Social interactions of bottlenose dolphins which behaved aggressively towards harbour porpoises in the Cardigan Bay SAC in 2014

Transcription

1 Social interactions of bottlenose dolphins which behaved aggressively towards harbour porpoises in the Cardigan Bay SAC in 2014 Rebecca Bakker Student Coastal and Marine Management Van Hall Larenstein, University of Applied Sciences 3

2 Social interactions of bottlenose dolphins which behaved aggressively towards harbour porpoises in the Cardigan Bay SAC in 2014 Cardigan Bay Marine Wildlife Centre New Quay, Wales (UK) Van Hall Larenstein, University of Applied Sciences Leeuwarden, the Netherlands Rebecca Bakker Student Coastal and Marine Management Supervisor CBMWC: Sarah Perry Supervisor Van Hall Larenstein: Tjibbe Stelwagen Front-page picture: Milly Metcalfe, CBMWC. New Quay

3 Acknowledgements Firstly I would like to thank the volunteers of the Cardigan Bay Marine Wildlife Centre of 2010, 2011, 2012, 2013 and 2014 for collecting and processing the data used in this study. I would also like to thank current volunteers Manon Chautard and Anna Stevens for helping to check the photo identification data from 2014, and adding it into the database. I am also grateful for Tjibbe Stelwagen, my supervisor from Van Hall Larenstein, for giving feedback on the reports and actively helping with statistical issues. Finally, I would like to thank Sarah Perry, my supervisor from CBMWC, for giving me the opportunity to do this interesting project and supplying feedback on the project proposal and final report.

4 Abstract The Cardigan Bay Special Area of Conservation (SAC) is inhabited by a semi resident population of bottlenose dolphins (Tursiops truncatus). In the summer of 2014 four attacks of bottlenose dolphins on harbour porpoises (Phocoena phocoena) were recorded and witnessed by staff and volunteers from the Cardigan Bay Marine Wildlife Centre (CBMWC). During the study period from 2010 to 2014 land- and boat based surveys were carried out, during which photographs were taken for photo identification, as well as additional data collected including group size. During the four surveys in 2014 when the attacks were recorded, a total of 26 individuals were seen, of which ten were identified as dolphins already existing in the catalogue. Social network analysis has been applied on three of the individuals with well-marked dorsal fins which were involved in these attacks, #007, #015 and #036; to investigate the social interactions these individuals have with each other, and other members of the population. Data analysis showed that the three individuals did not occur in outstanding group sizes, with a mean group size of 5.61 (SD=5.31) for all three individuals. The association between the three individuals is regarded as being casual (HWI s: #007 - #015= 0.24, #007 - #036= 0.20, #015 - #036= 0.29). However, comparing these associations to associations between the three individuals and other members of the populations, the associations among the three individuals are relatively strong. The three individuals are part of the same cluster based on the modularity, and are relatively closely connected in the hierarchical cluster. It cannot be proven that the behaviour of attacking harbour porpoises is transmitted horizontally throughout the population, however the relative strong association among the three individuals does support this theory. Key words: Tursiops truncatus, group size, social analysis, community structure, harbour porpoises, aggression, socprog.

5 Table of contents 1. Introduction Methods Study area Data collection Photo identification Data analysis Individual identification Group size Social associations Results Individuals Survey efforts Group size # # # All individuals Associations among individuals Associations within the population Dataset population # # # Population analysis Permutation test Discussion Data collection Porpoise attacks Group size Social associations Conclusion Recommendations References... 33

6 Appendix Appendix I: Sighting sheet Appendix II: Bottlenose dolphin encounter form Appendix III: Grading criteria Appendix IV: Population Appendix V: HWI coefficients of association Appendix VI: community division by modularity Appendix VII: Permutation test... 44

7 1. Introduction Bottlenose dolphins (Tursiops truncatus) belong to the class of Mammalia, the order of Cetaceans and the family of Delphinidea. They are part of the Odontoceti class (toothed whales), and are opportunistic feeders, feeding on a wide range of fish species, and occasionally cephalopods, crustaceans and elasmobrancs (Connor et al, 2000). They are found throughout all tropical and temperate seas and oceans, in both coastal and offshore waters (Gregory & Rowden, 2001). Cardigan Bay, Wales, is inhabited by a semi-resident population of bottlenose dolphins throughout the year (Evans et al, 2003). It is known that at least for some individuals this bay does not cover their entire home range (Pesante et al, 2008). Currently there are 178 well-marked individuals in the CBMWC catalogue, this includes animals that have distinctive markings on their dorsal fins and could easily be recognised from photographs on a subsequent occasion. A number of these have only been photographed once or twice, and might not been photographed since then for years. In addition there are a number of less well marked animals that are not so easily recognisable and therefore may be duplicated in the catalogue. The current estimated population size is over 300 individuals (Cardigan Bay SAC, n.d.). The adult bottlenose dolphins here in the bay, which are one of the biggest within their species, have a length which ranges from 3.5 to 4.1 metres (Lockyer & Morris, 1985). Cardigan Bay is also used by Harbour porpoises (Phocoena phocoena), which also belong to the order Cetacean, but to the family Phocoenidae. The porpoises are much smaller than the bottlenose dolphins with a length ranging from 1.4 to 1.7 metres (Hebridean Whale & Dolphin Trust, n.d.). In 2011 it was estimated that there live approximately 565 porpoises in Cardigan Bay (Feingold & Evans, 2013). Simon et al (2010) revealed that when the two species use overlapping locations which leads to inter-specific interactions which can be fatal for the porpoises. In the summer of 2014, four attacks on harbour porpoises by bottlenose dolphins were recorded by the CBMWC, and in all these cases at least two individuals were involved. There have been earlier reports of attacks of bottlenose dolphins on porpoises, which were in some studies related to infanticide (Perrtree et al, 2015), regarding the fact that the size of a harbour porpoise is comparable to the size of a bottlenose dolphin calf (Patterson et al, 1998). There are also researchers suggesting the aggressive behaviour of the dolphins towards the porpoises is due to the food competition among the two species (MacLeod et al, 2007; Spitz et al, 2006), that it is part of the mating process or play behaviour as a practice for fighting (Neale, 2013). However, these are all assumptions, no true reason has been assigned yet. During attacks in the Pacific 92% of the involved dolphins were (expected to be) males, and remarkably attacks occurred during the height of the mating season when males have higher testosterone levels (Cotter et al, 2011). 6

