The behavioural ecology of the bottlenose dolphins (Tursiops truncatus) of northeastern New Zealand: a population exposed to tourism

Abstract

Bottlenose dolphins (Tursiops truncatus) are frequently brought into contact with humans, due to their coastal habitat use. Over the last decade there has been considerable growth in the commercial dolphin-based tourism industry. This growth exposes coastal populations to boats and swimmers. New Zealand has allowed commercial operations offering swimming with wild dolphins since 1989. Little is known about the effects of boats and swimmers on dolphin behaviour. The present study examines a population of bottlenose dolphins that utilise the Bay of Islands as part of their range along the northeastern coast of New Zealand, and their behavioural response to commercial swim-with-dolphin operations. Data were collected from an independent research boat and the commercial dolphinwatching boats from December 1996 to February 2000. Photo-identification showed a closed population of 446 (95% C.I. 418-487) dolphins, with an 81% resight rate of the 378 catalogued individuals. Group size ranged from 2 – 50 dolphins, with 80% of groups (n = 160) containing 2 - 20 dolphins. The number of groups with calves present increased from 32% (n = 17) in 1997, to 63% (n = 47) in 1999. This could be explained by the increased number of neonates born; from four in 1997, to nine in 1999. Photo-identification in areas outside the Bay of Islands showed that individuals ranged minimum distances of 82 km north, and 388 km south of the Bay of Islands. All photographs taken outside the Bay of Islands were matched to known animals, suggesting that the majority of the northeastern coastal population is seen in the Bay. With the next nearest population of bottlenose dolphins approximately 1,000 km south, these dolphins probably form an isolated population ranging along the east coast of the northeastern North Island. Habitat use by the dolphins was examined using Geographic Positioning System (GPS) points simultaneously linked to behavioural states. In total, 31 groups were followed, and 847 GPS points were recorded. Position of initial encounter was recorded for 138 groups of dolphins. There was a significant correlation between water depth and sea-surface temperature, with dolphins found in deeper waters in summer when water temperature was highest, and in shallow waters in winter when the water temperature was lowest. Only one behavioural state, slow travel, was significantly different by season. Although all behaviours were observed in all zones, dolphins had preferential habitat use throughout the four zones within the Bay. To determine the level of impact on dolphin behaviour due to boat traffic, focal-group follows using a scan sample methodology were conducted from an independent research boat. A CATMOD analysis was used to determine dolphin behaviour by season, number of boats, and group size. Dolphin behaviour differed by boat number; in particular, resting behaviour decreased as boat number increased and milling behaviour increased as boat number increased. Behaviour differed when the research boat was the only boat present. Closer examination of boat type, i.e., permitted dolphin watching boats, non-permitted tour boats, and recreational boats, found a difference in dolphin behaviour in the presence of the permitted boats when compared to the other types. Dolphins rested less and engaged in more milling behaviour when the permitted boats were present. Responses to swim attempts from the commercial boats were compared from 1994 - 1995 to 1997 - 1998. In total, 255 groups were encountered of which 36% (n = 93) were exposed to at least one swim attempt. Successful swims (i.e., where at least one dolphin remained within 5 m of at least one swimmer) decreased from 48% in 1994 - 1995 to 34% in 1997 - 1998 and avoidance responses increased from 22% to 31%. Dolphin response varied according to swimmer placement from the boats. The ‘in path’ placement had the highest level of avoidance. Only ‘line abreast’ placement resulted in a decrease in avoidance. Based on the photo-identification sighting records, it was estimated that an average dolphin was exposed to 31 swim attempts per year. This suggests that individual dolphins have, with cumulative experience, become sensitised to swim attempts. If a swim attempt was successful, on average, it involved 19% of the group. Juveniles were more likely to interact with swimmers than adults. The present study suggests that the wide-ranging population of bottlenose dolphins that frequents the Bay of Islands is exposed to levels of tourism which can affect the dolphins behaviour. In particular, the permitted boats’ presence results in decreased resting behaviour by the dolphins. The apparent sensitisation of this population to swim attempts, and the relatively small number of individual dolphins that interact with swimmers, are grounds for concern about the long-term impact of interactions. Ongoing monitoring of this populations’ response to swimmers and boats, combined with a precautionary approach to management of boats and swimmers, are recommended.