Characterising sounds associated with Atlantic salmon (Salmo salar) surface activity in tanks and their implications for behavioural monitoring using passive acoustics

Affordable and reliable automated methods for monitoring fish behaviour in recirculating aquaculture systems (RAS) are needed. Atlantic salmon (Salmo salar) surface activities have been related to a wide range of different settings and causes, and could be indicative of both negative and positive welfare states. Salmonids are known to produce a range of sounds related to surface activity, indicating that passive acoustic monitoring (PAM) could be used to differentiate between behaviours and their potential drivers. This study investigated sounds related to Atlantic salmon surface activities in RAS tanks. Video from two tanks was manually analysed using an ethogram to classify all instances of surface activities. Audio recorded with hydrophones was then manually analysed for sounds at the times of the identified surface activities. The sounds were manually classified according to primary and secondary descriptive characteristics, and 25 acoustic features were measured for each sound. Dimensionality reduction and unsupervised clustering with UMAP and HDBSCAN was then performed and compared with manual annotations of audio and video, to discern sounds associated with different surface activities. 2928 sounds were found in 579 analysed audio segments, manually classified as “Other” (45.22%), “Pulse” (33.06%), “Surface” (20.87%) and “Frequency modulated” (0.85%). These were assigned to 9 clusters, which were mostly aligned with manual classification and helped further separating similar sounds. The results indicate that several sounds are commonly associated with the surface activities investigated in this study. Among the interesting sounds were a low frequency pulse sound, associated with fast collisions with the tank walls (which pose a welfare risk) and large jumps. The different surface activities displayed by Atlantic salmon in this study, and the associated sounds, facilitate future advancements of using passive acoustics for behavioural monitoring in RAS tanks.