8 The main reasons that bottlenose dolphins live in groups are predator avoidance, more efficient foraging, reproduction and social interactions for sharing information (Lott, 2004). Analysis on the social structure of bottlenose dolphins shows the social network of the populations which can give an insight into how certain information or behaviours can spread among the population. Examples of these information and behaviours are genetic structure (Wiszniewski et al, 2009), the spread of disease (Magileviciute, 2006) and the use of tools (Mann et al, 2012). In the case of the attacks on harbour porpoises, this might also be a kind of information which spreads among the population, instead of it being a collective spontaneous behaviour. Several projects globally have conducted research on the social structure of bottlenose dolphins (e.g. Connor et al, 2000; Titcomb et al, 2015; Lusseau et al, 2003). A characteristic which seems to be applicable to all populations of bottlenose dolphins is the fission-fusion society, in which individuals associate in small groups which change regularly in composition, often on a daily or hourly basis (Connor et al, 2000; Lott, 2004). However, an individual seems to have some relatively permanent associations with other individuals of the population, based on sex and age (Shane et al, 1986). In addition to this social preference for certain individuals, the dolphins also show avoidance of certain individuals (Titcomb et al, 2015). Very strong associations occur between females and their dependent calves, and within some populations there are quite strong associations among males as well, which cooperate by forming super-alliances to gain access to females (Connor et al, 2001; Lott, 2004). Besides these studies on relations among individuals another popular research subject is the learning ability of bottlenose dolphins. In some cases information is spread vertically (also known as oblique) among a population, which means that certain information is only transferred from mother to offspring. When certain information is spread horizontally among the population, this means that the individuals are learning from one another, this is not related to kinship (Sargeant et al, 2005). Some individuals of the population of bottlenose dolphins in Shark Bay, Western Australia, have learned to use sponges as a foraging tool (Krützen et al, 2005). However, DNA analysis show that this behaviour is only spread vertically in the population. Within the same population some individuals developed the skill of beach hunting, which was again transferred vertically, but also expected to spread horizontally among the population (Sargeant et al, 2005). More south along the west coast of Australia, dolphins conditioned to food handouts of people, and research showed that it was more likely for an individual to become conditioned to this when they spent more time in high boat density areas and spent more time with dolphins which were already conditioned to it, suggesting horizontal learning among the population (Donaldson et al, 2012). In Florida bottlenose dolphins showed individual role specialization during group hunting, which is extremely rare for mammals (Gazda et al, 2005). In this case one individual herded the fish into a barrier the other 7

9 individuals formed. It is also known that bottlenose dolphins are capable of vocal learning (Janik, 2013), from their mother and from other members of the population (Fripp et al, 2004), indicating horizontal and vertical learning. To give an insight in the social interactions of the bottlenose dolphins that behaved aggressively towards harbour porpoises in the summer of 2014, the research question is as follows: what social interactions do the bottlenose dolphins have, which behaved aggressively towards harbour porpoises in the summer of 2014? The sub questions are: 1. Which individuals have shown aggressive behaviour towards harbour porpoises in the summer of 2014? 2. What are the sizes of the groups in which these bottlenose dolphins occur? 3. Are the dolphins which have shown the aggressive behaviour associated with each other, and to what extent? 4. What other associations do the dolphins have within the population of Cardigan Bay? 8

10 2. Methods 2.1 Study area Cardigan Bay is enclosed by the Welsh counties Pembrokeshire in the south, Ceredigion to the east and Gwynedd at to the north and is connected to the Irish Sea on the western side. In this area the sea temperature ranges from 4.9ºC in winter to 15.2 ºC in summer (Eliassen, 2014). Surveys were mostly conducted within the Cardigan Bay Special Area of Conservation (SAC), which stretches from Ceibwr in Pembrokeshire (52º 13 7 N, 5 º 0 15 W) to Aberarth in Ceredigion (52 º 25 6 N, 4 º W)(Lott, 2004), and extends approximately 12 miles offshore (Ceredigion County Council et al, 2008), covering an area of about 1,000 km 2 ( 2 Cardigan Bay SAC, n.d.). One of the aims of the management scheme of the SAC is to manage activities taking place within and near the SAC in order to protect the dolphins and their habitat from any adverse effects that human activities may have on them (Ceredigion County Council et al, 2008). Fig. 1: Cardigan Bay SAC (Rent Cardigan, n.d.). 2.2 Data collection Individuals are identified using photo identification techniques of photographs taken during research surveys. Photo identification is a technique using naturally occurring permanent and semi-permanent marks of nicks and notches along the edge of the dorsal fin, as well as anthropogenic marks to identify an individual (Simon et al, 2010). The surveys which were conducted during the study period of 2010 until 2014 include boat based surveys in the Cardigan Bay SAC, and land based surveys from the harbour wall in New Quay harbour. Boat based surveys were conducted from one of the two charter boats during 9

