Saturday, 17 July 2010

Murder mystery on the Ock...

The river Ock, like most of the rivers of England, have been invaded by Signal Crayfish and last year I found a dead one along the river bank:

Whilst surveying for water voles on the upper Ock I encountered another one near Drayton Road Bridge, but this one has suffered a more violent fate - ripped in half and partially eaten:


When I returned the following day, the carcass had completely gone.
So far, Wild About Britain and Twitter have drawn blanks, so I am left to carry on the tradition of Oxfordshire detecives and investigate who the culprit might be.  I have several suspects to chose from:

People: At the time I suspected the first corpse was a victim of a fishing incident. Discarded when caught by accident.  But  it is unlikely to have been caught by a fisherman as the bank at this part of the river is steep and the undergrowth is high and restrictive.
Also, catching crayfish is illegal without a permit from the environment agency - as it is known for animals (including otters) to be killed in crayfish cages and why would anyone leave a partially eaten crayfish corpse by the river?
Therefore, I suspect the crayfish was not killed by a person.

Rats: I have seen rats further downstream and there are  almost certainly several in the area.  However  I have not found any evidence of a rat killing a crayfish, but it is not outside the realm of possibility.
Although I suspect a rat would go for an easier target - a moorhen chick or a discarded take-away.

Mink: It is highly likely that a Mink would kill a Crayfish in this manner, given they will attack and kill most creatures they encounter.  But there is no evidence to support the idea of mink on the upper Ock: there are no obvious scats; there are water voles and young moorhens nearby - often the first victims when a mink moves into an area. 

Birds: Several birds are known to catch and kill crayfish - Herons (who would swallow it whole), Owls & Buzzards - but a heron would swallow it whole, whilst the others would eat it on a perch away from the river.

Otter: There are cases of otters killing and eating crayfish and there has definitely been at least one otter on the river, as I found an otter spraint  back in February

But the spraint was found over a mile stream upstream where the river is deeper (and before it splits into the upper and lower ocks) and here the river is flowing under the bridge before it becomes it flows through the ock valley walk - popular with dog walkers and children, something an otter would have previously shied away from. Although otters are now being recorded in urban areas - Reading, Leicester & Birmingham - it is possible that they have ventured this far downstream.

Like the best murder mysteries each of the suspects have their albies, but base purely on circumstantial evidence my chief suspect would be an otter.  Hopefully the planned use of mink rafts later in the year should give further evidence of mustelid activity.

I welcome any readers have any insights or suggestions into the death of the crayfish 


  1. I agree with your suspect list, Richard. However you might add badger, as they must be pretty desperate for food at present since the worms for which they usually forage must be in short supply at the moment because of the dry weather.
    When they eat hedgehogs they leave a spiny shell similar to the way they have left the crayfish shell

  2. Hi David,
    Badgers? An interesting and intriguing suspect and something I certainly had not considered - I think I might have do another post on the subject.
    Thanks for the suggestion.

  3. I actually watched numerous rats eat numerous LIVE crayfish last night in a flash... the crayfish didn't stand a chance.