Group Characteristics, Site Fidelity and Seasonal Abundance of Bottlenose Dolphins (Tursiops Aduncus) in Jervis Bay and Port Stephens, Southeastern Australia

Australian Mammalogy 24(I):11-21

S.J. Allen and R.G. Harcourt,
Marine Mammal Research Group,
Graduate School of the Environment,
Macquarie University, Sydney, NSW, Australia, 2109.

L.M. Möller - Corresponding author.
Department of Ecology and Evolutionary Biology,
Yale University,
New Haven, CT, 06520-8106, USA.

Möller, L.M., Allen, S.J. and Harcourt, R.G. (2001).
Group Characteristics, Site Fidelity and Seasonal Abundance of Bottlenose Dolphins (Tursiops Aduncus) in Jervis Bay and Port Stephens, Southeastern Australia

Social organisation and abundance of bottlenose dolphins (Tursiops aduncus) in Jervis Bay (JB) and Port Stephens (PS), NSW, were investigated through behavioural/photo-identification surveys between May 1997 and April 2000. Mean group size was significantly larger at JB (12.3 ± 0.87, n =167) compared to PS (6.8 ± 0.37, n = 218). At both sites, groups were significantly larger when calves were present. Group size varied with activity, being smallest when feeding and largest when socialising. While mean group size of feeding dolphins did not vary between sites, travelling and socialising groups were significantly larger in JB. Site fidelity was assigned based on sighting rates and presence across seasons. Sighting rates varied significantly between areas, but the proportion of dolphins categorised as residents, occasional visitors and transients did not. Minimum abundance by season, based on mark-resighting of recognisable individuals, ranged from 61 ± 3.2 to 108 ± 7.1 in JB and 143 ± 8.1 to 160 ± 8.1 in PS. Differences in group size at the two sites may relate to social factors and/or human impacts, while differences in abundance may be associated with habitat size and complexity. The lack of dolphin matches between areas suggests that they represent distinct populations.

Key words: bottlenose dolphin, Tursiops aduncus, social organisation, site fidelity, seasonal abundance, calving season, Jervis Bay, Port Stephens, southeastern Australia

BOTTLENOSE dolphins (genus Tursiops) are found in all temperate and tropical waters (Leatherwood and Reeves 1983). There are two currently recognised species, T. truncatus and T. aduncus (Rice 1998; LeDuc et al. 1999; Wang et al. 1999). Tursiops truncatus inhabits a wide range of habitats, including bays, lagoons, estuaries, open coasts, pelagic waters and the coasts of oceanic islands (e.g., dos Santos and Lacerda 1987; Wells et al. 1987; Scott and Chivers 1990; Acevedo and Würsig 1991; Bearzi et al. 1997; Defran and Weller 1999), while T. aduncus appears to be restricted to coastal environments of the Indo-Pacific, Indian and Western Pacific Oceans, including southeastern Australia (Ross and Cockcroft 1990; Rice 1998; Wang et al. 1999; Möller and Beheregaray 2001).

Studies on bottlenose dolphin social ecology in different habitats, primarily on coastal T. truncatus, have revealed considerable variation in average group size, site fidelity, home range and the size of local populations. This variation has been related to differences in predation risk, food availability and dolphin foraging strategies (Würsig 1978; Shane et al. 1986; Ballance 1992; Defran et al. 1999). In protected habitats, such as small, shallow bays and estuaries, bottlenose dolphins usually associate in small groups, show a degree of site fidelity, feed primarily on scattered prey, and belong to relatively small local populations (Shane et al. 1986; Wells et al. 1987). In less protected habitats, such as deep bays, open coasts, and pelagic waters, individuals will aggregate in larger groups, show reduced levels of site fidelity, and belong to larger populations (e.g., Würsig 1978; Ballance 1992; Defran and Weller 1999).

While limited information is available on the social organisation of offshore T. truncatus populations, coastal populations of both T. truncatus and T. aduncus appear to exhibit fission-fusion grouping patterns, ie. they associate in groups that change frequently in size and composition (Wells et al. 1987; Smolker et al. 1992). Habitat structure and activity patterns are reportedly the main factors influencing group size, while group composition is primarily based on age, sex, reproductive condition and kinship (Shane et al. 1986; Wells et al. 1987; Duffield and Wells 1991; Smolker et al. 1992; Möller 2001).

