Evaluating the potential role of extreme dives by swordfish (Xiphias gladius)

Movements of swordfish ( Xiphias gladius ) typically follow the diel vertical migration of mesopelagic animals, spending daylight hours at ~ 600 m and nights near the surface. However, years of satellite tagging data have also revealed infrequent extreme dives exceeding 1000 m, the motivation for which remains unknown. Here, we investigated the movements of 34 North Atlantic swordfish using data from pop-up satellite archival transmitting tags deployed for one to six months. Statistically “extreme” maximum daily depths were identified on 39 of 3666 deployment days, from which 29 extreme dives were confirmed using depth-time series. These dives were brief, predominantly daytime events, and occurred across the full geographic range of the dataset, spanning temperate to tropical waters and from shelf-break regions to the Mid-Atlantic Ridge. We evaluated competing hypotheses for these dives, including foraging, predator avoidance, and navigation. Dive geometry and mixed-effects models of environmental conditions indicated that extreme dives were most likely to occur in association with elevated sea surface height and variability in sea surface temperature, conditions associated with dynamic oceanographic features such as frontal systems and major currents, with weaker associations with bathymetric and magnetic field features. Together, these findings are most consistent with a role in environmental profiling, whereby swordfish intermittently sample vertical structure or physical cues during large-scale movements. While this profiling may contribute to navigation, the underlying mechanisms for such behavior remain unresolved. Our results highlight the behavioural complexity of swordfish and underscore the importance of the deep ocean in shaping the ecology of highly migratory pelagic predators.