Benthic invertebrate management in plastic-lined fish culture ponds
The use of plastic-lined ponds in place of earthen ponds for fish culture is becoming common among agencies; however limited information is available on their management. Since plastic lining significantly reduces the amount of organic sediment typically found on the bottom of culture ponds there is also a limited amount of organic substrate available to potential benthic macroinvertebrates, which are important prey for larval fish. Previous studies on plastic-lined ponds have focused on fertilization and management of zooplankton populations. Zooplankton populations are an important prey for larval fish, but as fish grow, their diet switches from zooplankton to benthic invertebrates, most commonly Diptera. Therefore, this study looked at the effects of fertilization and supplemental formulated fish feed on benthic populations and fish production at the Iowa Department of Natural Resources' Rathbun Fish Culture and Research Facility, Moravia, Iowa. In both years of the study, walleye (Sander vitreus) were stocked into six 0.04-ha ponds. In 2002, effects of organic fertilization on the benthic community and fish production were determined. It was found that benthic communities were not significantly (P<0.10) affected by the fertilization regimes, inorganic vs a mix of inorganic plus organic fertilizers; however ponds in the mix treatment had more organisms. At harvest, walleye in ponds in the mixed treatment were significantly longer, heavier, and had greater biomass. These results suggest that organic fertilizer is important for the benthic food base as well as growth of fingerling walleye. In 2003, effects of supplemental fish food on fingerling growth and survival, as well as the benthic invertebrate community were investigated. It was found that supplemental feed did not significantly increase the benthic invertebrate community or fish growth and survival. Furthermore, only a small portion of the walleye had consumed formulated feed at harvest; this occurred when the natural food base of the larval walleye, zooplankton and benthic invertebrates, was depressed. Therefore, for the culture of walleye fingerling in plastic-lined ponds we recommend managing for zooplankton and benthic invertebrate food base with organic fertilizers. The application of supplemental commercial feed may be useful when the natural food base is depleted.