Physiological effects in fish exposed to effluents from mills with six different bleaching processes

Immature rainbow trout (Salmo gairdneri R.) were continuously exposed in a flow-through system to a total of six different bleached kraft mill effluents (BKME) at two exposure concentrations (400 and 2000 times dilution) for 7 weeks, during three consecutive years (1982-1984). The fish were exposed in outdoor tanks connected to the outgoing water from a parallel mesocosms study simulating the Baltic littoral (Fucus vesiculosus) zone. After exposure, hematological, osmoregulational) and mixed function oxidase (MFO) parameters were studied. The results showed that conventional choline bleaching (C95 + D5)EHDED, with and without treatment in aerated lagoons, had the strongest effects on the fish exposed. Oxygen prebleaching followed by high (50%) substitution of the active chlorine as chlorine dioxide, as well as oxygen prebleaching and normal (85%) chlorine levels in the CD step and treatment in aerated lagoons, had the fewest effects on the fish. The results imply a complex pattern of effects where the relative prevalence of MFO-stimulating and -inhibiting substances may give different biological results in fish exposed to these kinds of effluents.