Migrations of elvers and juvenile European eels, Anguilla anguilla L., in the River Thames

The up‐river migration of eels in the R. Thames has been studied at a number of locations since 1985. The run occurs between April and October, with the majority of eels moving over a distinct period averaging 47 days in May–June. The timing of the run varies and its commencement appears to be mainly dependent on water temperature. The number of unpigmented elvers migrating from the estuary to fresh water is variable and most appear to spend at least one year in the estuary. The average size and age of migrants increases with distance upstream. Migrants leaving the estuary were mainly 1–3 river‐year (<14 cm) pigmented juveniles, though individuals of up to 45 cm were encountered. At a site 15 km from the estuary the majority of migrants were between 4 and 8 river‐year eels (20–30 cm). Mark–recapture studies showed variable tendencies to display migratory behaviour among individuals, with some taking up to 2 years to reappear in a trap when released a few hundred metres below it. The implications of the results with respect to the commercial fishery in the estuary, populations in the catchment, and the contribution of Thames eels to international stocks and recruitment are discussed.