The byelaw requires the mandatory release of all salmon and sea trout caught by anglers on the River Wye in Wales and England.
Releasing 100 per cent of salmon and sea trout will enable more fish to survive and spawn upstream and produce more juvenile fish to boost the declining population.
The byelaw was approved by Welsh Government and Defra ministers this week. It could be in place for up to 10 years but results will be reviewed annually, with a full review midway through.
Anyone found to be contravening these byelaws could face prosecution and a substantial fine.
The decision to introduce the byelaw was driven by the continuing decline of salmon and sea trout stocks in the River Wye.
Although many anglers on the Wye are already returning salmon and sea trout, too many continue to kill their catch.
This byelaw will enable anglers to continue with their sport whilst safeguarding the fish.
A lot of work has been done to improve habitats for fish and it is important that the fish survive to spawn in these improved areas.
The Agency, and its main partner the Wye and Usk Foundation, have also invested in building fish passes and removing barriers to improve access for migratory fish and improving water quality.
Pete Gough, Environment Agency Wales, said: The majority of anglers are very responsible and do release the salmon and sea trout they catch. However, there are some that continue to kill the fish before they have a chance to spawn upstream.
Any of these fish that are killed will have an impact and slow down the recovery of the population in these rivers which will take many years.
We will be monitoring the stocks closely and will lift the measure in the future if the stocks recover sufficiently.
Questions are now being raised on how this may affect salmon-only fishermen.