Marine biota are redistributing at a rapid pace in response to climate change and shifting seascapes. While changes in fish populations and community structure threaten the sustainability of fisheries, our capacity to adapt by tracking and projecting marine species remains a challenge due to data discontinuities in biological observations, lack of data availability, and mismatch between data and real species distributions. To assess the extent of this challenge, we review the global status and accessibility of ongoing scientific bottom trawl surveys. In total, we gathered metadata for 283,925 samples from 95 surveys conducted regularly from 2001 to 2019. We identified that 59% of the metadata collected are not publicly available, highlighting that the availability of data is the most important challenge to assess species redistributions under global climate change. Given that the primary purpose of surveys is to provide independent data to inform stock assessment of commercially important populations, we further highlight that single surveys do not cover the full range of the main commercial demersal fish species. An average of 18 surveys is needed to cover at least 50% of species ranges, demonstrating the importance of combining multiple surveys to evaluate species range shifts. We assess the potential for combining surveys to track transboundary species redistributions and show that differences in sampling schemes and inconsistency in sampling can be overcome with spatio‐temporal modeling to follow species density redistributions. In light of our global assessment, we establish a framework for improving the management and conservation of transboundary and migrating marine demersal species. We provide directions to improve data availability and encourage countries to share survey data, to assess species vulnerabilities, and to support management adaptation in a time of climate‐driven ocean changes.

In recent years, Arctic and sub-Arctic fish communities have shown extensive reorganization on shelves and in shallow waters, but little is known about the ecological impact of environmental changes in deeper waters. We examined temporal changes (1998-2016) in fish diversity and community structure based on research survey data from East Greenland, over a depth gradient spanning 400 to 1500 m. A northern and a southern continental slope region, 360 km apart, were analysed for temporal changes in water temperature and fish community structure. The bottom water temperature increased by up to 0.2 and 0.5°C, respectively. Contrary to expectations, there was a concomitant loss of species richness of up to 3 and 5 species, respectively, and a decrease in total abundance in both regions. Abundances of individual species displayed different trends between regions, with 3 species of wolf fishes (Anarhichas spp.) and American plaice Hippoglossoides platessoides decreasing in the north and blue antimora Antimora rostrata, Agassiz’ slickhead Alepocephalus agassizii and the roundnose grenadier Coryphaenoides rupestris decreasing in the south. The regional differences may reflect different oceanographic characteristics, as the northern region is more influenced by colder Arctic water, whereas the southern region is primarily influenced by the Subpolar Gyre (SPG). However, the observed temperature increase is expected to be due to an intensifying Atlantic Multidecadal Oscillation and/or anthropogenic climate change and not to SPG changes. The observed changes in biodiversity and community structure associated with warming are likely to affect community dynamics and alter ecosystem functioning.

The most prominent impact of climate warming on marine ecosystems are distributional shifts in fish, which influence species interactions and food web organization. For shallow continental shelf seas, this usually implies a poleward shift or movement to deeper waters to retreat in cold water refuges (Dahlke et al., 2018;Fossheim et al., 2015;Pinsky et al., 2013). Although more than 90% of the habitable oceans' volume lies below 200 m, long-term studies of biodiversity in slope and deep-sea regions are rare (Danovaro et al., 2020;Howell et al., 2020;Levin and Bris, 2015). It has often been proposed that the rapidity of species' responses to climate change decreases with depth because deeper waters are thermally more stable (Levin and Bris, 2015;Van der Spoel, 1994;Yasuhara et al., 2014). However, rapid responses in the abundance of deep-sea biotas, such as fish, nematodes and amphipods, to changing environmental conditions at the surface indicate that processes such as changes in primary productivity can trigger unexpectedly fast responses in the deep