Channel catfish polyculture with fathead minnows or threadfin shad: effects on pond plankton communities and catfish fillet flavor, color, and fatty acid composition

Threadfin shad, Dorosoma petenense, or fathead minnows, Pimephales promelas, were co-cultured with channel catfish, Ictalurus punctatus, in earthen ponds to determine the effects of planktivory on plankton community dynamics and catfish fillet quality. Fathead minnows had no effect on the plankton community structure or catfish fillet flavor, color, and fatty acid composition. Fillet color was also unaffected by the presence of threadfin shad. Small differences were found in fillet fatty acid composition for catfish from ponds with shad, but these differences probably have no biological significance. Threadfin shad did, however, have important impacts on the plankton community structure and catfish flavor. Size-selective filter-feeding by shad reduced cyanobacterial abundance relative to ponds with catfish-only and fathead minnows. Relative abundance of smaller phytoplankton in the groups Chlorophyta, Cryptophyta, Bacillariophyceae, and Euglenophyta increased in ponds with shad. Relative abundance of small zooplankton (rotifers) also increased in shad ponds. Reduced abundance of large, colonial cyanobacteria that are known to produce odorous metabolites caused a corresponding reduction in off-flavor prevalence and intensity in catfish from ponds with threadfin shad when sampled in September. Although threadfin shad dramatically reduced catfish off-flavor prevalence during the warm season, they apparently caused a high prevalence of “fishy” off-flavors in the February sample. This undesirable flavor appeared to be caused by catfish foraging on shad killed during a preceding period of exceptionally cold water temperatures. Use of threadfin shad for phytoplankton biomanipulation therefore presents a dilemma: catfish–shad polyculture reduces prevalence of cyanobacteria-related off-flavors in warm months but may cause undesirable foragerelated off-flavors in the colder months. Catfish farmers must consider these benefits and risks when deciding to use threadfin shad as a management tool.