Observations on the Death of a Northern Resident Killer Whale

Deaths of wild cetaceans are rarely observed, particularly when not associated with acute anthropogenic interactions, stranding, or entrapment. Even for closely monitored populations, such as the exclusively fish-eating “Northern Resident” killer whales (NRKW; Orcinus orca ater) of the eastern North Pacific, mortality can typically only be assumed when well-documented individuals go missing from otherwise consistent social groupings (Towers et al. 2020). Consequently, the social behavior and ecological context surrounding the majority of deaths remain unknown, though killer whales from multiple discrete populations have been documented exhibiting epimeletic behavior toward deceased conspecifics (Bisther and Vongraven 2022; Shedd et al. 2020). We present the following observations on the death of an adult male NRKW, I76, in Johnstone Strait, BC, Canada. I76 was a regularly seen individual in the region during summer and had been documented every year since his birth in 1997; he traveled with his mother I4 (born in 1980), his younger sister I102 (born in 2003), and I102's son, I172 (born in 2022). This group, collectively known as the I4s, is considered part of the I11 pod in G clan (Towers et al. 2020). Along with the rest of the I4s, I76 was documented in robust condition and actively foraging on July 27, 2024, at Goose Bank, B.C. The family was next documented on August 28, 2024, in Blackfish Sound, BC, where I76 presented with a minor depression behind the blowhole indicative of reduced body condition. He and his kin were not documented again until August 16, 2025, when community members observed Pacific white-sided dolphins (PWSD; Aethalodelphis obliquidens) and two humpback whales (Megaptera novaeangeliae) interacting with I76 in Blackfish Sound (Figure 1A). Photographs taken at that time indicated that he was in visibly poor body condition. On August 17, 2025, both authors set out from Alert Bay, BC, with the intention of documenting I76 and his kin. Working from a 7 m rigid-hulled inflatable boat (RHIB), we encountered I76 at 1054 at 50°32.52′N, 126°42.48′W in Johnstone Strait, where he was conducting short, shallow dives (30–60 s) within 2–3 m of the water's surface. Boat-based photographs taken from a distance of ≥ 30 m at that time indicated a pronounced loss of adipose tissue at I76's nuchal crest, consistent with severe emaciation in killer whales and other cetaceans (Figure 1B; Fearnbach et al. 2018; Joblon et al. 2014). No other vessels were within 500 m of I76, though several private and commercial boats were in Johnstone Strait that morning. At 1112, we launched a DJI Mini Pro 4 drone and conducted a short (< 20 min) photogrammetry flight, not lower than 20 m, to collect still images and videos of I76 (Figure 1C). I102 and I172 were concurrently observed traveling along the southern shoreline of Hanson Island, approximately 2 km away, in the vicinity of 3–4 PWSD. I4 was not initially observed. Two humpback whales were also noted about 2 km from I76. At 1211, I76 was approached and surrounded by a group of 8–12 PWSD. The dolphins were persistently surface active (porpoising, rapid turns) in close vicinity to I76, who made no evasive movements but was observed lifting his head from the water at an approximately 45° angle amidst the interaction. Almost immediately, I4 porpoised toward the interaction at high speed from about 300 m away and spent several minutes chasing the PWSD near her son. I102 and I172 also came into close range. At 1220, all four killer whales were approached by the two humpbacks observed in the area previously. At 1221 we turned off the engines, deployed a 20 m hydrophone below the RHIB, relaunched the drone, and observed both humpbacks in a brief apparent pursuit of I76. They were positioned behind and oriented toward I76; all three swam increasingly quickly for approximately 30 s before I76 turned sharply to his left, evading the humpbacks (Video S1). No vocalizations from the PWSD or the humpbacks were heard throughout the interaction, though both humpbacks produced trumpeting calls while exhaling at the surface. One brief pulsed call attributed to a killer whale was heard, though not recorded, during the interaction. All three species moved apart, and we returned the drone and the hydrophone to the vessel at 1233. For the next 90 min, we observed I102 and I172 foraging along the Vancouver Island shoreline while I4 and I76 made slow progress eastward nearby. At this time, two other vessels were nearby: a ~15–20 m private vessel and a 6 m RHIB conducting boater education. Of these, only the private vessel and our own research RHIB came within 300 m of the killer whales. At 1410, a larger group of 30–40 PWSD approached I76 in Johnstone Strait and began persistently and energetically swimming above and around him. As before, I4, I102, and I172 traveled to and grouped closely with I76 shortly after the interaction began. At 1411, we launched the drone and observed for 30 min as the dolphins rapidly swam, dove, porpoised, and otherwise maneuvered around the four killer whales, often within a single body length (Figure 2; Video S2). The killer whales assumed a close (< 1 body length) formation abreast of one another and moved slowly or not at all within 2–4 m of the surface while the dolphins continued swimming in front of