The grey seals died over the weekend after being shot in the head at point-blank range. Detectives described the massacre as "horrendous" and began house-to-house inquiries in an effort to find the killer.
The first blood-stained carcasses were uncovered on Saturday when a member of Orkney Seal Rescue found 10 seals dying on rocks at Green Head, Burwick, South Ronaldsay. Their suckling mother was also found dead near by.
One member of the group, Maureen Bain, described how each seal had been shot through the eye. "It was a harrowing sight and the whole experience has been devastating. The seals were so young their umbilical cords were still visible. At that age they could barely have moved. They were totally vulnerable. This is a cowardly act."
Northern Constabulary said 15 more pups were found dead in a bay north of Green Head. All the young were just a few days old. A spokesman said: "These pups were new-born. They would have just been lying on the rocks being fed by their mothers - it is a quite horrendous attack."
Detectives do not know why the seals were shot. Under the Conservation of Seals Act, fishermen can shoot seals which stray close to fishing nets and fish-farming equipment. However, police said none of the dead pups could swim and had not posed a threat to the local fishing industry.
Although fishermen have targeted seals in the past, fishermen's leaders said it would be "unfair" to presume that a fisherman was responsible this time. However, Geordie Costie, a spokesman for the Orkney Fisheries Association, said fishermen were "deeply frustrated" that seals were raiding lobster pots and fish farms. He called for a humane cull of seals to protect local fish stocks.
Post-mortem examinations on the seals were carried out yesterday. Mike Lynch, Orkney inspector for the Scottish Society for the Prevention of Cruelty to Animals, said the recovery of three of the bullets used could lead police to the killers. Two years ago, six adult female grey seals were shot dead in the same area of South Ronaldsay. The killer has not been found.