ESR prepress abstract   -  DOI: https://doi.org/10.3354/esr01263

ABSTRACT: Humpback whales (Megaptera novaeangliae) undertake extensive annual migrations, have complex migratory patterns, and have held several mammalian long-distance movement records. Here we report on a whale known to feed in the Russian Far East that was sighted in breeding areas on either side of the North Pacific, the Mariana Islands and Mexico, in less than a year (357 days apart). This is the longest published distance (11,261 km great-circle route) between two unique sightings of a photo-identified humpback whale. To understand the context of this movement, we investigated records of whales that had been sighted in Russian feeding areas and Mexican breeding areas using historic and newly available photo-identification data. We found 117 humpback whales documented in both countries between 1998 and 2021, revealing a substantial increase from the previously only 11 known matches. These whales exhibited high site fidelity to Mexico, with one-third seen in multiple years, and up to 10 years. However, we also found that they changed breeding areas more frequently than Mexico whales matched to other feeding areas, illustrating how the Mariana Islands-Mexico movement may have occurred. We document the first complete round-trip migrations between Mexico and Russia, a journey of >16,400 km, the longest known migration of Northern Hemisphere humpback whales. Our data demonstrate regular trans-Pacific movements of humpback whales in the North Pacific, highlighting the importance of Mexico for the species ocean basin-wide and the need for effective local management to aid in the conservation of multiple at-risk distinct population segments.