Tursiops truncatus

Bottlenose Dolphin

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Bottlenose Dolphin Bottlenose Dolphin

 

Description

Bottlenose dolphins have widely spaced eyes, relatively long flippers, a rounded forehead (called a melon), a relatively short, broad snout, and a mouth that seems permanently twisted into a grin. Inside the mouth are as many as 100 teeth. Highly social, bottlenose dolphins often swim in groups of several hundred individuals, and are famous for racing alongside watercraft. Some stay in coastal waters and others swim offshore. In the Atlantic, the coastal dolphins feed mostly on sea trout, croakers, and spot. The offshore population follows the Gulf Stream and feeds on deep-water fish and squid. Three different populations have been identified in the North Pacific: a temperate-water group, a tropical-water group, and a coastal group.

Geography

The Fiordland Bottlenose Dolphins comprise a regional subpopulation inhabiting the coastal fiords and bays of Fiordland, a mountainous, rainforest-covered World Heritage Area in the southwest of New Zealand’s South Island. The subpopulation is located at the southern limit of the species’ worldwide range and is genetically and geographically isolated from other coastal New Zealand subpopulations (Tezanos-Pinto et al. 2008). The nearest subpopulation seen regularly in coastal waters is located more than 500 km north in the Marlborough Sounds. Sightings of Tursiops in other South Island areas are uncommon and sporadic.

The subpopulation is subdivided into three discrete local units: one local unit ranges among the smaller fiords and bays of the northern Fiordland coast while two local units reside within the complexes formed by Doubtful and Thompson Sounds, and Dusky and Breaksea Sounds (Williams et al. 1993, Bräger and Schneider 1998, Lusseau and Slooten 2002, Currey et al. 2007, Currey et al. 2008a). These local units are largely isolated from each other, but there have been records of exchange between units (Lusseau et al. 2006), suggesting the units require management as a collective subpopulation.

The Doubtful-Thompson Sound local unit has exhibited a consistent, high degree of site fidelity since 1990 (Currey et al. 2007, Currey et al. 2008b). The Dusky-Breaksea Sound local unit has also shown a high degree of site fidelity in the three years of study to date (Currey et al. 2008a). Members of the Northern Fiordland local unit have a larger distribution, having been photo-identified in Milford Sound, Sutherland Sound, Bligh Sound, George Sound, Caswell Sound, Charles Sound and Lake McKerrow, which connects to the sea (Bräger and Schneider 1998, Lusseau and Slooten 2002, Lusseau 2003b, Boisseau 2004, Currey 2006, Currey 2009).

The minimum extent of occurrence (EOO) of the subpopulation is approximately 450 km², representing the sum of the established home ranges of the local units. A reasonable upper estimate of EOO for the subpopulation is approximately 2,400 km², encompassing the inshore waters of Fiordland and a coastal range that extends 325 km from Puysegur Point, just south of Dusky Sound to Jackson Bay, north of Lake McKerrow and offshore for 5 km.

Ecosystem

Habitat and Ecology

The Fiordland bottlenose dolphins exhibit many characteristics that appear to reflect constraints imposed by their cold-water habitat. They are physically larger than coastal conspecifics found in warmer waters, with rotund bodies and comparatively shorter flukes, fins and rostrum (Schneider 1999). The dolphins are found in large groups, sometimes comprising an entire local unit, and show strong, long-lasting associations within and between sexes (Lusseau et al. 2003), unlike the fission-fusion societies typical of many other bottlenose dolphin subpopulations (Connor et al. 2000).

The Doubtful-Thompson Sound local unit’s habitat use changes seasonally in apparent response to water temperature (Schneider 1999). Water temperatures in the inner parts of the fiords are cooler than open coast in winter and warmer in summer. In winter, the dolphins avoid the inner regions of the fiord, remaining closer to the open ocean, while in summer the dolphins are found in the inner sections of the fiord where they calve in the warmer waters (Schneider 1999, Haase and Schneider 2001). Seasonal patterns in bottlenose dolphin distribution relating to water temperature are unusual and are typically only observed in subpopulations in cool-temperate latitudes (e.g. Wilson et al. 1997, Ingram and Rogan 2002).

Stable isotope studies of exfoliated skin show that the dolphins are reliant on local productivity, feeding primarily on sub-tidal reef fish (Lusseau and Wing 2006). The dolphins have been shown to dive beyond 200m in the deep waters (max depth 434m) of Doubtful Sound (Schneider 1999).

Conservation

A recent quantitative threat assessment concluded the Fiordland Bottlenose Dolphin subpopulation qualifies as Critically Endangered on the basis of the number of mature individuals and the predicted rate of subpopulation decline over three generations. The subpopulation contained 205 individuals in 2008, of which 123 were mature. Further, a majority of Population Viability Analysis (PVA) model runs (67.6%) indicated a decline of more than 25% over one generation, with a significant number (36.2%) also exhibiting a decline of more than 80% over three generations (Currey et al. 2009).

 

Threats

Major Threats

The Fiordland Bottlenose Dolphins are known to be exposed to: 

  1. disturbance and boat strikes associated with boat-based tourism in Milford and Doubtful Sounds; 
  2. increased freshwater discharge into Doubtful Sound from hydroelectric power generation;
  3. reduced prey availability caused by environmental degradation and overfishing throughout Fiordland.

Given that the effects of these threats are largely indirect, their influence on population trends is technically challenging to quantify. However, the cumulative impact appears likely to result in ongoing population declines within the Doubtful-Thompson Sound local unit (Currey et al. 2008b, Currey et al. 2009).

