The Haddington-based rocker, 48, has spoken of his heartbreak at the "sad" split with Heather Findlay, who is the lead singer with the band Mostly Autumn and is 19 years younger than him.
It is the second recent break-up for Fish, who split from his long-term former wife, Tamara Nowy, five years ago.
Speaking of the breakdown of his relationship with Miss Findlay, Fish was reported as saying: "It's been very painful for both of us. We had to make a decision as to the long term of our relationship, with particular regard to having a family together.
"Heather is a young woman who one day wants a family and I am a 48-year-old father of a teenage daughter. We are both heartbroken but have accepted each other's wishes and desires."
The couple met last December at the Classic Rock Society awards, where Fish presented one of the awards, and they have been together ever since. But Miss Findlay has been spending much of her time at home in York, while Fish lives with his 15-year-old daughter Tara on his East Lothian farm grounds.
Following his marriage break-up in 2001 he was on the brink of bankruptcy and needed to sell his main farmhouse and move into a smaller house on the grounds.
He has recently returned to touring and releasing albums, as well as appearing on TV in Rebus, The Bill and Taggart.