Williamson says this is a ‘name given by fishermen to porpoises’, perhaps in dialect.  It doesn’t appear in the massive OED.

See also ‘vuz-peg’ for another quirky animal name.

Source: Henry Williamson, Tarka the Otter: His joyful water-life and death in the two rivers, illus. C.F. Tunnicliffe (Harmondsworth: Puffin Books, 1976 (1927)), p. 180