The Minister of Agriculture ordered the slaughter at the Farne Islands Nature Reserve , off Northumberland because the seals' appetite for fish has been menacing the rich salmon industry of the tweed . The operation approved by the Royal Society for the Prevention of Cruelty to Animals is intended to reduce the seal colony from 3500 to 2500 over the next five years . And all together today 145 seals - mainly pups , were killed . Seen here Mrs Grace Hickling , who has devoted years to studying and protecting the seals was on the spot to see that they do not suffer . 5 December 1963