Indo-Pacific bottlenose dolphin (Tursiops aduncus) habitat preference in a heterogeneous, urban, coastal environment
© Cribb et al.; licensee BioMed Central Ltd. 2013
Received: 13 July 2012
Accepted: 7 January 2013
Published: 1 February 2013
Limited information is available regarding the habitat preference of the Indo-Pacific bottlenose dolphin (Tursiops aduncus) in South Australian estuarine environments. The need to overcome this paucity of information is crucial for management and conservation initiatives. This preliminary study investigates the space-time patterns of habitat preference by the Indo-Pacific bottlenose dolphin in the Port Adelaide River-Barker Inlet estuary, a South Australian, urbanised, coastal environment. More specifically, the study aim was to identify a potential preference between bare sand substrate and seagrass beds, the two habitat types present in this environment, through the resighting frequency of recognisable individual dolphins.
Photo-identification surveys covering the 118 km2 sanctuary area were conducted over 2 survey periods May to August 2006 and from March 2009 to February 2010. Sighting frequency of recognisable individual Indo-Pacific bottlenose dolphins established a significant preference for the bare sand habitat. More specifically, 72 and 18% of the individuals sighted at least on two occasions were observed in the bare sand and seagrass habitats respectively. This trend was consistently observed at both seasonal and annual scales, suggesting a consistency in the distinct use of these two habitats.
It is anticipated that these results will benefit the further development of management and conservation strategies.
Cetacean habitats are typically heterogeneous, comprising a mosaic of patches which differ in their biological and physical properties . Understanding the space-time movement patterns and distribution of organisms within their environments can provide insight into the preference of specific areas ; information considered essential in the development of management and conservation initiatives . In this context, bottlenose dolphins (Tursiops spp.) are no exception. They occur globally in both temperate and tropical waters [3, 4], and are common in coastal waters, in particular estuaries, over a wide range of habitat types, such as seagrass beds, sandy substrates and reefs [5–8]. The occurrence of bottlenose dolphins in different habitats illustrates the ecological plasticity and adaptability of this species [2, 9–11]. This highlights the need to understand at the individual and population level the key habitat types and locations they preferentially frequent . This is especially critical for populations frequenting coastal environments, which are increasingly impacted by anthropogenic activities, such as tourism, chemical and noise pollution, habitat degradation, commercial and recreational fisheries and aquaculture [13–19], thus making them more susceptible to threats [20, 21].
The Indo-Pacific bottlenose dolphin (Tursiops aduncus) is a prime example of a coastal dolphin species with many populations throughout the Indo-Pacific region , and more specifically Australia, where they are found in a range of coastal environments such as bays, gulfs, lagoons and estuaries that are often highly urbanised [8, 16, 23, 24]. However, little is still known about this species habitat preference in estuarine locations .
In South Australian waters, T. aduncus is a known resident, especially in the Port Adelaide River – Barker Inlet estuary, where animals have been recorded year-round over the past 18 years . This area supports a small population of approximately 30 resident individuals as well as visiting non-regular transient animals [25, 26]. Field observations indicate no other marine mammals, specifically delphinids, living in direct sympatry with this population. Fur seals and sea lions were, however, punctually observed hauled out within the study site. The Port Adelaide River – Barker Inlet estuary is situated in close vicinity to the city of Adelaide, hence it is highly urbanised and subjected to a variety of anthropogenic activities such as industrial and sewage pollution, recreational and commercial vessel traffic, dredging, urban development and habitat degradation [19, 27–31]. As a result this area was proclaimed the Adelaide Dolphin Sanctuary (ADS) in 2005 in order to protect both the resident dolphins and their habitat .
Baseline habitat information is, however, still scarce and limited to the presence of bottlenose dolphins being independent of environmental features . This potentially limits the development and implementation of effective conservation and management strategies, hence the long term-survival of this population. This also stresses the need to further understand and monitor the preference of habitats within this area at both the seasonal and annual scales, and to identify potential areas of high occurrence of specific individuals. In this context, the objective of this paper was to use photo-identification to assess whether recognisable individuals were consistently sighted in the same benthic habitat type at both seasonal and annual scales.
Number of survey days shown as a function of both season and photo-identification survey periods
Photo-Identification Survey Periods
Survey Period 2006
Survey Period 2009 – 2010
Photo-identification of bottlenose dolphins relies on the matching of distinctive dorsal fin features, such as nicks and notches present on both the trailing and leading edges of the fin, and tip [36, 37]. Photographs were assessed for photographic quality (e.g. focus, clarity, contrast, angle, portion of the fin visible and the percentage of picture filled by the fin) and graded according to quality (excellent, average, poor) using Adobe Photoshop Elements 5.0 imaging software. Only those photographs considered to be of excellent quality were included in the analysis. Poor quality photographs were always discarded from the analysis. Photographs were checked systematically against each other to develop a master catalogue of recognisable individuals and to determine the number of re-sights. The individuals not matched with animals previously recorded were given a unique identification number and added to the catalogue.
The statistical package PWAS for Windows, version 18, was used for all statistical analysis. As the data failed to meet the assumptions of normality (Kolmgorov-Smirnov test, p < 0.05), non-parametric tests were therefore used to make comparisons between data sets. In order to explore the habitat preference of bottlenose dolphins in the ADS the resighting frequency of individuals (i.e. the sighting frequency of recognisable individuals seen at least on two or more occasions) was estimated for each benthic habitat type. Resighting frequencies were also assessed to identify potential habitat preference between seasons, defined as spring (September - November), summer (December - February), autumn (March - May) and winter (June - August), and years. Additionally, the resighting frequencies were examined to identify habitat preferences on an individual level. Sighting frequencies between habitats were compared using the χ2 test . Specifically, our survey equally covered the two habitat types; hence we compared the observed habitat preference frequencies to theoretical frequencies (50% – 50%).
