Killer whales’ impact on their prey: mortality and beyond

Killer whales are cosmopolitan apex predators that eat other predators, often also at the top of the food web: fish, sharks, seals, whales, and sea birds. Around the world, sub-populations of killer whales, or ‘ecotypes’, can specialize in particular prey types. The famous example is the ‘resident’ killer whales in British Columbia that feed mostly on salmon, in contrast to the ‘transients’ (Bigg’s) that exploit mostly mammalian prey (seals, dolphins, even whales). These specialized diets have led to distinct vocal and social behaviours, even genetic differences between the ecotypes.

In the North Atlantic however, the picture is slightly more mixed: killer whales that are known in Iceland to eat herring, move to coastal waters of Scotland where seal predation by killer whales is frequently observed. Photo-identification efforts by collaborating researchers, naturalists and citizen scientists led to the discovery of several individuals that visit both countries, and appear to be seasonally opportunistic in their diet. For the latest – now much longer – list of individuals visiting both countries, check the catalogues maintained by Orca Guardians Iceland and Orca Survey Scotland.

At the same time, harbour seal populations are showing regional declines in north and east coasts of Scotland whereas grey seal populations are relatively stable. The causes for the different trends and the potential for any role of killer whale predation in this remains poorly understood. The seasonally generalist diet could be taken to mean there will be less impact on seals, as the whales share the burden of their nutritional requirements among different prey, not only fish and seals but also others, such as harbour porpoise. But does the impact end there?

Predators contribute to a ‘landscape of fear’ for their prey animals: the mere presence of predators in a habitat can alter prey behaviour, reproduction and future survival. In this landscape, prey must balance the risk between being eaten and not having anything to eat themselves. When the resulting changes in the prey population are substantial enough to affect what they eat, these risk effects can lead to ecological consequences even at the lowest levels of the food web.

Brown & co influential paper from 1999

The risk effects on the food web are also called ‘non-consumptive’ effects, because they are the result of interactions between predators and prey that do not involve direct consumption (mortality). Essentially, when prey survives an encounter with their predator, the energy spent in the interaction, i.e., predator hunting effort and prey anti-predator behaviour, is lost rather than being transferred from the prey to the predator/consumer.

One of the two adult males of the 27s group (#72 I believe) in Breiwick, Shetland, 02.08.19

How about the non-consumptive effects of killer whales?

Playback studies show that a range of marine mammal species (sperm whales, humpback whales, pilot whales, harbour seals, Risso’s dolphins) respond to killer whale sounds. The behaviour response usually involves strong avoidance, but in pilot whales, also attraction. Marine mammals may also respond differently depending on the type of killer whale sound used – familiar/unfamiliar, ecotype and call type. An ability to recognize predator ecotype or hunting intent could reduce the risk effects on prey. On the other hand, an incorrect identification could be fatal; perhaps for the prey of an opportunistic predator it is not a risk worth taking.

Fewer studies have been able to demonstrate the effects of actual killer whales on their prey. In a unique study using satellite telemetry, Breed and colleagues showed that the habitat use and distribution of tagged narwhals was disrupted by the presence of concurrently tagged killer whales. In Scotland, the Sea Mammal Research Unit outfits harbour and grey seals with satellite tags that can be used in a similar way – killer whales have not been tagged in the country, but thanks to a large network of citizen scientists and collaborators, some of the seals’ movements can now be related to presence/absence of killer whale sightings as part of the ECOPredS project.

Satellite telemery data sheds light on the movement behaviour of harbour seals in Shetland. Pages from Shetland Sea Mammal Report, 1993-2003, available at Shetland Museum and Archives.

To address the true impact of killer whales on local seal populations in the North Atlantic, both consumptive and non-consumptive effects should be considered. Many questions remain open, including the scale of impact of killer whales on seal time and energy budgets, and whether killer whales prefer adults vs. pups, or harbour vs. grey seals.

While these top predator-prey interactions can be challenging to monitor in the wild, citizen science has a huge potential to collect data on predation events in coastal areas. In the UK, groups such as Shetland Orca Sightings and Orca Survey Scotland were established by naturalists and killer whale enthusiasts alike to collect sightings and photographs to identify individual killer whales and study their movements. Often these observations come from land-based vantage points and benefit from the whales’ coastal foraging strategy, as well as advancing camera technology. Such observations are already contributing to the scientific knowledge of killer whale movements in the North Atlantic. Continued collaboration and knowledge sharing will be key to providing insights into this enigmatic predator-prey interaction far into the future.

The mere presence of killer whales might be having impact on seals

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