Some observations on salmon smolt migration in a chalkstream

Observations on smolt migration in a chalkstream over a period of four years indicate a relationship between number of fish migrating during the day and maximum afternoon water temperature. Few fish move below an apparent threshold temperature. This threshold appears to be different each year, varying with the general water temperature of the period proceeding the main run. Tests made with dummy telemetry tags indicate that their use to observe natural migratory behaviour may be justified.