Crayfish in Britain

Signal crayfish are a non-native species which cause considerable harm to the ecosystem and potentially the economy. They are carriers of the crayfish plague, (a water mould that infects crayfish), to which they are resistant but other crayfish species are not, and it is pushing the White Clawed Crayfish closer to extinction. The White Clawed Crayfish is already classed as endangered due to this.

Signal crayfish have a faster growth rate than the native White Clawed Crayfish and this means that resources get taken by the non-native species before the White Clawed Crayfish has chance to mature. There are numerous ways to spot the differences between the two species, as the White clawed crayfish has a white underside to its claws compared to the singular white spot on the claw hinge of the Signal Crayfish. Both crayfish have different colours, the White Clawed Crayfish is an olive-brown colour while the Signal Crayfish is a blueish-brown to reddish-brown colour. The general sizes of the crayfish can be very similar as the native species can grow up to 12cm, but sizes under 10cm are far more common, while the Signal Crayfish can grow from anywhere between 6cm and 18cm. The habitats for both species are very similar, as they both live in freshwater rivers, lakes or streams where there is little to none salt content.

Signal Crayfish can cause economic damage as they burrow up to 2 metres into the riverbank which weakens it, but it only gets worsened as they can have multiple burrows intersecting with each other which causes the river bank to be weakened significantly more.  This can cause riverbanks to collapse which then makes it a flooding risk to buildings on the riverbank and can be a risk to livestock safety.

Both of these species of crayfish are present within the River Don and this is problematic due to the endangerment of the White Clawed Crayfish.

White clawed cray fish (Austropotamobius pallipes):

crayfishWhite-clawed Crayfish. Photo by Natural England/Jenny Wheeldon / CC BY

  • Appearance – olive/brown colour, pale undersides to the claw (pallipes translates to pale footed), can grow up to 12cm/5 inches but sizes below 10cm are common.
  • Habitat – Lives in lakes, rivers or streams usually about 1m deep where it hides amongst rocks and submerged logs, comes out to forage for food.
  • Conservation status – Falls under threatened as signal crayfish kill them off as it is a carrier for crayfish plague. Insecticides are also a major factor in white claw crayfish population declines.


Signal crayfish (Pacifastacus leniusculus):

crasyfish 2A Signal Crayfish. Photo by Andrew Gray  / CC BY

  • Appearance – 6-9cm (2.4-3.5 inch long) can grow up to 16-18cm (6..3 – 7.1 inch), blueish-brown to reddish-brown colour with large claws. They have a white to pale blue-green patch near the claw hinge.
  • Habitat – Rivers, streams or lakes, places that White Clawed Crayfish live and this cause them to die due to crayfish plague.
  • Conservation status – Least concern
  • Effects as an alien species – Signal crayfish are driving White Claw Crayfish and other native crayfish species towards extinction as they spread the crayfish plague and compete for resources. They grow faster and are far more aggressive than WCC, they can tolerate a much larger variety of conditions as well. Diet consists of fish and amphibian eggs, tadpoles, detritus, aquatic invertebrates, juvenile fish and aquatic vegetation which can decline populations of other species. Signal crayfish also have an economic impact as their burrows can cause river banks to collapse which then need replacing before flooding occurs which can damage livestock safety and stability of structures that are built upon the banks. Crayfish take refuges from salmonid fish and predate their eggs, which can reduce the value of fisheries.

Blog by
Josh Laidlow


We’re recruiting!

We are looking for people help deliver our Heritage Lottery Funded ‘Living Heritage of the River Don’ project:

24 hours per week
£19,000 pro rata

Project Assistant Job Description

40 hours per week

Community Engagement Officer Job Description

The deadline for both posts is 12 noon on Wednesday 29th March 2017.

To apply please download and complete an application form (no CVs please)

Application Form

Application Form (pdf)


An Ocean of Plastic

I read a startling prediction the other day. By 2025 it is estimated that the world’s oceans will contain 1 tonne of plastic for every 3 tonnes of fish, and by 2050 the accumulated plastic may well outweigh the fish1.


Beach strewn with plastic debris in the Hawaiian Islands National Wildlife Refuge. Photo by U.S. Fish and Wildlife Service / CC BY

Of course these figures are rough predictions that assume current trends continue, but despite the uncertainty, it’s indisputable that a tremendous amount of plastic is entering the oceans.

This plastic can have deadly consequences for aquatic life. A big conservation problem facing turtles for example is that they eat floating carrier bags, which they mistake for their jellyfish prey. Plastic can also get caught around the necks and limbs of sea creatures, cutting into flesh and often killing the animal as it grows.


