Fifty thousand fish being reared to stock a reservoir for fishermen have been freed in what is feared could be an attack by animal rights activists.
Howard MacKenzie and Robert Barden at Bewl fishery
The rainbow trout should have been released gradually this year into Bewl Water on the Kent/Sussex border but many are now likely to die.
It would have taken several people with a boat to lower the heavy nets where the fish were kept.
Sussex Police said vandals or animal rights activists may be the culprits.
"I would like to think it wouldn't be environmentalists because a lot of the smaller fish that have been released will certainly perish," said Howard MacKenzie, Bewl's estates manager.
"We will lose an awful lot of the fish between 6in and 10in in size."
Managers are looking at alternative ways to stock the reservoir, but it could cost tens of thousands of pounds.
Workers fear for the future if they have become a target.
"We have had reports in the past of fishermen being approached by people and abused and it is worrying if that sort of thing is going to happen here," said senior ranger Robert Barden.