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To
study the distribution, abundance and dynamics of an animal population, it is
essential to be able to follow individuals within a population through time.
Where species are only visible for a brief moment, photo-identification (or
photo-ID) studies must be used. In these cases, individuals are photographed
in the field and the resulting photographs provide a permanent record of the
encounter which can later be compared with a catalogue of existing pictures.
Photo-identification techniques have been used on a wide variety of cetacean
species, and a few studies now extend for periods of over twenty years.
Together, these studies have greatly improved our understanding of the distribution and abundance of cetaceans, and photo-ID has provided a basis for expanding studies of other aspects of cetacean biology, for example social organisation and foraging behaviour.
For many years it has been recognised that the Moray Firth was one of the few places in UK waters where bottlenose dolphins could be seen on a regular basis. On the other hand, little was known about the ecology of these animals.
In response to concern over the status of these animals, the University of Aberdeen and the NERC Sea Mammal Research Unit (AU/SMRU) started a joint study of this population in 1989. Photo-ID techniques have allowed us to estimate the size of this population, which appears to number around 130 individuals. Observations of the movements of recognisable dolphins have shown that the same individuals may be seen in the Moray Firth year after year, but that others spend some of their time along the Grampian coast. Studies of associations between different individuals have highlighted both similarities and intriguing differences in the social behaviour of these bottlenose dolphins and populations of the same species in tropical areas.
It is now over seven years since this study began, and our understanding of this population has much improved. But bottlenose dolphins are long-lived mammals. Females reach sexual maturity at 5-12 years of age, after which they may produce a calf only every 2-3 years throughout their 40-50 year life span. Clearly, assessments of the population's status and the factors which affect the success, or otherwise, of these animals require long-term studies. For example, whilst we know that there are around 130 dolphins in the population, we also recognise that it could be into the next century before we can determine whether their numbers are increasing or declining.
The Moray Firth is currently listed as a possible Special Area of Conservation (pSAC) under the EC Habitats Directive. Designation as an SAC will require a management scheme to be established to protect and enhance the dolphins' habitats within the Moray Firth. Whilst research must continue to assess the status of this population and the extent of the threats which it may face, it is also timely to review the existing data on the distribution, behaviour and life-history patterns of the Moray Firth dolphins. The primary aim of this report is to outline the methodology used in our photo-ID study of this population, and to summarise the data for those individual dolphins which have been observed most regularly. These data should provide an important basis for the development of management and monitoring programmes for the Moray Firth pSAC. Furthermore, we hope that the information on individual dolphins will provide a useful resource for the environmental education programmes which are likely to form a crucial component of management schemes.
On the face of it, photo-ID is a relatively straightforward process: photograph the dorsal fins of the dolphins, decide whether the markings match those from previous photographs and then store that information. However, there are many factors that complicate this simple scheme, notably the difficulties in obtaining pictures, the ambiguous nature of the markings on the dolphins and the huge number of matching decisions that this process generates. High quality pictures must be obtained, the analysis and cataloguing techniques must be simple and accountable, whilst being flexible enough so that mistakes in matching or record keeping can be traced and rectified. Finally, the resulting information must be easily accessible so that further analyses can be carried out.
Most data have been collected during monthly boat surveys which follow a standardised route from Cromarty, to a point just north of the mouth of the Cromarty Firth, and then west to Inverness before returning to Cromarty (Fig. 1). This route was chosen because it passed through several areas where dolphins were seen frequently (for example the Chanonry Narrows), but was short enough so that it could be completed within the shortest of winter days. Surveys were generally made from a small 5.5m rigid hull boat with an outboard motor, and were only carried out on calm days when the sea state was Beaufort 3 or less and when light conditions were suitable for photography. In 1989, photo-ID surveys were less regular and mostly conducted around the mouth of the Cromarty Firth. Standardised surveys were started in the summer of 1990, and have been made once or twice each month since February 1991.
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A map of the inner Moray Firth showing the standard route followed for photo-ID surveys. |
Conducting one of the dolphin photo-ID surveys. |
After starting a survey, the standard route was followed until a group of dolphins were sighted (referred to as an encounter) We then slowed the boat and carefully manoeuvred onto a track parallel to, and approximately 20-50m from, the dolphins. At this point we recorded our position using local landmarks, Decca, or a Global Positioning System (GPS) and estimated the number of dolphins in the group. Subsequent boat movements and changes in engine speed were minimised in line with the Dolphin Awareness code.
Where possible, we then photographed each of the dolphins in the group. However, this was not always successful for larger groups, in poorer weather conditions or if groups dispersed during the encounter.
Material arising from surveys consisted of several exposed films of dolphin photographs along with notes describing the exact route on which the survey was carried out and separate sheets with general information about each group of dolphins encountered.
Once slide films were processed, the information from each boat survey could be analysed. Each film was numbered, and stored in transparent sheets with an associated sheet detailing the material on each frame. Pictures were then examined with an 8x magnifier to determine whether dolphins in the photograph had marks which permitted them to be recognised. For each encounter, the best picture of each individual dolphin was then mounted in a 35mm slide mount. These could then each be compared with existing pictures in the photo-archive using the slide magnifier or a projector. If a match was found, it was added to the appropriate hanging file for that dolphin. If there were distinctive marks which could not be matched with an existing animal, it was given a new identification number. General information (eg. date, time and location) for each encounter, together with data on which individual dolphins were present in the group, were then recorded on a PC-based relational database.
We have used a wide variety of identification marks to recognise different dolphins from the Moray Firth. These include some obvious and permanent marks, such as pieces missing from the trailing edges of the dorsal fins (nicks), others which may or may not be permanent, such as de-pigmented areas, and a range of scratches and skin lesions which might last for periods of only days to a year or two (Fig. 2). Some of these marks (such as nicks) remain useful throughout a long- term study. In other cases (such as rake marks), they can be used only to identify the animal over a period of a year or so.
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(a) Dorsal fin nicks |
(b) De-pigmented areas |
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c) Skin lesions |
(d) Rake marks |
Fig. 2. Examples of some of the main types of natural marks used to identify individual bottlenose dolphins in the Moray Firth population.
If that animal is photographed regularly, it may be possible to keep track of it by following the gradual loss of old marks and gain of new ones. Inevitably, some animals may lose their marks before we manage to re-photograph them but may then be photographed once again with new marks. As there would be no way of linking the two sets of pictures, they would then be classified as a new individual. Thus, over the years we have built up a photo-archive of over 395 "individuals", but we know that some dolphins may appear in that archive more than once due to the problems outlined above. Similarly, it may not always be possible to photograph both left and right sides of a dolphin's dorsal fin, and thus the same individual may appear in the archive twice, once for each side, until we eventually have pictures from an encounter which allows us to link the two. Furthermore, new animals are born, and recruit into the population, while others die. Consequently, the number of animals in the archive does not provide an estimate of the number of dolphins in the Moray Firth population. Instead, a minimum estimate of population size can be produced by looking at the pictures from a particular time-period, and counting the number of different individuals with photographs of either their left or right side. These data can also be applied to a mark-recapture model which uses the proportion of photographed individuals in several separate samples (time periods) to provide an estimate of abundance. Our current population estimate of 130 was obtained in this way by analysing data from dolphins with nicks in their dorsal fin.
Lighthouse Field Station
University of Aberdeen · George Street · Cromarty · Ross-shire IV11 8YJ · Scotland
Telephone +44 (0)1381-600548 · Fax +44 (0)1381-600548 · Email: lighthouse@abdn.ac.uk
School of Biological Sciences
Last modified: Monday, 05-May-2008 17:04:49 BST
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