Assessment of reproductive effects in largemouth bass (Micropterus salmoides) exposed to bleached/unbleached kraft mill effluents

This study evaluated the potential effects of different concentrations of b leached/unbleached kraft mill effluent (B/UKME) on several reproductive end points in adult largemouth bass (Micropterus salmoides). The kraft mill stu died produces a 50/50 mix of bleached/unbleached market pulp with an estima ted release of 36 million gal of effluent/day. Bleaching sequences were C(9 0)d(10)EopHDp and CEHD for softwood (pines) and hardwoods (mainly tupelo, g ums, magnolia, and water oaks), respectively. Bass were exposed to differen t effluent concentrations (0 [controls, exposed to well water], 10, 20, 40, or 80%) for either 28 or 56 days. At the end of each exposure period, fish were euthanized, gonads collected for histological evaluation and determin ation of gonadosomatic index (GSI), and plasma was analyzed for 17 beta -es tradiol, 11-ketotestosterone, and vitellogenin (VTG). Largemouth bass expos ed to B/UKME responded with changes at the biochemical level (decline in se x steroids in both sexes and VTG in females) that were usually translated i nto tissue/organ-level responses (declines in GSI in both sexes and in ovar ian development in females). Although most of these responses occurred afte r exposing fish to 40% B/UKME concentrations or greater, some were observed after exposures to 20% B/UKME. These threshold concentrations fall within the 60% average yearly concentration of effluent that exists in the stream near the point of discharge (Rice Creek), but are above the < 10% effluent concentration present in the St. Johns River. The chemical(s) responsible f or such changes as well as their mode(s) of action remain unknown at this t ime.