Socialising

Approximately 7% of the activity budget for Hauraki Gulf common dolphins was attributed to social behaviour. This is consistent with that previously observed by Neumann (2001c) and similar to that reported by Cañadas & Hammond (2008) for the southwestern Mediterranean Sea. Neumann (2001c) recorded similar behavioural events to those observed during the present study e.g. belly-to-belly contact (both with and without copulation), rubbing of the pectoral flippers and biting of the tail flukes. Presumed copulation (Figure 4.7) was observed during most bouts of social activity in the Hauraki Gulf and occasionally involved repetitive copulation with the same female by different males during a single observation. The significance of this remains unclear, although life history studies of common dolphins from the northeast Atlantic suggest promiscuous mating systems and sperm competition in this genus (Murphy et al., 2005). In the Hauraki Gulf, socialising was most evident during autumn and winter and frequently recorded in smaller group sizes and in groups with calves as the youngest group component. This suggests that social behaviour exhibited by common dolphins in this region may also be important for bonding and/or cultural learning (e.g. Rendell & Whitehead, 2001; Kuczaj & Highfill, 2005).

4.5.6 Multi-species groups

Approximately 70% and 80% of foraging common dolphin groups observed in the Hauraki Gulf occurred in the presence of Bryde’s whales and Australasian gannets, respectively. A bias in both the amount of foraging behaviour and multi-species groups recorded from Dolphin Explorer was anticipated since both whales and gannets were frequently used as sighting cues to find dolphin groups from this platform. Surprisingly however, despite differences in search methodologies, the proportion of foraging activity recorded from Aihe remained similar, thus negating the possibility of bias resulting from the presence of associated species. Conversely, this suggests that the high degree of foraging behaviour observed for common dolphins in the Hauraki Gulf represents a true reflection of habitat use. Mixed-species aggregations have frequently been reported in mammals and are known to serve various functions including among others, increased predator protection and foraging opportunities (see to Stensland et al., 2003 for a review). Delphinids frequently form multi-species groups with other cetaceans (e.g. Frantzis & Herzing, 2002; Herzing et al., 2003) but are also observed in

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Figure 4.7 Two common dolphins (Delphinus sp.) observed in the Hauraki Gulf during presumed copulation.

multi-specific groups containing large tunas Thunnus sp. (e.g. Das et al., 2003; Rogan & Mackey, 2007) and various avian species (e.g. Martin, 1986; Clua & Grosvalet, 2001). Further research conducted on other associated marine taxa, particularly fish species, would be helpful to our understanding of these multi-species feeding aggregations.

4.5.7 Study limitations

The systematic following of groups or individuals in a population can contribute greatly to our understanding of the behavioural ecology of a species. Since boat-based surveys were necessary, I adopted protocols proven to minimise the potential impact of the observation vessel (Stockin et al., 2008b) and used a four-stroke outboard engine for reduced noise and exhaust emissions.

While the frequency of multi-species groups was greater from Dolphin Explorer, and undoubtedly overestimated owing to the use of binoculars to detect birds and whales, less bias was anticipated from Aihe. This was due to the employment of a random survey methodology, where areas searched were neither dependant upon previous dolphin encounters or the presence of indicator species. Moreover, the conscious decision not to use binoculars to detect indicator species from Aihe helped balance the probability of observing both single- and multi-species groups.

I acknowledge that conspicuous surface activity and the presence of indicator species could arguably have biased the activity budget. Dolphin Explorer frequently used feeding Australasian gannets (Figure 4.8) as indicator species to locate common dolphins, thus providing bias towards feeding groups of common dolphins. However, comparison of the initial and lagged behavioural states revealed no significant difference in the prevalence of foraging or other more conspicuous surface behaviour in the activity budget. Thus, it would appear that the importance of the Hauraki Gulf as a feeding ground for common dolphins is not merely an artefact of the way the dolphins were detected but an accurate representation of how important these waters are for foraging.

Figure 4.8 Australasian gannets (Morus serrator) observed feeding in the Hauraki Gulf, New Zealand. Note: Presence of prey in the mouths of two of the gannets, marked by arrows. Photo: Karen Stockin.

4.5.8 Management implications

Within New Zealand, the Hauraki Gulf is already acknowledged as a significant coastal, marine and island ecosystem via the Hauraki Gulf Marine Park Act (2000). However, there is a strong argument for specific management strategies to be developed and applied for species that rely on this region for important biological processes e.g. feeding. Herein, strong evidence that common dolphins utilise the Hauraki Gulf as an important feeding ground is presented. Foraging activity in this region is considerably higher than that reported in a comparable study by Neumann (2001c) for a different region. This is likely the result of a combination of factors, including prey availability and the need to meet energetic demands relating to calving and/or lactation (Bernard & Hohn, 1989), especially since the Hauraki Gulf is used as a nursery and potential calving area for this population (Stockin et al., 2008a; Chapter Three). The use of coastal waters for feeding is of notable management importance, particularly given the apparent susceptibility of foraging common dolphins to tour boat disturbance (Stockin et al., 2008b; Chapter Seven).

4.6 Conclusions

This study provides strong evidence to suggest the Hauraki Gulf is a unique habitat which provides significant opportunities for common dolphins, allowing them to spend less time travelling and searching for food and, consequently more time foraging, resting, socialising, and raising calves. The Hauraki Gulf is clearly an important ecosystem for common dolphins, thus wise management of its marine resources is critical. This is of notable importance given the increasing demands being placed upon these waters by nearby urban population growth and increasing recreational use. Thus, it is imperative that specific management strategies for the common dolphin be implemented for Hauraki Gulf waters.

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