ABSTRACT
Fish populations of walleye and white sucker in the vicinity of a kraft pulp mill on the North Saskatchewan River, Saskatchewan, were compared to fish from a pristine reference river, the Cowan River in northwest Saskatchewan, on behalf of Weyerhaeuser Canada Ltd. in 1991 and 1992. The target sample size for the two sentinel species was 20 per sex per sample area. There were no indications of significantly slower or faster growing walleye in the North Saskatchewan River. Exposure indicators such as EROD activity and organochlorine body burdens in North Saskatchewan walleye were in the range found in the reference river. Livers were not enlarged. North Saskatchewan River white suckers were younger on average and matured earlier than Cowan River white suckers. Condition factors were nearly identical between the two rivers. The rate of growth between sites was equal or only marginally higher in the North Saskatchewan River. The main differences were a higher mean EROD activity (although the ranges were similar in both rivers), younger fish, smaller GSIs (because the fish were younger and some partially spent) and a higher incidence of pale livers and parasites in the North Saskatchewan. There were no detectable levels of organochlorine compounds in white suckers from the North Saskatchewan River. There were no correlations between indicators of exposure to bleached kraft mill effluent and population-level parameters in either species. A greater range of age classes observed in the walleye population of the North Saskatchewan River may have been a consequence of a difference in exploitation rates in the two rivers. Very low river flows affected the success of the main sampling year (1992) and thus the ability to interpret data on reproductive potential of walleye. Comparison of 1991 and 1992 data was difficult because of differences in spawning condition (pre-spawning versus spawning) in the two years. The cause of younger, faster maturing white suckers in the North Saskatchewan is not apparent, since the overall pattern observed is not “typical” of population responses observed in other studies. It appears that the usefulness of whole organism and population parameters as early indicators of stress from specific sources such as pulp mills depends on a strong understanding of the effects of other factors (e.g., exploitation and river flows). In these large rivers, detecting incremental population-level effects from one particular stressor in the absence of a strong contaminant gradient seems unlikely.
