Williamson uses – and invents – a number of words to convey the sounds of otters.  Whistling can be in affection or playfulness or fear, grief or warning.  Other sounds include tissing, yikkering and yinnying.

‘For she was young, and calling to the dog, with a soft, flute-like whistle, she swam to the high-arched bridge upriver.’

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. 19-20