Doubtful Sound Research Articles

Evidence of geographic variation in the non-signature whistle repertoires of two isolated populations of New Zealand common bottlenose dolphin

ABSTRACT Dolphins are social animals that depend on sound to communicate, navigate, and find food. The aim of our research was to describe dolphin whistle characteristics and compare whistles between two isolated populations of bottlenose dolphins in New Zealand. We focused on three aspects of dolphin bioacoustics (1) classification of whistles; (2) comparison of the whistles of two discrete bottlenose dolphin populations; and (3) comparison of whistles of dolphin populations from around the world. At Great Barrier Island, the most common type of whistle was upsweep followed by sine, while at Doubtful Sound, convex and sine whistles were the most common whistle types. Random Forest models discriminated between whistles from the two populations with an estimated accuracy of 90%. The key differences were whistle-type contour, duration, and end frequency. Linear Mixed models show that whistle acoustic parameters were influenced by location but not by group size or the presence of immatures. Whistle parameters of both populations fell within the range of multiple populations from around the world. This studyprovides a benchmark for future research into drivers of geographic variation in dolphin vocal communication.

Assessing the utility of marine filter feeders for environmental DNA (eDNA) biodiversity monitoring.

Aquatic environmental DNA (eDNA) surveys are transforming how marine ecosystems are monitored. The time-consuming preprocessing step of active filtration, however, remains a bottleneck. Hence, new approaches that eliminate the need for active filtration are required. Filter-feeding invertebrates have been proven to collect eDNA, but side-by-side comparative studies to investigate the similarity between aquatic and filter-feeder eDNA signals are essential. Here, we investigated the differences among four eDNA sources (water; bivalve gill-tissue; sponges; and ethanol in which filter-feeding organisms were stored) along a vertically stratified transect in Doubtful Sound, New Zealand using three metabarcoding primer sets targeting fish and vertebrates. Combined, eDNA sources detected 59 vertebrates, while concurrent diver surveys observed eight fish species. There were no significant differences in alpha and beta diversity between water and sponge eDNA and both sources were highly correlated. Vertebrate eDNA was successfully extracted from the ethanol in which sponges were stored, although a reduced number of species were detected. Bivalve gill-tissue dissections, on the other hand, failed to reliably detect eDNA. Overall, our results show that vertebrate eDNA signals obtained from water samples and marine sponges are highly concordant. The strong similarity in eDNA signals demonstrates the potential of marine sponges as an additional tool for eDNA-based marine biodiversity surveys, by enabling the incorporation of larger sample numbers in eDNA surveys, reducing plastic waste, simplifying sample collection, and as a cost-efficient alternative. However, we note the importance to not detrimentally impact marine communities by, for example, nonlethal subsampling, specimen cloning, or using bycatch specimens.

Mechanisms of Lateral Spreading in a Near-Field Buoyant River Plume Entering a Fjord

Observations collected from a fast-flowing buoyant river plume entering the head of Doubtful Sound, New Zealand, were analysed to examine the drivers of plume lateral spreading. The near-field plume is characterised by flow speeds of over 2 ms−1, and strong stratification (N2 > 0.1 s−2), resulting in enhanced shear which supports the elevated turbulence dissipation rates (ϵ > 10−3 W kg−1). Estimates of plume lateral spreading rates were derived from the trajectories of Lagrangian GPS surface drifters and from cross-plume hydrographic transects. Lateral spreading rates derived from the latter compared favourably with estimates derived from a control volume technique in a previous study. The lateral spreading of the plume was driven by a baroclinic pressure gradient toward the base of the plume. However, spreading rates were underestimated by the surface drifters. A convergence of near-surface flow from the barotropic pressure gradient concentrated the drifters within the plume core. The combination of enhanced internal turbulence stress and mixing at the base of the surface layer, and the presence of steep fjord sidewalls likely reduced the rate of lateral spreading relative to the theoretical spreading rate. The estimates of plume width from the observations provided evidence of scale-dependent dispersion which followed a 4/3 power law. Two theoretical models of dispersion, turbulence and shear flow dispersion, were examined to assess which was capable of representing the observed spreading. An analytical horizontal shear-flow dispersion model generated estimates of lateral dispersion that were consistent with the observed 4/3 law of dispersion. Therefore, horizontal shear dispersion appeared to be the dominant mechanism of dispersion, thus spreading, in the surface plume layer.

