Loadings of nitrogen and phosphorus from a Northern Ireland fish farm

Expressed per tonne of rainbow trout produced, loadings measured over a 12-month period from a N. Ireland fish farm were 25.6 kg total phosphorus (TP) and 124.2 kg total nitrogen (TN). These values were, for TP, 97.6% and, for TN, 112.6% of estimated loadings derived as the difference between nutrient added in the diet and nutrient retained by the fish. There were large day-to-day variations in measured TN and TP loss rates and only weak non-linear relationships between temperature and loss rates could be established. TN loss rates tended to increase with temperature within the temperature range 4–12°C, which was also the range in which a linear relationship between monthly feeding rates and temperature occurred. At higher temperatures, monthly feeding rates were not correlated with temperature but there was a tendency for TN loss rates to decline as temperature increased. This decline was also evident for daily TP loss rates, but was largely due to the inclusion of two loss rates measured at 22°C and 19°C. At low temperatures there was no evidence of a relationship between TP loss rate and temperature, and overall mean monthly TP loss rates showed little variation throughout the year. This lack of seasonal variation was attributed to the impact of the variability in the TP content of the diet which had a mean TP content of 1.66%P. In February, when feeding rate was lowest, TP content was highest at 2.2% P and the converse occurred in September when feeding rates were highest but the TP content of the diet was 1.0% P. In contrast, the TN content of the diet was relatively constant at 7.4% N.