Shifts and expansion in a spatially structured fish stock spanning international borders in the Northwest Atlantic.

Marine commercial fish species can shift their geographic distributions as environmental conditions change. Distribution shifts of commercial species across borders pose a challenge because individual jurisdictions often conduct stock assessments using data from within their own borders only. This can lead to fisheries managers reacting to local trends in abundance that do not represent stock-wide status. Atlantic halibut (Hippoglossus hippoglossus) spans across Eastern Canada and extends into the United States, across a French territory and into the High Seas. Here, we integrate survey data from the United States and Canada to test for distribution shifts across Atlantic halibut's range. In Canada, the halibut fishery is certified as sustainable, whereas in the United States, the same population is designated as "overfished". We used a spatiotemporal modeling approach to integrate US and Canadian fisheries-independent surveys so as to examine a suite of distribution shift indicators at both regional and smaller (focal area) scales, before and during a period of enhanced warming (1990-2005 and 2006-2023, respectively). Abundance increased in both the United States and Canada over the full time series. The proportion of halibut abundance in Canada ranged from 89 to 96%, averaging 94% over the time series. As abundance increased during the 2006-2023 period, the center of gravity (COG) shifted at a rate of 4 km/year north (95% CI: 3-4.3 km), and 12 km/year east (95% CI: 10-15 km) in Canada. During the same period in the United States, the COG shifted southwest at 3 km/year south (95% CI: 1.5-4 km) and 1 km/year west (95% CI: 0.5-2 km). As the population increased between 2006 and 2023, the leading edge of the range expanded northward and predominantly eastward ~270 km, while the trailing edge shifted northward and predominantly eastward 70 km, consistent with the shelf topography and overall northern expansion. Our results are consistent with the spatial expansion of suitable thermal habitat and an increase in the length of the growing season for Atlantic halibut throughout the region. Here, we recognize the need to operationalize distribution shift indicators for fisheries management and other fisheries-focused decision-support frameworks.