A gannet with plastic caught on its bill. Photo by Bo Eide / CC BY

Overtime plastic debris breakdown into microscopic particles that accumulate in ocean gyres or rain down upon the sea bed. The impact of these microparticles is hardly understood, but there is no doubt that they are infiltrating almost every part of the marine environment including the fish we eat.


Very sad that some sea birds inadvertently kill their own offspring by feeding them plastic, as the parents of this albatross chick must have done. Photo by U.S. Fish and Wildlife Service / CC BY

While the greatest inputs of rubbish into the oceans generally come from poor countries that have inadequate systems for dealing with waste, you only need to take a look at the detritus dumped by the River Don after high flows to appreciate that we British are very much part of the problem. Just last month an autopsy on a Cuvier’s beaked whale that had died off the coast of Norway found 30 plastic carrier bags in its gut, which probably caused its death2. Tellingly, labelling on the plastic contents of its stomach was largely in Danish and English.


Even a fish as sleek as a salmon can fall victim to rubbish. Photo by Caz VanDevere / CC BY

The sad story of the whale and alarming predictions made me realise that I have underestimating the importance of the river cleanup work that the Trust has been doing. I had tended to focus only on the aesthetic improvements, which is very important in making riverside spaces pleasant places for local communities, and a good enough reason alone to clear up rubbish. But next time I see the bags of rubbish collected by Trust staff and volunteers I will feel doubly satisfied to know that waste won’t end up harming river or marine life.

If you are interested in volunteering with the Trust see our Get Involved Page.



Ed Shaw

Ed resized


Five poor excuses & five great reasons to pick up some litter!

Volunteer, David Burton, talks to you about becoming a Litter Champion:

In a group or on your own, picking up litter is more than OK, it’s actually an enjoyable, safe and satisfying thing to do. Pick it up! It won’t hurt and nobody will laugh at you. Pick up some more litter, there’s lots of it out there.

David tying up the last of the litter bags for collection after clearing the footpath by the River Don at Dalton

Litter is a complex problem and there are no quick, easy solutions, it boils down to two key issues;   A significant proportion of the population still think it is OK to drop litter, a lot from cars, and hardly anyone picks it up.

Stopping people littering is very difficult.  Years of poster campaigns, TV adverts, and a few high profile prosecutions, have had little effect on littering behaviour. More resources are going into litter wardens and on the spot fines, but it is difficult to catch people in the act of littering. In the mean time, we need to do a better job of picking up litter. This is not something that can be left to Local Authorities to solve. We all need to get into the habit of picking up litter.

The five lamest excuses for not picking up litter

1) “I don’t see why I should pick up other peoples litter” A well known saying is “All that is necessary for the triumph of evil, is that good men do nothing” and that’s what is happening right now. By default we are allowing the people who drop litter to win because we can’t be bothered to do anything about it.

2) “It’s the councils job” That is a good attitude to take if you like paying Council Tax. In England and Wales Local Authorities already spend hundreds of millions of pounds of our taxes on picking up our litter. Getting to grips with litter would require a significant increase in this amount. The truth of the matter is that local councils cannot solve this problem on their own. This is something we can do and should do for ourselves.

3) “It’s dirty” Not true. It’s no dirtier than a little light weeding – just wear some gloves. How dirty can you get just picking up a can of coke?

4) “It’s demeaning” Yes, you may feel a bit self-conscious at first. But the feeling lasts about 5 minutes. People won’t think you are nuts and your friends will still speak to you. Be Brave! Getting started litter picking can be a bit daunting – it does feel a bit embarrassing at first. My advice is just go for it – the feeling of embarrassment soon passes. And actually it’s quite important that people see you at it: firstly, so they know it is being done by a volunteer and not the council and secondly because seeing you pick it up will deter others from dropping litter.

5) “I might get hurt picking up something nasty” There is really very little risk. You very rarely come across anything dangerous or unpleasant. It is almost all plastic bottles, cans, crisp packets, cigarette packets and sweet wrappers.


Five good reasons why you should go out and pick some litter.

1) It’s enjoyable! Honest it really is. It is a great excuse to get out in the fresh air, get some exercise and do something useful.

2) You’ll make a big difference! Twenty minutes litter picking can really transform an area. Today’s litter is so in your face that removing it makes a big difference.

3) You’ll feel good! Litter picking is surprisingly rewarding. It’s very satisfying to look back on a clean, rubbish free area and know it is down to your efforts.

4) You’ll help prevent further littering. People are less likely to drop litter in areas which are litter free. By helping keeping somewhere clean, you’ll be helping to discourage littering.

