Effects of Improved Sewage Effluent Management and Urbanization on Fish Associations of Toronto Streams

Abstract

Data on fish associations and on water quality collected for a variety of purposes since the 1940s were used to test whether improvements in sewage treatment in and near Toronto, Ontario, had led to improvements in the ecological quality of the previously stressed stream reaches. The fish associations at eight locales in the Toronto area were compared upstream and downstream of water pollution control plants (WPCPs) before and after major alterations in sewage management at the plants. In some of those locales, urbanization occurred during the period in which sewage loading was being reduced. A species association tolerance index with respect to water quality (SATI-WQ) was created for application to the problem. With paired data from the sites where water quality data were collected, the SATI-WQ was negatively correlated with 5-d biochemical oxygen demand. The background variance of the SATI-WQ was estimated by applying it to five unaltered or reference locales where no notable changes had occurred in WPCP outfalls or urbanization. At six of the eight altered locales, urbanization in the vicinity of sampling sites upstream of sewage outfalls increased; at the upstream sites of each of these six locales the SATI-WQ score decreased to a significant degree, which implied that urbanization had a negative impact on the fish association. At seven of the eight locales at which land use downstream of WPCP outfalls remained relatively constant (either settled or nonsettled), the downstream scores increased to a significant degree, which implied that better wastewater treatment and management allowed sensitive species to colonize. Improved management of sewage was offset by urbanization at the eighth downstream locale, and the a priori hypothesis was indeterminate.