In a previous Nature Notes (Sept, 2019), I discussed the presence of Signal Crayfish, a non-native species in the river Wharfe, and some of the reasons they are widely considered to be an ecological problem. These include displacing the native White-Clawed Crayfish, in a manner not dissimilar to the displacement of Red Squirrels by Greys. No, not by hanging around in the same treetops – but by being a vector for a lethal disease, to which they are relatively immune, living in greater densities, and outcompeting them when it comes to food and shelter. But could it be that there also be some positive outcomes associated with the presence of Signal Crayfish?

One sunny October morning, I was sitting on the riverbank, close to the water’s edge. From some distance, I was watching a small group of Goosanders as they made their way downstream, towards me. These aren’t everyone’s favourite bird. Sometimes they are referred to as ‘sawbills’, because the serrated edges to their bills facilitate catching small, slippery fish. So, some sections of the angling community are concerned about the effect they may have on fish stocks.

The Goosanders that I was watching did seem to be feeding. They were repeatedly diving beneath the surface. However, their behaviour when surfacing was unusual. When they were holding prey, they shook it before swallowing it. Goosanders often ‘snaffle’ small prey items underwater. Larger fish may be brought to the surface, but I hadn’t seen this behaviour before. With the naked eye, it was difficult to identify what they were feeding on. Fortunately, I had taken a few photographs and greater magnification was available on the display on the back of my camera. It was possible to see that they were feeding on Signal Crayfish (see photograph). The shaking was presumably either to kill the Crayfish and/or to remove its claws.

Although it was novel to me, after a bit of investigation, it turns out this wasn’t a ‘ground-breaking’ observation. There are relevant records, including this video (I’m not sure what species of Crayfish it is, but you’ll get the idea):

Signal Crayfish tend to be nocturnal in their activities, and hide away during the day, in underwater crevices, perhaps between or under rocks. So, presumably, Goosanders either reach into these hiding places and/or have learnt to turn over underwater stones to uncover the resting Crayfish. Could it be that Goosanders feeding on Crayfish will reduce pressure on fish numbers?