In Australia, bottlenose dolphins appear to be distributed continuously around the continent and are also found in offshore waters (Ross and Cockroft 1990; Hale et al. 2000). Nevertheless, published information on specific populations is available for only a few coastal areas (e.g., Corkeron 1990, 1997, T. cf aduncus in Moreton Bay and off Stradbroke Island, southeast Queensland (QLD); Connor et al. 1992a, b; Smolker et al. 1992, T. cf aduncus in Shark Bay, Western Australia (WA); Mandelc and Fairweather 1995; Möller and Harcourt 1998, T. aduncus in Jervis Bay, New South Wales (NSW)).

In this paper we describe and compare group characteristics and site fidelity of bottlenose dolphins from two large bays in NSW; Jervis Bay and Port Stephens. We also estimate, by season, minimum abundance of the two geographically separated populations. Genetic evidence indicates that both populations belong to the species T. aduncus (Möller and Beheregaray 2001).

METHODS

Study Areas

Jervis Bay (35°07'S, 150°42'E) is a relatively enclosed, kidney-shaped embayment located approximately 200 km south of Sydney in NSW (Fig. 1b). With an area of 102 km2, it is characterised by shallow waters (generally < 20 m) and a shelving bottom, which gradually slopes towards the entrance (West 1987; Cho 1995), reaching maximum depths of approximately 40 m (Holloway 1995).

Port Stephens (32°42'S, 152°06'E, hereafter referred to as the Port) is a drowned river valley about 200 km north of Sydney in NSW (Fig. 1a). It represents an area of over 140 km2 of water surface and is also characterised by relatively shallow waters (generally 2 to 8 m deep), with a few deeper channels reaching 40 m depth. Estuarine processes dominate the western Port, while the eastern Port is typically marine.

Figure 1. Study areas in New South Wales, southeastern Australia. (a) Port Stephens, (b) Jervis Bay.

Boat Surveys

At Jervis Bay 86 boat surveys were conducted between May 1997 and May 1999 for approximately 463 h of survey time. Surveys were carried out in either a 4.7 or a 5.8 m aluminium boat powered by 40 hp and 90 hp outboards, respectively. Surveys were concentrated along the periphery of the bay, with limited transects crossing the middle of the bay (details in Möller and Harcourt 1998).

At Port Stephens 43 boat surveys were conducted between December 1998 and April 2000 for approximately 244 h of survey time. The research boat was a 6.1 m Hydrofield, powered by twin 115 hp outboards, or the same 5.8 m boat as used in Jervis Bay. Transects within the Port were run parallel and perpendicular to the coastline along a preset route between specific landmarks. Transect direction was alternated with each survey.

Sighting Protocol

The sighting protocol previously described in Möller and Harcourt (1998) was used at both research sites. When a dolphin group was encountered, we approached to a distance of approximately 30 - 50 m and recorded the following information: time of sighting, estimated group size, location (using a hand-held Global Positioning System) and predominant behavioural activity.

A group was defined as all dolphins sighted within a 100 m radius and that appeared to be coordinating their activities (it can be represented by a single animal) (Irvine et al. 1981). Group size was estimated by visual counts during each sighting. Numbers of adults, calves and newborns were also recorded. Calves were defined as animals from one half to two thirds' the length of an adult and individuals less than half the size of an adult were categorised as newborns. Activity patterns were defined according to Hanson and Defran (1993) and Shane (1990) with five categories: travelling, feeding, socialising, milling, and resting.

After the initial encounter, the group was approached to a distance of about 10 m for photo-identification of individual dolphins. Dolphins were photographed with a SLR camera fitted with an autofocus 100-300 mm zoom lens using 200 ISO slide film. Individuals were photographed opportunistically during a sighting, attempting to obtain at least four times the estimated group size in photographs. Individual dolphins were identified by natural marks on the trailing edge of their dorsal fins (Würsig and Würsig 1977).

Data Analysis

Group characteristics

Group sizes, including young, are presented as means (± standard errors) to facilitate comparisons with other studies and as medians (25 and 75%, interquartile range) because the distribution of group size was skewed and sample sizes were relatively small. Non-parametric tests were used for group size comparisons. Calves and newborns were pooled as 'young' for analysis.