and around the group. I4 and I172 were both observed swimming directly beneath I76. At 1442, all four killer whales dove, and we returned the drone to the vessel. At 1445, the PWSD were observed porpoising westward away from the area. After approximately 8–10 min underwater, I4, I102, and I172 surfaced in the same location without I76. They conducted 1–3 min of abrupt surfacings, then all dove for a second 8–10 min dive, followed by a 1–2 min surface interval during which each of the three whales took 8–10 breaths. I4, I102, and I172 subsequently conducted a 7.5 min dive in the same location, then finally resumed a more typical dive pattern (2–4 min dives, 1–3 min surface intervals) but did not travel away from the location in which I76 was last seen. We redeployed the hydrophone at approximately 1520. At 1530, several PWSD returned and briefly approached I4, but departed after 2–3 min. Vocalizations were first detected at 1538 and continued until 1559. We recorded output from the hydrophone speaker on an Apple iPhone 13 Mini and an Apple iPhone 16 Pro. The whales remained in the same location while vocalizing; I102 and I172 were grouped closely and I4 remained within 50 m. Additional vocalizations were detected from 1620 to 1625, but not thereafter. At 1655, after more than 2 h spent in the location of I76's last surfacing, the three remaining killer whales began slow travel eastward. We retrieved the hydrophone and followed at ~100–200 m, but turned back westward at 1708 to participate in a short ceremony with research colleagues led by a 'Namgis First Nation hereditary chief at the site of I76's disappearance. At 1814, we observed the three remaining I4s back near the same location, but by 1855 they had entered Blackney Pass and were transiting west in Blackfish Sound. In the following days I4, I102, and I172 were documented in the area traveling closely with unrelated families of NRKW, including the A34s and C10s. Temporary associations between unrelated NRKW families are typical in summer in Johnstone Strait (Ford et al. 2000). Following the encounter on August 17th, we used photogrammetry of aerial images collected via drone to quantitatively evaluate the body condition of the I4 group. Still images and videos were collected using the DJI Mini Pro 4's built-in 48 MP/4K camera positioned parallel to the water's surface and centered over the whales at 20 m altitude. Still images were automatically extracted from 4K video using VLC Media Player (Bierlich, Hewitt, et al. 2025) at a frame rate of 6 images per second. All images were graded for quality and positioning, and the five best images of each individual were selected for measurement. Measurements were made in ImageJ (https://imagej.net/ij/) on a 3024 × 1964 Liquid Retina XDR display. Body condition was evaluated using the eye patch ratio (EPR), a unitless metric that quantifies the robustness of a killer whale by taking the ratio of two measurements across the cranium (Fearnbach et al. 2019). I76's mean EPR was estimated at 0.922 (SD = 0.005; n = 6), considerably lower than mean EPR values for adult male Southern Resident killer whales (SRKW) and Bigg's killer whales (BKW), which have been estimated at 1.24 (SD = 0.03) and 1.27 (SD = 0.04), respectively (Kotik 2020). I4's and I102's mean EPRs were estimated at 1.222 (SD = 0.027; n = 5) and 1.213 (SD = 0.032; n = 5), respectively; both were within typical ranges for adult female SRKW and BKW (Kotik 2020). I172's mean EPR was estimated at 1.120 (SD = 0.021; n = 5), which was lower than the mean EPR of a BKW calf (ages 0–3) but within range of the mean EPR of a SRKW calf (Kotik 2020). Of the I4s, only I76's body condition was notably poor. Cetacean body condition is influenced by a variety of environmental and individual factors, including prey availability, age/sex class, reproductive status, social position, group composition, and overall health (Bierlich, Pirotta, et al. 2025; Kay 2024; Kotik et al. 2022; Miller et al. 2012; Raverty et al. 2020; Robinson and Visona-Kelly 2025; Stewart et al. 2021; Van Aswegen et al. 2025). Nutritional stress and emaciation linked to reduced prey quality and availability are well documented in SRKW but are not believed to impact NRKW population health as significantly (Ayres et al. 2012; Ford et al. 2010; Hanson et al. 2021; Kay 2024; Murray et al. 2021; Stewart et al. 2021; Ward et al. 2013). The drivers of I76's decline are unknown, but the comparatively healthy body conditions of his kin suggest that his circumstances were individual (i.e., resulting from illness or injury) rather than environmental (i.e., resulting from poor prey availability). We did not observe any physical or behavioral evidence that I76 had sustained any injuries that might have interfered with his ability to forage or feed, though the possibility cannot be entirely excluded. His decline could also have been symptomatic of an underlying disease process, but we did not attempt to collect any samples from I76 or his kin and cannot comment on his physiological state beyond the observations already presented herein. While the underlying etiology of I76's poor body condition remains uncertain, we suggest that his immediate cause of death may have been related to the stress and energetic expenditure of several successive interactions with other species. Interactions between killer whales and other cetaceans