Investigating fear in domestic rainbow trout, Oncorhynchus mykiss, using an avoidance learning task

The capacity of rainbow trout to experience fear was assessed using an avoidance learning task. Each of 13 fish was placed individually into a two-chambered shuttle tank where it could be subjected to the putative frightening stimulus of a plunging dip net in either chamber. The fish could escape from the stimulus by swimming through a doorway to the other chamber. The fish escaped from the plunging net by swimming through the doorway, some on the first occasion and all after a few exposures. Each fish was then presented with a neutral stimulus of a light that went on 10 s before the net plunged into the water. Over a 5-day period, all fish learned to avoid the plunging net by swimming through the doorway when the light was illuminated. All fish showed evidence of longer-term memory by showing the learned avoidance response on the first occasion they were tested after 7 days of no testing. Whereas the escape responses to the plunging net were immediate and reflexive-like, the avoidance responses to the light going on were delayed a few seconds and more deliberate in nature. This evidence suggests that trout can experience fear and that they can learn to avoid frightening stimuli. It implies that they are sentient animals, more complex than previously thought. © 2004 Elsevier B.V. All rights reserved.