A crackdown on fish poaching across Wales has seen 139 people prosecuted in the first four months of the year, says Natural Resources Wales (NRW).
The legal action has seen those involved fined £18,000 in total.
On Tuesday two men were the latest arrests for illegal fishing, after being found on a dinghy with a net on the River Usk in Monmouthshire.
But officials say the issue is a problem across Wales, and threatens an angling industry worth £150m a year.
Since the start of the year, individuals have been caught fishing illegally or poaching from the Loughor estuary in west Wales to the Menai Strait in Gwynedd and the River Dee in Flintshire.
NRW said its biggest concern was “foul hooking”, which involves dragging hooks through the water at high speed in an attempt to impale fish.
This often leaves more fish damaged and dying in the river than are brought to shore, a spokesperson said.
The fish most threatened are wild salmon and sea trout.
The spokesperson said: “Illegal fishing has far-reaching consequences as angling is worth more than £150m to the Welsh economy.
“Illegal anglers can damage the whole ecology of a river as well as impacting on angling as a sport.
“Angling helps protect the environment and is a big draw for tourism.
“It’s important that we continue to crack down on illegal fishing activity so that it remains sustainable for licensed fisherman.”