Submitted to: Aquaculture Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/22/2010
Publication Date: 9/24/2010
Citation: Green, B.W., Perschbacher, P., Ludwig, G.M., Duke, S.E. 2010. Threadfin shad impacts phytoplankton and zooplankton community structures in channel catfish ponds. Aquaculture Research. 41:e524-e536.
Interpretive Summary: Very dense populations of algae develop in ponds during the summer months when channel catfish are fed large quantities of formulated feed. Typically, the pond algal population comes to be dominated in mid-summer by a small number of genera of blue-green algae. This can be problematic because algal populations composed of a small number of genera can die-off suddenly, for reasons that are understood incompletely, and create an acute dissolved oxygen crisis that may cause fish to die from lack of oxygen. Additionally, certain nuisance bloom genera of blue-green algae produce compounds that impart an earthy-musty flavor to catfish flesh. In this experiment, we impacted significantly algal population composition in channel catfish ponds during the summer months by stocking threadfin shad in ponds with catfish. By consuming zooplankton, which are micro/macroscopic crustaceans that feed on algae, the threadfin shad modified the pond ecosystem. In the absence of grazing by zooplankton, algal species that normally are out-competed by blue-green algae are able to flourish. We found that the algal community composition shifted from dominance by blue-green algae to diatoms and that the total numbers of algae were reduced in ponds stocked with threadfin shad. The results of this experiment demonstrate that threadfin shad stocked in catfish production ponds do impact the pond algae populations in a manner considered positive by fish farmers: reducing the dominance of blue-green algae and the abundance of certain genera of blue-green algae known to cause off-flavor.
Technical Abstract: Plankton community structure and chlorophyll a concentration were compared in 12 0.1-ha earthen ponds co-stocked with channel catfish (Ictalurus punctatus Rafinesque, 1818) in multiple-batch culture (initial biomass = 5,458 kg ha-1) and a planktivore, threadfin shad (Dorosoma petenense Güther, 1867; initial biomass = 449 kg ha-1), during the April-November growing season. We used a completely randomized design in 2 x 2 factorial arrangement to test planktivore level (presence or absence of threadfin shad) and channel catfish feeding frequency (daily or every 3rd d). Channel catfish were fed a 32% protein feed to apparent satiation on days fed. The presence of threadfin shad affected phytoplankton and zooplankton community structure more than did feeding frequency, and the impact in ponds was more pronounced after 1 July. Numbers of all major groups of zooplankton were lower in ponds with threadfin shad, but were unaffected by feeding frequency. Chlorophyll a concentration before 1 July was higher in ponds with threadfin shad and unaffected by feeding frequency, whereas after 1 July it was higher in ponds without threadfin shad and that were fed daily. Phytoplankton community structure after 1 July was dominated by nuisance algal bloom genera of cyanobacteria in ponds without threadfin shad and by Bacillariophyceae in ponds with threadfin shad.