Reproductive and histopathological effects in wild fish inhabiting an effluent-dominated stream, Wascana Creek, SK, Canada

► Wild fish collected downstream of sewage effluent discharge in Wascana Creek, SK, demonstrated reproductive impairment. ► Exposed fish demonstrated altered gonadal development, delayed spawning, alter sex steroid production and induction of vitellogenin. ► Exposed fish express reproductive impairment and damage to the kidneys and gills. During the winter low flow periods, Wascana Creek, Saskatchewan, Canada, can be 100% treated municipal wastewater downstream of the City of Regina's Wastewater Treatment Plant. The objective of this study was to determine if exposure to municipal effluent affects the health and reproductive development of fish in an effluent-dominated stream. Field studies were conducted on post-spawning (August 2006), spawning (June 2007), recrudescent (October 2007) and pre-spawning (May 2008) sentinel fish [Fathead Minnow Pimephales promelas and Brook Stickleback Culaea inconstans] to assess responses in terms of growth (condition factor), reproduction (in vitro sex steroid biosynthetic capacity, and gonadosomatic indices, histology) and survival associated with the effluent outfall. Sentinel species demonstrated varying responses depending on the season of field collections. While Stickleback collected downstream of the sewage discharge were often longer, heavier and had greater condition, Fatheads from the same site were shorter and lighter. Exposed fish of both species exhibited delayed spawning and altered gonadal development depending on the season. Exposed male Fathead Minnows also had significantly lower scores of secondary sexual characteristics (fewer nuptial tubercles, little or no development of the dorsal pad, and the lack of presence of a dorsal fin dot). Histopathology of exposed Fathead Minnows revealed thickening of the gill lamellae and alterations in structure of the kidneys (inflammation of the proximal tubules and Bowman's capsule). It is not known if the effluents are affecting natural reproduction and recruitment into this population or if these impacted populations rely on immigration from upstream reaches to sustain the populations. Climate change and human population growth will further challenge this effluent-dominated stream's ability to assimilate nutrients and contaminants which may further impair the performance of fish in this arid environment.