Abstract
The effects of chronic un-ionised ammonia (UIA-N) exposure on growth and food conversion efficiency in juvenile Atlantic cod, Gadus morhua, were examined. Fish with a mean (SD) initial weight 16.7 (3.4) g were exposed to four concentrations of UIA-N, ranging from 0.0005 mg l −1 (control) to 0.17 mg l −1, for 96 days at 13 °C. Compared to the control, growth was significantly reduced at UIA-N concentrations above 0.06 mg l −1, but the fish experiencing the most severe UIA-N concentrations still maintained high growth rates (>1% day −1). The growth reduction is attributed to a decrease in food intake. Fish from the exposed groups displayed an acclimatory response to UIA-N, as growth rate reduction in these groups, compared to the control, diminished as the experiment progressed, and in the last period, growth rates in the UIA-N exposed groups were not significantly different from that of the control. Concurrently, a reduction in plasma urea-N content was seen in these groups at the end of the experiment, indicating an increased efflux of urea in response to high ambient UIA-N.
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