An Alien in the Arctic: Risso's Dolphins ( <i>Grampus griseus</i> ) in Northern Norwegian Waters

The authors declare no conflicts of interest. The data are openly available (https://doi.org/10.5281/zenodo.17241250). The code for data analysis is available on GitHub (https://github.com/aleck111/Risso-s-dolphins-Northern-Norway). Additionally, a photo-identification catalog relating to the manuscript was published (https://doi.org/10.5281/zenodo.15384625). Figure S1: Study area. Tracks of the Whale2Sea whale watching RIBs (orange lines) as recorded in the automatic information system (AIS) and downloaded from the Kystdatahuset online tool of the Norwegian coastal authority (Kystverket 2025) for each summer season (June 1st to September 30th) from 2019 to 2024. Darker shades of blue indicate deeper waters and correspond to the Bleik Canyon, for which depth contours are shown in Figure 1. Figure S2: Distinctiveness scoring of photo-identified individuals. Scoring was based on (a) presence, size, and number of nicks in the dorsal fin along with dorsal fin shape/outline, that is, Score 0: no discernable nicks, Score 1: moderate number and size of nicks, Score 2: clear and distinctive features, for example several or big nicks; and (b) Scaring patterns, that is, Score 0: no to little scaring, Score 1: moderate scaring, Score 2: clear and distinctive scaring. Figure S3: Previous Risso's dolphin sightings in the Northeast Atlantic. The map shows locations of all Risso's dolphin sightings compiled from databases held by Artsdatabanken (Artsdatabanken no. 2025), Faroe Island Whale Watch (FIWW), the Ocean Biodiversity Information System databank (obis.org 2025), the Norwegian Institute for Marine Research (IMR), and Chosson et al. (2023). Countries with existing photo-ID catalogs are shaded in a dark gray. Catalogues exist from Ireland (Felce 2012; Seán O'Callaghan pers. commun.), from the United Kingdom around North Scotland and the Shetland Islands by Whale and Dolphin Conservation (WDC) (WDC 2022a, 2022b), and from France off the Brittany coast (Oihana Olhasque, Association AL LARK, pers. commun.). The blue shaded area in the southwestern part of the map shows the distribution range according to the International Union for Conservation of Nature and Natural Resources (IUCN, Kiszka and Braulik 2018). Table S1: Number of trips, number of documented Risso's dolphin sightings, sighting percentage, number of occasions on which photographs were taken, and total number of photographs taken for each season. Table S2: All individuals that were seen on more than one encounter. Longest stay is counted from first to last encounter within a year. Encounter number refers to the Risso's dolphin encounter on the relevant day. Table S3: Age classes and sex of the study individuals. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.