The fish with a ring in its mouth is a salmon and the ring was a present from Hydderch Hael, King of Cadzow, to his Queen Languoreth.
The Queen gave the ring to a knight and the King, suspecting an intrigue, took it from him while he slept during a hunting party and threw it into the River Clyde. On returning home, the King demanded the ring and threatened Languoreth with death if she could not produce it. The Queen appealed to the Knight who, of course, could not help and then confessed to St Mungo who sent one of his monks to fish in the river, instructing him to bring back the first fish caught. This was done and St Mungo extracted the ring from its mouth. The scene is represented on the counter seal of Bishop Wyschard, made about 1271.