Effects of ammonium sulfate on stress physiology and innate immunity of Western mosquitofish (Gambusia affinis).

Fertilizers increase yield of crops but may have unintended negative effects on fish as a byproduct of runoff into bodies of freshwater. The objective of this study was to determine if environmentally relevant concentrations of an ammonium fertilizer impacts stress and innate immunity in Western mosquitofish (Gambusia affinis). The mosquitofish were exposed to different concentrations of ammonium sulfate fertilizer: 0 ppm, 40 ppm, and 80 ppm. To test the effects of ammonium sulfate on stress physiology, cortisol released into water by individual fish was collected after 1 week of exposure and again after 2 weeks of exposure and quantified with an enzyme immunoassay. Cortisol levels in the 0-ppm group were not significantly different over the course of the study, but we found a significant increase in cortisol levels in the fish exposed to 40 ppm and 80 ppm. We found reduced survival in fish from the 40 ppm and 80 ppm of ammonium sulfate groups compared with the 0-ppm group. We also used blood samples to complete a lysozyme assay as a measure of innate immune defense. Higher concentrations of ammonium sulfate correlated with significantly lower lysozyme activity in the fish. Overall, our results suggest that relatively low amounts of ammonium sulfate runoff into bodies of water are likely to have negative sublethal and lethal effects on small fishes.