Contrasting the effects of immobilisation and anaesthesia on the stress physiology and behaviour of juvenile lake sturgeon (Acipenser fulvescens).

Anaesthesia and/or immobilisation are typically required to facilitate experimental procedures on fishes such as the surgical implantation of electronic tags. Yet, optimal anaesthetic or immobilisation methods have primarily been developed and tested in teleosts, with relevant information on efficacy and suitability of these methods lacking for basal ray-finned fishes such as imperilled sturgeons (Acipenseridae). Here, we investigated the behavioural and physiological responses of hatchery-origin juvenile lake sturgeon (Acipenser fulvescens) to four immobilisation or anaesthetic methods. We conducted an experiment wherein fish were held in a fashion designed to induce tonic immobility, exposed to electroanaesthesia with a TENS unit, anaesthetised with MS-222, or anaesthetised with clove oil, and then subjected to a simulated intracoelomic tagging surgery. Following reflex indicator scoring, fish were "released" into a circular tank arena for behaviour trials and blood sampling at 0.5, 2, and 4 h post-release. Both tonic immobility and electroanaesthesia were ineffective at immobilising fish, and surgery under these methods led to relatively little behavioural impairment but the highest plasma cortisol peaks. Fish anaesthetised with MS-222 or clove oil had greater post-release behavioural impairment but lower cortisol levels. We discuss the implications of our findings for the practical application of these methods, and the challenges with identifying the most suitable method for immobilising or anaesthetising sturgeons in laboratory and field settings.