Abundance and predatory impact of killer whales at Marion Island

• Main Author: Reisinger, Ryan Rudolf
• Other Authors: Bester, Marthan Nieuwoudt, De Bruyn, P.J. Nico
• Format: Doctoral or Postdoctoral Thesis
• Language: unknown
• Published: University of Pretoria 2013
• Subjects: Diet; Apex predator; Foraging; Killer whale; Marion Island; Occurrence cycle; Orcinus orca; Penguin; Photo-id; Population size; Predator-prey interactions; Program mark; Seal; Southern Ocean; Subantarctic; Top-down control; UCTD; Indian; Elephant Seals; Killer Whale; Orca
• Online Access: http://hdl.handle.net/2263/27643; http://upetd.up.ac.za/thesis/available/etd-08302011-165011/

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 ...