Populations of mountain whitefish (Prosopium williamsoni) and longnose sucker (Catostomus catostomus) exposed to bleached‐kraft mill effluent (BKME) in the Wapiti/Smoky river system in northwestern Alberta were compared to similar populations in a reference river system (the upper North Saskatchewan River). Fish species distribution and relative abundance patterns were linked to natural events such as floods and low flows, and to habitat quality. There was no evidence that fish were avoiding the effluent plume area; relative abundance was often highest near the effluent discharge due to use of this area for overwintering and rearing. Radiotelemetry and recapture studies indicated that fish movements could be both rapid and extensive, occurring during spawning periods and at other times of the year. Thus, exposure could not be assumed to be related to capture location alone; separate exposure measures were necessary. Populations of exposed mountain whitefish and longnose sucker were growing and reproducing successfully. Higher condition factors in exposed longnose suckers and greater mesenteric fat stores in exposed individuals of both species were not accompanied by any apparent disruption in allocation of energy to reproduction or growth. The age structure of the populations showed that there had been no losses of age classes through reproductive or recruitment failures, and growth curves were indistinguishable from reference curves. There were no correlations between indicators of exposure and population‐level effects. Important natural phenomena, including a one‐hundred‐year flood and natural upstream/downstream habitat gradients, affected species distribution and were correlated with some population parameters such as condition factors and relative gonad size.

A suite of biochemical, physiological, and pathological measures was used to assess possible effects of exposure to bleached‐kraft mill effluent (BKME) on wild longnose sucker (Catostomus catostomus) and mountain whitefish (Prosopium williamsoni) in the Wapiti/Smoky river system, as compared to similar populations in a reference river system without BKME inputs. Individual fish body burden data (i.e., metals, polychlorinated dioxins and furans, fatty and resin acids, chlorophenolics) were examined for correlations between chemical exposure and biological response. General incidence of gross pathology and histopathology showed no relationship with exposure to BKME, and no neoplastic or preneoplastic lesions were observed in either exposed or reference fish. The few significant differences observed in longnose sucker blood parameters were not correlated with exposure to BKME and appeared to reflect, at least in part, habitat gradients. Liver somatic indexes were higher for female BKME‐exposed longnose sucker, but were not significantly different in male sucker nor in mountain whitefish. Some differences in circulating sex steroid levels were observed in longnose sucker exposed to BKME (but not in mountain whitefish, the species with higher contaminant body burdens). Steroid profile differences may have been related to natural differences in duration of spawning periods in the two fish populations. Other measures of reproductive capacity (relative gonad size, fecundity, young‐of‐the‐year) showed no reductions in exposed fish. The detoxification enzyme cytochrome P4501A was induced in both species, with greater induction in whitefish than in longnose sucker. Whitefish P4501A induction correlated well with some BKME exposure measures, but not with liver or gonad weights, pathology, reproductive capacity, or population‐level parameters. Increased liver size and apparent differences in sex steroid profiles in longnose sucker did not translate to other health effects or population‐level effects. Thus, exposure to this biologically treated BKME produced one consistent biochemical marker of exposure (P4501A) in the two fish species that was not associated with any discernible adverse effects on individual fish health.

A study was initiated in the spring of 2001 to determine if hormonally active chemicals are bioavailable to white sucker (Catostomus commersoni) residing in the Wabigoon River, downstream of the bleached kraft pulp mill and municipal sewage outfall located in Dryden, Ontario. Females collected during spawning migrations in May 2001 at a near-field exposure site exhibited increased condition factor, liver size and fecundity relative to those collected from a reference site. Liver samples within each site were pooled, soxhlet extracted and fractionated by gel permeation chromatography and HPLC according to octanol/water partition coefficient (Kow). Fractions were then tested to determine if fish accumulated ligands which bind to the goldfish testicular androgen receptor (AR) and sex steroid binding protein (SSBP). The levels of hormonally active substances were low in males whereas females had accumulated significant amounts of several chemicals capable of binding to the hormone receptors. The levels of ligands detected in pulp mill effluent samples were substantially reduced after secondary treatment. Fractionation of final effluent extracts showed only ligands for the AR in the hydrophobic region (log Kow >5), which corresponded to the activities detected in fish tissues from both sexes. It can be concluded that ligands for fish sex steroid receptors are present in final effluent after treatment at the pulp mill and chemicals with the same Kow are bioavailable to fish under spring freshet conditions. Further work at this site is necessary to determine the contribution of pulp mill and municipal sewage effluents to the physiological performance of fish in the Wabigoon River and to their burden of accumulated compounds.