What an otter ate

I have a favourite flat rock to sit on, on the banks of the river Tees where it meets the river Greta downstream from Egglestone. It's the perfect place to watch kingfishers as they flash past. Last week when we arrived there we discovered that an otter had decided to use it as a latrine and had left a substantial pile of poo, or to be more correct, a spraint.

There was no mistaking what it was - it had that characteristic oily, sickly smell - even though a few days of heavy rain had washed away its greasy coating. When we had a close look we could see the indigestible remains of the otter's meals embedded in it. 

A close look revealed some white circular objects and also the jointed antennae of a signal crayfish, confirmed by the presence of ...

... two large crayfish claws nearby which had been cracked open to extract the flesh.

I took the spraint home, soaked it in detergent overnight and extracted some of the remains of the food items.....

... Most of the objects on the left hand side are fragments of crayfish limbs and the larger object in the middle is part of its telson - the end of the abdomen that is usually curled under the crustacean and has flat fan-shaped paddles on the end. With one powerful flick of this it can propel itself backwards if threatened - although it didn't do this animal much good when it met the otter.

The long, thin object is one of the crayfish's antennae, still articulated after its passage through the otter's gut.

What interested me most though were those six pale objects down the right-hand side of the picture. They are otoliths - the 'earbones' of a fish that are part of its system for perceiving gravity and acceleration when it swishes its tail fin and shoots forward - not fast enough, in this case. The largest otolith is about seven millimetres in diameter, suggesting that the otter had caught quite a large fish.

The otoliths, seen under a magnifying glass, are fascinating structures made up of layers of calcium carbonate. You can see the concentric layers quite clearly here in two planes, because this otolith has been broken when the otter chewed the fish. 

The calcium carbonate comes from the water and the different coloured layers reflect differences in the chemical composition of waters that the fish swam in during its lifetime.

So, this little pile of smelly poo held some interesting clues to the otter's diet. Now I know that it has a territory here we'll be going back to see if we can watch the animal in action.