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Intersex in fish downstream of municipal wastewater treatment plants (MWWTPs) is a global concern. Consistent high rates of intersex in male rainbow darter (Etheostoma caeruleum) have been reported for several years in the Grand River, in southern Ontario, Canada, in close proximity to two MWWTPs. The larger MWWTP (Kitchener) recently underwent upgrades that included the conversion from a carbonaceous activated sludge to nitrifying activated sludge treatment process. This created a unique opportunity to assess whether upgrades designed to improve effluent quality could also remediate the intersex previously observed in wild fish. Multiple years (2007-2012) of intersex data on male rainbow darter collected before the upgrades at sites associated with the MWWTP outfall were compared with intersex data collected in postupgrade years (2013-2015). These upgrades resulted in a reduction from 70 to 100% intersex incidence (preupgrade) to <10% in postupgrade years. Although the cause of intersex remains unknown, indicators of effluent quality including nutrients, pharmaceuticals, and estrogenicity improved in the effluent after the upgrades. This study demonstrated that investment in MWWTP upgrades improved effluent quality and was associated with an immediate change in biological responses in the receiving environment. This is an important finding considering the tremendous cost of wastewater infrastructure.

Small-bodied fish species are commonly used for the assessment of environmental effects because they are short lived, abundant, and they mature early. Although they are generally considered to be less mobile than larger bodied species, relatively little is known about their movement patterns. In this study, we tagged 3,001 rainbow darters (Etheostoma caeruleum) (≤76 mm) in the upper Grand River of southern Ontario with visible implant alpha tags and elastomers in 3 riffles. Five hundred sixty-five fish were recaptured over 4 recapture events (including spawning and nonspawning periods) over a spatial extent of 1900 m. The rainbow darter demonstrated high site fidelity having a median movement of 5 m and with 85% staying within the riffle in which they were originally tagged. Most movements occurred during the spawning period, where males moved at a greater frequency and had a tendency to move longer distances (up to 975 m). There was also a bias in the direction of movement, which was dependent on the recapture season. Overall, the high site fidelity of the rainbow darter makes it a candidate, sentinel species for the assessment of environmental effects.

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