The demography and sociality of killer whales Orcinus Orca at subantarctic Marion Island

Abstract

Inter- and intra-annual variation of environmental conditions arises from climatic variability, resulting in the temporal variation of resources. Variations in the distribution and abundance of food resources affect animal survival, sociality and reproductive rates. In this thesis, I analysed 12 years of identification photographs taken of killer whales (Orcinus orca) at subantarctic Marion Island to identify the potential drivers of demography and social structure. I first obtained baseline demographic statistics by estimating the abundance, survival and growth rate for this population. This was achieved by fitting multistate and single-state capture recapture models in the program MARK. An annual survival probability of 0.98 (95% CI = 0.96-0.99) was obtained, which did not vary substantially over time or between sexes or age classes. Population size (n = 54 [54-60] individuals), growth rate (𝝀�� = 1.012 [0.987-1.037]) and calving rate (0.11 [0.05-0.17] calves born per year per reproductive female) were also estimated. This demographic information is useful for comparison with other killer whale populations, and as a baseline for future studies at Marion Island. Despite seasonal variation in the availability of prey resources at Marion Island, killer whales are present throughout the year with seasonal differences in relative abundance. I investigated the social structure of killer whales in two seasons which correspond to periods of high and low prey availability. Social structure varied between these seasons with fission and fusion of social structure occurring during periods of low and high prey availability, respectively. This is indicated by significant changes in group size and in social measures at the dyad and network level. To investigate whether environmental variability influenced killer whale survival, I estimated survival as a function of covariates of at-island prey availability, climate proxies of prey availability, measures of social structure and fishery effort. Social structure had the greatest impact on survival followed by measures of fishery effort, both positively influencing survival. The absence of a correlation between at-island prey variables and survival may be attributed to possible predation saturation and a Holling type II functional response. Lastly, I investigated the response of measures of social structure and calving rate to environmental variability such as at-island prey availability, climate proxies of prey availability and measures of fishery effort. Sociality was both positively and negatively affected by at-island measures of prey availability but not by toothfish fishing effort. Calving rate was unaffected by all measures. I suggest that Marion Island killer whales adjust social structure to the type and to the abundance of prey Cumulatively, these results suggest that Patagonian toothfish Dissotichus elegenoides fisheries operating in the waters surrounding Marion Island are not facilitating access to toothfish prey but could influence survival through mortality events. Furthermore, at-island prey abundances may not be the only attraction and reason for seasonal aggregations of killer whales at Marion Island suggesting that this location is important for social reasons and/or population maintenance. These results illustrate how different areas in animal home ranges serve different functions, which should be considered when planning and implementing conservation areas.