Dusky dolphin foraging habitat: overlap with aquaculture in New Zealand

Abstract

Abstract 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. 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. 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 ⩾1 h. 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. 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 18plusmn1;0.5 of 436 groups would be encountered in existing inner Admiralty Bay farms; however, no dolphin groups were first encountered in farms. 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. 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. 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. Copyright © 2004 John Wiley & Sons, Ltd.