Interactions between harbour porpoises and aquaculture on the west coast of Scotland
The nature and extent of interactions between cetaceans and aquaculture are largely unexplored. Individuals may face a trade-off between potential benefits, such as foraging opportunities, and the possibility of negative consequences from exposure to industrial activities. In Scotland, the expanding Atlantic salmon (Salmo salar) farming sector is mainly located on the west coast and among the northern and western isles, which also host some of the highest densities of harbour porpoise (Phocoena phocoena) in Europe. The region’s importance for the species is recognised by the Inner Hebrides and the Minches Special Area of Conservation (SAC). The present study conducted long-term Passive Acoustic Monitoring (PAM) at eight salmon farms within the wider Firth of Lorn and Sound of Mull, to determine whether porpoises interact with farms on Scotland’s west coast. Results indicated that although porpoise presence around farms was site-specific, detection probabilities most strongly correlated with sun altitude, diel hour and water temperature. Simultaneously, the salmon farm soundscape was explored to identify and characterise the sector’s noise emissions. Analyses revealed the distinctive acoustic characteristics of various operations, including air-driven fish feeder systems, electricity generators, acoustic deterrent devices (ADDs), and net cleaning activities. These added additional background noise which negatively affected click logger monitoring effort, and subsequent bespoke configurations were successfully optimised so these widely used tools could be applied to this specific application. Baited remote underwater vehicles (BRUVs) and imaging sonar were shown to effectively survey pelagic and demersal wild fish, and the most frequently observed species was juvenile whiting (Merlangius merlangus). Finally, hydrophone arrays were deployed and found to be successful in monitoring fine-scale movement of echolocating harbour porpoises amongst farm infrastructure. The present study affirms that interactions between salmon aquaculture and porpoises commonly occur on the west coast of Scotland, and offers a broad examination of the sector’s relevance to this vulnerable cetacean species in an internationally important conservation region.
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