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Investigating the movement patterns of sharks and the significance of potential shark predation attempts on bottlenose dolphins (Tursiops aduncus) in the waters of south-western Australia

King, Carissa (2014) Investigating the movement patterns of sharks and the significance of potential shark predation attempts on bottlenose dolphins (Tursiops aduncus) in the waters of south-western Australia. Honours thesis, Murdoch University.

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Abstract

Despite their prominence as species of great interest and conservation significance, currently our understanding of shark movement patterns and their ecological role as predators on small Odontocete populations is far from complete. The first component of my research investigated the movement patterns of the four main commercial species (sandbar, Carcharhinus plumbeus; dusky, Carcharhinus obscurus; whiskery, Furgaleus macki; gummy, Mustelus antarcticus) and several species occasionally caught off the coast of Western Australia (Chapter 2). I analysed long-term (1994 to 2013), conventional tagging data collected by the Department of Fisheries, Western Australia to provide a detailed account of observed movement patterns. The second component of my research investigated the predatory role of sharks on bottlenose dolphins (Tursiops aduncus) off Bunbury, Western Australia (Chapter 3). I examined the prevalence of shark bites on systematically photographed dolphins over an eight year period (2007 to 2014). I also attempted to identify the shark species responsible for the attacks by seeking the expert opinions of three independent reviewers on the bite marks.

The analyses of movement patterns from the long-term tagging data (Chapter 2) found mean displacements of < 250 km for the four main commercial species. However, occasional displacements of more than 2,000 km were also observed for all four species. Displacement was positively correlated with time at liberty, size (fork length) and release condition. The swimming speed varied significantly between species, with faster movements observed for larger Carcharhinid sharks. Furthermore, significant differences in sex ratios at release and recapture were observed for gummy and whiskery sharks. A significantly higher proportion of females were released and recaptured for both species. Although, fork length was found to significantly vary for non-recaptured and recaptured sandbar and dusky sharks, results from the analyses were disregarded due to bias gear-selectivity and inaccurate measurements reported by fishermen. The highest recapture frequency for all other species occasionally caught was less than ten. Maximum displacements ranged from 0.70 to 1,143 km, these displacements were observed by wobbegongs (Orectolobus sp.) and copper whaler (Carcharhinus brachyurus) sharks, respectively. Maximum time at liberty ranged from 4 to 5,245 days for nervous (Carcharhinus cautus) and tiger (Galeocerdo cuvier) sharks, respectively. The estimated maximum swimming speed for occasionally caught species peaked at 0.42 km/hr for copper whaler (Carcharhinus brachyurus) sharks. Swimming speeds for all other species were less than 0.03 km/hr. No statistical analyses were completed for these species.

Analyses of the prevalence of external injuries believed to have been inflicted by sharks (Chapter 3) found that the total bite mark frequencies of systematically photographed dolphins varied significantly among age classes (calves, juveniles, adults) with the highest frequency of injuries sustained by adults (X2 = 38.436, P = < 0.001). Bite frequencies did not differ significantly between sexes or between mothers with or without calves (X2 = 0.111, P = 0.738; X2 = 1.316, P = 0.251, respectively). The total frequency of shark bites was significantly lower in the austral autumn and winter months than in the spring and summer (X2 = 15.333, P = 0.002). However, the frequency of fresh injuries was higher in the summer of 2013 (n = 8), than in other seasons (range = 0 - 1). When the data for 2013 were removed from the seasonal analyses the frequency of bites in summer (pooled over years) was higher but not significantly (X2 = 3.889, P = 0.274). The total frequency of injuries did not significantly differ among years (2008 – 2013) (X2 = 4.948, P = 0.422). After evaluating the photographed bite marks on dolphins, three independent reviewers suggested that the shark species responsible for the injuries were most likely white (Carcharodon carcharias), tiger (Galeocerdo cuvier), and several smaller Carcharhinid species (e.g., bull, Carcharhinus leucas; whaler, Carcharhinus brachyurus; dusky, Carcharhinus obscurus).

The results from Chapter 2 of the study provide a complete and meaningful representation of the movement patterns for the four main commercial species (sandbar, Carcharhinus plumbeus; dusky, Carcharhinus obscurus; whiskery, Furgaleus macki; and gummy, Mustelus antarcticus) and several species occasionally caught off Western Australia. The described movement patterns presented in this study can be incorporated into future conservation and management plans increasing their effectiveness and precision, ensuring west Australian shark stocks remain at target thresholds. The results from Chapter 3 refine our understanding of the interactions between sharks and small living Odontocetes (i.e., bottlenose dolphins, Tursiops aduncus). The results from the research confirm that sharks not only scavenge but actively hunt and predate living bottlenose dolphins. The ability and significant influence of these predatory sharks to shape and alter dolphin population dynamics is likely to be more important than previously recognised.

Publication Type: Thesis (Honours)
Murdoch Affiliation: School of Veterinary and Life Sciences
Supervisor: Loneragan, Neil
URI: http://researchrepository.murdoch.edu.au/id/eprint/25039
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