Occurrence of pilot and killer whales
In total, 8090 days of effort were recorded from 2007 to 2020 across all locations, including 183 days (2.3%) with pilot whale and 647 days (8.0%) with killer whale sightings. However, effort and occurrence of the two species varied considerably between sites (Table 1).
Pilot whales were only encountered from June to September (Fig. 2). Pilot whale occurrence increased over the study period in four out of six locations (all except Steingrímsfjörður and Skjálfandi), but its correlation with year was statistically significant only in Vestmannaeyjar (r(9) = 0.85, p < 0.01) and Faxaflói (rs = 0.69, p < 0.01). Pilot whales were sighted most regularly in Vestmannaeyjar, Breiðafjörður, and Steingrímsfjörður. In all other locations, sightings were irregular and infrequent (Table 1 and Fig. 2).
Killer whales were encountered year-round but occurrence varied between seasons and locations (Fig. 2). They were sighted regularly in the summer months in Vestmannaeyjar. In Breiðafjörður, occurrence was highest in winter and spring. In all other locations, killer whale encounters were infrequent and there was no clear seasonal pattern in their occurrence. There was no significant positive or negative correlation in occurrence of killer whales with year for any location. In the two areas with the highest occurrence of killer whales, Vestmannaeyjar and Breiðafjörður, occurrence of pilot whales was also high (Table 1 and Fig. 2). These were also the only two locations where both species were sighted on the same days and behavioural interactions between the two species were observed (Table 1).
Interactions between pilot and killer whales
Although both killer whales and pilot whales occurred in the same area on 44 days of effort (Table 1), some of these sightings occurred at different times of the day such that both species were not simultaneously present. In 34 of these 44 days, pilot and killer whales were observed simultaneously, providing the potential for interactions to occur (Table 2). Interactions were observed on 70.6% of these days, or on a total of 24 days, 20 in Vestmannaeyjar and four in Breiðafjörður. The difference between these two locations may be due to a greater temporal overlap of the two species in Vestmannaeyjar, where both were common in summer. In contrast, in Breiðafjörður pilot whales were mostly seen in summer and killer whales were most common in winter and spring (Fig. 2). The mean duration of the observed interactions was 62 ± 71 min (mean ± standard deviation; range 6–286). The mean number of killer whales and pilot whales involved in the interactions was 27 ± 27 individuals (range 4–100) and 52 ± 39 individuals (range 20–150), respectively.
In all interactions, pilot whales moved towards killer whales, causing the killer whales in most cases to move directly away from the pilot whales. Pilot whales typically approached killer whales at high speed, often porpoising, a behaviour used as an indicator of high-speed travel (Weihs 2002). The interactions could be divided into three categories, based on the behavioural response of the killer whales: no response, avoidance, and high-speed avoidance. In very few cases, the pilot whales approached and the killer whales showed no visible response (Table 2). Regular avoidance included instances where killer whales moved away from pilot whales at low to moderate speed, showed minor evasive behaviour, or disappeared from view for a few minutes and were then sighted some distance away. High-speed avoidance was assigned when killer whales were observed porpoising out of the water, with the pilot whales chasing the killer whales at high speed (Fig. 3, Video in Supplementary Material). Obvious aggressive physical contact between the two species was never observed. The interactions either ended with the pilot whales leaving the area or, in the case of high-speed avoidance behaviour, with both species slowing down and becoming less directional in their movements.
The majority of interactions were categorised as regular avoidance (68.0%), but this should be considered a maximum estimate as the absence of porpoising was not always confirmed (Table 2). High-speed avoidance was confirmed in 28% of interactions. Pilot whale group size differed between regular avoidance, high-speed avoidance, and when no interaction was observed (Kruskal–Wallis test, x2(2) = 8.06, p < 0.05, Fig. 4). No interaction reflected times when both species were present in the same area simultaneously but there was no interaction between them. Observations of pilot whale approaches where killer whales showed no response (i.e. an interaction with no response) were too few to include in the comparison (n = 1). Pilot whale group sizes were highest during high-speed avoidance (85 ± 46, range 40–150, n = 5) compared to regular avoidance (34 ± 20, range 20–80, n = 8) and no interaction (31 ± 15, range 10–50, n = 5) but due to the small sample sizes, no further tests were performed. There was no evidence that the group size of killer whales varied across the three categories (Kruskal–Wallis, x2(2) = 1.09, p > 0.5, Fig. 4), although group sizes were largest for regular avoidance. However, usually several groups of killer whales were in the area and it was not always clearly recorded in the field notes whether all or only some groups responded. We could therefore only compare the total number of individuals observed.
Examples of interactions between pilot and killer whales
Examples with movement tracks of three subsequent interactions can be seen in Fig. 5 and are briefly described here. These visual observations were collected on 23rd July 2019 in the Vestmannaeyjar archipelago and were made by boat, or from land with binoculars and a theodolite. Throughout the day, several groups of killer and pilot whales were observed. Visual observation of killer whales from land was obscured by islands during the second and third interaction events, but the boat was in proximity of killer whales throughout the entire observation period (10:51–17:03 UTC).
During the first approach, the pilot whales were only observed from the land station and no clear response of the killer whales was observed. At 11:29, 2–3 subgroups of pilot whales with 11 individuals in total were sighted. The groups were noted to merge and split several times while travelling fast towards the killer whales. Between 12:30 and 12:46, five killer whales were seen near one of the islands. At 12:45, the pilot whales made a sharp turn and started heading away from the killer whales. The pilot whales split and changed direction several times and then all started moving southwest at 12:51 and again towards the killer whales at 13:11. The killer whales changed direction frequently but stayed in the same area (‘milling’). At 13:35, the pilot whales were very close to the killer whales and changed direction a few times but then stopped travelling. The killer whales were logging at the surface. The pilot whales were not seen again until 14:00 when they were heading away from the killer whales.
The second approach was also observed from land and began at 14:45 when a group of 50+ pilot whales was sighted coming from the area where the previous group of pilot whales was last seen. Five killer whales were seen at 14:46 feeding close to one of the islands. At 14:50, the pilot whales split into two groups, both of which started approaching the killer whales at high speed and tracking of the two groups began at 14:56 and 15:07. At 15:24, one position measurement could be taken of the killer whales before they disappeared from view behind an island. Soon after (at 15:27), one of the two groups of pilot whales appeared to stop for a short time, and then at 15:36, the two subgroups merged. At 15:51, a group of eight killer whales was sighted close to the island but no coordinates could be measured. The pilot whales started to head away from the killer whales and were last seen at 16:15. As the killer whales were located close to an island that obstructed the view during parts of the approach, it is unclear whether they actively avoided the pilot whales.
The last approach by pilot whales was observed from the boat only. Five groups of killer whales with 26–28 individuals in total were observed from the boat between 16:01 and 16:36. At 16:35, 80–100 pilot whales were seen approaching from the east. A group of killer whales initially turned towards the pilot whales but then turned away, and at 16:41, the pilot whales were pursuing this group of six killer whales at high speed, with both species porpoising (Fig. 3, Video in Supplementary Material). The boat closely followed the killer whales for a short period (Fig. 5), allowing us to estimate a minimum swimming speed of at least 8 knots (15 km/h) and a maximum of 13.5 knots (25 km/h). At 16:50, the distance between the two species increased and both were slowing down. At this point, it is noted that several killer whales had stayed behind and had not been part of the group showing high-speed avoidance.