Table 1 . Annual use of selected chemicals according to Danish Environmental Protection Agency (DEPA 2004) and antibiotics in Danish freshwater aquaculture according to Vetstat. Year 2001 2002 2003 Chemical additives Formaldehyde (37 %) (L) 136, 608 134, 751 151 , 284 Coppersulphate (kg) 7, 294 8, 772 7, 747 Chloramine-T (kg) 7, 352 8, 769 7, 1 47 Hydrogen peroxide (L) 4, 1 78 7, 210 5, 271 Sodium percarbonate (kg) 1 1 , 696 23, 703 3, 598 Antimicrobial agents Amoxicillin (kg) 45 0 0 Oxolinic acid (kg) - 192 755 706 Sulfadiazin/ Trimethoprim (kg) 1 , 963 1, 775 1 , 625 Fish farm A l '/ :J i Fig. 1. Overview of fish farm A. Florfenicol (kg) national legislation with regard to water quality criteria (WQC), as required by EU water quality directives (TGD 2003). The WQC are assessed for individual agents by DEPA, according to toxicological values from international scientific studies and using application factors as a maximum permissible average discharge concentration. It is an administrative tool to ensure that safe levels are not exceeded. A prerequisite for environmental approval of the aquaculture industry is that common treatment practices do not conflict with WQC. Otherwise alternative treatments or improved cleaning and drug removal must be applied if the fish production is to be maintained. Freshwater aquaculture production in Denmark takes place on about 360 fish farms, with total rainbow trout production of approximately 32,000 t/yr. Annual uses of chemicals and antibiotics have to be reported to the national environmental authorities, and are shown in Table 1. 58 MARCH 2007 97 73 48 What Did We Want to Know? This Danish study was launched to investigate discharge magnitude from fish farms, identify environmental fate of chemicals/antibiotics and to develop a model to estimate flow and removal, breakdown, adsorption and precipitation of chemicals through fish farms to predict discharge cbncentrations. To investigate these topics, laboratory studies were performed using sediment and water samples that were collected in Kajak tubes from fish farms (Rysgaard et al. 1995), incubated and aerated. Subsequently, chemicals were added to the tubes and water samples were taken with fixed time intervals and analyzed to quantify elimination kinetics in water and sediment and to identify the effects of temperature and pH. Also field experiments were important and two traditional rainbow trout farms with earthen ponds were used (Figures 1 and 2a, b). Prior to all dosage experiments, volume and flow were measured in the ponds and channels concerned. Automatic water samplers were installed upstream of the treatment pond, in the treatment pond, in succeeding channels before and after drum filters, after the settling pond (effluent) and in the receiving water. The simple conservative hydraulic model was validated using a sodium chloride addition to a pond and analysis of successive water samples. Similarly, formaldehyde, hydrogen peroxide,, chloramine-T and copper were all applied to a pond, and water samples were collected and analyzed. At three different disease outbreaks and subsequent antibiotic treatments with oxolinic acid, sulfadiazine/ trimethoprim and florfenicol, water samples were collected during a two-week period. Measured concentrations of chemicals and antibiotics were compared to the concentrations predicted by a conservative model and a modified model taking elimination contributions into account.
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