I liked the alliteration of this triologism, and the recurrence of bubbles in the water-washed world of otters.  Bubbles blown by an otter can be a signal of joy or fear.  Here they are created by a line of human hunters and signal danger.

‘Way down the river was stopped by a line of upright figures, standing a yard apart and stirring the water with poles.  Tarka heard the noise underwater, but swam down until he saw before him the bright-bubbled barrier.  He swung round and swam upstream as fast as three and a half webbed feed could push him.’

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. 218