I had a report of a mink-like animal a week and a half ago in the fields by the Prees Branch nature reserve. I checked at once and found nothing. But today I found this otter spraint (top photo) on the tow path. The distinguishing feature of otter poo is that it smells fishy/not unpleasantly scented/of jasmine tea. It's a really distinctive odour.
Otters are a mixed blessing for water voles. They do predate on voles, but on the other hand, they're native and so their impact balances out (a mink will go through a colony of water voles like a blow-torch through butter). More importantly, otters drive mink out, and will kill them. I was sent a photo just this week of a mink that had probably been dispatched by an otter, but I won't post it here as it's pretty gruesome.
Hearteningly, there was a lot of water vole feeding (photos 2-5) right along the whole length of the canal, and I was interested to see an example of a water vole leaving droppings on top of food. I knew field voles did this, but I've never seen it in water voles.