Acquisition of polarized-light orientation in salmonids under laboratory conditions

We examined orientation responses of juvenile salmonids to a polarized-light stimulus under laboratory conditions. juvenile rainbow trout, Oncorhynchus mykiss, steelhead (anadromous), O. mykiss, and brook char, Salvelinus fontinalis, were trained using an operant conditioning methodology to orient relative to the axis of a linear-polarized light field. On average, rainbow trout responded correctly relative to the orientation of the light stimulus approximately 70% of the time within five training sessions. The proportion of correct responses increased further under an intermittent reinforcement schedule. We released trained and untrained fish individually in a circular tank and quantified orientation responses using a digital image-tracking system. Experimentally naive rainbow trout had no directional tendency, in contrast to trained rainbow trout, steelhead and brook char, which oriented relative to the plane of polarized light. Trained fish showed no orientation response when a diffuser was used to depolarize the light source. Rainbow trout trained to orient parallel to a polarization axis in the laboratory were tested under natural skylight before sunset. These fish oriented parallel to the bearing of maximally polarized light in the celestial hemisphere. (C) 2003 Published by Elsevier Science Ltd on behalf of The Association for the Study of Animal Behaviour.