Whales of the deep: analyzing movement and diving of humpback whales to understand oceanic breeding congregations in New Caledonia

Humpback whales (Megaptera novaeangliae) are known for their nearshore distribution during the breeding season, but their pelagic habitat use patterns remain mostly unexplored. From 2016 to 2018, 18 humpback whales were equipped with depth-recording satellite tags (SPLASH10) to shed light on environmental and social drivers of seamount association around New Caledonia in the western South Pacific. Movement paths were spatially structured around shallow seamounts (<200 m). Indeed, two males stopped over the Lord Howe seamount chain during the first-ever recorded longitudinal transit between New Caledonia and the east coast of Australia. Residence time significantly increased with proximity to shallow seamounts, while dive depth increased in the vicinity of seafloor ridges. Most of the 7,986 recorded dives occurred above 80 m (88.5%), but deep dives (>80 m, max 616 m) were also recorded (11.5%), including by maternal females. Deep dives often occurred in series and were characterized by U-shapes suggesting high energy expenditure. This study provides new insights into the formerly overlooked use of pelagic habitats by humpback whales during the breeding season. Given increasing anthropogenic threats on deep sea habitats worldwide, this work has implications for the conservation of vulnerable marine ecosystems.

Humpback whales (Megaptera novaeangliae) are known for their nearshore distribution during the breeding season, but their pelagic habitat use patterns remain mostly unexplored. In New Caledonia, an archipelago located in the western South Pacific, several offshore seamounts and banks are visited by humpback whales. Yet, the reasons why whales would aggregate and move between these offshore waters remain poorly known. From 2016 to 2018, 18 humpback whales were equipped with depth-recording satellite tags to shed light on environmental and social drivers of seamount association around New Caledonia. Shallow seamounts (< 500 m deep) spatially structured movement paths at small and large scale. Indeed, two males stopped over the Lord Howe seamount chain (3 to 7 days) during the first-ever recorded longitudinal transit between New Caledonia and the Australian East coast. Movement paths were modeled with Correlated Random Walks to estimate the position of dive events. Residence time was found to significantly increase with proximity to shallow seamounts, while dive depth increased within 100 km of these seabed features. Most of the 7,986 recorded dives occurred above 80 m (88.5%), but deep dives (> 80 m, max 616 m) were also regularly recorded (11.5%), including by maternal females. Deep dives often occurred in series and were characterized by U-shapes suggesting high-energy expenditure. Based on observed patterns of vertical and horizontal movements, we hypothesize that seamounts could 1) constitute navigational landmarks, 2) provide favorable conditions for opportunistic feeding, and 3) have acoustic properties facilitating male song propagation. At a broader scale, humpback whale use of seamounts is predicted from habitat models based on boat-based surveys conducted over the entire Oceania breeding range. Given increasing anthropogenic threats to deep sea habitats worldwide, these new insights into the use of seamounts by humpback whales during the breeding season have both ecological and conservation implications.