Durban, J. W., et al. “Quantifying a stopover of killer whales preying on gray whales rounding the Alaska Peninsula.” Marine Ecology Progress Series 724 (2023): 1-15.
Over the past few decades, depleted baleen whale populations have been recovering and growing due to conservation efforts which mainly have focused on a bottom-up approach of understanding how the environment and climate change are affecting whale populations. However, baleen whale individuals are still low in number – which is why conservation efforts are now taking a top-down approach to understanding baleen whale population dynamics. This top-down approach aims to understand predator-prey interactions and their behavioural and morphological effects on baleen whale populations.
Orcas vs. Gray whales
Orcas (Orcinus orca), also known as killer whales, are the natural predators of baleen whales, such as Gray whales (Eschrichtius robustus). Gray whales in the Eastern North Pacific have recently been removed from the US List of Threatened and Endangered Wildlife with a population of over 20,000 individuals. From April to October, Gray whales migrate north from North Pacific waters off the coast of Mexico into colder Arctic waters – which is their source of food. Due to warming oceans and early retreat of sea ice, the Gray whales have access to their feeding ground for larger spans of time and has led to increased reproductive success – meaning more Gray whale calves.
Even with many Gray whale calves being born, their population ultimately depends on the survival of these calves to adulthood – which is where the Orcas come into play. Orcas prey on Gray whales as they migrate north – and are known to be the most vulnerable to predation near Unimak Island in Alaska. These Orcas are known as ‘transient’ or Bigg’s killer whales – which are known to eat mammals.
More Calves, More Orcas
Using mark-recapture and satellite tracking data, the authors in this study monitor predator-prey dynamics between the two species of whales to understand how predator-prey dynamics play a part in limiting the recovery and growth of the Gray whale population.
They found approximately 200 different Orca individuals in the Unimak Island area in May and early June – when the Gray whales are migrating to their feeding grounds. These Orca individuals returned every year to the same area over a period of 6 years. They also stayed in the area for several weeks – as they were able to feed on the submerged Gray whale carcasses in the shallow waters around Unimak island. Additionally, more orcas spent more time in the area when they were more Gray whale calves migrating. When there were lesser Gray whale calves – there would be a large number of Orcas which would spend a shorter amount of time in the area.
Why do we care?
Quantitative data on the predator-prey dynamics of Gray whales and Orcas provides an important reference point for monitoring and understanding how predation pressure can affect populations of previously threatened species. Taking a top-down approach along with a bottom-up approach will no doubt add to our conservation efforts of marine megafauna and other marine species in the future.
I am a PhD student at the University of Chicago and the Marine Biological Lab, currently studying germline development and regeneration in the amphipod crustacean, Parhyale hawaiensis. I did my undergraduate degree in India and did my Masters in Oceanography at the University of Massachusetts (during which I participated in multiple month-long research cruises out in the Pacific and Atlantic Ocean!). I am broadly interested in integrating ecology with developmental biology in marine organisms and I hope to comprehend the fundamental interconnectedness of the mysteries that swim in the Earth’s oceans. I am also an illustrator and a PADI certified diver.