Unravelling the site fidelity and movements of killer whales around Iceland

Our latest research on Icelandic killer whale movements within and outside herring grounds show that their occurrence and movement patterns around Iceland are complex and dynamic, which is true even within a single area.

A female and male killer whale surface in Vestmannaeyjar (South of Iceland). Photo: Tatiana Marchon

Although killer whales are present all-around Iceland, information on the connectivity between different regions had mostly been restricted to herring grounds. Thanks to the increase in whale watching activities in different regions and through collaboration with these and many other partners around the country, we have been able to extend data collection to areas outside herring grounds. In this study, we used photo-identification data collected from 2002 to 2022 to determine site fidelity and connectivity between areas inside (South and West) and, for the first time, outside (Southwest, Northwest, Northeast and East) Icelandic herring grounds. In addition, we conducted the first long-term study (10 years) on the residency patterns of killer whales in a summer herring spawning ground (South).

Map of Iceland divided into regions (South, Southwest, West, Northwest, Northeast, East) where photographs of killer whales were collected. Map: Google Earth

In the marine environment, the importance of different regions can be driven by several factors, such as mating opportunities, breeding or food resources. In the context of apex predators, such as killer whales, it is mainly prey distribution and abundance that define their site preference and movement patterns. While Icelandic killer whales closely associate with herring, recent studies have demonstrated that their foraging behaviour is more complex than previously thought. Indeed, some individuals are fish-eating killer whales while others have a mixed diet, including both fish and marine mammals. Moreover, some individuals have also been observed feeding on other species of fishes, cephalopods, birds and marine mammals suggesting that other dietary preferences may occur within this population. The majority of the attacks on marine mammals have been observed in areas outside herring grounds.

Left: A photo of a killer whales feeding on herring in Vestmannaeyjar (South of Iceland). Photo: Filipa Samarra. Right: A group of killer whales attacking a minke whale in Skjálfandi bay (Northeast of Iceland). Photo: Marianne Rasmussen

Identifying site fidelity, residency and movements of marine predators is of great importance to determine their ecological role and dependency on specific area(s), both crucial for future management and conservation plans. Thanks to the research of Dr. Michael Bigg and colleagues in the 1970s in the Northeast Pacific, we know that individual killer whales can be identified through natural markings in their dorsal fin and saddle patch, making photo-identification a powerful and non-invasive method to study their life history, population dynamics and behaviour.

Photo identification picture of a female (left) and male (right) killer whales showing the natural markings on their dorsal fin and saddle patch used for identification. Photos: Filipa Samarra

Using this powerful technique, we found out that, depending on the area, Icelandic killer whales display different site fidelity, as well as a mosaic of movement patterns. The site fidelity to herring grounds was high as was movement between herring grounds. The whales moving between herring grounds are believed to be herring specialists, following the herring year-round. In addition, resightings occurred both within and between regions outside herring grounds. Outside herring grounds, the Southwest of Iceland seems to be an important route for individuals moving between herring grounds but also to individuals moving elsewhere in the North Atlantic. In the Northwest of Iceland, there were only a few sightings, which may suggest that killer whales are rare visitors in this area; however, further continuous monitoring of this region is needed to confirm this. In contrast, the Northeast of Iceland appears to be visited by an offshore population that may target marine mammals or alternatively Norwegian herring, similarly to what appears to happen in the East of Iceland. Both areas showed the highest proportions of unique individuals, i.e., individuals not seen elsewhere, and are of high interest due to the new arrival of Norwegian herring, with the possibility of a separate killer whale community associating with Norwegian herring, and thus possibly connected to Norway. Future research in these regions would be of great interest to better understand the connectivity between killer whales across the North Atlantic. The population, thus, appears to include fish-eating killer whales moving between herring grounds and individuals with potentially diverse, and mixed, foraging strategies elsewhere.

In the South of Iceland, a herring ground, long-term occurrence patterns showed that there can be large variations from year to year on the number of whales sighted and identified. The year 2014 was a year with a high influx of new individuals that joined those previously identified and returned to the areas the years after. It is likely that changes in the local marine environment occurred in 2014. We cannot pinpoint a single cause for this change in killer whale occurrence patterns, but this was the year when pilot whales (Globicephala melas) started to occur regularly in the South of Iceland, when mackerel (Scomber scombrus) was also present in large abundances and the wintering distribution of the main herring fishery changed. There might have also been other un-monitored changes in the ecosystem at this time that we are unaware. Therefore, collecting information on prey availability and combining it with observations of feeding events will, in the future, allow for an assessment of the ecological determinants of killer whale site fidelity patterns.

This study represents our largest photo-id collaboration to date and it would not have been possible without all the individuals and organisations that contributed their photos to build this giant puzzle. Team work really makes the dream work!

This study was led by our PhD student Tatiana Marchon under the supervision of Dr. Filipa Samarra and Dr. Marianne Rasmussen at the University of Iceland. The full paper is available in view-only here or you can download it here