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Effective methods of producing instantaneous and irreversible unconsciousness at the time of slaughter are crucial for ensuring animal welfare in commercial aquaculture. However, the traditional method of visually evaluating unconsciousness has been shown to be insufficient and may lead to misjudgements of stunning efficiency. In this study, we developed a non‐invasive technique that measures brain activity in fish as an alternative to traditional invasive, brain implants and used it to determine when a change in consciousness occurs in trout during anaesthesia induction. Nine rainbow trout (Oncorhynchus mykiss) were equipped with a custom designed silicone cup fitted with electrodes and submerged in 10°C water with dissolved MS‐222. During anaesthesia, the state of consciousness was assessed by recordings of electroencephalogram (EEG). The EEG recordings were analysed both by visually evoked responses from light stimulation (VERs) and from changes in signal amplitude, median frequency and relative signal power. According to the loss of VERs and decrease in signal amplitude, trout transitioned to surgical depth of anaesthesia within 5 min. Our results show that consciousness, or loss of, can be determined using a non‐invasive system to record EEG in fish.

Bio-logging devices can provide unique insights on the life of freely moving animals. However, implanting these devices often requires invasive surgery that causes stress and physiological sideeffects. While certain medications in connection to surgeries have therapeutic capacity, others may have aversive effects. Here, we hypothesized that the commonly prescribed prophylactic treatment with enrofloxacin would increase the physiological recovery rate and reduce the presence of systemic inflammation following the intraperitoneal implantation of a heart rate bio-logger in rainbow trout (Oncorhynchus mykiss). To assess post-surgical recovery, heart rate was recorded for 21 days in trout with or without enrofloxacin treatment. Contrary to our hypothesis, treated trout exhibited a prolonged recovery time and elevated resting heart rates during the first week of post-surgical recovery compared to untreated trout. In addition, an upregulated mRNA expression of TNFα in treated trout indicate a possible inflammatory response 21 days post-surgery. Interestingly, the experience level of the surgeon was observed to have a long-lasting impact on heart rate. In conclusion, our study showed no favorable effects of enrofloxacin treatment. Our findings highlight the importance of adequate postsurgical recovery times and surgical training with regards to improving the welfare of experimental animals and reliability of research outcomes. Across a wide range of disciplines, approximately 11.5 million experimental animals were used for research purposes within the member states of the European Union in 2011 1. While the vast majority of these animals are rodents (~80%), the proportion of ectotherms, including teleost fishes, is rapidly increasing 1. One expanding area within experimental fish research concerns the use of novel implantable electronic tags (e.g. bio-logging and bio-telemetric devices). The recent technological developments and use of bio-logging and bio-telemetric devices in aquatic organisms has been proposed to open up a 'panoramic window into the underwater world' 2. The use of these devices in freely swimming fish allow the continuous collection of high-resolution physiological and behavioural data (e.g. heart rate, blood flow and muscle activity) over long periods of time 3-9. Moreover, data from implanted fish swimming amongst conspecifics (i.e. focal animals) can provide important insights into relationships between physiological and behavioral traits across different social contexts in both natural and aquaculture settings 3,10. However, there are still challenges associated with the use of these implants in order to produce reliable high-quality data, as well as to safeguard the health and welfare of the experimental animal in accordance with the 3 R guidelines. First of all, introducing a foreign body into an animal may lead to expulsion or encapsulation of the implant, and secondly, the protective barrier of the epithelium is breached during surgery where after the wound can act as an entry-point for pathoge...

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