Organic and Mixed Organic–Inorganic Fertilization of Plastic‐Lined Ponds for Fingerling Walleye Culture

Abstract

AbstractTreatment with organic fertilizer was compared with treatment involving a mix of organic and inorganic fertilizers for culture of walleye Stizostedion vitreum fingerlings in 0.04‐ha plastic‐lined ponds. Organic fertilization consisted of weekly additions of alfalfa and cottonseed pellets. Each type of organic fertilizer was applied to each pond at 2.3 kg/week. Nitrogen (36–0–0 as N–P–K) and phosphorus (12–49–6) fertilizers were used weekly to adjust the ratio of nitrate‐nitrogen to total phosphorus ratios in the mixed‐fertilizer treatment to 7:1. Nutrient values in ponds treated with organic fertilizer were similar to those in ponds treated with the organic–inorganic mix. However, the water used to fill the ponds was initially high in both nitrogen and phosphorus, which may have reduced the nutrient differences between treatments. There were also no differences in food base densities (zooplankton and benthos) or fingerling production between treatments. Concentrations of dissolved oxygen at pond bottoms were sometimes less than 1 mg/L in the morning in both treatments; however, this had no apparent effect on fingerling survival in either treatment. Daphnia spp. and copepod populations sharply declined in both treatments after fish were stocked, suggesting that larval walleyes immediately fed on these crustaceans. Zooplankton populations remained low until near the end of the season. Benthic invertebrate densities were high throughout the season in both treatments and showed no differences between treatments. Stomach content analyses revealed walleye fingerlings consumed Daphnia spp. and copepods throughout the experiment, whereas dipteran larvae became increasingly important once walleyes reached 20 mm, about 2 weeks into the experiment. Both fertilization treatments produced relatively large fish (0.57 g) with good survival rates (>50%). The results from this study suggest that dipteran management is important for providing food items for larval walleyes and that high initial nutrient levels from water sources may reduce the requirements for inorganic enrichment in plastic‐lined ponds for walleye culture.