11 the field season (April to November), the 10m vessel MV Sulaire or Anna Lloyd (figure 2). Observations were made and verified by at least one research volunteer from the CBMWC, who was most often located approximately 3m above sea level with a 360º view, or by the skipper, positioned in the wheelhouse. During each survey an effort/environmental form was filled in by the volunteer, but that data is not used for this study. When bottlenose dolphins where seen the sighting sheet was completed, see appendix I, which includes data about the group size and composition and their behaviour. This sheet was also completed when porpoises or seals were observed. When photo identification was carried out, a bottlenose dolphin encounter form was also completed, see appendix II. During an encounter positional and environmental information is recorded, as well as an estimation of the number of dolphins, their behaviour and any additional information. During the longer trips (up to 8 hours) photographs are taken when a group of dolphins are encountered and it is appropriate for a photo-identification encounter to follow, a photo-identification licence, issued by Natural Resources Wales may be invoked, where and when appropriate in order to capture the images required, while during the shorter trips (up to 2 hours) opportunistic photo identification is carried out. Most photographs were taken using Canon 20D and Canon 30D Digital SLR cameras with a mm lens. The photographer attempted to capture images of all individuals present, preferably from both sides. Fig. 2: The two research vessels Sulaire (left) and Anna Lloyd (E.G.R. Bakker). Land based surveys are conducted from the harbour wall of New Quay. During the normal land-based surveys data on group size and composition as well as behavioural data was recorded throughout the entire study period, but no photographs of the dolphins were taken, making this data unsuitable for this study. However, in 2014 a new project was introduced, the ShoreFin project, which has the aim to acquire a greater understanding of the bottlenose dolphin population residing in Cardigan Bay and in particular the individuals 10

12 frequenting New Quay bay. This project is designed to be an on-going project, which will continue in future years (Metcalfe et al, 2014). This will be achieved by applying the photo identification technique on individuals photographed from the shore, and that data can be used for the social analysis of this study. During the initial ShoreFin pilot study in 2014 no consistent data on group size was recorded and therefore the data and photographs collected as part of this project were used for social analysis only. 2.3 Photo identification The photographs taken during the surveys are uploaded onto a dedicated computer, they are then sorted by individual photographer. Then photographs are graded with a 1, 2, 3.1, 3.2 and 3.3, based on the quality of the photograph (focus and light) the size of the fin within the frame and the angle of the photo towards the fin. In table 1 you can see the definitions for the grades and you can find the grading process overview in appendix III. Only photographs graded with a 3.1, 3.2 and 3.3 are used for photo identification analysis. The photographs with these grades are assorted per individual, adding all the photographs of the same individual into a specific folder. Grade Definition 1 Not in focus and less than 1cm above water. 2 Not entire fin in frame and fin not perpendicular. 3.1 Dull light. 3.2 Backlit/silhouette. 3.3 Bright light, clear image. Table 1: Grade definitions. The identification is executed by comparing the photographs taken during the surveys with the photographs in the catalogue, and try to match every individual to an individual in the catalogue. Dolphins in the catalogue all have a number, and some dolphins which are seen regularly also have a nickname. The photographs in the catalogue are ordered by the side of the fin which is photographed. Once an individual on a photograph was matched to an individual of the catalogue, a second volunteer analysed the photograph to reduce the possibility of false identification. After this second check all encounter data, including the identification of the individuals, is entered into a bespoke Microsoft Access database, ready to use for analysis. 2.4 Data analysis Individual identification Individuals were identified using the photo identification technique. To identify the individuals which were involved by the attacks on the harbour porpoises, the photographs 11

13 taken during these four encounters were analysed. Afterwards, only individuals with wellmarked fins are used for further analysis, to reduce bias during the identification process Group size The group size is the maximum number of individuals recorded in a group, including calves. A group is defined as individuals in close proximity, within about 10 body lengths of another individual and behaving in a similar manner. If an exact same group is seen more than once on the same day, only the first encounter of that day will be used for analysis (Owen et al, 2002). For these analysis only data of the boat surveys are used, since no consistent data on the group size is recorded during the ShoreFin project. The difference in group sizes among the years and among the three individuals was tested using Kruskal-Wallis tests (Daura-Jorge et al, 2005; Coscarella et al, 2011; Lott, 2004), which were executed with the statistical program R. Only P values smaller than 0.05 were used to reject H 0 hypothesises. When a significant difference in group size of an individual was found among the years, all years were independently tested against each other with a Mann-Whitney-Wilcoxon test ( , , etc.) Social associations The social associations of the individuals with each other and other members of the population are analysed using the program SOCPROG, which is developed for analysing animal social structures (Whitehead, 2014). For calculating the coefficient of association (COA) between individuals in this study the half weight index is used, because this index introduces a bias to correct any missed individuals within a dyad (Lott, 2004) and it is used in several other studies on bottlenose dolphins (Lusseau et al, 2003; Möller et al, 2006; Gero et al, 2005), so results can be easily compared with other studies. A dyad is a pair of dolphins (Lott, 2004). Although the Simple Ratio index has also been used in several cetacean studies (Rogan et al, 2000; Shane, 2004; Rossbach & Herzing, 1999), this one is known to be used when researchers had achieved complete identification of individuals of a group during most encounters (Lott, 2004). All dolphins in the same group are regarded to be associated (Chilvers & Corkeron, 2002). The formula for the HWI is: In which: X HWI= X + ½ (Y a + Y b ) X = the number of encounters that included both dolphin a and b. Y a = the number of encounters that included dolphin a, but not dolphin b. Y b = the number of encounters that included dolphin b, but not dolphin a. The HWI ranges from 0.00 to 1.00, in which 0.00 indicates that they have never been observed together and 1.00 indicates that they have always been observed together. The 12

