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Endocrine modulating responses from non-persistent substances have not been as well documented as those from the persistent organic pollutants. Because of their lack of persistence, these substances present a special case with respect to endocrine modulation as consideration of the probability of exposure and sensitivity becomes more important. Endocrine modulation has recently been observed in fish exposed to pulp mills effluents. Until installation of treatment facilities, effects on fish in pulp mill receiving waters were acute and lethal. Large concentrations of resin acids, highly chlorinated phenols, and biological oxygen demand acted in concert to reduce fish populations to small numbers in receiving waters with
small to moderate dilutions (Owens 1991). With appropriate treatment of effluents, toxicity has been reduced to below the threshold of acute and chronic lethality. Under these conditions, fish have returned to pulp mill effluent receiving waters but now show a syndrome of symptoms related to effects on reproduction and induction of metabolic enzymes such as mixed function oxidases (MFOs) in the liver. The MFO-related responses observed in these fish include increases in MFO activity and increased liver (Munkittrick et al. 1992b, Munkittrick et al. 1994, Robinson et al. 1994). Responses associated with reproductive effects include delayed age to maturity, decreased gonad size, decreased secondary sexual characteristics, smaller plasma steroid concentrations and reduced rates of steroid synthesis in the gonad (Munkittrick et al. 1992a, Van der Kraak et al. 1992). These responses have been observed at mills regardless of the use of primary or secondary treatment of effluents or chlorine bleaching of the pulp (Munkittrick et al. 1994, Robinson et al. 1994), suggesting that these responses are caused by naturally produced organic compounds released from the wood during pulping and not from chlorinated substances that may be formed during bleaching. That endocrine modulating substances from natural sources can produce reproductive responses in fish is not surprising as they have been reported to cause effects in other organisms (Leopold et al. 1976). However, these natural substances are not thought of as being persistent in the same sense as the organochlorine pesticides, such as DDT. Nevertheless, in the case of these natural substances, prolonged exposures do result from continuous release into the receiving environment. Even though these substances may be easily degraded and are non-bioaccumulative, their continuous presence in the environment can result in essentially the same exposure scenario as for persistent bioaccumulating substances as discussed above (Figure 3-27). Thus, regardless of the cyclical nature of sensitivity, continuous presence of the substance ensures that responses will be observed. The exact identity of the substances in pulp mill effluent responsible for the reproductive responses or MFO induction is not known, however, retene type polyaromatic and chlorinated stilbenes have been implicated as possible MFO inducers (Parrott et al. 1995, Burnison et al. 1996) and phytosterols, such as β-sitosterol have size