English river fish communities did not shift toward warm-water species as local air warmed 0.28 C per decade in 1990-2025; warm summers raised the next-year index only slightly

Question: did the community temperature index (CTI) of English river fish follow local warming, and does a warm summer change the CTI in the next year? Approach: we used 24,945 Environment Agency electric-fishing surveys at 2,297 sites in six regions, 1990-2025. The species thermal index (STI) is the mean 1991-2020 E-OBS air temperature of the European cells where GBIF density maps show the species. Models had site, month, method and strategy fixed effects. The short-term test used the previous June-August anomaly and a next-summer placebo, with standard errors clustered by year. Results: local air temperature increased 0.276 C per decade, but the presence CTI decreased 0.021 C per decade (95% CI -0.028 to -0.013). The European eel decrease caused most of this; without eel the trend was +0.005 (-0.001 to +0.012), 2% of the warming rate. Regional trends without eel were from -0.052 to +0.056 C per decade, and the trend changed with survey season and period. A 1 C warmer previous summer raised the CTI by 0.009 C (0.002 to 0.016); the placebo was 0.004 (-0.007 to 0.014). Without eel the values were 0.012 and 0.000. Warm-water cyprinids (bleak, barbel, gudgeon, bream) carried this response, but across 25 species the response did not clearly increase with STI. This short-term slope predicts only +0.002 to +0.003 C per decade. Eel collapse, salmon decrease and increases of small benthic species changed the CTI more than warming. Limitations: air temperature is a proxy for water temperature; the short-term test has about 34 independent years and weak regional replication; the abundance-weighted CTI failed the placebo; water quality was not modelled.

river fish · community temperature index · thermophilization · climate change · summer temperature · European eel · cyprinid recruitment · observation bias · placebo test · Environment Agency · NFPD · E-OBS · GBIF · England · regional replication · climate debt