Research investigating the influence of temperature and flood events on juvenile Atlantic salmon found that warmer winters and cooler springs could be driving a fall in their numbers.

Experts from the Game & Wildlife Conservation Trust (GWCT) studied the River Frome in Dorset and found numbers of juveniles were lower following unseasonal temperatures, which could be associated with climate change. Their findings highlight the fragility of chalk streams and the salmon populations that inhabit them, to such changes.

Atlantic salmon stocks have suffered declines in recent decades, leading researchers to try and understand the threats to both juvenile freshwater and adult marine habitats in order to manage these habitats better.

This survey, one of only a few conducted in the past year due to Covid-19 and completed under strict health and safety protocols, recorded the second lowest numbers of juvenile salmon, only beaten by the widespread juvenile salmon crash in 2016.

Both the 2016 and 2020 autumn juvenile sampling followed unusually mild and wet winter conditions and, in 2016 in particular, spring temperatures were unseasonably cold.

Dr Jessica Marsh, who led the GWCT research, said: “Our findings highlight that changes to seasonal temperatures in chalk streams – groundwater-fed rivers that are considered relatively stable in their thermal regime compared to more dynamic rain-fed rivers – can have detrimental impacts on juvenile salmon recruitment in these systems”.

Other GWCT-led research found that warmer winters and wetter spring conditions contributed to lower numbers of juvenile salmon in numerous Welsh rivers.

Dr Stephen Gregory, a GWCT fisheries scientist who contributed to both studies, added: “That findings from Welsh rivers also applied to a southern English chalk stream suggests that the effects of unusual weather conditions are more widespread than we first thought, and could therefore be usefully incorporated into future salmon recruitment models.”

This study, led by the GWCT and Centre for Environment, Fisheries and Aquaculture Sciences (Cefas), used data on juvenile (young-of-the-year) salmon collected at multiple sites throughout the River Frome catchment from 2015 to 2020.