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Putting prey into the picture: improvements to species distribution models for bottlenose dolphins in Doubtful Sound, New Zealand

Species distribution models (SDMs) often rely on abiotic variables as proxies for biotic relationships. This means that important biotic relationships may be missed, creating ambiguity in our understanding of the drivers of habitat use. These problems are especially relevant for populations of predators, as their habitat use is likely to be strongly influenced by the distribution of their prey. We investigated habitat use of a population of a top predator, bottlenose dolphins Tursiops truncatus, in Doubtful Sound, New Zealand, using generalised additive models, and compared the results of models with and without biotic predictor variables. We found that although habitat use by bottlenose dolphins was significantly correlated with abiotic variables that likely describe foraging areas, introduction of biotic variables describing potential prey almost doubled the deviance explained, from 19.8 to 39.1%. Biotic variables were the most important of the predictors used, and indicated that the dolphins showed a preference for areas with a high abundance of a reef fish, girdled wrasse Notolabrus cinctus. For the dolphins of Doubtful Sound, these results show the importance of prey distribution in driving habitat use. On a broader scale, our results indicate that making an effort to include true biotic descriptors in SDMs can improve model performance, resulting in better understanding of the drivers of distribution of marine predators.

Long‐term stability in core habitat of an endangered population of bottlenose dolphins (Tursiops truncatus): Implications for spatial management

Abstract A small population of approximately 68 bottlenose dolphins, Tursiops truncatus, resident in Doubtful Sound, New Zealand, is subject to physiologically challenging conditions, and is exposed to anthropogenic pressure from tourism. A voluntary Code of Management incorporating dolphin protection zones (DPZs), in which tour boat access is limited, was established in 2008. Kernel density estimation (KDE) was used to quantify dolphin habitat use over a 13‐year period in order to describe seasonal variation in habitat use and consistency of habitat use over a decadal period, and to provide quantitative estimates of the extent of overlap between DPZs and core areas (50% volume contour) of habitat use. Habitat use varied seasonally, with the inner fjord area used more frequently in warmer months, and with a shift in use to the outer fjord in colder months. Patterns in habitat use were highly consistent over the 13‐year duration of the study. The spatial overlap between the area of core dolphin habitat and DPZs was low (<18%) overall, and some DPZs were rarely used during colder periods. Consistency in habitat use through time vindicates spatial management, but low overlap between core habitat and current DPZs suggests that an expansion of the DPZ areas would confer greater protection.

The role of turbulence and internal waves in the structure and evolution of a near-field river plume

Abstract. An along-channel momentum budget is quantified in the near-field plume region of a controlled river flow entering Doubtful Sound, New Zealand. Observations include highly resolved density, velocity and turbulence, enabling a momentum budget to be constructed over a control volume. Estimates of internal stress (τ) were made from direct measurements of turbulence dissipation rates (ϵ) using vertical microstructure profiles. High flow speeds of the surface plume over 2 m s−1 and strong stratification (N2∼10-1 s−2) resulted in enhanced turbulence dissipation rates (ϵ>10-3 W kg−1) and internal stress (τ>10-2 m2 s−2) at the base of the surface layer. Internal waves were observed propagating along the base of the plume, potentially released subsequent to a hydraulic jump in the initial 1 km downstream of the plume discharge point. The momentum flux divergence of these internal waves suggests that almost 15 % of the total plume momentum can be transported out of the system by wave radiation, therefore playing a crucial role in the redistribution of momentum within the near-field plume. Observations illustrate that the evolution of the momentum budget components vary between the distinct surface plume layer and the turbulent, shear-stratified interfacial layer. Within the surface plume, a momentum balance was achieved. The dynamical balance demonstrates that the deceleration of the plume, driven by along-channel advection, is controlled by turbulence stress from the plume discharge point to as far as 3 km downstream. In the interfacial layer, however, the momentum equation was dominated by the turbulence stress term and the balance was not closed. The redistribution of momentum within the shear-stratified layer by internal wave radiation and other hydraulic features could account for the discrepancy in the budget.

Dolphins and Boats: When Is a Disturbance, Disturbing?

