Demersal community thermal indices followed year-to-year sea surface temperature changes in five US East Coast trawl surveys but not in ten other North American surveys, 1970-2020

Question: when the sea is warmer than usual, does the demersal shelf community change to warmer-water species in the same year, and is this response the same in all regions? Data: 15 North American bottom trawl surveys in the OceanAdapt compilation (121,118 hauls, 1970-2020), species thermal indices from the WOA23 climatology, and HadISST1 and ERSSTv5 sea surface temperature (SST). Method: we used fixed strata and calculated a community temperature index (CTI) for each survey year. The main statistic is the slope of the CTI change between consecutive surveys on the change in the 12-month SST anomaly (C per C). We used block-bootstrap intervals, phase-randomized SST surrogates and random-effects means. Results: the five US East Coast surveys had slopes of 0.15-0.35 (mean 0.25, 95% CI 0.17-0.33). The ten other surveys had a mean of 0.013 (-0.008 to 0.034). The East Coast result repeated in two periods, with two SST products, with consistent taxa, with gear-change pairs removed, and with thermal indices from other regions only. Biomass weights made two Southeast US slopes small. The response came in the same year. We found no clear warm-cold asymmetry. Slopes became smaller with depth (Northeast US fall 0.65 at 0-50 m, 0.06 at 200 m or more). Outside the East Coast, strata at 0-100 m had slopes of only 0.04-0.05. In the Northeast US, warm-affinity species carried the response. In the Southeast US fall and summer, cold-affinity species carried it. Thus range-edge species of each community responded. Interpretation: fast thermal re-sorting was a property of the East Coast shelf, not of North American shelves in general. A poor SST link to sea-floor temperature explains part of the regional contrast, but not all of it. Limitations: we used SST, not sea-floor temperature, and nominal survey months because the data have no haul dates. Thus we cannot separate range change from a change in migration timing. We formed the East Coast group after the first per-survey results, and it contains only two independent regions. STI is a climatological mean.

community temperature index · species thermal index · demersal fish · bottom trawl survey · OceanAdapt · sea surface temperature anomaly · thermal tracking · species distributions · range edge · tropicalization · deborealization · depth · observation bias · regional replication · Northeast US shelf · Southeast US shelf