A rare relative of the herring, the allis shad, is returning in large numbers to the Tamar estuary, the Environment Agency says.
The fish, one of the UK's rarest, spends most of its time at sea, only returning to fresh water to breed.
The agency said officers had noticed an increase in numbers in the valley on the Devon and Cornwall border, with some believed to be bigger than normal.
At least three spawning areas had been identified in the area, it added.
One fish caught in the Tamar by officials as part of a monitoring programme was 20ins (52cm) long and weighed 4.2lb (1.9kg).
The Tamar estuary and the Solway Firth - on the Scotland-England border between and Dumfries & Galloway and Cumbria - are currently the only known sites in the UK where fish regularly spawn, the agency said.
Outside the breeding season, the fish are mainly found in shallow coastal waters.
The agency said 2011 "will certainly be remembered as a year when this species was present [in the Tamar] in abundance".