Otters are voracious piscivores so it’s appropriate to liken the black marks they have left to fish-bones. And let’s not forget the Greek-based unpronounceable answer to the Latinate piscivore: ichthyophagous. Save that one for your next spelling test.
‘On the grey boulder at his feet the wan light showed a black mark, as of tar on bitten fish-bones – the spraints of an otter.’
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. 101
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