Globally, exploitation of marine mammals has shifted from hunting to viewing over the last few decades. While refraining from actively killing animals may have a positive effect on marine mammal populations, whale and dolphin watching can induce changes such as displacement from preferred habitat and disruption of foraging that may also have severe fitness costs. Under some circumstances, this non-lethal disturbance may affect populations in a manner similar to directed mortality. Here, we focus on inshore dolphin populations that are known to show short-term behavioural responses to boat approaches. Long-term fitness effects have only been clearly identified in a small number of these populations, and all share certain characteristics, i.e. closed, small and food limited. This raises the question of importance of context when considering the long-term effects of disturbance, since many dolphin populations may be open, large, and/or free from resource restriction. We explored the effect of disturbance based on the characteristics of populations using the population consequences of disturbance (PCoD) framework. PCoD was developed to link short-term changes in individual behaviour and physiology to presumed long-term effects on population dynamics. To ensure our scenarios were biologically plausible, they reflected the ecological context of four well-studied populations of dolphins, Doubtful Sound NZ, Sarasota Bay USA, Durban Bay SA and Jervis Bay AUS, in terms of their size, closure and food resources. We found that the characteristics of the populations being disturbed are important with regards to the level of disturbance that could be tolerated. Closed populations were most sensitive, while large, open populations with no food limitation appeared to be able to withstand a higher probability of disturbance. This implies that population characteristics should be accounted for when determining the suitability of whale and dolphin watching operations in a given area.

Turbulent Scales Observed in a River Plume Entering a Fjord

AbstractObservations of turbulent mixing are quantified in the near‐field region (first several kilometers) of a controlled river flow entering Doubtful Sound, New Zealand. Measurement techniques include direct turbulence measurements using microstructure profilers in both Eulerian and quasi‐Lagrangian perspectives and turbulent overturn analysis. Over 500 profiles of vertical microstructure were collected within the inner fjord. With high flow speeds of the surface plume over 2 m/s and some of the strongest stratification (N2∼10−1 s−2) recorded in the coastal ocean, turbulent overturning (Thorpe, LT) scales are used to indirectly infer turbulence dissipation rates (ϵ) and compared with direct measurements in a quasi‐idealized laboratory setting. There exists an inverse relationship between directly measured estimates of ϵ and distance from the plume discharge point. Peak values of ϵ>10−2 W/kg are observed within 500 m from the discharge point, among the highest turbulence dissipation rates recorded in oceanic shear flows. Thorpe scales range from below 1 cm in the pycnocline to a few meters in the weakly stratified ambient water below the plume. Stratification within the surface layer constrains the vertical dimension, limiting LT. Therefore, LT is an unreliable estimator of L0, where LO is the Ozmidov scale and is used to infer ϵ. Care must thus be taken when applying this method of overturn analysis in an environment where stratification or physical boundaries limit the vertical extent of overturns.

PATTERNS OF UNREPAIRED SHELL DAMAGE IN RECENT BRACHIOPODS FROM FIORDLAND (NEW ZEALAND)

In order to provide quantitative data concerning patterns of shell breakage and repair in rhynchonelliform brachiopods, we studied undisturbed death assemblages from a New Zealand fiord complex where three species of terebratulide and one rhynchonellide occur in dense mixed patches on the near vertical walls. Proportions of damaged (both repaired and non-repaired) individuals varied between both taxa and sampling site. However, the general observation was that few individuals show signs of having been able to repair damage but the proportion of individuals showing unrepaired, and hence presumably fatal, breakages was higher (up to 76% in Magasella sanguinea from one sample from Tricky Cove in Doubtful Sound). Damage was mostly concentrated around the anterior margins and affected both valves and is consistent with having been clamped between a set of either jaws or claws. Potential culprits include fish (wrasse), rock lobsters and echinoids. As yet it is unclear whether the damage results from deliberate feeding activity or as collateral damage from grazers feeding on other organisms on the fiord walls which may allow secondary predation by asteroids. The net effect is, however, the same, in that the damage appears to have been fatal. More structured sampling is now required to understand the spatial variation in this damage and mortality, and also to establish the culprits with more certainty.

A time-based method for defining associations using photo-identification

Photo-identification is an invaluable method for documenting associations. Based on the assumption that individuals photographed close together in time are physically close in space, the metadata associated with digital photography offers an opportunity to base association analyses on time between images. This was tested via analysis of associations within a population of bottlenose dolphins (Tursiops truncatus) in Doubtful Sound, New Zealand. We compared the widely used group-membership method and an alternative time-based method. Overall social structures between methods were similar; high degrees of association among all individuals and little support for sub-groups. Results also indicated an increase in the precision of pairwise indices for the time-based method. This study validated the approach of using time as a basis for analyses of associations. Importantly, this method can be retrospectively applied to any photo-ID data set in which images of uniquely identifiable individuals are time-stamped by the camera.

