Newly published research using a novel “triple drawdown” (TDD) technique for surveying has shown that signal crayfish can exist at astonishing densities – over 100 individuals per square meter counting juveniles. In the face of this revelation, the research also suggests that trapping, as a method of control, is relatively futile: something the scientific community has been saying for some while. The three study reaches were trapped in a conventional way, before being surveyed again using the TDD method. The numbers caught using TDD showed just how many conventional trapping left behind. 

The headlines so far on social media, including from the Angling Trust, have focussed on this latter point, the relative inefficacy of trapping. In fact I believe the study and some of the data suggest a more nuanced position on the efficacy of trapping as a means of control. At the very least it highlights an important area for further research if we are to limit the destruction by crayfish of our rare chalk streams.

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My issue with the bald “trapping is futile” position is threefold: 

• there is currently no form of biocidal or allowable genetic control and yet signal crayfish cause enormous damage to the physical structure and biodiversity of globally rare chalk streams like the River Bure in Norfolk – we ought therefore to be looking for a form of trapping that can help control their numbers, even if outright elimination is currently impossible (the analogy with the government’s current Covid policy is too obvious to miss)

• the so-called futility / ineffectiveness is rarely contextualised against a desired outcome as above. If numerical control of the adults which damage the habitat were the desired goal then a form of trapping may well be found to be “effective”: this is something that has not yet been tested with a bespoke study programme.

• unlicensed, ad hoc and recreational trapping is likely to be one of the major vectors of the spread of invasive crayfish from one waterbody to another: the prevention of this spread is a major motivation – and a very laudable one – behind the message that trapping is futile as a means of control. But it may also mean that the message is subject to a touch of confirmation bias.

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First of all it is worth emphasising that this new study is mostly about a novel method of sampling crayfish numbers, one that is far more effective at revealing the numbers of animals that exist than previous techniques. The literature review points out the deficiencies of all previous methods of trapping, electrofishing and even biocidal control, none of which get close to revealing the true numbers in the way that the triple-drawdown (TDD) method does.

It’s a brilliantly simple idea: the authors built a dam across the stream and used a pump to bypass and thus dry out a reach of streambed below the dam. The dried-out reach was then searched by hand, after which flow was allowed to resume, encouraging any crayfish that had managed to hide away during the first search to re-emerge. The process was repeated until no more crayfish could be found. Stop nets at the upper and lower limits prevented other crayfish from entering the study reach when it was re-watered.

The results of the TDD survey were compared with hand-searching and baited funnel trapping carried out on the exact same study reaches before the TDD trial.

The study was conducted on a small stream in Yorkshire, the Bookill Gill Beck in the Ribble catchment: a small stream, 5km long and 0.5 to 2 meters wide. This stream once held juvenile trout and salmon, as well as native crayfish. Now, following an illegal introduction of signal crayfish in 1995, all it seems to hold are signal crayfish in enormous numbers.

Three sites (DGB, PAD and CON) were selected for the study. In 2016 DGB and CON were surveyed and in 2017 DGB and PAD were surveyed. CON, therefore, was surveyed only once.

DGB and PAD were close to the each other quite high up the stream while CON was very close to the confluence with the Long Preston Beck.

The results unequivocally show that trapping tends to catch the larger individuals (average carapace length of approx 30 to 40mm) and that very few sub-adults were caught in the traps and virtually no juveniles. It’s worth pointing out that sub-adult crayfish are still sexually mature.

The hand-searching, on the other hand, caught roughly the same number as trapping (883 versus 721) but of a smaller size, mostly juveniles and sub-adults.

The TDD method, by contrast, caught vastly more crayfish and of all size classes, a total of 4,803, and revealed that a very large proportion of the crayfish population comprised juveniles, averaging 55%, ranging from 36% to 72%. 

These are the crayfish not caught by trapping, seeming to suggest – as the conclusion states – “unequivocally that trapping cannot be used as an effective control method for invasive crayfish populations at least in conditions resembling our study system”. Note that caveat to the “unequivocal” conclusion, italicised by me.

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The River Bure, the river I get so animated about, (and in fact most chalk streams beset by signal crayfish), does not resemble the study system. The study system (at least the upper two sites accounting for five out of the six surveys) is devoid of predatory fish. It would have been a spawning tributary once upon a time, perhaps it still is. But when one appreciates that at the upper sites there were up 100 crayfish per square meter (!), it is easy to see why there aren’t any juvenile trout and salmon in the Bookill Gill Beck anymore.

