Interaction between bottlenose dolphins and fish farms: could there be an economic impact?

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Despite a number of studies focusing on the impact of aquaculture on marine mammals, the interaction between common bottlenose dolphins (Tursiops truncatus) with fish farms has been the subject of few investigations. In this paper we report the results of our research on the interaction between bottlenose dolphins with a fish farm on the Sardinian coast (Italy) from 1999 to 2004. In order to know the interactions between the dolphins and fish farm, to conduct behavioural studies and to collect photographic data, fish farm based observations were regularly undertaken throughout the research period. The year was divided into seasons to assess differences in frequency of occurrence and behaviour of the bottlenose dolphins. A total of 293 sightings of bottlenose dolphins interacting with the fish farm were carried out in 234 sighting hours over 218 days at sea. Bottlenose dolphins were observed year round, but there was a seasonal variation in frequency of sightings (seasonal cycles). Group sizes ranged from 1 to 17 dolphins. There was a high presence of groups of females with immatures. The nourishment coming from the fish farm increased the presence of 'wild' fishes in the surrounding area favouring bottlenose dolphin opportunistic feeding. The most direct impact of fish farming on marine mammals is its need to control predators. Bottlenose dolphin attacks on farmed fish could present a problem to the industry in terms of financial loss. Double walls of netting are the most often used deterrent, but present problems of fouling and reduced water circulation.