Silver eels are the maturing adult eels that are migrating down the rivers and into the sea. They swim across the ocean to the Sargasso Sea to spawn. Silver eels typically migrate in the autumn using high flooded rivers on dark stormy nights to help them on their way down the river to the sea.
Silver eel have been monitored in Burrishoole since 1959. Trapping of the downstream migrating silver eels became possible following the construction of permanent upstream and downstream fish traps on the Mill Race. With the completion of the Salmon Leap Trap in 1970, full trapping facilities have been in place for over 40 years, the longest time series for mature European Eel, yielding a unique dataset on silver eel downstream migration, including daily counts, size, sex and migration patterns.
Catches of silver eel between the years 1971 (when records began) and 1982 averaged 4,400 eels, fell to 2,200 eels between 1983 and 1989 and increased again to above 3,000 eels in the 1990s. There was an above average catch in 1995, possibly contributed to by the exceptionally warm summer. The average weight of the eels in the catches has increased from 95g in the early 1970s to 215g in the 1990s and 2000s.
There has been a change in the sex ratio from 94% males in 1962 to 37.5% in 1988, so there are more females in the migration now compared to in the earlier years. A decrease in elver recruitment into freshwater and changes in the environmental status of the system are discussed as being possible reasons for these changes.