Killer whales in the Subantarctic: long-term demographic insights at Marion Island

Notches and scratches make it possible for researchers to identify individual killer whales. This is M002 closely followed by her 3-year old calf, M062 born into the population in 2014. © Rowan Jordaan

By Rowan Jordaan

Since 2006, over-wintering researchers from the Marion Island Marine Mammal Programme (MIMMP, University of Pretoria, South Africa) have conducted dedicated killer whale research at Marion Island in the Southern Ocean. Killer whale research at this small island is unique in that all observations are shore-based with killer whales observed swimming close to shore either in deep water close to cliffs or in shallow waters leading onto beaches where potential seal and penguin prey haul out.

An observer takes photos of a killer whale swimming passed the observation point. Shore based observations makes Marion Island killer whale research unique. ©Yinhla Shihlomule

A photographic identification catalogue started in 2006, has been maintained to date through structured dedicated observation sessions and opportunistic sightings of killer whales.  Killer whale identification is enabled by unique notches and scratches on killer whale dorsal fins. Currently, this catalogue is made up of 67 individuals including 19 calves born into the population since 2006 and 8 individuals who have also been observed at the nearby (~950km) Îles Crozet. In a new study published in Wildlife Biology, these sightings data (nearly 90,000 photographs taken over 4,739 sightings) were used to assess abundance, survival and population growth of killer whales at subantarctic Marion Island.

 Analyses was performed in the program MARK through the implementation of Multistate capture recapture models, and Pradel Survival-Lambda and POPAN single-state models. The authors report that killer whales at Marion Island have an annual survival probability of 0.98 which equates to an approximate lifespan of 48 years. This population has a healthy mean population growth rate of 1.012 and a calving rate of 0.13 calves born per year, per reproductive female. These parameters are similar to those of other killer whale populations in the Eastern North Pacific, Norway and Îles Crozet. Observed variation are attributed to dietary difference and resource abundance, historical negative impacts on social structure (e.g. mortalities caused by illegal fishing vessels), environmental conditions and the presence and scale of legal and illegal fisheries.  Fisheries may provide opportunities for direct interactions with vessels resulting in variable impact on survival and reproduction rates.

Full Gold Open Access paper here

A mother and young calf swim along the coastline observed from the cliffs above. © Rowan Jordaan

These results increase our knowledge of killer whales in the Southern Ocean, an area that is surprisingly understudied considering the abundance of killer whales in the region. Additionally, these results highlight the importance of long-term data sets and continuous monitoring. This study would not have been possible without the continued effort put in by the MIMMP and the numerous field assistants that have contributed to this data set over the years.

A sneak peak to see what is happening above the water surface. © Rowan Jordaan

For more information on the MIMMP and its other research please visit https://www.marionseals.com/ or our Facebook https://www.facebook.com/marionseals83 and Instagram pages @_marionseals

The authors through Rowan Jordaan

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