Fingerling walleye production in plastic-lined ponds
In 2000, inorganic and organic fertilization regimens were evaluated for production of fingerling walleye Stizostedion vitreum in plastic-lined ponds. The control treatment was no fertilizer. Inorganic fertilizer was applied to maintain a targeted 7:1 nitrate-nitrogen to total phosphorus ratio. Organic fertilizer was an equal mix of alfalfa and cottonseed pellets applied weekly. Dissolved oxygen readings were significantly lower in organically fertilized ponds, and pH was higher in inorganic ponds, possibly due to less decomposition of organic matter or increased photosynthesis. There were no significant differences in nitrogen or phosphorus concentrations among treatments. Planktonic crustacean population trends were also similar among treatments; however, copepod populations increased late in the season in fertilized ponds. Ponds treated with organics had significantly larger fish compared to fish from control and inorganically treated ponds. No treatment differences were observed in survival, which was low in all treatments. Results from 2000 suggest organic fertilizers facilitate better walleye fingerling production compared to inorganic or no fertilizer in plastic-lined ponds. In 2001, organic fertilizer was compared to a mix of organic and inorganic fertilizers. Organic and inorganic fertilizer applications were similar to 2000. All ponds were treated with organic fertilizer; ponds in the mix treatment received supplemental inorganic fertilization. Organically treated ponds had similar nutrient values when compared to fertilizer mix; however, water used to fill ponds was initially high in both nitrogen and phosphorus, which may have reduced nutrient differences between treatments. There were also no differences in food base densities or fingerling production between treatments. Planktonic crustacean densities sharply declined in both treatments following fish stocking, indicating larval walleye immediately fed on small crustaceans (Daphnia spp. and copepods). Benthic invertebrate densities were high throughout the season in both treatments, with no differences between them. Stomach content analyses revealed walleye fingerlings consumed small crustaceans throughout the season, with dipteran larvae becoming important once walleyes were [Greater than or equal to symbol]20 mm. Both fertilization treatments produced large fish with good survival. Results from this study suggest benthos management is important in plastic-lined ponds, and high nutrient influx from water sources may reduce requirements for inorganic enrichment in fingerling walleye plastic-lined culture ponds.