WARMER winters and cooler springs have been linked to lower numbers of juvenile salmon in Dorset's River Frome.

New research probing 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 and found numbers of juveniles were lower following unseasonal temperatures, which, they say, could be associated with climate change.

Dr Jessica Marsh, who led the research for GWCT, 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."

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.

The GWCT says Atlantic salmon stocks have suffered declines throughout their range in recent decades, leading their researchers to pursue a better understanding of threats in both juvenile freshwater and adult marine salmon habitats.

With this understanding, the GWCT believes freshwater habitats could be managed to produce lots of large juvenile salmon – giving them the best chance of surviving the marine journey to return to their natal river to spawn.

Most juvenile salmon monitoring programmes nationally were cancelled in 2020 due to the Covid-19 pandemic.

Under strict health and safety protocols, GWCT were able to undertake their long-term monitoring of juvenile salmon in the River Frome successfully in autumn.

This 2020 survey recorded low numbers of juvenile salmon, second lowest only to the widespread juvenile salmon crash in 2016.

Recent research led by the GWCT 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, said: “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.”