Review of Sportfish Pond Fertilization Studies at Auburn University Claude E. Boyd are poorly buffered and pH fluctuates widely in response to daily changes in carbon dioxide concentration caused by the combined effects of photosynthesis and respiration by the plankton community. In addition, availability of carbon dioxide is low and acidic bottom soils strongly adsorb phosphorus applied in fertilizer, making it unavailable to phytoplankton in the water column. The increase in phytoplankton photosynthesis caused by fertilization in low-alkalinity ponds typically is less than that realized in ponds with greater alkalinity and less acidic bottom soils, but it can exacerbate daily pH fluctuation. Moreover, growth of benthic organisms important as natural fish food is inhibited by low pH in bottom soil. Unless waters are extremely acidic (pH < 5), pH is not a reliable indicator of whether or not a pond should be limed. Thomaston and Zeller (1961) reported that ponds with less than 20 mg/L total hardness should be limed to assure good response to fertilization and this recommendation is often used today. In the area where Thomaston and Zeller made their observations, total alkalinity and total hardness concentrations were approximately equal. Also, agricultural limestone is made by pulverizing limestone rock that consists of calcium carbonate, or more commonly, a mixture of calcium and magnesium carbonates. When agricultural limestone dissolves in water without mineral acidity (pH > 4.5), the increase in alkalinity resulting from bicarbonate and the increase in hardness resulting from calcium and magnesium will be roughly equal: CaMg(CO3)2 + 2CO2 + 2H2O = 4HCO3 - + Ca2+ + Mg2+ (CONTINUED ON PAGE 42) FIGURE 1. Application method for liming sportfish ponds. Liming material is loaded onto a small barge and washed into the pond with a high-pressure water hose. Photo: Kedric Nutt, SE Pond Management.
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