ABSTRACT1. Marine farms have the potential to affect dolphin foraging in the coastal environment, yet this issue has been largely omitted from aquaculture management models. Data on the subject are minimal. This study was conducted to examine potential overlap between dusky dolphin habitat use and New Zealand's growing green-lipped mussel farming industry.2. Data on dusky dolphin occurrence, distribution, abundance, and behaviour were collected from small vessels over five successive winters in the Marlborough Sounds, the centre of New Zealand's mussel farming industry.3. Locations and movements of dolphin groups were recorded at 2 min intervals with a global positioning system receiver to examine the overlap of dusky dolphin use of coastal areas with existing and proposed marine farms. All cases of dolphins entering the boundaries of mussel farms and total time spent in farms were recorded. Over 8500 dolphin dorsal fin photographs were analysed to develop a catalogue of 421 marked individuals utilizing the area. All instances of dolphin feeding were noted, and focal group behaviour was recorded at 2 min intervals for groups observed 51 h.4. Within the Marlborough Sounds, dusky dolphins were most often encountered during the winter in Admiralty Bay, the area with the greatest density of proposed farming activity in the region. Mark-recapture data indicate that more than 1000 dusky dolphins used Admiralty Bay over the course of the 5 year study, with an average of 220 individuals inhabiting the bay on any given week during the winters of 1998-2002. As many as 55% of individuals returned to Admiralty Bay in consecutive winters.5. Overlap of dusky dolphin habitat use with proposed marine farms is high, and dolphins rarely used areas within the existing farms. If dusky dolphin distribution with respect to farms were random, an expected 18AE0.5 of 436 groups would be encountered in existing Inner Admiralty Bay farms; however, no dolphin groups were first encountered in farms.6. In 5 years, only eight of 621 dusky dolphin groups monitored in Admiralty Bay were observed to enter the boundaries of mussel farms at any point. Dolphins entering mussel farms moved rapidly up the lanes between rows of lines and floats. Dolphins were observed a total of 14.2 min inside farms versus 147.5 h outside of farms in Admiralty Bay. Correcting for area, dolphins were observed spending significantly less time per survey inside than outside of farms. *Correspondence to: Tim M. Markowitz, Marine Mammal Research Program, 4700 Avenue U, Building 303, Galveston, TX 77551, USA. E-mail: tim markowitz@yahoo.com 7. Most dusky dolphin groups in Admiralty Bay were observed feeding on small schooling fish, often associated with seabirds and/or fur seals. Movement and diving patterns indicate much foraging when not actively feeding.8. Regular seasonal migration of dusky dolphins and frequent feeding associations with other apex predators make management of marine farming a wider socio-economic and ecological issue.

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Little is known of foraging habits of sound-scattering layer consumers. A 200-kHz echosounder was used to survey dusky dolphins and the sound-scattering layer in winter 2002, in Kaikoura Canyon, New Zealand. Visual observations of dolphin surfacings occurred 84% of the time that dolphins were acoustically detected, confirming identifications from the acoustic data. Dusky dolphins were within the layer at 2000 h (about 1.5 h after dusk), within 125 m of the surface. As the layer rose to within 30 m of the surface at 0100 h, the observed depth of dolphins decreased presumably as the dolphins followed the vertical migration of their prey. The mean depth of dolphins was within the scattering layer except when the top of the layer was deeper than 125 m. Dusky dolphins often forage within large groups. Acoustically identified subgroups of coordinated animals ranged from 1 to 5 dolphins. Subgroup size varied with time of night, minimum depth of the scattering layer, and the variance of the food resource. The largest subgroups occurred when the scattering layer was closest to the surface, and when the layer was most heterogeneous. Time, depth of layer, and layer variance contributed significantly to predicting foraging dusky dolphin subgroup size.