14 HWI was found by producing an association matrix by Socprog. The COA were categorized as infrequent ( ), casual ( ), fair ( ), moderate ( ) and strong ( )(Garcia-Vital et al, 2015). The standard error was found by producing an SE association matrix using the bootstrap method. The total strength of associations which is manually calculated for every individual is the sum of all the COA which the individuals has with the other members of the population, including the two from the case study. When considering the association among the three individuals, only the encounters in which at least one of the individuals was observed, were used for the analysis. This decision was made because experimental analyses with just a part of the dataset showed, as could be expected, that adding all encounters, also the ones which did not include any of the three individuals, did not affect the association among the three individuals. For the final analysis the associations of the three individuals with other members of the population were analysed. Other individuals of the population which have been seen only once or twice in the study period were excluded from the analysis (Chilvers & Corkeron, 2001; Pereira et al, 2013), as well as calves which are born within the study period, because of their unique and strong association with their mother (Blasi & Boitani, 2014), which lasts for about five years or more (Dolphin Research Institute, 2005). Individuals which were never seen together with either one or more of the three individuals were removed from analysis as well. As the same for the group size analysis, when a group was seen more than once on the same day, with the exact same group composition, only the first sighting was used for analysis (Owen et al, 2002). An estimate of social differentiations is produced to analyse the variety in the social system, and an estimate of correlation is produced to analyse the power of the analysis to detect the true social system. Results are displayed in sociograms and hierarchical clusters. In a sociogram all individuals are represented by dots which are arranged in a circle. The lines between the dots represent associations, and the thickness of these lines indicate the strength of these association. A hierarchical cluster presents the individuals on the y-as and the COA on the x-as (Whitehead, 2009). The tree which appears shows the different clusters and displays the different groups within the population. This will be based on as well as preferred partnership, as on least preferred partnership (Lusseau et al, 2003). The cophenetic correlation coefficient is automatically calculated when preforming a hierarchical cluster analysis with Socprog. The community division by modularity is another analysis which is performed by Socprog, calculating the modularity as well as the number of clusters and which individuals are involved (Whitehead, 2009). To test the reliability of the association results by testing whether the associations were not random, a permutation test was executed by Socprog (Blasi & Boitani, 2014). With this permutation test individuals were randomly permuted within the encounters, while the 13

15 group sizes of each encounter stayed the same, as well as the number of times each individual was seen (Lusseau et al, 2003). Table 2 gives an example of such a dataset, and as you can see the group sizes stay the same, as well as the amount of times one individual is observed. The permutation test determines whether the associations from the real dataset differs significantly from the permuted dataset. The standard deviations of the association indices of the permuted dataset were compared to the ones of the real dataset (Barnes, 2011), as well as the coefficient of variation (CV) of both datasets. Real dataset Permuted (random) dataset Encounter Group compositions Encounter Group composition 1 #007, #036 1 #004, #007 2 #015 2 #094 3 #004, #036, #453, #094, #583 3 #015, #453, #007, #684, #004 4 #684, #947, #321 4 #321, #036, #583 5 #007, #004 5 #947, #036 Table 2: Example of a real and permuted dataset. The real dataset in this table is just an example, not based on data used for this study. The amount of permutations can be adjusted manually in the program. For a reliable outcome the dyadic p-values has to be stable. By other studies this p-value stabilized by 20,000 permutations (Lott, 2004 & Payton, 2011; Bejder et al, 1998; Welsh & Herzing, 2008; Dungan et al, 2012), so that is the amount of permutations used in this study, and 1,000 trials per permutation were performed (Kelley et al, 2011). The null hypothesis of this test is that all calculated associations are random. When the standard deviation of the real dataset is significantly higher than the standard deviation of the permuted dataset, this null hypothesis can be rejected (Augusto et al, 2012), as well as when the coefficient of variation (CV) of association indices is significantly higher or lower in the real data set than in the random data. 14

