Since the late 1980s several studies o f fish taken from habitat receiving effluents from bleached kraft pulp mills in Canada and northern Europe have shown a consistent pattern of increases in a series of liver enzymaticactivities. Theseresponses,calledbyavarietyofnames(monooxygenases,mixed-function oxidases or cytochrome P-450) have been found to respond to a number of chemical compounds includingcomponentsofbleachedkraftmilleffluent. Morerecentstudyindicatesthatcomponentsof effluents from non-bleaching mills also cause this response. The responses arise from the synthesis of increased amounts of new enzyme by the exposed animals. These enzymes are often thought of as defense mechanisms used by the fish in an effort to detoxify chemical compounds and excrete them through bile or urine. There are a number of statistical relationships between these enzyme activities and other biological processes, notably those of reproduction and immune responses. Some evidence suggests that these processes are controlled independently, however, the statistical linkages exist, and so the enzyme activities can be used to signal the need for examination of other biological processes. The enzyme response is very sensitive, and so it offers an 'early warning' to signal the need for studies at higher levels o f biological activity. However, if the enzyme activity fails to show a response, then other, more costly biological studies may not be required.
We applied two enzyme assays to the fish taken by Environmental Management Associates in the spring andfallof1992aspartoftheRepresentativeAreaProgramoftheNorthernRiverBasinsStudy. The spring samples of mountain whitefish showed a response downstream from Hinton. The reach-by-reach pattern of enzyme activities was strikingly similar to the pattern shown by chlorinated dioxin and fioran residues. Statistically, the enzyme activities were correlated with residue concentrations in the same fish (Pastershank and Muir, 1995). In comparison with other sites in western Canada, the response below Hinton was small and the activities found were all quite low. The fall collection (at spawning time) of mountain whitefish did not show a consistent pattern of induction. (Previous studies with other species have shown that enzyme activities are often low during spawning activity.) However, there were enough longnose sucker females taken in the fall to permit analysis for differences among reaches and these indicated a consistent pattern o f elevated enzyme activities at the downstream sites, the same as that found in mountain whitefish in the spring. High enzyme activities in fall longnose suckers were associated with low levels o f circulating steroid hormones (Brown et al., 1993) in the same individuals.There were too few samples o f northern pike or white suckers to support firm conclusions about site differences.