Taking a longer and more detailed look at the findings and especially if we compare the CON survey site at the confluence (the only site where there are predatory fish, surveyed only once in 2016) with the upper sites, we see a more nuanced picture with regard the effectiveness of trapping as a means of control.

The authors of the report state that TDD gave a very good picture of the real numbers of crayfish in a given reach, and it does indeed seem as if this novel method of surveying surpasses other methods giving a more reliably “robust and representative information on the signal crayfish populations including estimates of density, biomass, male:female ratios and size‐class distribution”.

The total number of crayfish at the CON site, therefore, we can take as probably close to the TDD result of 538. Of these trapping caught only 75, roughly 14%, of the total number. 

If we look closer at the numbers we can see that:

• 37% of the crayfish caught in traps at CON had a carapace length greater than 35mm. Roughly 28 individual crayfish.

• whereas only 3% of the crayfish caught by TDD at CON had a carapace length greater than 35mm.  Roughly 17 individual crayfish.

• 62% of the crayfish caught in traps at CON had a carapace length of between 26 and 34mm. Roughly 46 individual crayfish.

• whereas 12% of the crayfish caught by TDD at CON had a carapace length of between 26 and 34mm.  Roughly 65 individual crayfish.

Trapping caught virtually no crayfish below this size, which TDD revealed to be 85% of the total number present.

In other words trapping caught hardly any juvenile crayfish, but it did catch significantly more large crayfish (40% more) even than TDD and what would appear to be 70% of the medium-sized adult crayfish, suggesting that trapping is quite effective at removing adult crayfish and very effective at removing the largest crayfish.

The results from the upper sites bear the same thing out, certainly as far as the larger crayfish are concerned. In 2017 trapping accounted for 153 large crayfish at PAD (82% of 187) whereas TDD accounted for 65 (5% of 1319) and it accounted for 90 large crayfish at DGB (38% of 236) while TDD accounted for only 13 (1% of 1290). 

For medium sized adults TDD is more effective: again in 2017 trapping accounted for 24 medium adult crayfish at PAD (13% of 187) whereas TDD accounted for 132 (10% of 1319) and it accounted for 134 at DGB (57% of 236) whereas TDD accounted for 425 (33% of 1290).

It might be more accurate to say, therefore, that trapping is ineffective as a means to control smaller and especially juvenile crayfish numbers, and therefore as a means to control sheer overall numbers. On the other hand it does appear to be potentially effective as a means to control larger crayfish – within an isolated reach at least.  

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It is also worth taking a look at the crayfish population demographics of CON versus the other sites. It is notable that the raw density of crayfish at CON was a quarter that at DGB and half that at PAD, entirely because the numbers of smaller and juvenile crayfish were much lower. In fact in the year CON was surveyed, 2016, the number of juvenile crayfish at DGB was 1192 (72% of 1656) versus 193 at CON (36% of 538).

As the report states: “what is clear from all sites is the large number and overall dominance of juveniles in all the populations (36%–72%), with the relatively smaller population of juveniles at CON2016 potentially linked to greater predation pressure from fish”. 

As stated CON was the only site of the three where predatory fish were present: bullhead, Atlantic salmon (parr presumably), trout and eels. Predation at the upstream site would have been limited to otters and heron and the vast numbers of crayfish there, exceeding any densities heretofore recorded “could represent highly successful populations thriving under potentially optimal conditions.”

The italicised caveat in the concluding sentence I quoted above is an important one therefore, in that the study site represented optimal conditions for a numerically enormous population of crayfish, dominated by juveniles, with no predation. Faced with these conditions, if the goal of trapping was to remove all the crayfish, trapping clearly won’t work: because trapping doesn’t catch the numerous smaller crayfish, some of which are sexually mature.

But what if, on the other hand, one were to make the conditions less optimal, and what if there were predatory fish present? The data seems to suggest that trapping is actually quite an effective way, perhaps the most effective way, to capture larger crayfish. It also seems to suggest that predatory fish keep the population of juveniles significantly trimmed, by as much as 80% based on the comparison between sites in 2016.

