Behavioral and Physiological Responses of Largemouth Bass to Rain-Induced Reductions in Dissolved Oxygen in an Urban System

Waters in urban areas often experience hypoxic events due to combined sewer overflows, which have the potential to negatively affect aquatic biota. Despite these hypoxic events, many urban areas have diverse fish assemblages, suggesting hypoxia has a minimal impact. Data to quantify the impacts of aquatic hypoxia in urban systems are currently lacking. The current study sought to define how rain-induced hypoxia affected the movement, distribution, and physiology of individual Largemouth Bass Micropterus salmoides residing in the Chicago Area Waterway System (CAWS), an urban area prone to episodes of hypoxia. Following the onset of hypoxic events, the likelihood of Largemouth Bass remaining in hypoxic water was reduced, but fish did not completely avoid hypoxic areas. This suggests that hypoxia exerts only a moderate influence on the movement of Largemouth Bass. Field sampling showed that Largemouth Bass from the site prone to hypoxia were not in poor nutritional condition and were not suffering from chronic stress, relative to compared with those from reference sites. Field sampling also showed that fish from the CAWS displayed an improved capability to transport oxygen in the blood compared with individuals from control sites. Following a low-oxygen challenge in the laboratory, fish from the CAWS also displayed elevated levels of oxygen transport capabilities compared with fish from some control sites. Together, results suggest that hypoxic events have limited behavioral consequences for Largemouth Bass, and in fact, Largemouth Bass in our study may have developed an improved ability to tolerate hypoxia, which would allow them to persist in hypoxia-prone areas.