Characterisation of waste output from flow-through trout farms in France: comparison of nutrient mass-balance modelling and hydrological methods

Abstract

-Water quality assessment is a key factor in the environmental management of freshwater networks, especially those including fish farms, which need cost-effective operational tools to monitor and control their waste output. In France, current legislation specifies limits in concentrations of dissolved compounds and suspended solids at fish-farm outlets. Despite the development of mass-balance modelling tools, chemical analysis of water (hydrological method) remains the most widely used method. To understand better the environmental impact of trout farms on aquatic ecosystems and to compare waste assessment methods, we monitored 20 commercial flow-through trout farms for 24 h, and we compared data obtained with the two methods (hydrological method and mass balance modelling) by linear regression. For total nitrogen and total phosphorus, the correlation between the two methods was high; thus, considering the uncertainty of both methods, this study was not able to determine which was more accurate. The high correlation between observed ammonia concentrations and predicted total nitrogen emissions provides a coefficient for estimating ammonia emissions at the farm level. The same approach is proposed for the evaluation of phosphate emissions. In conclusion, this study confirms the utility of simulation modelling for assessing nutrient release from fish farms.

Waste loading into a regulated stream from land-based trout farms

“…Ammonia nitrogen can form 53 to 69% of total nitrogen wastes in the effluent of rainbow trout farms (Kajimura et al 2004), but the ratio may increase up to 79% in some in stances (Dalsgaard & Pedersen 2011). The ratios of TAN/TN in effluents in our study were unexpectedly lower than the literature values, suggesting that nitrification of ammonia and temporal variations in the samplings most likely played a significant role, as reported previously (2008), and Aubin et al (2011). Although urea, amino acids, and nitrogen excretion via the gills and/or skin and mucus may comprise a considerable amount of the soluble fraction of organic nitrogen (Kajimura et al 2004), both soluble and particulate fractions may reach up to 36% of TN (Foy & Rosell 1991).…”

“…Although there could be seasonal variations in farm parameters such as fish stocks and feeding rates, we used the average annual feed conversion based on interviews with the farmers to estimate the average daily feeding rate. The hydrological method was based on concentration differences (ΔC) of parameters and flow rate measurements (Roque d 'Orbcastel et al 2008, Aubin et al 2011.…”

“…To estimate total waste loads many efforts have been made to date (e.g. Cho et al 1991, Kelly et al 1996, Aubin et al 2011). The main purpose of load estimations is to predict the existing and future discharge of suspended solids, organic compounds, and nutrients, thereby allowing authorities to quantify the environmental impacts of activities with the greatest accuracy (Frier et al 1995).…”

“…The hydrological method is based on the measurement of selected indicators between the inlet and the outlet of fish farms by taking the flow rate into consideration (Roque d'Orbcastel et al 2008, Aubin et al 2011). However, there may be uncertainties arising from changing farm practices and sampling methodologies, which can result in temporal variations in the suspended solids and nutrient concentrations in the effluent (Cho et al 1991, Papatryphon et al 2005.…”

“…The principle is based on the assessment of the difference between the nutrients and digestible energy supplied to fish and their body nutrient and energy gains. The proportion not retained by the fish for growth is released into the water and constitutes the waste emissions of the fish farm (Cho & Bureau 2001, Aubin et al 2011.…”