Eleven sea trout which were “poached” from a North East river were ready to spawn with about 35,000 eggs, the Environment Agency has said.
An agency spokesman said 13 trout were illegally fished from Waskerley Beck in Wolsingham on Tuesday.
A 3ft (1m) long metal hook called a gaff was allegedly used to catch the fish, the agency said.
A 27-year-old man has been arrested on suspicion of poaching and two other suspects are being sought.
The Environment Agency’s Kevin Summerson said: “This level of illegal activity could have a serious impact on the sustainability of future stocks of sea trout in the River Wear.
“At this time of year salmon and sea trout are returning to rivers to spawn and protecting returning fish is vital to maintaining a healthy fish population.”