16 3. Results 3.1 Individuals The attacks of the bottlenose dolphins on the porpoises occurred four times in 2014, all on different dates. The dates on which the attacks have been recorded are the following: 22/05/2014, 07/07/2014, 30/07/2014 and 09/09/2014. During these encounters a total of 26 animals were recorded, and 10 of which were identified. Date Survey Number of individuals Identified base Land 3 # Boat 3 #409, # Boat 2 #007, # Boat 18 (11 adults, 3 calves, 3 juveniles, 1 new born) #413, #367, #067, #082, #133 Table 3: Details of the sightings in which porpoises were attacked. However, during the last encounter a huge number of animals were around (18), but not all of them were involved in the attacks. There is only proof of one individual, #413 (possible male), who actually attacked, and this animal has not been observed often (6 times) during the study period. The other individuals of this encounter are excluded for further analysis because of the uncertainty about which individuals were actually involved in the attack. Individuals #409 (unknown sex) and #687 (unknown sex) are excluded for further analysis as well, because they have only been observed seven and two times respectively during the study period. From the 10 individuals identified during the attacks on porpoises, the three remaining and also well-marked individuals are #007 (Bond), #015 (Nick) and #036, you can find their details in table 4. In the report they will be referred to as their catalogue number. Catalogue number Nickname Sex Age class First seen Calves #007 Bond Possible male Adult 2005 Unknown #015 Nick Female Adult (#183, #220) #036 - Possible male Adult 2005 Unknown Table 4: Details of study individuals. In figure 3 on the next page you can see photographs of the fins of the three individuals to give you an idea of the individuals which are discussed in this report, and as you can see they all have well-marked dorsal fins. 15

17 Fig. 3a: #007 Fig. 3b: #015 Fig. 3c: #036 16

18 Number of encounters 3.2 Survey efforts From 2010 until 2014 a total of 427 boat encounters have occurred and in 2014 a total of 226 land encounters were recorded in which photo identification took place, which makes in total 653 encounters during the study period. In the graph below the amount of encounters is displayed per individual per year, and as you can see most encounters occurred in 2014, when photo identification also took place from the harbour wall of New Quay #007 # # Year Fig. 4: Number of encounters per individual per year. During the boat based surveys, #007 is encountered 44 times throughout the study period. #015 was also encountered 44 times and #036 was only encountered 14 times during these five years. During the ShoreFin project #007 has been observed 56 times, #015 has been observed 40 times and #036 has been observed 33 times. In table 5 you can see in a short overview the amount of encounters per individual during the study period. ID Boat Land Total encounters # # # Table 5: Amount of encounters for the three individuals during the study period. 17

19 3.3 Group size The data collected as part of the ShoreFin project were not used for any analysis on group size #007 Individual #007 is mostly seen in 2010 (16), and least in 2011 and 2012 (5). The mean group size over the entire study period is 4.65 (SD=3.90). 19% (8/43) of all the encounters with this individual during the study period, the individual was seen on its own. In 2010 it was seen in the largest group, containing 22 individuals. Table 6: Group size data of # N Mean (SD) 5.56 (5.11) 7.00 (2.00) 2.40 (0.89) 2.40 (1.65) 1.87 (3.22) Max Min There is a difference between the group sizes of #007 among the years the study was conducted (X 2 = 13.23, df = 4, p < 0.05). When all years were independently tested against each other, differences were found between (W = 119.5, p < 0.05), (W = 25, p < 0.05), (W = 48, p < 0.05) and (W = 12.5, p < 0.05). However, take into account that the sample sizes per year, as mentioned before and as you can see in table 6, are not large, especially for 2011 and Fig. 5: The group size of #007 throughout the study period. Horizontal lines inside boxes represent the median and the whiskers above and below indicate interquartile ranges. 18

20 3.3.2 #015 This individual is mostly seen in 2010 (15), and least in 2011 (5). The mean group size over the entire study period is 6.61 (SD=6.46). Only at 9% (4/44) of all the encounters with this individual during the study period the individual was seen on its own. The largest group in which this individual was seen was also in 2010, containing 25 individuals. Table 7: Group size data of # N Mean (SD) 5.05 (6.62) 5.20 (2.95) 5.29 (4.82) 3.82 (1.78) 1.37 (3.56) Max Min There is no difference between the group sizes of #015 among the years the study was conducted (X 2 = 3.47, df = 4, p > 0.05). Fig. 6: The group size of #015 throughout the study period. Horizontal lines inside boxes represent the median, whiskers above and below indicate interquartile ranges and the circle symbol shows outliers. 19

21 3.3.3 #036 This individual is mostly seen in 2010 (4), and least in 2012 and 2013 (2). The mean group size over the entire study period is 5.43 (SD=4.78). At 14% (2/14) of all the encounters with this individual during the study period the individual was seen on its own. The largest group in which this individual is seen was in 2014, containing 17 individuals. Table 8: Group size data of # N Mean (SD) 9.33 (6.50) 3.33 (2.52) 4.50 (2.12) 1.5 (0.71) 8.33 (7.77) Max Min There is no difference between the group sizes of #036 among the years the study was conducted (X 2 = 4.759, df = 4, p > 0.050), the sample size is also too small for these kind of analysis. Fig. 7: The group size of #036 throughout the study period. Horizontal lines inside boxes represent the median and the whiskers above and below indicate interquartile ranges. 20

22 Group size All individuals The mean group size off all three individuals over the entire study period is 5.61 (SD=5.31). #015 has been observed in the largest group, containing 25 individuals. During 14% (14/101) of the encounters with one of the three individuals, the animals were observed being on their own. #007 #015 #036 N Mean (SD) 4.65 (3.90) 6.61 (6.46) 5.43 (4.78) Median Max Min Table 9: Group size data of all three individuals. There is no difference in group sizes between the three individuals throughout the study period (X 2 = , df = 2, p > 0.05). Group size all three individuals #007 #015 #036 Individual Fig. 8: The group size of all three individuals throughout the study period. Horizontal lines inside boxes represent the median, whiskers above and below indicate interquartile ranges and the circle symbol shows outliers. 21

