Abundance and predatory impact of killer whales at Marion Island

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dc.contributor.advisor Bester, Marthan Nieuwoudt
dc.contributor.coadvisor De Bruyn, P.J. Nico
dc.contributor.postgraduate Reisinger, Ryan Rudolf en
dc.date.accessioned 2013-09-07T11:55:46Z
dc.date.available 2011-09-09 en
dc.date.available 2013-09-07T11:55:46Z
dc.date.created 2011-04-07 en
dc.date.issued 2011-09-09 en
dc.date.submitted 2011-08-30 en
dc.description Dissertation (MSc)--University of Pretoria, 2011. en
dc.description.abstract Killer whales are the oceans’ apex predator and are known to have important effects on ecosystems. At Subantarctic Marion Island, southern Indian Ocean, they have only been studied opportunistically, resulting in limited knowledge of their ecosystem impact here. This dissertation describes the prey and seasonal abundance, estimates the population size and assesses the predatory impact of killer whales on seals and penguins at Marion Island, using dedicated and opportunistic shore-based observations and photographic identification, from 2006 to 2009. During 823 sightings of killer whales at Marion Island (2006 to 2009) 48 predation events were recorded; in only 10 cases could prey be identified. Killer whales fed on fur seals, elephant seals and penguins. Constant effort (dedicated) observations (259 hours, 2008 to 2009) showed that killer whale abundance, which peaked in September to December with a secondary peak in April to May, is linked to the abundance of seals and penguins. Mark-recapture analyses were performed using nearly 10 000 photographs taken from 2006 to 2009. Following careful quality control criteria 37 individuals were identified and a population size of 42 (95% CI = 35-50) individuals estimated using the open population POPAN parameterization in the software program MARK. The analytical approach is more rigorous than that used in any previous population size assessment at Marion Island. Finally, the above data were integrated to assess whether top-down control of seal and penguin populations at Marion Island is generally plausible using a simple process of elimination. Based on published data I predicted the energetic ingestion requirements of adult male and female killer whales as 1 394 MJ.day-1 and 1 028 MJ.day-1, respectively. Expanding these requirements to the 37 killer whales photographically identified at Marion Island, the population requires 40 600MJ.day-1. Based on available energy density and mass data, I predicted the energy content of available seal and penguin prey and calculated the rates at which killer whales would consume these prey in various scenarios. Penguins and Subantarctic fur seals are relatively insensitive to killer whale predation owing to their large population sizes (10 000s to 100 000s), conversely, the smaller populations (100s to 1 000s) of Antarctic fur seals and southern elephant seals are sensitive to predation, particularly the latter as they have a high energy content (approximately 2 000 to 9 000 MJ). Populations of these seals are currently increasing or stable and I conclude that presently killer whale predation is not driving population declines, although they clearly have the potential for regulation of these smaller populations. Thus, if population sizes were reduced by bottom-up processes, if killer whale diet shifted, or if prey availability changed, top-down control by killer whales could become significant. This study provides baseline information for the informed management and conservation of killer whales at Marion Island, identifies avenues for further research, and provides a foundation for the continuation of structured and dedicated killer whale research at Marion Island. en
dc.description.availability unrestricted en
dc.description.department Zoology and Entomology en
dc.identifier.citation Reisinger, RR 2011, Abundance and predatory impact of killer whales at Marion Island, MSc dissertation, University of Pretoria, Pretoria, viewed yymmdd < http://hdl.handle.net/2263/27643 > en
dc.identifier.other E11/449/gm en
dc.identifier.upetdurl http://upetd.up.ac.za/thesis/available/etd-08302011-165011/ en
dc.identifier.uri http://hdl.handle.net/2263/27643
dc.language.iso en
dc.publisher University of Pretoria en_ZA
dc.rights © 2011, University of Pretoria. All rights reserved. The copyright in this work vests in the University of Pretoria. No part of this work may be reproduced or transmitted in any form or by any means, without the prior written permission of the University of Pretoria. en
dc.subject Diet en
dc.subject Apex predator en
dc.subject Foraging en
dc.subject Killer whale en
dc.subject Marion Island en
dc.subject Occurrence cycle en
dc.subject Orcinus orca en
dc.subject Penguin en
dc.subject Photo-id en
dc.subject Population size en
dc.subject Predator-prey interactions en
dc.subject Program mark en
dc.subject Seal en
dc.subject Southern Ocean en
dc.subject Subantarctic en
dc.subject Top-down control en
dc.subject UCTD en_US
dc.title Abundance and predatory impact of killer whales at Marion Island en
dc.type Dissertation en


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