Photo-identification

Only high quality slides with clear identification of individuals were used for analysis. Repeated sightings of individuals on the same day were treated as a single sighting. Data from the first month of study in Jervis Bay (n = 2 surveys) were excluded from analyses because groups were photographed opportunistically during this month.

Site fidelity

Individuals were allocated to one of three categories according to the proportion of photo-identification surveys in which they were identified. Low sighting rates (LSR): sighted in less than 10% of the surveys with photographs taken; Moderate sighting rates (MSR): sighted on between 10 and 30% of surveys with photographs taken; High sighting rates (HSR): sighted on more than 30% of surveys with photographs taken.

In addition, individuals were assigned to three categories of residency status, according to their sighting rates and presence across seasons.

Residents (RES): Dolphins with moderate to high sighting rates and present in multiple seasons; Transients (TRS): animals with low sighting rates and present in only one season; Occasional Visitors (OCV): those with low sighting rates but present in multiple seasons. Calves and newborns were excluded from the above analyses. Chi-square was used to compare the proportion of individuals with different sighting rates and residency status between study sites.

Minimum abundance

The number of recognisable dolphins in the population, or a minimum abundance of the population, was estimated through a mark-recapture model using photo-identification data (including individuals from all residency status categories). Individuals were considered 'captured' in the first survey day (i.e., sample) they were photo-identified, and 'recaptured' when photo-identified in subsequent surveys (this method is also called mark-resighting). The estimation was conducted within program CAPTURE (Rexstad and Burnham 1991), using Model Mth, which allows capture probability to vary with time and individual animal (Chao et al. 1992). This model was chosen because the assumption of equal probability of capture is likely to be violated in mark-recapture studies of bottlenose dolphins (Williams et al. 1993; Wilson et al. 1999).

Population closure was investigated by plotting the cumulative number of individuals identified against the number of surveys completed, and by examining the re-sighting rate of individuals within seasons. Analyses of abundance were conducted on relatively short periods of sampling in order to avoid violation of the assumption of population closure (i.e., that the population is closed from emigration or immigration, births or deaths during the sampling period).

A minimum abundance was estimated for warmer (summer) and cooler (winter) months. Summer months were considered to be from November to April (average SST at both sites = 23° C), and winter months from May to October (average SST at both sites = 17° C, Holloway 1995). For Jervis Bay abundance was estimated for two consecutive winters and summers (Table 1). For the first summer period in Jervis Bay, more surveys were conducted than the maximum number of occasions (n = 18) supported by CAPTURE. Therefore a sample of surveys was selected to reflect similar starting and ending dates to the second summer. For Port Stephens, abundance was estimated for two consecutive summers (Table 1). Abundance was not estimated for winter in Port Stephens because the number of surveys (n = 5) was too small to give a reliable estimation.

Table 1. Number of surveys used for estimation of minimum abundance by season of bottlenose dolphins (T. Aduncus) in Jervis Bay and Port Stephens.

RESULTS

Group Characteristics

In Jervis Bay dolphins were observed in 81 of the 86 surveys conducted. A total of 167 groups were sighted and 2058 animals counted, including 142 calves and 14 newborns. Newborns were observed from December to May. Group size varied from a single animal up to 64 individuals (Fig. 2a), with a mean of 12.3 ± 0.87 dolphins and median of 8.0 (25% = 4.00; 75% = 18.75). In Port Stephens dolphin groups were observed on 42 of 43 surveys. A total of 1472 animals within 218 groups were counted, including 263 calves and 38 newborns. Newborns were observed from December to April. Group size varied from one to 30 individuals (Fig. 2b). Mean group size was 6.8 ± 0.37, while the median was 5.0 (25% = 3.00, 75% = 9.00).