23 3.4 Associations among individuals During 184 encounters at least one of the three individuals was identified within the study period. During nine of these encounters all three individuals were in the same group. As you could have read earlier #007 was encountered 100 times throughout the study period. In 22 of the groups in which he was photographed, #015 was also present. In 15 out of the 100 groups he was also seen with #036. Individual #015 was encountered 84 times throughout the study period and except the 22 encounters with #007 she was also photographed 16 times in the same group as #036, which was encountered 47 times throughout the study period. Table 10 below displays the information discussed above. Total All 3 present #007 #015 #036 # # # Table 10: The number of encounters individuals were seen together. Total indicates the total amount of encounters with that individual during the study period. All 3 present indicates the amount of encounters that all three individuals were seen together. In combination with another individual it shows the amount of encounters in which both individuals were photographed during the study period. Tests on the associations among the three individuals showed an HWI Coefficient of Association (CAO) of 0.24 between #007 and #015, an COA of 0.20 between #007 and #036 and an COA of 0.29 between #015 and #036 (table 11). #007 #015 #036 # (0.04) 0.20 (0.05) # (0.04) (0.05) # (0.05) 0.29 (0.05) - Table 11: HWI index of the three individuals (SD). This means that all three associations are regarded as being casual, in which the individuals are mostly seen without either one of the other two dolphins than with them. 3.5 Associations within the population Dataset population 231 individuals have only been photographed once, and 64 individuals have only been photographed twice during the study period. All these 295 individuals have been excluded from further analysis, which also causes the removal of 98 sightings. Seven calves which have been born within the study period have been excluded as well, resulting in the removal 22

24 of another two sightings. At last all 54 individuals which are never seen with either one of the three individuals during the study period were removed as well, resulting in the removal of another 54 sightings. Fig. 9: Discovery curve. The remaining 562 sightings with 85 different individuals, including the three individuals, have been used for the social analysis. In figure 9 you can see that after approximately ⅔ of the sightings the curve flattens off, meaning that no new individuals were identified, indicating that a significant part of the individuals with whom the three individuals associate are covered. Only individuals which were seen at least three times, and at least once with either one of the three individuals are included in analysis. Details about the 82 individuals can be found in appendix IV. The estimate of social differentiation of this population, which measures how varied the social system is, is (SE=0.222), which indicates a population with well to extreme differentiated societies. An estimated social differentiation between 0.5 and 2.0 indicates well differentiated societies and greater than 2.0 extremely differentiated societies (Whitehead, 2014). The estimate of correlation is a number between 0.0 and 1.0 in which 0.0 indicates a useless analysis, and 1.0 a perfect analysis. The estimate of correlation between true (the actual proportion of time, pairs of individuals spend associated) and estimated association is an indicator of the power of the analysis, to detect the true social system (Whitehead, 2009). The estimate of the correlation indices of this study using the Poisson approximation is (SE=0.012), which can be regarded as a fair representation of the population. An overview of all the calculated Coefficient of Association (COA) of the three individuals in combination with all 82 individuals can be found in appendix V. The total strength of #007 is 2.53, of #015 is 3.57 and of #036 is A high strength indicates that this individual has 23

25 either some very strong associations, or associations with a number of different individuals, or both situations are the case #007 Taking into account all other members of the population, still the strongest association of #007 is with individual #015 (COA=0.24), followed by #036 (COA=0.20). The third strongest associations are with the well-marked possible male #219 and female #376 (COA=0.17). Fig. 10: #007 (right) and #219 on the 8 th of July, #015 This female has the strongest association with #036 (COA=0.29), followed by the possible male #219 (COA=0.25) and in third position is #007 (COA=0.24). As you could have read earlier this individual has the highest total strength #036 This possible male has the strongest association with the possible male #219 (COA=0.31), followed by #015 (COA=0.29) and #007 (COA=0.20). As you could have read earlier this individual has the lowest total strength. Based on the results which show relative high associations between #219 and the three individuals of this study, his social interactions were analysed as well. His strongest association is with #036 (COA= 0.31), followed by #015 (COA=0.25). However, his third strongest association is not with #007 but with the female #177 (COA=0.18) and then on number four is #007 (COA=0.17). 24

26 3.5.5 Population analysis Sociograms can be quite confusing when all 85 individuals are included. Therefore, all individuals which have no COA of 0.05 or higher with either one of the three individuals were excluded, concerning 42 individuals. In figure 11 you can see the sociogram with the remaining 43 individuals, including the three individuals of interest. The thicker the line between two individuals, the stronger their association is. Fig. 11: Sociogram of the individuals which have a COA of at least 0.5 with either one of the three individuals. The thinnest lines indicate a COA between 0.20 and 0.25, the middle thickest lines indicate a COA between 0.25 and 0.30 and the thickest lines indicate a COA higher than

27 In figure 12 below you can see the hierarchical cluster of the 85 individuals, including the three individuals. As you can see the individuals #007, #015 and #036 are in the same cluster, approximately in the middle of the figure. Fig. 12: Hierarchical cluster. COA index The hierarchical cluster has a cophenetic correlation coefficient of This coefficient calculates the correlation between the actual associations among the individuals and the levels of clustering within the population (Whitehead, 2009). If this coefficient is larger than 26

