The Effect of Rain Induced Reductions in Dissolved Oxygen on Fishes in an Urban System

During large rain events, waters in urbanized areas often accumulate oxygen-depleting substances through combined sewer overflow discharges and urban runoff, as well as from the re-suspension of existing substrates.  Reductions in dissolved oxygen are believed to negatively affect biota in the receiving waters.  Despite this, many urban areas have surprisingly rich and diverse fish species assemblages.  The objective of this study was to quantify the behavioral and physiological responses of urban fish to acute exposure of hypoxia resulting from large rain events to improving our understanding of how fish populations may survive in degraded environments.  To accomplish this, a combination of field telemetry and laboratory experiments was employed focusing on fish from The Chicago Area Waterway System (CAWS).  The telemetry portion consisted of implanting acoustic tags into 20 largemouth bass that live in areas of the CAWS frequently experiencing rain-induced hypoxia and quantify the movement and activity of the fish in relation to changes in dissolved oxygen.  Results did not show a clear pattern of fish leaving these areas, suggesting preferred habitat selection motivates fish to remain. Laboratory trials consisted of shock experiments to determine tolerance to hypoxia for CAWS largemouth bass relative to control fish.  Largemouth bass from the study site had a similar physiological response compared to control site populations, indicating a lack of an improved tolerance to hypoxia.  Our results are valuable in designing policies that are protective of fish assemblages, and also for establishing regulatory limits for oxygen.