Responses of Coastal Largemouth Bass to Episodic Hypoxia

Abstract

AbstractThe river systems inhabited by coastal populations of Largemouth Bass Micropterus salmoides in North Carolina and along the Atlantic and Gulf coast regions exhibit episodic (i.e., several times per year) fluctuations in environmental conditions (e.g., dissolved oxygen [DO]). Laboratory studies have documented the effects of low DO (hypoxia) on Largemouth Bass, yet few field studies have examined these effects in open systems. The objective of this study was to determine the effects of episodic hypoxia on Largemouth Bass distribution, survival, and feeding success in an open coastal system. We collected 45 Largemouth Bass from four tributaries of the Chowan River and tagged them with acoustic transmitters. Fish movements were monitored using active tracking and passive receivers, and these data were compared with DO levels recorded in the tributaries and main‐stem Chowan River. We found that tagged Largemouth Bass exhibited avoidance behavior at DO concentrations below 1.8 mg/L, with some seeking higher DO in the main stem or near tributary mouths during hypoxic events in the tributaries. The natural mortality rate of Largemouth Bass was low compared with rates reported in other studies, indicating that Largemouth Bass in coastal systems are able to survive hypoxic events. Analysis of stomach contents collected during hypoxic and non‐hypoxic periods indicated that Largemouth Bass had less food in their stomachs under hypoxic conditions; however, the CPUE (fish/h of pedal time) of potential prey fishes was not lower during hypoxic periods relative to non‐hypoxic periods, and thus a change in foraging opportunities did not appear to drive Largemouth Bass movement.