The mean number of young (calves and newborns) per group in Jervis Bay was 1.2 ± 0.11 (median = 1.0). Groups with young (mean = 17.1 ± 1.3, median = 17.0, 25% = 7.75, 75% = 25.00) were significantly larger than groups without young (mean = 5.4 ± 0.51, median = 5.0, 25% = 3.00, 75% = 6.50) (Mann-Whitney, U = 2125, P < 0.01, n = 133). In Port Stephens a mean of 1.5 ± 0.12 young (median = 1.0) was encountered per group. Group size was significantly greater when young were present (mean = 8.3 ± 0.46, median = 7.0, 25% = 4.00, 75% = 11.00) than when young were not present (mean = 3.1 ± 0.26, median = 3.0, 25% = 1.00, 75% = 4.00) (Mann-Whitney, U = 3506, P < 0.01, n = 209). Group size was significantly smaller at Port Stephens than that at Jervis Bay (Mann-Whitney, U = 37557, P < 0.01, n = 385), whether young were present (Mann-Whitney, U = 11997.5, P < 0.01, n = 227) or absent (Mann-Whitney, U = 3743.5, P < 0.01, n = 115).

Figure 2. Frequency distribution of bottlenose dolphin group size in Jervis Bay (a) and Port Stephens (b).

Group size in Jervis Bay varied significantly with activity (Kruskal-Wallis = 26.498, d.f. = 3, P < 0.01, n = 142) (Table 2). Feeding groups were significantly smaller than socialising and travelling groups (Dunn's method, P < 0.05 for both comparisons). In Port Stephens group size also varied significantly with group activity (Kruskal-Wallis = 12.523, d.f. = 3, P < 0.05, n = 203) (Table 2). Feeding groups were significantly smaller than socialising groups (Dunn's method, P < 0.05). Travelling groups were larger in Jervis Bay than in Port Stephens (Mann-Whitney, U = 11416, P < 0.05, n = 215), as were socialising groups (Mann-Whitney, U = 84, P < 0.05, n = 21). The size of feeding groups did not vary significantly between sites (Mann-Whitney, U = 1116, P > 0.05, n = 72), nor did milling groups (Mann-Whitney, U =303.5, P > 0.05, n = 36).

Table 2. Characteristics of bottlenose dolphin group sizes according to activities in Jervis Bay and Port Stephens. s.e.=standard error, 25% and 75%=interquartile range; n=number of groups. (Only one resting group, in Port Stephens and with 5 dolphins, was observed in the 3 minute interval at the beginning of a sighting.)

Photo-identification and site fidelity

Dolphins in Jervis Bay were photo-identified on 65 out of the 81 surveys in which dolphins were observed. A total of 118 individuals were identified, including two calves. In Port Stephens dolphins were photo-identified on 39 out of the 42 surveys during which dolphins were observed. During these surveys 155 individuals were identified, including five calves. No individuals photo-identified in Jervis Bay were sighted in Port Stephens, or vice-versa.

The number of sightings per individual in Jervis Bay varied from one to 43, with a significant proportion of dolphins showing either low or high sighting rates (Fig. 3a). Mean number of sightings per individual in this area was 13.7 ± 1.19 and median was 7.5 (25% = 2.00, 75% = 27.00). In Port Stephens the number of sightings per individual varied from one to 21, with most dolphins showing low to moderate sighting rates (Fig. 3b). The mean number of sightings per dolphin in Port Stephens was 6.4 ± 0.36 and median was 6 (25% = 3, 75% = 9). The proportion of dolphins with low, moderate and high sighting rates varied significantly between areas (c2 = 35.514, d.f. = 2, P < 0.01), with more animals showing high sighting rates and fewer showing moderate sighting rates in Jervis Bay than in Port Stephens.

Figure 3. Number of sightings of photo-identified bottlenose dolphins, excluding calves, during 65 surveys in Jervis Bay (a) and 42 surveys in Port Stephens (b) (sighting rates: LSR = low, MSR = moderate, HSR = high).

Figure 4a shows the cumulative number of photo-identified individuals in Jervis Bay during the 63 surveys used for analyses. At the end of the first winter the curve seemed to plateau, but towards the middle of the first summer several new animals were identified. During the second winter more animals were identified for the first time, while in the second summer the curve approximated a plateau. In Port Stephens the cumulative curve of photo-identified individuals by survey shows that most of the animals were identified in the first half of the surveys. Few new identifications were then obtained until close to the end of the study, and included five calves which acquired marks during the second summer (Fig. 4b).

Figure 4. Cumulative number of photo-identified bottlenose dolphins by survey in Jervis Bay (a) and Port Stephens (b).