Biospheric and petrogenic organic carbon flux along southeast Alaska

Holocene fjords store ca. 11–12% of the total organic carbon (OC) buried in marine sediments with fjords along southeast (SE) Alaska possibly storing half of this OC (Smith et al., 2015). However, the respective burial of biospheric (OCbio) and petrogenic OC (OCpetro) remains poorly constrained, particularly across glaciated versus non-glaciated systems. Here, we use surface sediment samples to quantify the sources and burial of sedimentary OC along SE Alaska fjord-coastal systems, and conduct a latitudinal comparison across a suite of fjords and river-coastal systems with distinctive OC sources. Our results for SE Alaska show that surface sediments in northern fjords (north of Icy Strait) with headwater glaciers are dominated by OCpetro, in contrast to marine and terrestrially-derived fresh OC in non-glaciated southern fjords. Along the continental shelf of the Gulf of Alaska, terrestrial OC is exported from rivers. Using end-member mixing models, we determine that glaciated fjords have significantly higher burial rates of OCpetro (∼1.1×103 gOC m−2yr−1) than non-glaciated fjords and other coastal systems, making SE Alaska potentially the largest sink of OCpetro in North America. In contrast, non-glaciated fjords in SE Alaska are effective in burying marine OC (OCbio-mari) (13–82 g OC m−2yr−1). Globally, OC in fjord sediments are comprised of a mixture of OCpetro and fresh OCbio, in contrast to the pre-aged OC from floodplain river-coastal systems. We find that there may be a general latitudinal trend in the role of fjords in processing OC, where high-latitude temperate glacial fjords (e.g., Yakutat Bay, SE Alaska) rebury OCpetro and non-glacial mid-latitude fjords (e.g., Doubtful Sound, Fiordland) sequester CO2 from phytoplankton and/or temperate forests. Overall, we propose that fjords are effective in sequestering OCbio and re-burying OCpetro. Based on our study, we hypothesize that climate change will have a semi-predictable impact on fjords' OC cycling in the near future.

Boat-based tourism and bottlenose dolphins in Doubtful Sound, New Zealand: The role of management in decreasing dolphin-boat interactions

Indirect non-lethal effects of wildlife tourism have the potential to compromise the long-term health of animal populations. While appropriate management of impacts is clearly necessary, such management is rarely reported to be effective. Doubtful Sound (New Zealand) has boat-based scenic cruises running year-round. This fjord is also home to an endangered population of bottlenose dolphins, a natural asset for the local tourism industry. A voluntary code of management (COM) was implemented in 2008 to alleviate vessel impacts, establishing guidelines to leave dolphin encounters to chance and restricting vessel traffic in areas of critical habitat. The frequency and duration of interactions decreased substantially since the implementation of the COM. This evidence shows that a science-based voluntary agreement has the potential to mitigate tourism impacts. Nevertheless, due to the small size of the population and its history of low calf survival, a precautionary approach is necessary to further reduce current anthropogenic impacts.

Thermochronology of extensional orogenic collapse in the deep crust of Zealandia

The exhumed Fiordland sector of Zealandia offers a deep-crustal view into the life cycle of a Cordilleran-type orogen from final magmatic construction to extensional orogenic collapse. We integrate U-Pb thermochronologic data from metamorphic zircon and titanite with structural observations from >2000 km 2 of central Fiordland to document the tempo and thermal evolution of the lower crust during the tectonic transition from arc construction and crustal thickening to crustal thinning and extensional collapse. Data reveal that garnet granulite facies metamorphism and partial melting in the lower crust partially overlapped with crustal thickening and batholith construction during emplacement of the Western Fiordland Orthogneiss (WFO) from 118 to 115 Ma. Metamorphic zircons in metasedimentary rocks yield 206 Pb/ 238 U (sensitive high-resolution ion microprobe–reverse geometry) dates of 116.3–112.0 Ma. Titanite laser ablation split stream inductively coupled plasma–mass spectrometry chronology from the same rocks yielded complex results, with relict Paleozoic 206 Pb/ 238 U dates preserved at the margins of the WFO. Within extensional shear zones that developed in the thermal aureole of the WFO, titanite dates range from 116.2 to 107.6 Ma and have zirconium-in-titanite temperatures of ∼900–750 °C. A minor population of metamorphic zircon rims and titanites in the Doubtful Sound region yield younger dates of 105.6–102.3 Ma with corresponding temperatures of 740–730 °C. Many samples record Cretaceous overdispersed dates with 5–10 m.y. ranges. Core-rim traverses and grain maps show complex chemical and temporal variations that cannot easily be attributed to thermally activated volume diffusion or simple core-rim crystallization. We interpret these Cretaceous titanites not as cooling ages, but rather as recording protracted growth and/or crystallization or recrystallization in response to fluid flow, deformation, and/or metamorphic reactions during the transition from garnet granulite to upper amphibolite facies metamorphism. We propose a thermotectonic model that integrates our results with structural observations. Our data reveal a clear tectonic break at 108–106 Ma that marks a change in processes deep within the arc. Prior to this break, arc construction processes dominated and involved (1) emplacement of mafic to intermediate magmas of the Malaspina and Misty plutons from 118 to 115 Ma, (2) contractional deformation at the roof of the Misty pluton in the Caswell Sound fold-thrust belt from 117 to 113 Ma, and (3) eclogite to garnet granulite facies metamorphism and partial melting over >8 m.y. from 116 to 108 Ma. These processes were accompanied by complex patterns of lower crustal flow involving both horizontal and vertical displacements. After this interval, extensional orogenic collapse initiated along upper amphibolite facies shear zones in the Doubtful Sound shear zone at 108–106 Ma. Zircon and titanite growth and/or crystallization or recrystallization at this time clearly link upper amphibolite facies metamorphism to mylonitic fabrics in shear zones. Our observations are significant in that they reveal the persistence of a hot and weak lower crust for ≥15 m.y. following arc magmatism in central Fiordland. We propose that the existence of a thermally weakened lower crust within the Median Batholith was a key factor in controlling the transition from crustal thickening to crustal thinning and extensional orogenic collapse of the Zealandia Cordillera.

