Modelling of nitrogen loads from the farming of yellowtail kingfish Seriola laiandi (Valenciennes, 1833)

Farming of yellowtail kingfish (Seriola lalandi, Valenciennes, 1833) in the coastal waters of Australia is a relatively new aquaculture industry, and little is known about the magnitude of nutrient discharges from individual pens. In this work, we modelled the flow of nitrogen for each of two commercial pens in Fitzgerald Bay, upper Spencer Gulf, South Australia. The fish were fed commercial pellets with feed conversion ratios (FCRs) between 3.0 and 3.2 (dry weight feed/wet weight growth). These high values of FCR were reflected in the high nitrogen loads to the environment (176-195 kg N tonne(-1) growth) and a small retention of nitrogen in fish growth (14-16%, of feed inputs). Considering an annual production of 2000 tonnes. total loads to the environment can reach 391 tonnes N year(-1). Eighty-two per cent of these loads are expected to be lost to the water column as dissolved wastes. The high nutrient loads and the importance of dissolved wastes compared with other aquaculture species, such as salmon and trout, reflect the distinctly higher metabolic rates of this pelagic predatory species. The nature of the wastes suggests low localized impacts at current production levels. but regional effects remain unknown.