The number of resightings in Jervis Bay steadily decreased by season. Nevertheless there was a relatively large proportion of animals identified in the first winter (1997) that were resighted within the Bay in all subsequent seasons (Table 3). By contrast, more than half of the animals identified for the first time during summer 1997/1998 were not resighted in the bay in later seasons (Table 3). In Port Stephens the majority of individuals identified in the first summer were also present during the second summer, and a moderate proportion of those were also resighted during the less intensive winter surveys (Table 4). Few new animals were identified in winter and all were resighted in the subsequent summer (Table 4). Within seasons in Jervis Bay between 76.5% (Winter 1998) and 85% (Winter 1997) of animals were resighted (Table 3). Within summers in Port Stephens 72.1% (1999/2000) to 75.2% (1998/99) of individuals were re-sighted. Only 40% of animals were resighted during the winter (Table 4), but the low number of surveys (n = 5) precludes comparisons with other seasons or site. In Jervis Bay 57 dolphins were classified as residents, 27 as occasional visitors, and 32 as transients. In Port Stephens 87 dolphins were classified as residents, 32 as occasional visitors, and 30 as transients. The proportion of residents, occasional visitors and transients did not differ between the sites (c2 = 2.670, d.f. = 2, P > 0.05).

Table 3. Number of bottlenose dolphins identified and re-sighted among and within seasons in Jervis Bay by season of first identification. New Ids: new identifications (in bold).

Table 4. Number of bottlenose dolphins identified and re-sighted among and within seasons in Port Stephens by season of first identification. New Ids: new identifications (in bold).

Minimum abundance

The estimated number of recognisable dolphins in Jervis Bay, by season, varied from 61 ± 3.2 in Summer 98/99 to 108 ± 7.1 in Summer 97/98 (Table 5). There was overlap in the confidence intervals of the estimates for season pairs Winter 97 - Summer 98/99, and Summer 97/98 - Winter 98 (Table 5). In Port Stephens the population of known individuals was similar in each summer surveyed, with a minimum estimate of 160 ± 8.1 in 98/99, and 143 ± 8.1 in 99/00 (Table 5).

Table 5. Results of mark-recapture analysis for Jervis Bay and Port Stephens (Model M(th), Chao et al. 1992). p-hat = average probability of capture, N-hat = estimate of the number of recognisable dolphins in the population, SE = standard error, CV = coefficient of variation, 95% CI = 95% confidence interval.

DISCUSSION

Variability in dolphin social organisation in general, and in group size in particular, has been attributed to variance in physical characteristics of the environment, to predation pressures, to foraging strategies, to social factors and possibly to methodological differences between studies (e.g.,Würsig 1978; Shane et al. 1986; Smolker et al. 1992). However most studies have not compared populations. In this study, using identical methods, we found that two populations of bottlenose dolphins (T. aduncus) inhabiting two embayments in relative close proximity (approximately 400 km apart) differed in a range of group characteristics, yet still showed commonalities.

Worldwide group size for Tursiops is highly variable, ranging from a mean of about 5 individuals (Irvine et al. 1981; Weigle 1990; Smolker et al. 1992) up to 140 animals (Saayman and Tayler 1973). Average group size at both study locations was intermediate between those found elsewhere and was significantly higher in Jervis Bay than in Port Stephens. In Jervis Bay, dolphins coalesced in groups whose mean size was similar, albeit a fraction smaller, to those of open coasts, such as California (T. truncatus) and southern Queensland (T. cf aduncus) (Ballance 1990; Corkeron 1997). In Port Stephens, group size was comparable to that found elsewhere in shallow bays, such as in Florida (T. truncatus) and Western Australia (T. cf aduncus) (Wells et al. 1987; Smolker et al. 1992).

It has been proposed that larger groups of Tursiops occur in open and deeper habitats (either coastal or offshore), while small groups usually occur in shallow protected areas and that this is a function of differences in predation pressure and foraging strategies in the two environments (reviewed in Shane et al. 1986, though see Scott and Chivers 1990). In open, exposed environments, larger groups of dolphins would be favoured due to increased vigilance (Norris and Dohl 1980). Concomitantly, schooling fish would have a greater likelihood of being detected and an increased probability of being successfully herded by large dolphin groups foraging cooperatively (Norris and Dohl 1980).

