The activity of hepatic glutathione-S-transferase (GST) was analysed in 3 different fish species with respect to fish sex and infection with parasites. In both sexes of laboratory bred three-spined sticklebacks (Gasterosteus aculeatus) experimentally infected with Schistocephalus solidus (Cestoda), a significantly lower GST-activity was found for infected fish compared to control. After field sampling of roach (Rutilus rutilus) from Lake Müggelsee (MS) and the Reservoir Listertalsperre (LTS), the GST-activity showed significantly lower values for males infected with Ligula intestinalis from MS (25%) and for infected females from LTS (55%). L. intestinalis-infected female chub (Leuciscus cephalus) from LTS also appeared to have a lower GST-activity. Thus, it could be shown that the presence of parasites significantly affects GST-activity in different fish species resulting in a decreased GST-activity due to infection. Our results therefore emphasize the need for more integrative approaches in environmental pollution research to clearly identify the possible effects of parasites in an effort to develop biomarkers for evaluating environmental health.

Acute toxicity of ammonia was investigated in four life stages of juvenile chub, Leuciscus cephalus (cyprinid fish): 1, 10, 20 and 30 days after the first feeding. The fish used for the toxicity test were reared intensively in a closed recirculation system. Each acute toxicity test duration was 96 h and lethal concentration LC1, LC50 and LC99 values were calculated for 24, 48, 72 and 96 h. The susceptibility of chub to acute ammonia toxicity decreased linearly with age and stage of development. The LC50 (48 h) values ranged from 0.62 mg L-1 of unionized ammonia nitrogen for one day after first feeding larvae to 1.73 mg L-1 for 30 days after first feeding ones. A significant linear relationship between chub larvae susceptibility to ammonia toxicity and both body weight and length was found. The critical level of unionized ammonia nitrogen for chub larvae was suggested as 0.49 mg L-1.