The response of common minnows, Phoxinus phoxinus, to visual cues under flowing and static water conditions

While fixed visual cues provide reliable reference points for navigation in static water, it remains unclear whether fish energetically benefit from their presence in flowing water. Furthermore, benefits of visual feedback from fixed sources may be less for group members that can gain additional information from others. Using an open-channel flume, this study investigated how the response of fish to stationary visual stimuli is influenced by flow and group membership under two treatments: vertical black and white stripes placed on (1) both side walls of the channel or (2) one side wall only compared to a control where both walls were uniform white. The responses were compared in flowing and static water, and between individuals and groups of five. Fish exhibited a positive affiliation for the visual cues, travelling more slowly and spending more time closer to the striped walls. Fish were more edge oriented under flowing conditions, presumably utilizing the lower velocities at the wall boundary to reduce energy expenditure. When only one wall presented visual cues, individual fish spent more time associated with it in flowing water, suggesting some energetic benefit in lotic conditions. This may result from a greater ability to maintain station or control position relative to a reference point and/or the use of visual stimuli as a proxy indicator of physical structure which may provide drag-reducing refuge. A lesser association with the striped wall in static water suggests that visual cues provide other nonhydrodynamic benefits, such as physical refuge from predators or opportunities for crypsis. Conversely, less association of shoals with the striped walls may reflect a greater dependence on information provided by conspecifics or increased security associated with being part of a group. This study indicates that fixed visual cues likely provide several benefits that vary depending on flow and group membership.