MARINE ECOLOGY PROGRESS SERIES Vol. 295: 265–272, 2005 Published June 23 Mar Ecol Prog Ser Residency pattern of bottlenose dolphins Tursiops spp. in Milford Sound, New Zealand, is related to boat traffic David Lusseau* Department of Zoology, University of Otago, PO Box 56, Dunedin, New Zealand Present address: School of Biological Sciences, University of Aberdeen, Lighthouse Field Station, George Street, Cromarty, Ross-shire IV11 8YJ, UK ABSTRACT: A population of bottlenose dolphins inhabits 7 of the 14 fjords that compose Fiordland, New Zealand. One of these fjords, Milford Sound, supports a large tourism industry that results in intense boat traffic. Bottlenose dolphins regularly visited Milford Sound and tour boats interacted with them during these visits. I studied the factors affecting the frequency of visits to Milford Sound by relating the residency pattern of dolphins in this fjord to oceanographic parameters and variations in boat traffic between December 1999 and February 2002. Boat traffic was the only variable that could explain the frequency of dolphin visits to Milford Sound. Dolphins spent less time in Milford Sound during seasons of intense boat traffic. Moreover, when dolphins visited this fjord, they spent more time at the entrance of the fjord when boat traffic was intense, out of the reach of tour boats. It seems that dolphins avoid Milford Sound when traffic is heavy. This avoidance could have long-term implications for the demography of the population. KEY WORDS: Bottlenose dolphin · Tursiops · Tourism impact · Area avoidance · Habitat use Resale or republication not permitted without written consent of the publisher INTRODUCTION Anthropogenic impacts on the coastal environment are increasing. Human activities can affect the distrib- The movement of many marine animals is associ- ution and movement of whales and dolphins ated with changes in water temperature and produc- (cetaceans). Gray whales Eschrichtius robustus have tivity (Quinn & Brodeur 1991). Top predators such as been displaced from areas heavily used by humans bottlenose dolphins are also affected by these (Withrow 1983, Bryant et al. 1984), as have Indo-Pacific oceanographic parameters as they can alter the dis- humpback dolphins Sousa chinensis (Jefferson 2000). tribution of their prey. For example, the distribution Humpback whales Megaptera novaeangliae might and residency of bottlenose dolphins in many loca- also have been displaced by boat traffic on summering tions have been related to temperature (Kenney grounds (Baker & Herman 1989) and from their migra- 1990, Shane 1990, Wells et al. 1990, Barco et al. tion routes (Nishiwaki & Sasao 1977). Boat traffic also 1999, Bristow & Rees 2001). Bottlenose dolphins are affected the habitat use of the cetacean community in also more often encountered close to shore during the NW Atlantic Ocean (Sorensen et al. 1984). These summer months, and often use coastal bays and long-term displacements may be related to short-term estuaries as nursery areas (Wursig & Wursig 1979, avoidance strategies (Lusseau 2004). Short-term dis- Kenney 1990, Scott et al. 1990, Berrow et al. 1996, placement is a common response of cetaceans to inter- Wilson et al. 1997, Barco et al. 1999, Bristow & Rees actions with boats (Edds & MacFarlane 1987, Bejder et 2001). al. 1999, Constantine 2001, Nowacek et al. 2001, *Email: [email protected] © Inter-Research 2005 · www.int-res.com 266 Mar Ecol Prog Ser 295: 265–272, 2005 Williams et al. 2002, Lusseau 2003a, 2004). Cetaceans and assessed whether they were avoiding this fjord move away from boats when interactions become when boat traffic was intense on a seasonal and daily intrusive or too lengthy (Bejder et al. 1999, Williams et basis. I also looked at the effects of a range of oceano- al. 2002, Lusseau 2003b). Why they do so is seldom graphic parameters on their residency pattern. The known exactly, but there are many probable causes, habitat of this population is composed of several ranging from acoustic disturbance to risk of physical estuary-type zones separated by a coastal environment injuries. (Lusseau et al. 2002). The dolphins may have a cycle of We still have little understanding of how these short- occupancy of these fjords (for example they may visit term displacements are used and selected by the all of them every week). I, therefore, also looked for animals for varying boat traffic intensity. What any temporal cycle in the residency pattern of the happens if the probability of encountering another dolphins in Milford Sound. boat is so high that a short-term displace- ment only results in another boat interac- tion? Would dolphins start to avoid the area because the boat traffic in this area is too intense? To answer these questions, I assessed the relationship between the pres- ence of bottlenose dolphins in Milford Sound, New Zealand, and boat traffic. Boat traffic is intense in Milford Sound and based on tourism (scenic