Bottlenose Dolphin (Tursiops truncatus) Predation on a Marine Fin Fish Farm: Some Underwater Observations Bruno Díaz López Bottlenose Dolphin Research Institute (BDRI), V. Díaz 4, Golfo Aranci 07020 Sassari, Italy Abstract This paper reports on the results of underwa- ter observations of bottlenose dolphin feeding behaviour in a marine fin fish farm on the Sardinian coast in Italy from 2000 to 2005. During the study period, 178 underwater encounters were noted during 79 sightings of bottlenose dolphins at a fish farm. Total time spent underwater in the presence of dolphins was 284 min, with a mean encoun- ter duration of 1.6 ± 1.3 min. Bottlenose dolphins were primarily observed hunting both schooling and solitary prey around the fish farm cages, using seven cooperative and individual feeding strate- gies throughout the water column. The underwater observations suggest that the use of different feed- ing strategies is consistent with the hypothesis that bottlenose dolphins apply common decision rules in relation to prey availability, resulting in the use of different foraging techniques. The observed frequency of the feeding strategies employed by dolphins preying directly on farmed fish could be worrisome for aquaculture. Key Words: Tursiops truncatus, bottlenose dol- phin, aquaculture, behaviour, predation, marine fin fish farm, Mediterranean Sea Introduction Marine aquaculture and, in particular, intensive fish farming have shown a large expansion in most Mediterranean countries over the last ten years (UNEP/MAP, 2004). Coastal sea-cage fish farms have been introduced into an environment that has a natural complement of fish-eating predators. Würsig & Gailey (2002) concluded that interac- tions between the food being raised by humans in coastal aquaculture and the predators that attempt to take advantage of it should be studied. Most of the literature to date has focused on how aqua- culture influences dolphin distribution (Watson- Capps & Mann, 2004; Díaz López et al., 2005), but there is a lack of information on how dolphins interact with marine fin fish farms. In this paper, underwater observations of bottle- nose dolphin (Tursiops truncatus) feeding behaviour near a marine fin fish farm on the Sardinian coast in Italy from 2000 to 2005 are reported. This study focuses on the Gulf of Aranci where the presence of a floating marine fin fish farm, which raises sea bass (Dicentrarchus labrax), gilthead seabream (Sparus auratus), and corb (Sciaena umbra), has been linked to a change in bottlenose dolphin distribution as a result of high fish density around the floating cages in the farming area (Díaz López et al., 2005). Fish species such as common grey mullet (Mugil cepha- lus), salema (Salpa sarpa), and pilchard (Sardina pilchardus) are known to be attracted to floating fish farms; unused feed that falls through the cages may enhance the attraction (Dempster et al., 2004). Even though these data are only from one study site, it is possible to extrapolate to other areas where bottlenose dolphins have been observed interacting with fin fish farms. This study site pro- vides a unique opportunity for close-up and regu- lar observations of underwater bottlenose dolphin behaviour. Materials and Methods The Marine Fin Fish Farm Data were collected at a fin fish farm located in the Gulf of Aranci (40º 59' N, 9º 37' E) in the northeastern coast of Sardinia (Figure 1). This coastal sea-cage fish farm was set up in 1995 and consists of 21 floating cages. The floating cages were grouped into three rows of seven cages. Each floating cage was constructed of nylon mesh net- ting and was 22 m in diameter and 15 m deep. The cages were situated at approximately 200 m from the shore, with a minimum depth of 18 m and a maximum depth of 26 m. The fish farm covered 2.4 ha and contained 800 to 900 tons of ichthyic biomass, sea bass, sea bream, and corb. The water temperature underwent yearly variation, with sur- face temperatures ranging between 11º C (March) to 26º C (August). Water clarity, measured by Secchi disk, varied between 11 m (January) and 22 m (July). The sea bottom in the fish farm area Aquatic Mammals 2006, 32(3), 305-310, DOI 10.1578/AM.32.3.2006.305