Fish at cold English river sites did not decline in 1993–2019 as in US cold streams; a larger richness gain at warm sites was mostly in one region with more small-species records

Question: Do English river fish show the diverging trends reported for US streams in 1993–2019 (Rumschlag et al. 2025), where fish abundance and richness decreased in cold streams and increased in warm streams? Data: Environment Agency National Fish Populations Database. We used 481 long-term sites with electric-fishing surveys in May–October 1993–2019 (6,276 surveys). Method: We put the sites into three equal groups by July–August air temperature (E-OBS, primary index). We also used modelled water degree-days, altitude and latitude. We estimated class trends in first-run fish density and species richness with site fixed effects and catchment-clustered errors. A second model compared classes only within regions. We also tested homogenization with equal-effort species lists. Results: Cold sites did not decline. Density changed by +10% (95% CI −35% to +87%) and richness by +4% (−6% to +14%). The richness intervals exclude the US cold-stream decrease (−32%) in all variants. Density did not diverge consistently. The warm-minus-cold slope was +0.020 ± 0.012 log units per year (US +0.048), and its sign changed without the Anglian region. Richness increased more at warm sites (+39%; contrast +0.011 ± 0.002 per year), and warm sites became more similar. But 141 of the 160 warm sites are in the Anglian region. In that region the number of recorded small-bodied species per survey increased from 1.0–2.1 to 3.4–4.4, while large-bodied species did not change. Without that region the richness contrast was +0.005 ± 0.003. With the water degree-day index there was no contrast. European eel occurrence decreased by 17–23 percentage points in all classes. Limitations: Air temperature is a proxy for stream temperature. All English sites are below the US warm limit (23.8 °C). Thermal class and region are confounded. Changes in recording or catch efficiency of small fish can explain a part of the warm-site richness gain.

freshwater fish · river monitoring · climate warming · species richness · abundance trends · biotic homogenization · England · Environment Agency NFPD · replication · thermal regime · electrofishing · recording bias