Experimental evaluation of the impacts of reduced inorganic phosphorus fertilization rates on juvenile saugeye production

Optimizing fertilization rates to obtain suitable algal composition, water quality, and zooplankton forage base to produce stockable-sized juvenile fish remains a challenge for fish managers. Earlier research suggested a weekly fertilization protocol using liquid inorganic fertilizers to restore phosphate concentration to 30 μg/L and inorganic nitrogen to 600 μg/L optimized juvenile saugeye production. We conducted field experiments to test whether the fertilization rate of 30 μg P/L was higher than required and facilitated proliferation of inedible algal species leading to low zooplankton biomass and low fish survival. We compared the original phosphorus fertilization rate of 30 μg P/L with lowered fertilization rates of 20 μg P/L and 10 μg P/L, maintaining a constant inorganic nitrogen fertilization rate of 600 μg N/L. We found that lowering of phosphorus fertilization rate from 30 to 10 µg P/L neither affected the larval saugeye production (in terms of survival, yield, and growth) nor the maintenance of adequate zooplankton forage base throughout the culture duration. The phytoplankton biomass and species composition remained similar among treatments; the inoculum-weighted dominance of Cyanobacteria prevented proliferation of edible algal species in the initial weeks of fish culture. Lowering the phosphorus fertilization rate in percid ponds may reduce the risk of elevated pH, unionized ammonia, low dissolved oxygen concentration, macrophyte infestation and the discharge of nutrient-rich effluent into surrounding waters. Managers of Ohio's state fish hatcheries have implemented a juvenile percid pond fertilization protocol with a lowered phosphorus fertilization rate of 10 μg P/L based on the results of this study.