In the early 1990s, a variety of responses were reported in fish populations living downstream of bleached kraft pulp mills. This included delayed sexual maturity, smaller gonads, changes in fecundity, and a depression in secondary sexual characteristics. The changes in fish reproductive development were also associated with alterations in the endocrine mechanisms controlling the production of the sex steroid hormones. The early studies were conducted predominantly at large bleached kraft pulp mills with primitive pulping processes and little effluent treatment. Subsequent studies documented several of the changes at some sites with effluent treatment and without chlorine use, and the dominant reproductive changes (delayed maturity, reduced egg production, changes in secondary sexual characteristics) were replicated in laboratory fish exposed to secondary-treated effluent. Studies have continued to follow the responses of wild fish as these mills underwent process modifications and installed secondary treatment. Improvements in the performance of wild fish have been recorded at five North American sites after the mills modernized their waste treatment and pulping processes. However, the improvements are not complete and are not universal and the process changes resulting in the improvement in environmental conditions have not been defined and their identification remains a priority. Reductions in gonad size and depressions of sex hormones are still seen at one mill with 100% chlorine dioxide substitution and secondary treatment.

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