Salmon spawning in the River Taff are being threatened by poachers, the Environment Agency has claimed.
The salmon was virtually wiped out in the River Taff by the 1980s
Officers took some of the fish out of Blackweir pool in Cardiff's Bute Park on Friday in order to transfer them to the agency's hatchery in Brecon.
A lack of rain and recent frosts had led to them becoming stuck in the pool making them easy prey, officers said.
Peter Gough from the Environment Agency said "several people" had been arrested for poaching the salmon in the Taff.
Salmon fishing requires a special licence and it is illegal to remove salmon from pools like Blackweir.
But local fisherman Steve Phillips said that was not something the average fisherman would be aware of.
"There is a sign for Blackweir but it is over 100 yards away from the bridge which is misleading to people," he said.
"If you don't know the river 100% you may end up breaking the law without realising it."
Covert officers from the Environment Agency are currently monitoring the river and they have the power to arrest any poachers.
Mr Gough said two individuals had recently been fined £1,000 each at Cardiff Magistrates' Court after being caught poaching salmon from the Taff.
"Certain people have seen these poor fish jumping around and have decided they want to get their hands on them," he said.
"But these things are ready to spawn not eat.
"They are completely and utterly unpallatable.
"They are nothing like they were earlier in the year when they were silvery and fresh."
The Environment Agency has a yearly target to collect fish for their hatchery in Brecon to help them produce next year's youngsters.
Pollution from mining and heavy industry meant that by the early 1980s salmon were at very low numbers in the Taff.
But in April around 5,000 young salmon were released back into the river and experts hoped they would swim as far north as Merthyr Tydfil to spawn.
They are born in freshwater and migrate thousands of miles to the ocean before returning to fresh water to reproduce.
The Environment Agency is hoping to build a new fish pass which should make Blackweir less of an obstacle in the future.