5) Your Town and countryside needs you! We live on a small, overcrowded island. We do not have so much open space and countryside that we can afford to trash it with litter. Today’s litter is very long lived, it does not rot or degrade in a hurry. If we do not pick this stuff up it will still be there for a long, long time. So go on, try picking up some litter

So, come on, join the litter pickers and help MAKE A DIFFERENCE



All complete!

Thanks to funding from the Heritage Lottery Fund, Associated British Ports, Green Ports Hull, The Environment Agency, Yorkshire Water, The Wild Trout Trust and Biffa Award, DCRT have successfully completed the fish passage solutions on the five weirs in our Living Heritage of the River Don project.

It all began at Steelbank in July of 2016 and was finished at Brightside in December 2016.

This work complements a programme of fish passage work undertaken by others on the Don

There are 21 weirs on the Don between Doncaster and Beeley Wood Lower weir above Sheffield City centre.  At the time of our bid to HLF ten were already passable to salmon.  In December of 2015 salmon were seen and recorded jumping at Aldwarke weir on the Don.  This was the season after a hydroelectric scheme with a fish pass had been installed at Thrybergh weir, the next weir downstream.

Since then our project has addressed five of the ten remaining weirs; those that are considered ‘orphaned’ weirs. The project has reconnected 6.5km of the Don and opening up 10km of spawning grounds to migratory fish in the middle of Sheffield. Of the remaining 6, Aldwarke weir had a fish pass installed as part of a hydroelectric scheme in 2016; the Environment Agency demolished Beeley Woods weir and work commences on Ickles weir in May of 2017 as part of a biomass power plant.  There are plans for the remaining three weirs Jordan Dam (Yorkshire Water); Sanderson’s weir (Sheffield City Council) and Masbrough weir (Canal & River Trust).

Salmon to Sheffield by 2020?



Eels and the Don

Of all the species that frequented the River Don in medieval times, eels were by far the most sought after and most valued.  Salmon in comparison were regarded as fare only for the poorest in society, but eels were what graced the tables of the gentry.  Many of the most productive eel fisheries, for instance those in the fen areas in the tidal part of the river, were the preserve of monastic orders and those that exploited the fish stocks paid heavily for the privilege.


There is little doubt that the Don was an exceptional eel fishery. In his diaries written between 1701 and 1704 Abraham de la Pryme,the Vicar of Thorne, wrote the following words, referring to the lower part of the River Went which joins the Don close to Rawcliffe.

‘‘The river is no more than six yards wide but the crookedest and deepest that I have seen in my life. Every turn makes a great bog on the other side on which the water is thrown by the current and there is delicate fish therein. But such quantities of eels like was never seen. Sometimes there will break out or fall out from the hollow bank sides when the people are a fishing such knots of eels almost as big as a horse, that they break their nets’’.

He goes on to describe the annual elver runs which occurred at that time.

 ‘’Commonly every May such vast numbers of young eels comes over the water wheels with the waters and runs into the mills, that they are forced to give over working and send into town for the swine to devour them for they are as innumerable as sand grains on the seashore’’.

For those who are familiar with the lower Went de la Pryme’s description of the river could hardly be further from the truth. Land drainage works, carried out over the last couple of centuries, have reduced the river to little more than a featureless canal and whilst eels can be caught here, the numbers that he describes are almost beyond our imagination.OLYMPUS DIGITAL CAMERA

It is difficult to ascertain for how long after de la Pryme’s description was written that eel populations remained so abundant, but what we do know is that by the beginning of the 20th century, eels had  been all but eliminated from the River Don and its major tributaries.

The gross pollution created by the emergence of the Don Valley as one of the largest and most important industrial sites in Europe, created conditions which were untenable to fish life and reduced the river system to little more than an open sewer for more than a century. To compound the situation for eels, weirs, sluices and pumping stations were built all along the river in an attempt to control flooding and to provide conditions suitable for navigation.  These structures created a barrier to migration and as the efforts to control and reduce pollution began to take effect in the 1990’s, they prevented any natural recolonization from occurring.
Sargasso Sea.png

Elements of the life cycle of eels still remain a mystery to us but what we do know is that the juvenile eels (elvers) appear around our shores in early spring and begin to move up rivers to find conditions suitable for their further development.  They can remain in our rivers for up to twenty years and occasionally longer, but then begin their return migration to what is believed to be the Sargasso Sea of the coast of North America to reproduce.  This unique life cycle presents problems when it comes to trying to rebuild eel populations.  Whilst opening up opportunities for elvers to bypass obstructions is essential for the future of the populations, such action has to be recognised as a long term strategy.

