The otters lay the spraints in highly visible places to clearly mark their territory, often on rocks in the centre of rivers. This helps the spraint-hunter, as they can be easy to spot - if you know what you are looking for. When fresh, the spraints are a slimy dark green, becoming paler and more brown as they dry out. They smell very strongly of fish - and Otter's diet is exclusively fish. Minks have a more varied diet, so their scat will smell less fishy, and will contain mammal bones and other debris. This was important distinction, as we didn't want to be finding the wrong poo - I've seen Mink along this section of river before.
After a barren first mile, we found a fresh spraint on a rock in the middle of the river. Not easy to access, but with a long stick, we got a sample. Dark green, a similar to goose scat, but without any white. And I can confirm the smell.
Later, nearer Saltaire, we found an older, dryer spraint. Again, not easy to access, but after some clumsy scrambling down the riverbank, I got close enough to take a photo.
Otter spraint - River Aire, Saltaire, West Yorkshire
After mashing it up, the spraint's contents were revealed. Lots of tiny fish bones, although the smell had obviously faded over time.
Contents of the Otter spraint
A successful evening, and now I know what to look out for when walking along the river.