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

The objective of the present study was to elucidate the influence of treated wastewater from a sewage plant on the state of health of brown trout (Salmo trutta fario). Trout were kept in two cages in a moderately polluted river at a site where a sewage plant discharges. River water only flowed through 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 and wastewater (RWW) trout). As a reference, fish were kept in tap water (tap 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. hydrophila) were isolated. Parasites were found in fish from both cages, whereby 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 liver alterations and a higher lactatdehydrogenase (LDH) enzyme level in the blood.