Fluke-less pygmy killer whale spotted on the central Atlantic

The increase in fluke-less whale reports raises concerns about the negative impacts of fishing activities and shipping routes on cetaceans. In addition to natural causes, the main reason for tail mutilation is ship strikes and entanglements in fishing gear (Urbán J. et al. 2004; Zimmer 2018; Jahoda 2020; Kim et al. 2022). Gray whales (Eschrichtius robustus) and bottlenose dolphins (Tursiops aduncus) have been reported to survive these injuries by adapting their peduncle movement to match their group’s speed, participating in social activities, and reproducing (Urbán J. et al. 2004; Kim et al. 2022). Here, we report the sighting of a fluke-less pygmy killer whale (Feresa attenuata Gray, 1874), a rarely observed species of oceanic dolphin (McSweeney et al. 2009), in the central Atlantic. The injured pygmy killer whale was spotted on 01 November 2024 during RV METEOR expedition M205 at position 10° 32′ 15.96″ N, 26° 42′ 51.68″ W in the central Atlantic, ca. 540 km SW of the Cabo Verde Islands, as a member of a group of three animals. When diving, the fluke-less whale occasionally lifted the terminal end of its caudal peduncle above the water surface (Fig. 1). It was remarkable to see that it was keeping up relatively effortlessly with the group surrounding stationary RV METEOR in a loose formation for about 15 min before setting on again together with the group. Photos and video footage (Fig. 1a–l) revealed that the tail stump of the fluke-less individual seemed to be well healed. However, while the two other individuals showed much more reciprocal interaction in parallel swimming patterns, joint surfing, and small jumps, the amputated individual remained nearby but outside the cluster for most of these activities. While wild individuals can manage and survive the partial loss of body parts, complete fin amputation is usually life-threatening (Wells et al. 2008; Kim et al. 2022). The more remarkable it is that this pygmy killer whale used the same propulsion method observed for fluke-amputated gray whales and dolphins (Urbán J. et al. 2004; Kim et al. 2022). This indicates that the observed means of propulsion — moving the flat peduncle in lateral side-to-side movements (relative to the dorsal midline), partially turning its whole body to the side — is a shared strategy for dolphins and other whales that lost their flukes and survived the injury.