Stable isotope analysis of baleen from North Atlantic right whales Eubalaena glacialis reflects distribution shift to the Gulf of St. Lawrence

Seasonal migration of the Critically Endangered North Atlantic right whale Eubalaena glacialis along the eastern seaboard of North America has been well studied. Since 2010, however, right whales have shifted their summer foraging location from the Bay of Fundy to the Gulf of St. Lawrence. There is a need to better understand right whale distribution to manage anthropogenic activities and mitigate impacts on whales. Stable isotope ratios of baleen plates can provide details about migration and foraging behaviour of an individual over a period of several years. We interpreted delta 13C and delta 15N cycles, examined whether stable isotope ratios of baleen could detect the right whale distribution shift, and compared variation within and among individuals before and after 2010. delta 13C and delta 15N values were compared between 8 right whales that died between 1992 and 2005 (pre-2010) and 5 right whales that died in 2019 (post-2010). The correlation between delta 13C and delta 15N varied considerably between individuals, indicating no clear pattern of annual migration in delta 13C among whales. We observed a change in both isotope ratios after 2010, whereby the post-2010 whales were enriched in 13C and 15N relative to the pre-2010 whales (mean +/- SE: 0.5 +/- 0.1 and 0.9 +/- 0.2 parts per thousand, respectively). The isotopic variance among and within whales did not change after 2010. These results suggest that a range shift observed in sighting data is also reflected in the isotope ratios of right whale baleen. Detecting shifts in right whale migration is essential for protecting this species, and stable isotope analyses may be useful in future conservation efforts.