Dozens of salmon had to be rescued from the River Avon near Salisbury after a hatch was left shut, trapping them in shallow water.
The salmon were trying get to spawning grounds upstream
The fish, some weighing more than 20 lbs, were migrating to spawning grounds when their journey was halted.
The Environment Agency said a local landowner had failed to open a sluice gate used to help control river levels.
The fish, who were left vulnerable to poachers and predators, were netted in a special operation and taken upstream.
All the salmon were mature fish in prime breeding condition.
The largest weighed more than 22 lbs and measured nearly 3ft from nose to tail.
Officers also opened the hatch to increase the river flow and encourage the salmon to move up river towards their spawning grounds.
Adam Davis from the Environment Agency said: "Large salmon were highly visible in shallow clear water making them a tempting target for poachers.
"These fish are important breeding stock, so we decided to take immediate action after receiving unconfirmed reports of fish being illegally taken."
The agency is mounting regular anti-poaching patrols in the Salisbury area to help protect salmon stocks.