Survey and photo-identification effort
Individual habitat preference
Indo-Pacific bottlenose dolphin habitat preference in the ADS
Our observations of dolphin presence and significantly higher sighting frequency in the bare sand habitat (76%; Figure 3A) at both the seasonal and annual scales (Figure 3B,C) and the significantly higher resighting frequency in the same habitat (69%; Figure 4A) are consistent with the previously reported regular occurrence and preference of bottlenose dolphins in one habitat over another [1, 6, 7, 41, 42]. While further work is needed to specifically address this issue, our results suggest the presence of a seasonal pattern in habitat preference with an increase in dolphin frequency in the seagrass habitat in autumn and summer (Figure 3B). Seasonal shifts and variations in habitat preference by bottlenose dolphins have also been observed in other locations such as the San Luis Pass (Texas, USA; ), the Moray Firth (Scotland; ) and the Hauraki Gulf (New Zealand; ). However, the occurrence of nearly one-third of the individuals (31%; Figure 4A) over both the bare sand and the seagrass habitats may indicate that a non-negligible proportion of the T. aduncus occurring in the ADS have enough behavioural flexibility to use the seagrass beds found in the open waters of Gulf St. Vincent as well as the sheltered waters found in the inner estuarine part of the ADS (Figure 1). More specifically, respectively 72 and 28% of the resighted individuals were observed over the bare sand and the seagrass habitats (Figure 4B). This suggests that the bare sand habitat may be a core area for this population, in contrast to previous work stressing the vital role of seagrass beds for bottlenose dolphins [41, 42]. However, further investigation into the behavioural budget of this population is needed to determine how and why these habitats differ in their importance and use.
Estuaries as important dolphin habitats
On the influence of sex and social structure on habitat preference
The frequency of the same individuals within the same habitat over time (Figure 4) may be linked to other factors such as social organisation and association patterns . Specifically, bottlenose dolphin habitat preference has been explained by the home range of individuals and the social strategies which individuals or different sexes adopt [1, 57]. It has been suggested that protected, shallow and narrow waterways which are geographically further from the open ocean such as the bare sand environment in the present work (see Figure 1), generally promote limited movement patterns and therefore some degree of site fidelity [47, 58]. In contrast, individuals found in open habitats have more extensive home ranges and a lesser degree of site fidelity [47, 58]. The individuals resighted consistently over time in the bare sand habitat may hence potentially represent resident individuals, and therefore those sighted on fewer occasions in the seagrass habitat may be transients. Additionally, this sighting frequency may be related to foraging or social specific strategies of male and females. Females have smaller home ranges and frequent more areas which provide a higher concentration of resources, such as estuaries that are important for reproduction and calving and the avoidance of predators [47, 59]. In contrast, males cover wider ranges than females which has been attributed to female breeding cycles and accessibility [45, 59]. As a consequence, the animals sighted consistently in the bare sand habitat might be females utilising local resources, whilst those sighted on fewer occasions in the seagrass may be males searching for females.
Our results show that bottlenose dolphins in the Adelaide Dolphin Sanctuary occur predominantly in a bare sand habitat. The consistent occurrence and resighting of individuals at both the seasonal and annual scale clearly highlight the importance of the sheltered, bare sand habitat for this population. With a paucity of information available on dolphin habitat due to a lack of monitoring and research in this area, these results provide critical information, which can improve conservation and management strategies previously implemented in the ADS . Specifically, it is recommended to monitor future trends in dolphin spatial and temporal habitat preference, as initiated here through photo-identification surveys. Additionally, due to the presence and potential growth of anthropogenic activities in the vicinity of the ADS, it is critical to understand the details of the seasonal patterns of habitat preference and social activities of bottlenose dolphins that will ultimately help in objectively establishing restricted access to specific core locations of the Adelaide Dolphin Sanctuary. We also stress that the approach presented here may be applicable to other anthropogenically impacted coastal environments, where the identification of dolphin habitat preferences might have critical conservation and management implications. Finally, as the driving mechanisms influencing bottlenose dolphin habitat preferences may differ depending on the intrinsic properties of their environment, such as the nature of anthropogenic activities, coastal geomorphology and bottom topography, further studies are needed to understand habitat choice on both local and global scales.
Rob Laver, Verity Gibbs, Brett Williams and staff from the Department of Environment and Natural Resources are acknowledged for providing resources and support in the collection of data. Thank you to Clayton McCloud, Coraline Chapperon, Stephanie Duong, Peter Cribb, and Mike Bossley for their contribution to support in the field. We also thank William Allen for providing comment on an earlier draft of this manuscript. This research was financially supported under Australian Research Council’s Discovery Projects funding scheme (projects number DP0664681 and DP0988554), Marine Innovation South Australia (MISA), the Department of Environment and Heritage and the Flinders Collaborative Research Grant Scheme (FCRGS) from the Faculty of Science and Engineering, Flinders University. Professor Seuront is the recipient of an Australian Professional Fellowship (project number DP0988554). Field work was done under ethics approval from Flinders University (Project E269).
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