A Winter Survey of Fishes in the Vicinity of the WLSSD Sewage Treatment Plant: November 1991-April 1992

Abstract

The Western Lake Superior Sanitation District’s (WLSSD) waste treatment facility on the shore of the St. Louis River estuary contributes a large volume of warm effluent to the St. Louis River on a continuous basis. The effluent mixes with the river in a relatively shallow embayment adjacent to the treatment facility and forms an area approximately 0.5 kilometer in diameter in which water temperatures and conductivities may be considerably different than surrounding river waters. This mixing zone can also have chemical characteristics that contrast significantly with unmixed river water. During winter conditions, elevated temperatures within the mixing zone may provide a potential refuge for fish species that are unable to exist in the cold waters of the river, and, some native species may seek warmer temperatures during winter when they are available. Alternatively, fish species may avoid the elevated temperatures or water chemistry characteristics of the mixing zone. Either of these situations could result in altered fish community characteristics during winter months. The purpose of this study was to provide preliminary data on the types and relative abundances of fish that inhabit the mixing zone during several winter months. Surveys were only conducted within 0.3 km of the outfall pipe; results were compared to other winter fish abundance data from the St. Louis River estuary.