The results of this study are in part inconsistent with the hypothesis that predation and cooperative feeding results in the formation of larger dolphin groups. In our study, dolphins at Jervis Bay usually formed larger groups (mean group size: 12.3 ± 0.87 (Jervis Bay), 6.8 ± 0.37 (Port Stephens)). Jervis Bay is a large kidney shaped bay (Fig. 1b) which deepens towards the middle and the entrance. Consistent with predator avoidance the dolphins in Jervis Bay do not frequent deep waters, remaining in waters less than about 12 m deep nearly 90% of the time (Mandelc and Fairweather 1995). Instead they range back and forth in shallow waters on the periphery of the bay. However, large predatory sharks do not commonly occur in Jervis Bay (Pollard 1973), no shark wounds nor healed scars were spotted and cooperative feeding was rarely observed (Möller, unpublished data). By contrast, in Port Stephens group size was smaller. Port Stephens is a long, narrow habitat (Fig. 1a) with a few deep channels and dolphins ranged widely throughout the embayment. Yet in direct contrast to the predation hypothesis, shark attack occurred more frequently at Port Stephens with five fresh shark wounds photographed over the course of the study and many animals bearing scars from shark bites (Möller, unpublished data). Cooperative feeding is also unlikely to explain differences between the two areas. Critically, feeding groups were similar in size between Jervis Bay and Port Stephens; group formation differed only during other activities. This suggests that feeding behaviour is not a causative factor in the size of groups at other times. It seems that neither predation or cooperative feeding explain the larger size of groups found in Jervis Bay, unless the abundance of predatory sharks has increased in the Port in recent years, or if the larger groups at Jervis Bay are correspondingly more successful at avoiding shark attacks.

Alternatively, group size may be suboptimal in Port Stephens due to impacts from human activities. Dolphin watching is a very active industry in Port Stephens and about 10-12 boats run multiple trips daily on a year-round basis to see dolphins (Allen et al. 2000). Preliminary studies on the impact of dolphin-watch boats on dolphin behaviour in this area have suggested that dolphin groups often split into smaller units when operators employ intrusive boat-handling techniques (Allen and Möller 1999). It is therefore possible that repeated boat contact prevents large groups from staying together for any length of time.

Given that feeding groups are similar between the two sites, yet group size varies at other times, social factors may be responsible for the differences reported here. There was a considerable difference in the proportion of young observed between areas, around 8% in Jervis Bay and 20% in Port Stephens, and this may indicate a smaller proportion of reproductive females in Jervis Bay. During biopsy surveys in these areas, about twice as many males as females were sampled in Jervis Bay, while more females than males were sampled in Port Stephens (Möller 2001). If the difference in the sex ratio of sampled animals is indicative of the population as a whole, then there may be a marked difference in sex ratio between the two sites. If females are in short supply at Jervis Bay, males may need to aggregate with females at a relatively higher frequency in order to maintain access to females. This hypothesis awaits further investigation.

An increase in group size with an increase in the number of calves in the group was observed in both Jervis Bay and Port Stephens. Similar patterns have been found for T. truncatus (e.g., Wells et al. 1987; Weigle 1990; Bearzi et al. 1997). Increased dolphin group size when calves are present has been related to enhanced calf assistance, increased protection from predators, and reduced maternal investment (Norris and Dohl 1980). Observations of newborns from December to April in Port Stephens and December to May in Jervis Bay indicate that calving occurs in these areas during summer and early winter. Thermal constraints may restrict calving for Tursiops, as in other temperate areas calving occurs during the warmer months (Gruber 1981; Weigle 1990; Scott et al. 1990; Urian et al. 1996; Fernandez and Hohn 1998), while in subtropical areas, for example the coastal waters of southern Queensland, even though newborns were more frequent in summer, they were also observed in other months of the year (P. Corkeron, pers. comm). If the gestation period for T. aduncus is similar to that of approximately one year for T. truncatus (Perrin and Reilly 1984), mating should also occur in these areas during summer and early winter. Indeed, herding of females and mating behaviour was observed in Jervis Bay and Port Stephens only during this period (Möller et al. 2001, Möller 2001).

