Association between trichodiniasis in eel (Anguilla anguilla) and water quality in recirculation systems

Association between various water quality parameters and the infection leve l of Trichodina jadranica in cultured eels were examined in nine recirculat ion farms in Denmark. Production units suffering from trichodiniasis (n = 4 ) had relatively high contents of organic dry matter (13-65 mg l(-1)), low concentrations of nitrate (150-500 mg NO3 l(-1)) and high pH (6.2-7.4) in t he process water compared to farms without Trichodina problems (n = 5), whi ch had a relatively low load of organic dry matter (9-16 mg l(-1)), high ni trate concentration (250-900 mg NO3 l(-1)) and a low pH (5.3-6.9). More det ailed studies of the effect of water parameters on the infection level were conducted in two production units suffering from frequent outbreaks of tri chodiniasis. In one production unit, the high and highly varying content of organic dry matter (21-83 mg l(-1)) was significantly and positively corre lated (r = 0.37, p < 0.001) to the intensity of infection (1-150 trichodini ds per fish) during a 19-day period. In the other production unit (with a l ow and stabile content of organic dry matter (2-7 mg l(-1))), no correlatio n was seen neither between a high and highly varying concentration of nitra te (635-1104 mg NO3 l(-1)) and intensity of infection nor between a low and highly varying pH (5.35-6.39) and intensity of infection (2-300 trichodini ds per fish) during a 17-day period. It is suggested that the infection pre ssure from trichodiniasis in farms with a relatively high load of organic d ry matter (> 15-20 mg l(-1)) may be reduced by reducing the content of orga nic dry matter in the process water. (C) 2000 Elsevier Science B.V. All rig hts reserved.