cruise tour opera- tors). More than 8000 trips were under- taken in 2002. These tours operate year- round and seek to interact with dolphins when the animals visit the fjord. These interactions affect the behavioural state of these dolphins in the fjord, which tend to engage in a short-term horizontal avoid- ance strategy when interacting with boats (Lusseau 2004). Wide ranging photo-ID sur- veys (C. D. Rundgren unpubl. data) and incidental sightings show that the home range of the affected population of bot- tlenose dolphins spans over 7 fjords, from Lake McKerrow to Charles Sound (Fig. 1). Of these, only Milford Sound supports daily boat traffic. These fjords are relatively small (Fig. 1); Milford Sound is 15.7 km long and 1.6 km wide on average (Stanton & Pickard 1981). It is, therefore, easy to determine whether dolphins are present in the fjord or not on a daily basis. Four individuals out of between 45 and 55 dolphins in this population bear pro- peller scars resulting from collisions with boats (Lusseau et al. 2002). Of these 4 indi- viduals, 2 were hit during the present study in Milford Sound and one of them, a 2 wk old calf, disappeared after being hit. Boat interactions are, therefore, risky for bot- tlenose dolphins in this location. Associat- ing a risk with an area can lead to its avoid- Fig. 1. Map of the area where bottlenose dolphins visiting Milford Sound have been observed. Their home range spans from Lake McKerrow to ance (Whittaker & Knight 1998, Gibeau et Charles Sound. This home range is deduced from photo-identification of al. 2002). I examined the residency pattern dolphins in all 7 fjords (University of Otago Marine Mammal Research of the bottlenose dolphins in Milford Sound Group unpubl. data) Lusseau: Residency pattern of dolphins is related to boat traffic 267 MATERIALS AND METHODS the presence of dolphins in the fjord. The same analy- ses were conducted for the period December 1999 to Dolphin sightings. From December 1999 to Febru- February 2001 using the sighting information from my ary 2002, I conducted systematic surveys in Milford systematic surveys. In addition, using this latter Sound using a 4.8 m aluminium boat, powered with a dataset (1999 to 2001), seasonal averages were ob- 50 hp 4-stroke outboard engine. The same survey tained for each parameter and compared to a seasonal route was followed every day until a group of dolphins residency index (number of days dolphins were was encountered. The route allowed for a complete sighted in a season divided by the number of days of survey of the fjord and ensured uniform temporal and effort) using Spearman rank correlations. spatial effort. I then stayed with that group until it was A similar correlation analysis was conducted to lost or until weather conditions deteriorated. A code of detect any effect of boat traffic on the residency index conduct was established to minimise the effect of the of dolphins at a seasonal level. The northern shore of observing vessel (Schneider 1999, Lusseau 2003b). the fjord’s entrance can be considered as a ‘no boat Previous studies showed that this code of conduct was zone’ (Fig. 2) because tour boats do not visit it. I successful (Lusseau 2003b). assessed the likelihood of the dolphins using this zone In addition, tour operators provided dolphin sighting depending on the time of the day. Information was information from February 1996 to April 1999, along available on departures and lengths of scenic cruises with their sighting effort (Lusseau & Slooten 2002). It during the day and was compared to the likelihood of was, therefore, possible to determine days with and encountering dolphins inside the fjords. For each hour without dolphins in the fjord during this period. (from 06:00 to 20:00 h, New Zealand Summer Time), I Boat traffic. Tour operating companies provided calculated the amount of time dolphins spent in the information on the number of trips that they undertook ‘boat zone’ compared to the amount of time I observed between December 1999 and February 2002. Private them during that hour. In addition, I calculated the typ- boat traffic is restricted in the area because of the ical number of boats present in the fjord (calculated remoteness of the fjord. It represents a small fraction of from tour operators’ timetables) for each hour. I calcu- the daily boat traffic and occurs year-round; therefore, lated the Spearman rank correlation coefficient the information gathered on boat traffic in Milford between these 2 variables and its significance. Sound is conservative but represents a true sample of Boat traffic and oceanographic parameters. From the real boat traffic. December 1999 to February 2001, both oceanographic Oceanographic parameters. The Milford Under- parameters and boat traffic information were avail- water Observatory provided information on physical able. I, therefore, looked at the influence of each of parameters for the period June 1996 to February 2001.