Effects of diel un-ionized ammonia fluctuation on juvenile hybrid striped bass, channel catfish, and blue tilapia

The effects of variable and fluctuating sublethal ammonia concentrations on plasma ammonia concentration, growth, feed conversion ratio and survival o f juvenile hybrid striped bass Morone chrysops female X M. saxatilis male, channel catfish Ictalurus punctatus and blue tilapia Oreochromis aureus wer e evaluated in each of three sequential growth periods. Fish were cultured in a flow-through system in which pH was controlled to represent the diel p H variation known to occur in eutrophic aquaculture ponds. During the first growth period (29 days), pH was maintained at 8.0. During the second growt h period (22-43 days), pH was controlled to fluctuate daily from 7.75 to 9. 00, and during the third growth period (47-53 days), pH was controlled to f luctuate daily from 7.50 to 9.50. Four nominal total ammonia-nitrogen (TAN) concentrations (0, 1, 2.5 and 5 mg l(-1)) were established as treatment gr oups. Plasma TAN concentrations of each species responded rapidly to changes in e nvironmental NH3-N concentration. Channel catfish most effectively excluded ammonia from blood. Plasma TAN concentrations of blue tilapia were greater than those of hybrid striped bass and channel catfish. Of the three specie s evaluated, hybrid striped bass was the most sensitive to unionized ammoni a and died following brief daily exposure to 0.91 mg l(-1) NH3-N (2.5 mg l( -1) TAN at pH 9.00). Hybrid striped bass growth was affected by daily brief exposure to 0.65 mg l(-1) NH3-N, but was not affected by brief daily expos ure to 0.37 mg l(-1) MH3-N (P > 0.05). Brief daily exposure to 0.91 mg l(-1 ) NH3-N did not affect channel catfish or blue tilapia growth and feed conv ersion ratio. The results of this study, in combination with published repo rts of ammonia concentrations that are normally prevalent in eutrophic aqua culture ponds, suggest that sublethal ammonia exposure in such ponds has a minimal effect on fish growth. (C) 2001 Elsevier Science B.V. All rights re served.