What Do We Know About Hybrid Blue ( <scp> <i>Balaenoptera musculus</i> </scp> ) and Fin ( <scp> <i>B. physalus</i> </scp> ) Whales? A Comprehensive Review Across Ocean Basins

) whales have been reported from commercial whaling since the late 1800s. Even while species descriptions were still at their infancy, whalers described whales with intermediate characteristics between blue and fin whales. In more recent decades, hybrid reports have come mainly from sighting surveys or genetic analysis from biopsies or caught individuals. Over the past three decades, hybrids have been confirmed genetically with various methods, which have provided information related to maternal lineages. Furthermore, hybrids captured by recent whaling operations have enabled genetic verification and detailed assessment of their morphological traits. In this review, we synthesise available information on blue-fin whale hybrids across ocean basins, encompassing historical accounts and contemporary observations, including recently documented specimens. We document 46 possible hybrids, of which 17 have been genetically confirmed: 15 from the North Atlantic, one from the Mediterranean, and one from the north-east Pacific. We also highlight possible bias in current knowledge and recommend the collection of biopsies when atypical whales that exhibit characteristics of both species are encountered.