Otters have been found drowned in illegal crayfish traps in the River Wid in Essex.
The two animals, believed to be a mother and her cub, suffered what Essex Wildlife Trust said would have been a “long, drawn-out death”.
Darren Tansley, from the trust, said: “This is a tragic find, especially if it is a mother and her cub.”
Killing otters or native crayfish can result in a fine of up to £5,000 or up to six months in prison.
Anyone needing to set a trap must obtain a licence from the Environment Agency.
Mr Tansley said of the otters: “These are animals that are used to diving – they are semi-aquatic and can hold their breath, so to drown in this way is a long, drawn-out process.”
He added that the traps were now one of the biggest threats to otters with more dying this way than on the roads.