One of the first issues tackled by DCRT following its formation in 2006 was to look at how we could perhaps speed up this process.  The small numbers of elvers which would be entering the river initially would take 15 to 20 years to mature and it would be at least a further year before their progeny returned back to the rivers of the UK.  What is still unclear is whether these juveniles return to the river from which their parents emerged in the same way as salmon do but it was clear that any support for this process would be beneficial.

A further problem was that the runs of elvers entering Europe’s rivers had shown a marked decline to the extent that it was estimated the populations were down to only 10% of the historic levels. New legislation had to be introduced to control exploitation and one of the actions required a proportion of the commercial elver fishery catch to be made available for selective restocking of suitable waters. The Trust saw an opportunity to become involved in this process and selected a number of sites on the River Dearne including Old Moor Bird Sanctuary and Bolton Ings Nature Reserve.  These sites provided the conditions conducive to eel development, being quite shallow and productive with ample reed and weed cover.  In order to ensure the eels had freedom of movement,  when required, sluices which controlled water levels into and out of the sites were also adapted.

In total more than 100,000 elvers were introduced into sites on the Dearne and Don and subsequent capture of stocked eels has shown rapid growth rates. The population of eels in the River Dearne itself has also shown a marked improvement indicating that the adapted sluices are working as required.

Restocking of young eels presented an opportunity for the Trust to involve the public in its work and to re-engage people with their local rivers. During 2014/15 more than 20 schools were offered the chance to have tanks of elvers in their classrooms and to care for them and feed them for up to 2 months. The children were then invited to release them into local waters. A further 60,000 young fish were released this way. The enthusiasm shown by the children who participated was wonderful to witness and almost all of them took the opportunity to physically handle the fish as they were released into the waters. This experience will no doubt live with the children for the rest of their lives and help to engender a better appreciation of the value of their local river environments.p1000469-1024x575

The huge changes that have taken place to the physical structure of the River Don and its tributaries mean that we will never again witness the amazing sights described by de la Pryme but we like to think that our actions have helped to secure a future in our local rivers for these fascinating creatures.


Chris Firth MBE


Hidden Beauty

When you look upon a riverbed it can seem pretty drab, lifeless and uninteresting. But if you could take a pebble in your hand and somehow magnify its surface so that the microbes that covered it were visible, you may well be surprised by the beauty of what you saw. For throughout the slimy layer of life that covers the pebble you will find diatoms, a very pretty type of algae.

245311203_92a6a90302_b.jpgA boring brown riverbed? Photo by Matt Smith / CC BY

Like most plants and algae diatoms photosynthesise, deriving their energy from sunlight. They differ however in that they have yellow-brown rather than green chloroplasts, which contributes to the general brownish colour of riverbeds. The slimy layer in which most river diatoms make their home is known as a biofilm. This film coats most surfaces in rivers and lakes, and is comprised not only of diatoms but also a myriad of other microorganisms such as bacteria, protists, fungi and worms. Numerous invertebrate species such as certain mayfly larvae and freshwater snails graze upon biofilms, and are in turn prey for predators such as brown trout or dippers.

Diatoms are not restricted to freshwater ecosystems, and are in fact very widespread. Vast quantities float as plankton in the ocean. However they don’t necessarily need much water, and will live wherever there are moist soils or damp surfaces. There are an estimated 100,000 species worldwide.

Two plates of diatoms from Ernsrt Haeckel’s Kunstformen der Natur (Art Forms of Nature; 1904)

While diatoms are very abundant, they have an uncommon beauty. They come in a wide variety of symmetrical forms and have ornate and patterned cell walls. This is why they became a popular subject of study for those who pioneered the use of early microscopes. Their striking appearance led to their inclusion in Ernst Haeckel’s Kunstformen der Natur (Art Forms of Nature; 1904), a book that displays a collection of the famous biologist’s most beautiful drawings and watercolours.

davechandlerphoto1anotherworldOne of Dr Dave Chandler’s diatoms taken from his laboratory experiments (false colour image).

There is also a more mundane reason to study diatoms. Their communities change in response to human impacts such as pollution, as some species are more able to withstand contaminated waters than others. Environmentalists can therefore collect diatoms from a river and compare what species they have found to what should be growing if there wasn’t any pollution. The difference in the diatom community then gives us a good clue as to how polluted the river is. Dr Dave Chandler investigated the impact detergents had on diatom communities during his PhD at the University of Sheffield. During his research he took pictures of the diatoms he kept in his experiments, not because he needed to for his studies, but simply because they made such beautiful subjects.


Another of Dr Dave Chandler’s diatoms (false colour image).