Studying wide-ranging top predators requires a multidisciplinary and transnational approach to identify movement patterns, ecology and for the effective monitoring and conservation of populations. A workshop was held at the European Cetacean Society conference, Galway, Ireland, on 25 March 2012, on one of the most wide-ranging of top predators in the North Atlantic, the killer whale Orcinus orca. A selection of the presentations from this workshop was published in a special section of the Journal of the Marine Biological Association of the United Kingdom1.
By the late 1980s research on killer whales in the Pacific waters of British Columbia, Canada and Washington State, USA had made substantial progress. This was largely due to the pioneering work by the late Dr Michael Bigg, who developed the use of photographs of the dorsal fin and saddle patches to identify uniquely marked individuals (see Figure 1), allowing for the first time an annual census and insights into population dynamics, social structure, life history and movement. Dr John Ford was also reporting the first evidence of a complex pattern of group and population-specific call dialects. At this time, preliminary investigations of the distribution of killer whales in North Atlantic waters based on sighting data, whaling catch statistics or stranding data were just starting to be published. These reviews identified potential hotspots and seasonality in occurrence, and therefore provided the foundations which subsequent dedicated research could build upon. They also identified some of the major prey resources that killer whales appeared to be tracking in the North Atlantic, which included the Icelandic and Norwegian stocks of Atlantic herring (Clupea harengus).
The first photo-identification studies of killer whales in Norwegian waters had photographically recaptured a small number of naturally marked individuals between years on the herring wintering grounds in the Lofoten region of northern Norway, and between Lofoten and the herring spawning grounds in the Möre region of southern Norway. Dr Tiu Similä and colleagues have subsequently built upon this earlier work, expanded the photo-identification catalogue to include over 600 individuals, and further demonstrated the association between the movement and site fidelity of killer whales and the migration of the Norwegian spring-spawning herring stock.
Work carried out by the Marine Research Institute, Reykjavík and others, has found that some killer whale groups also follow the Icelandic summer-spawning herring stock, whilst some individuals photographed feeding on herring around Iceland were also reported feeding on the North Sea stock of Atlantic herring, off the coast of Shetland. Over 1,000 individual killer whales have now been photo-identified across the north-east Atlantic, and collaboration among institutions has allowed comparisons of photo-identification catalogues over greater spatial and temporal scales. A general pattern is emerging of site fidelity and association with a particular prey resource at several locations across the north-east Atlantic. However, some groups may switch between different prey resources depending on their seasonal availability.
North Atlantic researchers pioneered new approaches to study killer whales, by using underwater cameras and sonar, multi-hydrophone arrays and custom-built multisensor acoustic tags to record their movement and behaviour when foraging for herring. During what has become known as carousel feeding, the killer whales appear to work as a coordinated group, flashing their white undersides and releasing bubbles to herd a ball of herring from the school before tail slapping the ball to stun and then eat individual fish. Killer whales feeding on herring off Iceland and Shetland produce a distinctive low frequency call, just prior to tail slapping the herring. These ‘herding’ calls are very similar to those produced by humpbacks, which also feed on herring, and the low frequency of the call may resonate with the herring’s swim bladder, and therefore help herd the herring. In the Canadian Arctic, Steve Ferguson and colleagues have employed the use of tried and tested methods, in addition to novel and highly innovative approaches to investigate the ecology of killer whales in this rapidly changing ecosystem. As global temperatures rise and the Arctic sea ice retreats, many former ‘choke points’ are opening up and allowing killer whales to enter the bays and inlets of the Canadian Arctic, resulting in a significant increase in recent sightings (Figure 2)
Interviews with local Inuit hunters suggest that marine mammals are the main prey of killer whales in the Canadian Arctic. Killer whales are known to be able to have a population level effect on prey populations due to top-down effects, and therefore have the potential to significantly shape this rapidly changing Arctic ecosystem. To better understand the potential impact of killer whale predation and how persistent it is over time, Ferguson and colleagues satellite tagged a killer whale in the Canadian Arctic, and found that it moved between areas with known aggregations of marine mammals before heading out into the open North Atlantic in late autumn as the sea ice increased in concentration in the inlets. Therefore, predation on Arctic marine mammals may be seasonal.
We look forward with eager anticipation to the next twenty-five years of research on North Atlantic killer whales. We predict that the recent methodological advances in DNA sequencing technology, which have already been harnessed to sequence a dataset of complete mitochondrial genomes for North Atlantic killer whales, will be further applied to produce a complementary nuclear genomic dataset. The first high coverage marine mammal genomes have now been sequenced and the data made publicly available, and these include a North Atlantic killer whale. These genomic data will allow the comparisons of natural selection upon the genome among populations. The large acoustic datasets that have been collected for the past 25 years have allowed us to better understand the acoustic behaviour of north-east Atlantic killer whales. We anticipate that work currently undertaken will allow for broader geographical comparisons, as well as better understanding of the function of different signals, such as the still little understood high-frequency whistles produced by herring-eating killer whales in the north-east Atlantic. In addition, recent projects, deploying state-of-the-art multisensor tags in different locations in the north-east Atlantic, promise to allow for detailed comparisons of behavioural parameters. We expect that together these developments will improve our understanding of the biology, behaviour and ecology of killer whales in the North Atlantic.
Fig. 2. A group of killer whales in the Canadian Arctic, where they are being sighted more frequently coincident with the rising temperatures and melting of the Arctic ice sheets. Image: Gretchen Freund.
The past twenty-five years have seen global temperatures rise, including in the North Atlantic, and these changes have had biological consequences across a range of species, as well as for ourselves. In addition to the diminishing sea ice in the Canadian Arctic during this period there have also been natural shifts in the distribution of prey resources (e.g. herring and mackerel stocks), and subsequently the killer whale lineages that follow them. The next twenty-five years will likely lead to further and more rapid changes in climate, particularly in the Arctic as the sea ice melts. We therefore predict that the prey resources exploited by North Atlantic killer whales will shift their distribution, and they may undergo declines or increases as a result. Additionally, new prey resources may become available to North Atlantic killer whales. Although our understanding of killer whale ecology and evolution has come a long way, there are still many gaps in our knowledge about the extent of geographical movements and consequently connectivity between different locations, the prey preferences and diet composition, the population viability and status of killer whales in different locations in the North Atlantic. Without such information it is impossible to completely critically assess the threats faced by killer whales in these locations and their conservation status. Collaboration between researchers and long-term consistent monitoring effort will be critical to effectively assess these issues. The ecosystems of the North Atlantic are likely to be highly dynamic during the next twentyfive years, and both North Atlantic killer whales and the researchers that investigate them will need to adapt to these ongoing changes and challenges.
Andrew D. Foote1 , Sanna Kuningas2 and Filipa I. P. Samarra2,3.
1. Centre for GeoGenetics, Natural History Museum of Denmark, University of Copenhagen, Øster Volgade 5-7, DK-1350 Copenhagen, Denmark.
2. Sea Mammal Research Unit, Scottish Oceans Institute, University of St Andrews, St Andrews, Fife, KY16 8LB, UK.
3. Marine Research Institute, Skulagata 4, PO Bo
1 Journal of the Marine Biological Association of the United Kingdom, 2014, 94(6), 1256-1252.