Climate warming is a major aspect of climate change, directly impacting species, communities, and ecosystems. Ocean temperatures have increased by 1°C in the last 140 years, causing marine ectotherms to shift towards higher latitudes as their thermal niches change. These shifts vary among species depending on dispersal potential, thermal niche, and resource exploitation capacity, leading to altered community composition. Arctic and sub-Arctic ecosystems are warming rapidly, with some areas warming four times faster than the global average. Previous studies have reported increases in species richness in some areas, but consistent empirical evidence for Arctic fish communities is lacking. This study aims to investigate changes in demersal fish biodiversity across a wide latitudinal range (56°N to 82°N) over a long time period (27 years) in the Norwegian-Barents Seas and adjacent areas, examining changes at three scales: alpha (local species richness), beta (variability between sites), and gamma (total regional species richness).

Several studies have documented northward range expansions of various species in response to ocean warming, particularly in the North Sea and around North America. However, research specifically focusing on the Arctic Ocean, above 62°N, is limited, with only a few studies reporting increases in species richness in the Barents Sea. These studies, however, covered smaller areas, fewer species, and shorter time spans, lacking a long-term correlation with climate warming. The Norwegian and Barents Seas have experienced recent warming and increased Atlantic water inflow, along with sea-ice decline, impacting the distribution and productivity of commercial fish stocks. Predicted consequences include increased species turnover due to local extirpations and the arrival of warmer-water species. While there is evidence of northward expansions of Atlantic mackerel, European hake, and potentially bluefin tuna, and at least 11 boreal species in the Barents Sea, a comprehensive investigation of the wider trend in Arctic demersal fish fauna due to climate change is needed, accounting for both species gains and losses across a sufficiently large region.

This study analyzed data from 20,670 research trawls conducted between 1994 and 2020 in the North Sea to the Arctic Ocean. Data standardization excluded trawls with incomplete metadata or questionable data quality. The study area was divided into three regions: the main study area (Norwegian-Barents Sea) and two adjacent areas (Svalbard and North Sea). Environmental variables including sea bottom temperature (SBT), sea surface temperature (SST), salinity, ice concentration, sea surface currents, nitrate, iron, dissolved oxygen, productivity, bathymetry, and distance to coast were collected from various databases. Alpha diversity was assessed as the annual mean species richness per trawl using generalized additive models (GAMs) and boosted regression trees (BRTs). Beta diversity, including turnover and nestedness, was calculated using the pairwise mean Jaccard dissimilarity index. Gamma diversity was calculated using species accumulation curves. The temporal trends of individual species were analyzed using GAMs, and their Arctic affinity was assessed using their biomass-weighted mean latitude.

A total of 193 demersal fish species were recorded. In the main study area (Norwegian-Barents Sea), the average number of species per trawl (alpha diversity) increased by 66% from 1994 to 2020, correlated with changes in SBT. Adjacent areas also showed increases, though not all significantly correlated with temperature. BRT models, including 17 explanatory variables, projected increases in species richness up to 125% in some regions. The overall species richness in the main study area (gamma diversity) increased by 45%. Pairwise mean total beta diversity did not change significantly, but turnover (species replacement) increased by 16%. Of 193 species, 99 showed significant temporal trends in occurrence probability, with 71 showing only positive trends. Species declines were mostly observed in high-latitude species. The increase in species richness was concentrated in two periods, interrupted by a decline between 2007 and 2014.

This study demonstrates a significant increase in demersal fish biodiversity across a large Arctic region, concurrent with rising SBT. The findings support predictions of poleward species range expansions due to climate warming and are consistent with other studies in near-Arctic regions. While the increase in biodiversity is widespread, the BRT model only explains 36% of the deviance in species richness, suggesting other factors are involved. The study also revealed an increase in spatial heterogeneity of biodiversity. Although total beta diversity didn't change significantly, species turnover increased, indicating changes in community composition. The decline of some Arctic species is offset by the increase in other Arctic species and a substantial increase in lower-latitude species. This suggests a partial coexistence of boreal and Arctic species, leading to a net increase in biodiversity.

This study provides strong evidence for a widespread increase in Arctic and sub-Arctic demersal fish biodiversity associated with climate warming. The results highlight the complex interplay between species range shifts and overall biodiversity changes, emphasizing the need for continued monitoring and research to understand the long-term ecological and socioeconomic consequences of these changes. Further research should investigate the role of other environmental factors and food web dynamics in driving the observed patterns and assess potential future impacts on marine ecosystems.

The study relies on presence-absence data, potentially influencing alpha diversity estimates due to statistical dependence on abundance. While sampling effort was accounted for, variations in sampling efficiency could affect beta-diversity decomposition. The analysis is limited to demersal fish, excluding potential changes in pelagic communities. The time frame might not be long enough to capture the full extent of long-term changes.