A complete census for all upstream and downstream migrating sea trout is available from the Burrishoole traps from 1971. The young smolts migrate downstream towards the sea from March to May. Many trout return from the sea in the same year, known as finnock, and the larger trout return the year after they migrated as smolt.
Wild smolt migrations in the Burrishoole system historically ranged in number from 2337 to 6710, with a significant downward trend in smolt numbers after 1990 to a minimum recorded of 263 in 2010. The low smolt numbers since 2001 are indicating serious spawning and recruitment problems and are giving cause for concern.
Returns of finnock and adult sea trout have been low since the collapse of the stock in 1989 and 1990. Since that time the stock of adult fish has failed to recover. Before 1989, typically the stock consisted of approximately 1,000 to 2,500 sea trout, but this has fallen to less than 100 fish since the 1990s.