Atlantic bluefin tuna (Thunnus thynnus) in Greenland – mixed-stock origin, diet, hydrographic conditions and repeated catches in this new fringe area

Based on collaboration with the Greenlandic fishing fleet, we document the presence of Atlantic bluefin tuna (Thunnus thynnus) in most years from 2012 to 2018 in the waters east of Greenland (northern Irminger Sea). In total, 84 individuals have been registered as bycatch in the commercial fisheries in Greenland waters, which indicates that the first catch of three individuals in 2012 was not a single extreme observation, but that East Greenland waters have become a new outer limit of an expanded tuna habitat. Genetic analyses indicate that specimens from this region are mostly of Mediterranean origin with a small proportion originating from the Gulf of Mexico stock. Stomach content analysis suggests that the main prey is Atlantic mackerel (Scomber scombrus). The tunas ranged in size from 140 to 270 cm, corresponding to an estimated age range of 5–16 years; most were probably mature. The wide size–age range suggests that many year classes are participating in the migration to this region. Sea temperatures during summer have been above the long-term average in recent years of interest. Summer residence of bluefin tuna in the region could be due to a combination of increasing temperatures and higher overall abundances of both bluefin tuna and a key prey species (Atlantic mackerel).