Factors influencing heterogeneity in female reproductive success in a Critically Endangered population of bottlenose dolphins

For threatened species or populations, variation in reproductive success among females may be explicitly linked with vulnerability to extinction. Thus, an understanding of factors that may cause variability in reproductive success is important. The population of bottlenose dolphins in Doubtful Sound, New Zealand, has a recent history of rapid population decline and low calf survival rates. A previous study has shown high variability in calf survival among multi- parous females. This study addresses the factors that seem most important in explaining variation in calf survival and thus reproductive success among females in this population. Reproductive data were sourced from a long-term photo-identification dataset, which allowed tracking the fate of 49 calves born into the population between 1995 and 2012. General linear mixed models combined with model averaging were used to assess how birth timing, maternal size, age and potential anthropogenic impacts contributed to variation in calf survival. Models show that a female's size and her ability to give birth at an optimum time in the calving season are significant predictors of calf survival to an age of 1 and 3 yr. This is the first study to demonstrate how birth timing and mother size are correlated with female reproductive success in a cetacean species. These results confirm the importance of demographic stochasticity and reproductive hetero - geneity in small, threatened marine mammal populations.

Thompson and Bradshaw sounds, Fiordland, New Zealand: a relict, mid-latitude, temperate glacier system

Fjords are among the most spectacular glacial landforms on Earth (Holtedahl 1967; Syvitski et al. 1987; Benn & Evans 2010). New Zealand fjords are deep valleys cut into metamorphic basement rocks by former mid-latitude glaciers and flooded by seawater following glacial ice retreat (Pickrill et al. 1992). They are now ice free, 15–45 km long with maximum water depths of 200 to >400 m, and are surrounded by forested mountains rising to >2000 m. Their distinctive landscapes, geomorphology and sedimentary deposits reflect their past glaciation followed by deglaciation and marine transgression (Syvitski et al. 1987). Thompson and Bradshaw sounds comprise a single, 35 km long, 430 m deep fjord that is sinuous, very steep-sided with U-shaped morphology and is typically 1.5 km wide (Fig. 1). Precipice Cove (PC) is the only significant marine tributary, whereas a <50 m deep, rocky marine saddle joins the fjord to Doubtful Sound (DS) (Fig. 1a). Hanging valleys are common landscape features above the lower walls of the …