28 approximately 0.8, the cluster analysis is considered to indicate an effective representation (Whitehead, 2009). In this case it is just on the edge of being useful and not useful. Another method to calculate whether there are actual clusters within the population, and how many, is to calculate the community division by modularity. When the modularity is bigger than 0.3 the calculated clusters present useful clusters (Whitehead, 2009). These analysis showed in total 7 different clusters within the population with a modularity of The sizes of these clusters vary from seven individuals in cluster 2 and 6, to 19 individuals in cluster 4 and 5. Individuals #007, #015 and #036 were presented in the same cluster, together with #022, #219, #302, #664, #672, #676, #686, #689, #690, #691 and #694. These are three possible males one possible females and seven individuals of which the sex in unknown. For an overview of the other six clusters and the individuals they contain, see appendix VI. When looking at the hierarchical cluster in combination with the results of the community division by modularity, individual #664 is the furthest away from the three individuals (#007), covering in total 24 individuals. To create a better overview, figure 13 shows the part of the hierarchical cluster which is now discussed. The 24 individuals are much more than the 14 individuals which were presented by the community division by modularity. However, #664 seems like an outlier when comparing the results to the hierarchical cluster, when this one is not taken into account, #672 is the next furthest away, covering a total of 17 individuals. Fig. 13: Part of the hierarchical cluster of figure Permutation test Outcome of the permutation test showed that the results of the associations can be assumed to be reliable. The permutation test with 20,000 permutations and 1,000 trials per permutation calculated a standard deviation of for the real dataset, and a standard deviation of for the permutated dataset with a p-value of Since the standard deviation of the real 27

29 dataset is higher than the one from the permuted dataset, the null hypothesis can be rejected, meaning that the associations of the real dataset are not random (Welsh & Herzing, 2008). The numbers of the coefficient of variation (CV) also confirm that the null hypothesis can be rejected, since non-random social organisation is expected when the coefficient of variation (CV) of association indices is significantly higher or lower in the real data set than in the random data (Kelley et all, 2011; Welsh & Herzing, 2008). Since the real dataset had a CV of and the permuted dataset has a CV of with a p-value of , this number also shows the reliability of association tests. At last there is also evidence of avoidance since the proportion of non-zero elements is lower in the real dataset ( ) than in the random dataset ( )(Lott, 2004). For the direct output of the permutation test produced by SPSS, see appendix VII. 28

30 4. Discussion 4.1 Data collection Weather conditions influences the ability to spot and observe dolphins, and can therefore cause a bias in the data. For this study no sightings were excluded based on environmental data. However, for example the sea state can make it more difficult to estimate the total number of individuals present in a group and it can also influence the identification process of individuals, as well as light conditions. Especially photo identification from land can be difficult since the position from where photographs are taken is very limited, as well as the distance from the camera to the dolphins, which is fully dependent on the behaviour of the dolphins. Individuals which do not have well-marked fins can easily be missed or wrongly identified during poor environmental conditions, which influences the association results. Fortunately the three individuals mainly discussed during this study do all three have wellmarked fins. A large bias of the dataset of this study is the fact that not all individuals in the used encounters were identified. Actual association indices may therefore be higher than the presented results in this study. The sizes of the groups in which the dolphins occur, as well as the individuals present in the groups also depend on their behaviour (Gazda et al, 2011), which is not taken into account in this study. The temporal variances in the associations have not been taken into account as well, this because of the lack of data, especially for the first four years of the study period. 4.2 Porpoise attacks In the Moray Firth in Scotland researchers witnessed four attacks of bottlenose dolphins on porpoises (Ross & Wilson, 1996). The first two attacks were executed by groups with an unknown number of individuals, the third and fourth time the groups contained two or three individuals of which the sex was not mentioned, probably unknown. In California researchers witnessed three attacks which were executed mostly by males (61% males, 31% possible males)(cotter et al, 2011). In the first case one male was chasing a porpoise while nine other individuals were a bit further away. In the second case at first 16 individuals attacked a porpoise, and two continued with this. In the last case at first three males were attacking a porpoise while 13 others were more in the distance, and later six individuals of this group joined the threesome. In 2014 a member of the public published photographs of several bottlenose dolphins attacking a harbour porpoises at the east coast of Scotland (Weir, 2014). The bottlenose dolphins were later identified, by experts, as probably juvenile males (Williams, 2014). 29

