Time-space documentation of recovery/expansion of summer feeding habitat in the Northeast Atlantic by Bluefin tuna based on observations from multiple sources

Highly migratory species present challenges to fisheries management and sustainable exploitation, due to their movements within and across jurisdictions. These challenges are amplified by data limitations including reliance on commercial catch data gathered at times and places when the species is most abundant and a lack of research surveys for estimating stock distribution and abundance. Here, we use data from multiple sources from the Øresund to Denmark Strait and far north into Norwegian waters to describe a major change in the seasonal range, habitat use and migration of a large, highly migratory species, Atlantic bluefin tuna. The data we have compiled include citizen science data, including (1) direct sightings of bluefin tunas swimming and jumping at the surface, (2) stranding of tunas on beaches, and (3) some limited bycatch data. These observations provide new time-space insights to the recovery and expansion of the former habitat of bluefin tunas, and how this event might be related to fishery management decisions and large-scale climate-ocean variability. The findings demonstrate the value of multiple data sources for understanding species ecology and its dynamics.