The effect of sewage plant effluent on brown trout (Salmo trutta fario): acage experiment

The objective of the present study was to elucidate the influence of treate d wastewater from a sewage plant on the state of health of brown trout (Sal mo trutta fario). Trout were kept in two cages in a moderately polluted riv er at a site where a sewage plant discharges. River water only flowed throu gh one cage (river water only (RWO) trout), while treated sewage water was added to the river water (final concentration 5%) in a second cage (river a nd wastewater (RWW) trout). As a reference, fish were kept in tap water (ta p water reference (TWR) trout). After 66 days the mortality rate in group RWW was 87%, in group RWO 29% and in group TWR 0%. In RWW and RWO trout the infectious agent of furunculosis (A. salmonicida) and additionally in RWW another bacterial species (A. hyd rophila) were isolated. Parasites were found in fish from both cages, where by there were differences in species composition and degree of infestation between the two groups. Only in RWW trout were ulcerations of the upper and lower jaw found, together with significantly more severe histological live r alterations and a higher lactatdehydrogenase (LDH) enzyme level in the bl ood.