Silver eel have been monitored since 1959. Trapping of the migrating silver eels in the Burrishoole system 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 30 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, fell to 2,200 between 1983 and 1989 and increased again to above 3,000 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 been steadily increasing from 95g in the early 1970s to 215g in the 1990s.
The mean length of male eels has increased by 16.5% and females by 27.6% over the years, while the total weight of catch has not been affected by the drop in numbers. A change in sex ratio from 94% males in 1962 to 37.5% in 1988 also occurred. 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.