Effects of lowered inorganic phosphorus fertilization rates on pond production of percid fingerlings

Over-fertilization, low dissolved oxygen concentration, high pH, unionized ammonia, fish stocking density, and cannibalism are all possible causes of variations in larval percid survival and growth in earthen ponds. In this study, we examined the phosphorus fertilization effect over three different production years (2001–2003) by comparing fish raising success in ponds fertilized with phosphoric acid to restore phosphate concentrations to 30 μg P l− 1 each week with other ponds restored to only 20 μg P l− 1 each week, while restoring the inorganic nitrogen level in both sets to 600 μg N l− 1 weekly. At the end of the 5–6 wk production period, there were no significant differences between the two treatments in mean fish length, weight, survival, number harvested ha− 1, nor total biomass yield ha− 1 (P > 0.05). Phytoplankton and zooplankton densities were higher in 30 μg P l− 1 ponds than in 20 μg P l− 1 ponds (P < 0.05) in 2001, but not in 2002. Significantly lower Secchi depths and higher pH (P < 0.05) were observed in 30 μg P l− 1 ponds, while dissolved oxygen and temperature values were similar between treatments, as were soluble reactive phosphorus, total phosphorus, nitrate–nitrogen, and ammonium–nitrogen concentrations (P > 0.05). We conclude that weekly fertilization to the levels of 20 μg P l− 1 and 600 μg N l− 1 provides sufficient prey for larval percids, and it also reduces the risk of extreme pH and low dissolved oxygen that can cause increased fish mortality.