Dissolved oxygen fluctuations in organically and inorganically fertilized walleye ( Stizostedion vitreum) hatchery ponds

Abstract

We compared autotroph productivity and fish production in walleye ( Stizostedion vitreum) hatchery ponds under two fertilization regimes. Three ponds were fertilized with inorganic N and P (experimental) and three ponds were fertilized with a combination of organic chopped hay, soybean meal, alfalfa meal, and inorganic N and P (control). Daily dissolved oxygen (DO) changes were monitored at three depths. Chlorophyll a concentrations and Secchi depth measurements were taken twice per week. Mean DO concentrations were significantly lower ( P<0.0001, n=684) in the control ponds. Average productivity, calculated by either dawn–dusk–dawn DO changes, or diel DO changes measured at 15-min intervals, were not significantly different between the treatments ( P>0.05) however, chlorophyll a concentrations were significantly higher in control ponds (19.21 vs. 7.09 μg/l in experimental ponds). Secchi depth did not correlate with algal productivity or biomass. Higher alkalinity did not increase algal production or biomass. Even with cooler than normal temperatures, one of the control ponds required aeration to avoid a fish kill. Overall, fish yield and survival were higher in control ponds, suggesting that some other factors associated with organic fertilizers (increased protozoan production, slower release of nutrients) may make them necessary for optimum yields. Adjusting the ratio of organic to inorganic fertilizer could reduce cost and anoxia without reducing fish production.