31 No other attacks of bottlenose dolphins on harbour porpoises have been recorded. However, two Pacific white-sided dolphins, which are much smaller than bottlenose dolphins, have been observed attacking a neonate harbour porpoise at the west coast of the USA (Baird, 1998). One of them was a male, the other was a female. However, because of the difference in size and the fact that it is a different species, this example, as well as other possible attacks of dolphin species on porpoises are no further discussed. 4.3 Group size In coastal areas bottlenose dolphins tend to live in small groups (Connor et al, 1999). The mean group size of the three individuals which we found, 5.61 (SD=5.31), is comparable to the results of a study on the entire population of the Cardigan bay which showed a mean group size of 5.85 (SD=5.89)(Lott, 2004). In the Bahamas a study showed a mean group size of 3.45 (SD=3.65) for the coastal bottlenose dolphins (Rogers et al, 2004), while a population of coastal dolphins in Florida, USA, had a mean group size of 6.3 (SD=4.1) (Lewis et al, 2010). Two other populations of bottlenose dolphins not far from Cardigan Bay, the Shannon estuary in Ireland and the Moray Firth in Scotland, had a mean group size of 6.6 and 6.3 respectively (Lott, 2004), which is also comparable to the mean group sizes found in this study. In southern Italy the bottlenose dolphins have a mean group size of 7.91 (SD =3.75)(Blasi & Boitani, 2014) and within a coastal community in Mexico the groups are a bit larger with a mean of 8.5 (SD=8.6)(Garcia-Vital et al, 2015). In north east Ireland the coastal bottlenose dolphins had a mean group size of 10.4 (SD=11.1) which is quite a bit larger than the mean found in this study regarding it is geographically close to Cardigan Bay (Oudejans et al, 2015). Around the Galician coast (north west coast of Spain) the bottlenose dolphins occur in group sizes with a mean of 11.1 (Pierce et al, 2010) and a coastal community in Normandy even has a mean group size of 26, but the researchers of that study already discussed in their report that this is large compared to other coastal communities (Louis et al, 2015). 4.4 Social associations The results of this study showed associations with low strength between the three individuals, as well as between these individuals and other members of the populations. However, as already mentioned in the first chapter of this report, bottlenose dolphin populations live in fission-fusion societies, in which individuals associate in small groups which change regularly in composition, often on a daily or hourly basis (Connor et al, 2000; Lott, 2004). The strongest associations are between mothers and calves, and is some communities males form very strong associations as well, known as alliances. This makes it hard to make a firm statement about an individual s associations, and is probably also the reason why the COA indices are low with only infrequent and casual associations. 30

32 5. Conclusion During the four attacks of bottlenose dolphins on harbour porpoises in the Cardigan Bay in 2014, each time multiple individuals were involved, ranging from two to 18 individuals. However, as mentioned before it is unknown how many of the 18 individuals were actually involved in the attack. The limited availability of data of other attacks of bottlenose dolphins on porpoises also showed that mostly more than one individual is involved in these attacks, as well that the aggressive behaviour is mostly executed by (possible) males. Individual #007, #036 and the one individual of the fourth encounter which was definitely involved in the attack (#413) are all categorized as possible males. Individual #015 is a female, which is quite unusual compared to other studies. The sexes of the two individuals of the second attack (#409 and #687) are currently unknown. Only #007 showed a difference in the group sizes it occurred in during the study period, but only few data was available. The results did not show differences between the group sizes of the three different individuals. The mean group size of all three individuals is 5.61 (SD=5.31), and #015 has the biggest mean group size of 6.61 (SD=6.46) and #007 has the smallest mean group size of 4.65 (SD=3.90). The mean group size of 5.61 is comparable to other coastal populations, especially the ones in the northern hemisphere, and it is very similar to the mean group size of 5.85 of the entire population of Cardigan Bay, indicating that the three individuals do not stand out within their population regarding the sizes of the groups in which they live. Social analysis suggests that the individuals have preferred and avoided associations. The power of the association among the three individuals is low, categorized as casual associations. However, bottlenose dolphins are known for their relatively lose associations and when taking into consideration the associations of the three individuals have with other members of the population, the associations among the three individuals is relatively high. All three individuals have relatively strong associations with the possible male #219 as well, and the three individuals are part of his top four strongest associations. So far there is no record of this individual being involved in an attack on harbour porpoises. It cannot be proven that the attacks of bottlenose dolphins on harbour porpoises is transmitted horizontally throughout the population, however the relative strong association among the three individuals does support this theory. 31

33 6. Recommendations For this specific study it is important to keep track of possible aggressive behaviour towards harbour porpoises in the future, and to see whether the individuals which are involved in these attacks are associated to the three individuals mainly discussed in this study, or to individuals with whom these three individual associate. It is also important to stay up to date with the establishment of the sexes of individuals of the population so the relation between sex and involvement in attacks can be investigated more properly, as well as the relation between the associations an individual has and the sex of this individual and the individuals it associates with. The sex can be determined based on photographs of leaping individuals, as well as footage of underwater cameras from the boat(s). Social analysis should be performed in the future, when also more data of the ShoreFin project is available for analysis. During the first year of ShoreFin in 2014 no consistent data on group size was collected, which made it impossible to use this data for group size analysis. Fortunately during this second year of ShoreFin in 2015 data on group size is already being collected. It would also be interesting to investigate in the future the relation between the associations an individual has and their behaviour. Especially for the individuals which were involved in the attacks on the harbour porpoises it would be interesting to investigate during which behaviour the individuals are mostly observed together (e.g. foraging or socializing). At last the use of modern molecular genetic technology could reveal a whole new side of the population of bottlenose dolphins of the Cardigan Bay. The use of genetic samples of bottlenose dolphins would give a lot more and unknown information, for example about the health status of the dolphins, as well as their sex and the kinship within the population. However, it is very difficult to obtain a licence which allows you to execute this field work technique, and there is also the ethical question whether this invasive way of gathering data is worth the stress it potentially causes to the dolphins. 32

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39 Appendix Appendix I: Sighting sheet 38

40 Appendix II: Bottlenose dolphin encounter form 39

41 Appendix III: Grading criteria 40