What is a grilse?

A grilse is an Atlantic salmon which has spent only one winter at sea before returning to the river. Salmon grilse are often indistinguishable from multi sea winter (MSW) salmon except by scale reading. They are smaller on average (2-3lb in May, 5-7lb in July) but when they enter rivers in September often attain 8-10lb and in October 12-15lb.