Experimental evaluation of the use of vision and barbels as references for rheotaxis in green sturgeon

Rheotropism (the ability to detect and respond to a current) and rheotaxis (deliberate orientation relative to a current) are widespread in fishes and aquatic organisms, but the relative importance of different sensory modalities as references for the rheotaxis response in fishes is largely unknown. While mechanical stimuli (including water flows) have been used to evaluate rheotaxis behavior in fishes, comparison between sensory modalities is rare, and there has been little or no investigation into the mechanosensory role of barbels in rheotaxis for bottom-oriented fish. We conducted two experiments to evaluate the role of visual stimuli (in the form of an optomotor belt) and barbels in juvenile green sturgeon rheotaxis behavior. The green sturgeon did not exhibit a clear optomotor response, and spent a higher proportion of time positively oriented toward a flowing current than they did toward a moving background in the absence of flow. Removal of barbels increased the average individual tendency to orient positively in the presence of flow. While visual cues almost certainly play a role in rheotaxis behavior at large, individuals vary greatly in their degree of responsiveness to stimuli, and the optomotor stimuli used in our experiments were not as effective as the mechanosensory stimuli in provoking positive rheotaxis. Further, the barbels of green sturgeon do not appear to influence their ability to display positive rheotaxis in the presence of water current