The Effect of Rain-Induced Reductions in Dissolved Oxygen on Fishes in Urban Systems

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 is to quantify the behavioral and physiological responses of urban fish to acute exposure of hypoxia resulting from large rain events in hopes of improving our understanding of how fish populations survive in these harsh environments.  To accomplish this, a combination of field telemetry and laboratory experiments will be employed focusing on fish from The Chicago Area Waterway System (CAWS).  The telemetry portion will consist of implanting acoustic tags into 20 largemouth bass that live in areas of the CAWS frequently experiencing rain-induced hypoxia.  The movement and activity of these tagged fish in relation to changes in dissolved oxygen will be quantified.  Results have demonstrated a reduction of largemouth bass numbers in areas that experience rain-induced hypoxia, but fish do not depart from hypoxic areas completely.  Further, largemouth bass maintain small home ranges at the periphery of hypoxic areas, suggesting a behavioral shift to avoid hypoxic zones. Laboratory trials will consist of shock experiments to determine tolerance to hypoxia for CAWS largemouth bass relative to control fish.  In addition, largemouth bass in the laboratory will be acclimated to different DO levels for extended periods and then given an oxygen shock to determine if oxygen tolerance can be induced through plastic changes to phenotypes.  The results of this study will be valuable in designing policies that are protective of fish assemblages, and also for establishing regulatory limits for oxygen.