Transient river flow into a fjord and its control of plume energy partitioning

AbstractThe influence of variable inflows on near‐field plume dynamics and energy partitioning was examined using observations of a controlled flow into Doubtful Sound, New Zealand. The high temporal changes in flows passing through the Manapouri hydroelectric power station mimic the magnitude and variability seen in small mountainous river systems (SMRS) globally. The variable flow coupled with strong vertical density gradients akin to ambient conditions in coastal systems enabled plume behavior to be characterized for differing flow, wind, and tidal inputs in a quasi‐idealized “laboratory” system. Comparisons of the frequency distributions of energy for different forcing conditions showed that baroclinic and barotropic processes were closely intertwined for transient forcing. The periodicity of density due to tidal oscillations was initially absent; headwaters of the fjord absorbed the momentum when inflows were substantially increased from the mean of ∼420 m3 s−1. From the buoyancy frequency squared N2, six events were identified when N2 was greater than 0.07 s−2. Seven occurrences of supercritical flow (Froude number, &gt; 1) and associated transitions to subcritical flow were observed over the duration of mooring deployment. Transient inflows induced internal hydraulic jumps in the near‐field region which lead to a rapid breakdown of vertical stratification. The horizontal length scale of an internal hydraulic jump is O (1 km). Not all transitions from &gt; 1 to &lt;1 are explained by transient forcing and the role of strong, up‐fjord wind stress is likely to initiate supercritical flows via setup at the head of the fjord.

A pattern-oriented model for assessing effects of weather and freshwater discharge on black coral (Antipathes fiordensis) distribution in a fjord

Doubtful Sound and other New Zealand fjords are unique in that they provide suitable habitat for a black coral species (Antipathes fiordensis) at depths as shallow as 4m. Research suggests the upper depth limit at which black corals can survive in Doubtful Sound is controlled by gradients in salinity (as opposed to light levels) within buoyant low-salinity layers (LSL). The thickness of the LSL in Doubtful Sound varies along the fjord and fluctuates as a function of rain, wind and freshwater tailrace discharge from the Manapouri Hydroelectric Power Station. A pattern-oriented survivorship model was developed to predict mortality and upper-depth distribution of black corals in Doubtful Sound in response to changes in the LSL. Model parameters were estimated using observed patterns of black coral distribution and time-series salinity measurements over a range of depths. Predicted upper depth limits in Doubtful Sound and a reference fjord (Milford Sound) were validated against field observations of black coral depth distributions on the adjacent rock walls. The model accurately forecasted depths of corals in mid to outer regions of Doubtful Sound, but underestimated upper depth limits in the inner region of both fjords. The discrepancies in inner fjord locations suggest that other factors, such as sedimentation, are also limiting establishment of long-lived corals in some locations. Data from hydrodynamic simulations of the LSL in Doubtful Sound were fed into the survivorship model to investigate black coral distributions under varying rain, wind and tailrace discharge scenarios. Results indicate that weather, rather than tailrace discharge, drives long-term patterns in the vertical distribution of black corals in Doubtful Sound.

Photo-identification of bottlenose dolphins in the far south of New Zealand indicates a ‘new’, previously unstudied population

Declines in the abundance of bottlenose dolphins in the Bay of Islands and in Doubtful Sound have contributed to the species being classified as Nationally Endangered in New Zealand waters. Updated information on distribution and abundance nationwide is therefore a high priority. This study presents data from the first photo-identification surveys of Paterson Inlet, Stewart Island, conducted to document bottlenose dolphin presence, abundance and residency. Open-population mark-recapture models indicate that 18 (95%CI = 15–20) dolphins regularly use Paterson Inlet. Photo-identification of dolphins from unknown populations during two chance encounters in Otago Harbour and Dusky Sound included 11 individuals previously identified at Stewart Island. These results indicate that dolphins found regularly at Stewart Island are part of a larger, wide ranging southern population with a minimum population abundance of 92 (95%CI = 80–111) individuals.

Effects of boats on the surface and acoustic behaviour of an endangered population of bottlenose dolphins

Coastal cetaceans are increasingly being exposed to boats and noise as nature tourism grows. Such activity has a wide range of detrimental effects on the surface behaviour of cetaceans, but effects on their acoustic behaviour are poorly understood. We quantified the effects of tour boats and of the observing research boat on the group structure and vocal behaviour of bot- tlenose dolphins Tursiops truncatus in Doubtful Sound, New Zealand. Acoustic recordings and group follow data were collected from a 5 m research vessel, and analysed via an information- theoretic approach. Groups with mothercalf pairs were significantly less cohesive and coordi- nated when tour boats were audible. They were more vocal when boats were close and while moving away, presumably to re-establish group structure. Furthermore, groups with calves increased their whistle rates when tour boats were travelling faster, while groups without calves became quieter. Dolphins also responded to boat noise with alterations in whistle frequency and duration. These findings suggest that elevated boat noise affects communication, and groups with calves are particularly sensitive to boat presence and noise. Group structure and whistle parame- ters were affected by the research boat, highlighting the importance of accounting for observer effects in studies of tourism impacts. The particular sensitivity of groups with calves to boats has important implications for the management of impacts on this population due to its endangered status and history of low calf survival.