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The wild brown trout of the River Bure are surprisingly small for a chalk stream. Anecdotally, they used to be of a higher average size. The older anglers remember when the wild trout seemed to average closer to a pound, than a half-pound or less. This doesn’t really surprise me given what the crayfish have done to the river. 

The Bure is deeply incised for a chalk stream: it is quite a flashy river and it flows over a periglacial drift of sand and gravel into which it has cut a deepish channel which has also been dredged. The crayfish burrow into these steep banks, causing the banks to collapse along a fracture line at the extent of the burrows. The river thus gets wider and wider each year and progressively fills with silt and mud, which the widening stream cannot wash away. This, in turn, inhibits the growth of the kinds of plants which thrive in swift flowing chalk-streams. This silt and mud also smothers the habitat of the invertebrates on which the trout feed.

The crayfish are now present in such numbers that they churn up the silt, battling for territory and foraging for food and digging burrows: the opaque water further inhibits the growth of the plants. Thus everything is going down in a vicious cycle and it is difficult in this degrading habitat and diminishing larder, for trout to grow to the size one would expect on a chalk-stream. The crayfish have become the dominant species and their destructive behaviour only makes the habitat more and more favourable to the crayfish and less and less favourable to the trout.

Is it possible to turn this around? To tilt conditions back in favour of the trout and allow them to become a significant predatory impact on the young crayfish?

Far from suggesting that trapping is futile on a chalk stream so beset by crayfish as the Bure is, a careful reading of the study suggests that trapping could form a useful component for a carefully targeted programme of control designed to reset the balance and save the river from inexorable decline.

While the new research does indeed show that trapping is futile if the goal is to remove all the crayfish, including juveniles, it also shows that trapping is actually quite effective at capturing larger crayfish. And it suggests that predatory fish can have a significant impact on the numbers of juvenile crayfish, reducing the population by up to 80% in the 2016 comparison.

If you add the two together it is clear that trapping and predatory fish could together make significant inroads into either end of a crayfish population, provided the effort was sustained and provided the reaches were isolated in some way.

Now what if you added to this pincer three additional modes of control?

First, what if you made the habitat less favourable? Crayfish love to burrow, but they hate silty banks and reed-beds and they can’t burrow into banks armoured with gravel and geo-textile. About fifteen year ago Hunts Green on the River Lambourne was utterly plagued by crayfish and just like the Bure, had grown wider and wider. The keeper, Bruce Wheeler, could trap hundreds of crayfish in a night. Now he’d struggle to trap a dozen in a week. The difference? He has re-profiled the banks and armoured them with sloping gravel. That’s all. He hasn’t trapped. He has simply altered the habitat and made it less favourable.

Second, what if you neutered the large males (which will have been successfully singled out by the traps) and returned them? The larger males are territorial and predatory and cannibalistic. Research conducted by Nicky Green in Somerset (Barle Crayfish Project) has shown that neutering and replacing large male crayfish, while killing and removing all the others, can have a significant impact on the overall population. That research has been echoed in France and Italy.

Thirdly, what if you also used refuge traps, the kind (not used in the TDD study) that are more effective at capturing smaller crayfish and especially berried females?

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Unlicensed and unmanaged trapping is very likely a significant, perhaps even the major vector for the spread of the crayfish plague. We don’t want to encourage trapping of this sort. But to be honest the people who trap crayfish in an unlicensed and unmanaged way don’t read scientific papers on the efficacy of the method as a means of crayfish control. They are a different kind of problem requiring a wholly different kind of campaign.

Trapping as means of crayfish control would most likely be carried out by people who care deeply about the habitat they are trying to protect and are looking for a way to make a difference. It would be perfectly possible to licence any trapping programme with biosecurity conditions: for example by using traps dedicated for use on a single, named river and tagged as such.

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While this new Triple Drawdown method is novel in itself, and reveals a much better way of monitoring crayfish numbers and population dynamics and while the study does indeed back up the long-held claim that trapping is an ineffective form of eliminating a crayfish population, it also, I would argue, suggests that a targeted programme of control could well yield habitat-saving results. It certainly suggests that a research study could be fruitful.

We need to bring the two sides of this debate together, to unite in our condemnation of unlicensed and ad hoc or recreational trapping and also unite in an effort to find something that we river lovers can do in the face of the crayfish plague, not just what we shouldn’t do.