Effects of anaesthetic MS 222 on the orientation of juvenile salmon, Oncorhynchus tschawytscha Walbaum

Abstract

Juvenile salmon, Oncorhynchus tschawytscha Walbaum, were trained to orientate in a direction (270°) and then anaesthetized with ethyl m–aminobenzoate methane sulphonate (MS 222) in a test to determine whether anaesthesia affected the learnt orientation.Before anaesthesia the control group of twelve fish showed a mean unimodal orientation of 264° and a bimodal axis of orientation of 258°/78° with a confidence limit (second order analysis) of 218°←→285°. After administration of MS 222 nine out of ten fish showed marked changes in orientation and random behaviour, persisting in two fish for more than 14 days. The mean of means unimodal orientation, after anaesthesia, was 337° (random) and the bimodal axis of orientation was 337°/157° (random).Eight weeks after completion of the anaesthesia trial the fish were retested. Each fish, except one, showed a mean bimodal axis of orientation that fell within the confidence limit of the control. The mean axis of orientation for ten fish over a 3–day period was 270°/90°, (r) = 0.28.The results support the view that the reference orientation in this study (270°) was a learnt, not an innate directional preference. The effects anaesthesia may have on salmon behaviour during migration are discussed.