Sea trout have not migrated up a river because it is too dirty, an anglers group has said.
Fishermen have not seen the usual migration of sewin up the Towy River in Carmarthenshire, saying algae build-up is starving the water of oxygen.
Anthony Peynado of Llandeilo Angling Club, said: "I'd have expected to catch a couple by now, or at least have seen some, but there's been none."
Natural Resources Wales (NRW) said officers were investigating the issue.
Anglers usually expect to see sewin migrating up the river to spawn from May, but large quantities of algae have been spotted growing from Llangadog downstream to Carmarthen.
Mr Peynado, the club's vice-chairman, said: "I've been fishing this river for 30 years and never seen a build up like this, starving the river of oxygen.
"It's unprecedented and shows we are just not looking after the environment."
Anglers believe the algae blooms have been caused by agricultural pollution - high levels of nitrates in rivers can be caused by slurry run-off from farmers muck spreading in fields.
NFU Cymru spokesman Stephen James said: "More intensive farming may be part of the problem. But there are other issues too. We are working with farmers to find a practical solution."
Samples of the algae have been taken for analysis but Huwel Manley, operations manager for NRW, said it was not clear whether agricultural pollution was to blame.
"The algae growth is also more evident this year because of low water levels," he said.