Two healthy pregnant seals are believed to have been shot “within minutes” of each other, a wildlife boat tour operator fears.
Tony Haggis, from Great Clacton, said he found the dead seals on the Walton backwaters by Walton-on-the-Naze, Essex, on Monday.
He is “disgusted” and “angry” by what has happened and said the deaths were “immoral”.
The RSPCA has appealed for people to come forward with information.
“One had been shot clean through the head and the other shot in the stomach” said Mr Haggis, who has run wildlife boat trips off the coast for 20 years.
“As far as I know it is illegal and totally immoral” he said.
“There is a law which permits commercial fishermen to shoot seals, which is a law I don’t agree with, if the seals are annoying the fishermen and upsetting their livelihood.”
But the naturalist believes “anglers” could be to blame.
“Those two seals were extremely healthy and in good condition, and at breeding age,” he said.
“I will bet with my experience and knowledge these animals are three months pregnant.
“I am angry and disgusted by what has happened, if nothing is said, it will happen again. This might be just the tip of the iceberg.”
An RSPCA spokeswoman said: “At this stage we do not know the circumstances surroundings the death of these seals, we would appeal to anyone who might have any information regarding this to call our RSPCA inspectorate line on 0300 123 8018.”