During the first summer (mating season) in Jervis Bay there was a relatively large influx of animals into the Bay, including different age and sex classes. These dolphins associated in groups with individuals categorised as residents. On the other hand, during the second summer, the estimated population size was the smallest, and several resident animals were not sighted inside the Bay. The resident dolphins that disappeared were primarily large, heavily scarred individuals, including two pairs and one triplet male alliance (Möller, unpublished data). Two known resident females were also not sighted within the Bay that summer. In Sarasota Bay, Florida, resident male T. truncatus have disappeared for days to months from the area (Wells 1991). Genetic mixture occurs between adjacent dolphin communities in Florida, and roving males have been hypothesised to be the primary vectors for this exchange (Duffield and Wells 1991). The influx in one year and efflux in the second year of T. aduncus from Jervis Bay suggests that movements of both males and females outside their normal range may serve a similar function and would explain the inter-annual variation in population size.

Sighting rates of identified dolphins differed between Jervis Bay and Port Stephens. There were more individuals showing high sighting rates and fewer animals showing moderate sighting rates in Jervis Bay than in Port Stephens. These results contrasted with the residency status of individuals, as there were no significant differences in the proportion of residents, occasional visitors, and transient animals between areas. However, these differences possibly relate to the range of resident animals. In Jervis Bay, resident dolphins were usually found in an area that was the furthest away from the entrance of the Bay (Möller, unpublished data), where there is an extensive portion of shallow waters. During surveys, dolphins were seen to move close to shore around the Bay until close to the entrance, where they would then stop and return in a typical clock-wise / anti-clock wise movement pattern. Thus, the study area appeared to include their normal daily ranges. Shane et al. (1986) suggested that although Tursiops do not appear to be territorial, in certain areas they seem to define the boundaries of their range, and may consistently turn back at approximately the same location. Resident dolphins in Port Stephens were frequently seen leaving the Port. Opportunistically, the research team followed the dolphins outside the Port, where they tended to remain in an adjacent coastal bay. On a few occasions, however, they continued moving north along the coast. They were also seen to enter and leave some of the rivers within the Port, which were areas not normally surveyed during the study. Thus, Port Stephens resident dolphins appeared to range outside the study area more often than Jervis Bay resident dolphins, and were more likely to not be encountered during surveys. In fact, when high and moderate sighting rates were combined within sites and compared between sites, no significant difference in sighting rates was found between Jervis Bay and Port Stephens.

Photo-identified individuals totalled 118 in Jervis Bay and 155 in Port Stephens. Of those, residents accounted for 57 and 87 individuals, respectively. Port Stephens is a large and complex habitat, with both estuarine and marine environments, where dolphins are generally widespread. Jervis Bay is a typical marine environment that is smaller in size, and where dolphins frequently use a restricted portion of the habitat. In both areas there were similar, relatively large proportions of animals that were classified as either occasional visitors or transients. These dolphins, as well as resident individuals, were shown to belong to T. aduncus (Möller and Beheregaray 2001). If this species is truly restricted to coastal environments, then the considerable influx of dolphins must come from outside, adjacent coastal waters. Thus, Jervis Bay and Port Stephens dolphins appear to belong to populations that range across a larger geographic area than the study sites. However, the lack of matches of identified individuals between the two areas, in accordance with marked genetic structure at the nuclear DNA level between Jervis Bay and Port Stephens dolphins (Möller 2001), suggests that they represent two distinct populations.

ACKNOWLEDGMENTS

Jervis Bay research was funded/sponsored by AGFA Films, Australian Geographic, Booderee National Park/Environment Australia, Lowrance Australia, Graduate School of the Environment (GSE), Macquarie University. Research in Port Stephens was funded/sponsored by NSW National Parks and Wildlife Service, Port Stephens Commercial Dolphin Watch Association, GSE, and Linnean Society of NSW. Booderee National Park and HMAS Creswell provided fieldwork support in Jervis Bay, while NSW National Parks and Wildlife Service provided support in Port Stephens. Thanks to many volunteers for fieldwork assistance, in particular, L. Beheregaray, A. Mouland, L. Allen and L. Cotter. Thanks to C. Littnan, V.M. Peddemors and one anonymous referee for comments on the manuscript. L. Möller was sponsored by CAPES (Brazilian Ministry of Education).

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