Orientation of rainbow trout (Oncorhynchus mykiss) to multiple patches of linearly polarized light

Orientation responses of juvenile rainbow trout (Oncorhynchus mykiss) to tw o linearly polarized light patches were examined under controlled laborator y conditions. Fish were trained to swim the length of the training tank und er a polarized light field created by two linearly polarized stimuli that w ere oriented either parallel or perpendicular to the length of the tank. Tr ained fish were released in a circular tank and their angular responses wer e recorded. For each testing paradigm, the E-vector (electric vector) orien tation of one of the two linearly polarized light patches was varied by 15 degrees between 0 degrees and 90 degrees. Each fish was therefore tested in seven different paradigms in which the two E-vector orientations differed by 0 degrees, 15 degrees, 30 degrees, 45 degrees, 60 degrees, 75 degrees, a nd 90 degrees. Rainbow trout oriented in a bimodal distribution when the tw o E-vector orientations differed by 0 degrees, 15 degrees, 30 degrees, 45 d egrees, and 90 degrees. These results suggest that rainbow trout perceived the two stimuli as being the same when the two E-vector orientations differ ed by 45 degrees or less. Conversely, rainbow trout did not significantly o rient when the two E-vector orientations differed by 60 degrees and 75 degr ees. Rainbow trout may be able to discriminate two E-vector orientations th at differ between 60 degrees and 75 degrees, and therefore they do not sign ificantly orient, since they perceive two distinct E-vectors to orient to i nstead of one. When rainbow trout were exposed to a depolarized light field , they did not exhibit significant orientation subsequent to the E-vector c ue.