have been documented for decades, including nonpredatory interactions between NRKW, PWSD, and humpback whales (Jefferson et al. 1991; Visona-Kelly and Barrett-Lennard 2025). PWSD are frequently observed initiating interactions with NRKW in Johnstone Strait; these temporary associations may confer energetic benefits and antipredator advantages to PWSD through possible foraging advantages and mutual avoidance of mammal-eating killer whales (Baird and Dill 1995; Syme et al. 2021; Visona-Kelly and Barrett-Lennard 2025). However, these interactions do not appear to benefit NRKW, whose responses to PWSD range from neutral to antagonistic: NRKW have been observed disassociating from PWSD via long dives and frequently shift behavioral states during and after interactions (Visona-Kelly and Barrett-Lennard 2025). Less is known about interactions between humpback whales and NRKW, but nonpredatory interactions, including NRKW mobbing by humpbacks, have been documented (Pitman et al. 2017). While brief, I76's avoidance of the pursuant humpback whales likely had negative energetic consequences in his reduced state. Anthropogenic disturbance, including from research effort, also has the potential to negatively affect cetacean welfare (Nicol et al. 2020). Acoustic disruption from vessel presence can alter behavior and decrease foraging efficiency in NRKW (Konrad Clarke et al. 2024; Tennessen et al. 2024; Williams et al. 2006). Similarly, while drones are thought to be a relatively low-impact monitoring platform and NRKW typically do not exhibit strong reactions even to low-altitude drone flights (Durban et al. 2015), behavioral responses to drones by cetaceans may vary with context (e.g., Aubin et al. 2023; Castro et al. 2021; Lo et al. 2025; Giles et al. 2021; Ramos et al. 2018). Although vessel presence near the I4s was relatively limited during our observations and we did not observe any signs of behavioral response to drone presence, the potential impacts of our monitoring should not be discounted. In summary, we suspect that interspecies interactions critically impacted the welfare of I76 preceding his death. As has been documented in other interactions between PWSD and NRKW (Visona-Kelly and Barrett-Lennard 2025), the I4s may have attempted to dissociate from the PWSD via a long dive in relatively deep (~330 m) water. However, in his advanced state of emaciation, the stress and energetic expenditure associated with this and the preceding disturbances may have sufficiently exhausted I76 so that he was unable to return to the surface alongside his kin. NRKW are capable of conducting dives beyond this depth for durations up to 10 min, but are most often documented diving < 200 m for < 4 min (Wright et al. 2017). In his advanced condition, I76's ability to surface from depth was likely to have been considerably reduced. The repeated 7–10 min dives by I4, I102, and I172 in I76's last known location suggest his death took place below the surface during one of their initial long dives and that his kin were likely present for it. With very few exceptions, NRKW maintain close relationships with their maternal kin throughout their lives (Bigg et al. 1990; Ford 2019). Individual losses, in addition to potentially impacting inclusive fitness and foraging efficiency, may have significant social effects on surviving group members. We observed I76's mother, sister, and nephew conducting multiple long dives and lingering in the location of his last surfacing. While none of our observations included physically epimeletic behavior, the attendant behavior of I76's kin prior to and following his disappearance exemplifies the strong social bonds within NRKW families. Together with our photogrammetric analyses of body condition and notes on interspecies interactions, we present these observations as a rare glimpse into a poorly known aspect of the lives of wild killer whales. We extend our thanks to the community of whale-watchers and stewards that alerted us to I76's condition prior to his passing and in particular thank Scott Turton for the use of his photo. We also extend our thanks to chief Ernest Alfred of the ‘Namgis First Nation for conducting I76's farewell, Helena Symonds and Alexandra Morton for reviewing acoustic recordings, and the OrcaLab staff and volunteers for their support. All research operations described were conducted under Fisheries and Oceans Canada research license MML-01 and funded by Fisheries and Oceans Canada. The authors declare no conflicts of interest. The data that support the findings of this study are available from the corresponding author upon reasonable request. Video S1: Aerial footage of Northern Resident killer whale I76 being pursued by two humpback whales on August 17th, 2025. I76 is visible below the surface ahead of both humpbacks before making a sharp leftward pivot; his mother I4 approaches from the right side of the frame. Drone operations permitted under Fisheries and Oceans Canada marine mammal research license MML-01. Video: Jared Towers. Video S2: Aerial footage of the I4 matriline (I4, I76, I102, and I122) and 30–40 Pacific white-sided dolphins on August 17th, 2025. Drone operations permitted under Fisheries and Oceans Canada marine mammal research license MML-01. Video: Jared Towers. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.

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