St Nicks

Centre for nature and green living

Crayfish on the Foss

Surveying for crayfish
Surveying for crayfish: The upper reaches of the Foss are very natural with lots of very old alder trees sprawling across the banks both above and below the water. Many of the banks had undercuts which provide a perfect refuge for crayfish

Written by St Nicks’ Natural Habitats Manager Jonathan Dent.

This autumn, thanks to funding from the Postcode Local Trust, we have been privileged to assist wetland ecologist Martin Hammond to survey the upper reaches of the River Foss to determine to state of native white-clawed crayfish populations. White-clawed crayfish are the UK’s only native freshwater crayfish but their numbers have plummeted in recent years due to the spread of American signal crayfish. This American invader was first recorded in the south east of England in 1975 and has been slowly been spreading across our rivers and streams while out-competing our native crayfish along the way. Signal crayfish are more aggressive (taking over refuges), grow faster, and are much less fussy eaters (depleting food sources) than white-claws which and all this on its own has a huge impact. Combine this with the fact they carry a ‘crayfish plague’ (a fungal parasite) which is particularly deadly to white-claws while having little effect on themselves and they are in real trouble.

White-clawed crayfish prefer clean unpolluted rivers and streams, no deeper than 1 metre and that have plenty of suitable refuges. These refuges are essential for crayfish of all ages but juveniles are especially vulnerable to predation by fish, duck, other water birds, otters and mink, carnivorous dragonfly larvae (as described by Hogger, 1988, Hill & Lodge 1994). That’s a lot of hungry creatures that they need to protect themselves against. Adult crayfish can use their front claws to protect themselves against smaller predators but that doesn’t help them against bigger more aggressive fish species such as perch and eels, and birds such as herons.

Adult crayfish can use their front claws as protection

Adult crayfish can use their front claws as protection

Thankfully much of the upper reaches of the Foss are almost perfect habitat so we set out find out whether they are still found in the river, how healthy the population was if they were, and how far downstream they stretched. There have been no reports of signal crayfish in the River Foss and we hope this remains. The Foss is an isolated catchment with no other main rivers flowing in it and this offers hope for the Foss to be a real refuge for white-clawed crayfish in Yorkshire.

Our intrepid band of surveyors

Our intrepid band of surveyors

Long standing friend of St Nicks Martin Hammond has been assisting us with a range of river surveying on the Foss for the last few years. Helping us develop our plant and freshwater invertebrate ID skills. He has a licence to survey for crayfish and has been passing on his wealth of knowledge and skills to us with the hope that we can get our own licence to survey soon. For now we eagerly followed his instructions as we made our way into the river in waders, complete with nets and trays for surveying. We quickly learnt what made a good crayfish refuge (for example undercuts in the banks, build-up of leaf litter and mature tree roots crossing under the water) and we were soon turning out crayfish after crayfish into trays. We found a good range of both male and female including large seasoned veterans with misshaped claws, young soft shelled adults having just moulted and tiny juveniles which were often near impossible to see amongst the leaf litter.

A young adult crayfish with a soft shell having just moulted

A young adult crayfish with a soft shell having just moulted

Over two survey sessions we recorded large and healthy populations of crayfish, extending the known distribution about 1.5 km downstream of previous records.  The upper Foss is a lot more natural in its habitat than the rest of the river and it will be interesting to continue surveys further downstream next year to see if the populations extend further. A really nice good news story for a change when it comes to our rivers!

Surveying for crayfish can be done in a number of ways and should be completed between July and October after the end of the breeding season. These are:

The River Foss survey work is part of the Dales to Vales Rivers Network Catchment Management Plan. St Nicks have been working with the River Foss Society to train and support volunteers in conducting a range of surveys including riparian mammals surveying, walkover surveying, and next year we will look at conducting freshwater invertebrate surveys. All the findings from these surveys will build up a picture of the current state of the river and the potential to make habitat improvements in the future.

11 November 2019 | Categories: Spotlight on..., Wildwatch | Tags: crayfish, Dales to Vales Rivers Network, Foss, river survey, white-clawed crayfish, York, Yorkshire