Tourism in Fiordland is increasing (Lusseau and Higham 2004). Tour boats have been demonstrated to have direct impacts on the behaviour of dolphins in Doubtful Sound, increasing the dive interval of both males and females (Lusseau 2003c), as well as disturbing resting and socialising behaviours, resulting in increased travelling (Lusseau 2003a). Tourism also appears to have influenced the residency patterns of the northern Fiordland Bottlenose Dolphins, with the dolphins avoiding the fiord entirely when tour boat activity is at its peak (Lusseau 2005). In both Milford and Doubtful Sounds, dolphins have been observed with scars from boat strikes and in one case, a calf was killed by a boat strike (Lusseau et al. 2002, Boisseau 2003). This evidence led the International Whaling Commission Scientific Committee to issue a consensus statement that whale watching and vessel traffic have a significant impact on this subpopulation (IWC Scientific Committee report 2006).

In response to these observed impacts, in 2006 a voluntary code of practice was established for tour boats in Milford and Doubtful Sounds to improve boat behaviour around the dolphins. This was formalised into a Code of Management for Doubtful Sound in January 2008, including Dolphin Protection Zones (DPZs) in which vessel activity is limited. These zones extend 200m out from shore in regions of the fiord that include some of the local unit’s most frequently used habitats (Lusseau and Higham 2004, Lusseau et al. 2006). The Code of Management is voluntary at present, and of unknown efficacy. Non-compliance with the Marine Mammals Protection Act 1978 (i.e. excessive boat speeds, unsafe approaches to dolphin groups) has been frequently observed in Fiordland in the past (Lusseau 2003a, 2006), and voluntary codes of practice appear to be of limited effectiveness in other locations (Scarpaci et al. 2003, Scarpaci et al. 2004, Whitt and Read 2006, Wiley et al. 2008). Given these factors, the negative effects of boat-based tourism are therefore likely to continue in Milford and Doubtful Sounds.

The isolation of the Dusky-Breaksea local unit ensures that these dolphins are presently exposed to fewer tour boat interactions than neighbouring local units. However, as tourism activities have increased in the most accessible parts of Fiordland, they have begun to spill over to more remote regions, such as Dusky Sound. Large vessels and helicopters now regularly visit for multi-day hunting, fishing and sightseeing trips. The impacts (if any) from these activities are presently unclear, although the effects observed elsewhere in Fiordland suggest impacts are likely to occur, or may already be occurring.

In addition to tour boat activity, the Doubtful-Thompson Sound local unit is subject to the effects of freshwater discharge from the Lake Manapouri hydroelectric power station tailrace. The tailrace discharge into Deep Cove, Doubtful Sound, is c. 450-510 cumecs (cubic metres per second), two to three times larger than the mean inflow from precipitation, and results in a distinct low-salinity layer significantly deeper than found in neighbouring fiords (Gibbs et al. 2000, Gibbs 2001). The low-salinity layer shows significant seasonal temperature variation, from 14–16°C in summer to 8-10°C in winter at a depth of 1 m (and occasionally freezing over in winter), while the underlying marine waters maintain a relatively constant 13–15°C year round at 25 m (Gibbs 2001, Peake et al.2001).

A marked reduction in calf survival observed in the Doubtful-Thompson Sound local unit, from 0.8621 (95% CI: 0.6851–0.9473) to 0.3750 (95% CI: 0.2080–0.5782) coincided with the opening of a second tailrace tunnel for the hydroelectric power station (Currey et al. 2008b). That the second tailrace increased the mean discharge only marginally (9.7% greater between 2002 and 2007 than between 1969 and 2001, but only 2.7% greater than between 1972 and 2001; Merdian Energy unpublished data) has been used to argue that it was not the cause (DuFresne and Mattlin 2009). Circumstantial evidence suggesting an impact of increased freshwater input includes four times higher severity of skin lesions in the Doubtful-Thompson Sound local unit than in the Dusky-Breaksea Sound local unit, smaller calves and more restricted calving season in Doubtful-Thompson Sound, and a less clear seasonal pattern of habitat use in Dusky-Breaksea sound (Rowe et al. 2008), all of which are consistent with increased exposure to cold fresh water. The additional freshwater has had other ecological effects, altering sub-tidal community structure within Doubtful Sound, resulting in declines in species richness (Boyle et al.2001, Tallis et al. 2004, Rutger and Wing 2006).

Historical fishing practices have also affected Fiordland’s marine environment, resulting in significant declines in fish abundance throughout Fiordland (Beentjes and Carbines 2005). This was one of the main reasons for the establishment of eight new marine reserves in Fiordland in 2005, with two of the new reserves created in Doubtful-Thompson Sound and a further two in Dusky-Breaksea Sound. The cumulative effects of historical fishing practices and altered community structure on the resident dolphins may be significant, given that they rely on productivity from within the fiord (Lusseau and Wing 2006).

In addition to anthropogenic threats, the Fiordland Bottlenose Dolphins face the inherent risks associated with small subpopulation size and residing at the edge of the species’ range. Small subpopulation size increases vulnerability to stochastic effects such as demographic stochasticity, environmental stochasticity, catastrophes and genetic deterioration (Caughley 1994, Hedrick et al. 1996), all of which can result in increased extinction risk, even in the absence of further human impacts. Residing at the southern-most limit of the species’ range may also have significant implications for subpopulation viability. If the habitat is marginal, it may result in increased energetic costs – a factor that may help to explain the apparent energy limitation observed among females in Doubtful-Thompson Sound (Lusseau 2003c). Further, there is the risk that in time, the subpopulation’s existing range may be rendered suboptimal by climate change.

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