American Signal Crayfish- myth busters

warrenslaney

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We are in our third year of trapping after a river we took over was found to be infested with the things. Although very late coming out of hibernation this year we are finding our numbers to be well down on last year which in turn was well down compared to year one.

There is a wisdom that we have tested that says 'Trapping crayfish can increase numbers'. Although we have trapped intensively, we have completed exploded this myth. What is more, our insect populations have increased with the removal of crayfish.

They make wonderful compost for the garden too.
 

Tailing Loop

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We are in our third year of trapping after a river we took over was found to be infested with the things. Although very late coming out of hibernation this year we are finding our numbers to be well down on last year which in turn was well down compared to year one.

There is a wisdom that we have tested that says 'Trapping crayfish can increase numbers'. Although we have trapped intensively, we have completed exploded this myth. What is more, our insect populations have increased with the removal of crayfish.

They make wonderful compost for the garden too.
They make better eating - but you can get fed up with shelling them!

Time to get my traps out by the sounds of it...are your sizes going down as well as numbers?

Have you had to put in a blinder to reduce your mesh to trap the little ones?
 

BobP

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I wouldn't be so quick to claim success. Try it - stop trapping for two years and see what happens. If you're right the population won't increase; if you're wrong...........

Just out of interest, how many have you trapped in how many traps over what length of river? A river keeper friend of mine trapped over 8,000 in one trap from one pool no bigger that the average garage in one summer. And still the b*ggers kept coming!

They ain't THAT easy to get rid of.
 

BobP

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warren,

If you're so sure you've exploded the myth, put it to the test. You're certain you've sorted the crays out - the only open fishery I've heard of anywhere incidentally that has done it - then the only way to verify that is to stop trapping and see what happens.

Would you be kind enough to answer the question I put to you regarding the number of traps you used over what length of river and the number of crays caught. Surely you kept records? Also, did you trap all winter as well?

You see, about ten years ago they had reached plague proportions on a stretch of the Kennet - a coarse fishery as it happens, full of fish that munch signals for breakfast, lunch and dinner. It had got to the point at which the anglers couldn't use any bait for chub and barbel with any hint of meat or fish in it.

The club undertook a major trapping experiment and furnished me with the results. A range of different baits was tried to establish the best one, water temperature readings taken on a daily basis and traps lifted daily and the numbers recorded. We also looked at the mean size of the animals caught. The club also trapped throughout the winter months, only stopping for Christmas or if the river was in flood.

After three years they had caught in excess of 200,000 signals and had dropped the mean size by 30%. The anglers were not being bothered to the same extent so that was a success of sorts. BUT they didn't get rid of them. When we stopped to work it out the actual biomass per sq. metre was alarming!

There are a number of commercial trappers on the Thames and in order to avoid the boat traffic they tend to lay strings of traps up to 200 metres long down the centre of the river. Fisheries Enforcement guys told me that they had observed them lifting the traps and in one pass had filled to overflowing a 40 gallon drum with signals.

No, Warren, getting rid of them ain't easy!
 
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warrenslaney

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They make better eating - but you can get fed up with shelling them!

Time to get my traps out by the sounds of it...are your sizes going down as well as numbers?

Have you had to put in a blinder to reduce your mesh to trap the little ones?
I got fed up long ago as you say.

One of the indicators that we are reducing the population, along with our mark & recapture results and our records, is that numbers increase per kilo. I have custom made traps imported so we can catch the small ones too. The Swedish traps are harvest traps and are useless.

 

Steve Walker

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It would be interesting to see what you catch if you put out a trap which catches crays smaller than the minimum size caught by your current traps.

Reduced average size is what would be predicted by the theory that trapping reduces cannibalism.
 

Bourach

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Trapping with any form of mesh trap/creel will never eradicate crayfish. You are simply removing the larger cohort of the population. To think otherwise is simply folly.
 

warrenslaney

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It would be interesting to see what you catch if you put out a trap which catches crays smaller than the minimum size caught by your current traps.

Reduced average size is what would be predicted by the theory that trapping reduces cannibalism.

In fact, the larger crayfish come to the traps first. When they are on the compost heap the huge middle range come next. When they are on the compost heap, the babies come after. We get far, far more small crayfish in the late summer than the beginning of the trapping season. Please understand that we are catching around a 1000/day when we hit new areas. Our traps can catch everything above one summer larvae

---------- Post added at 12:33 PM ---------- Previous post was at 12:30 PM ----------





Trapping with any form of mesh trap/creel will never eradicate crayfish. You are simply removing the larger cohort of the population. To think otherwise is simply folly.
Correct. With the right type of trap you can certainly make in-roads into the 2 summer population. What is more, trapping in the autumn will catch the berried females, with 250 eggs under their tails...Well worth keeping the traps in as long into the early winter as you can. :thumbs:

 
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BobP

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So what is happening upstream and downstream of you? In the tributaries? In the river into which your river flows and the one after that?

They can travel far as fast.

A student did some mark recapture work on a Kennet sidestream. One marked animal was found 1.5km downstream three months later. Until then everyone thought they only went upstream - now we know different.
 

warrenslaney

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So what is happening upstream and downstream of you? In the tributaries? In the river into which your river flows and the one after that?

They can travel far as fast.

A student did some mark recapture work on a Kennet sidestream. One marked animal was found 1.5km downstream three months later. Until then everyone thought they only went upstream - now we know different.
Trapping upstream too of course. Everybody needs good neighbours, especially upstream ones.
 

BobP

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Can I suggest you google Stephanie Peay and make contact with her. She has had a long-standing interest in these animals and would be interested to review your findings.
 

warrenslaney

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I am acquainted with all of her work. As far as I can make out, she began the myth. Those who it suited to do nothing, such as our government agencies, eagerly followed. Now, instead of saying they ought not, are now saying they wont.
 
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jameswaine

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I think you may be getting ahead of yourself slightly when you claim to have exploded the 'myth'.

Three years is not a long time in terms of river eco-systems and it could well be other forces at work that have reduced your numbers? Also, how concentrated is the area in question? They could all be lurking 6 feet from you boundary line (in theory).

I'd be interested to see the reports with figures and dates etc. I hope you're right but as above, it'd be the first real success we've heard about, hence the scepticism....
 

warrenslaney

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Understood James. The myth, as explained in my first post, was that trapping crayfish increases their numbers. It does not when done in an intensive way, with good traps, plenty of them, a good bait and a time effective way of setting and emptying the traps. Our invert populations are on their up again. We are not claiming to get rid of signals. We only want to control their numbers to a manageable level.

More here...

INNSA - Controlling American Signal Crayfish on the River Derwent
 

graceman

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From a small stream in Oxfordshire we used to trap crayfish but they have no appeared this year yet and they were very much reduced last year.
Drop a bit of bacon on the bottom and just watch them all run for it all in a line but not anymore.

So are they declining we seem to think so
 

Tailing Loop

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Like any resource, if you put enough pressure on it, for long enough, it will suffer. As far as Signals go, this is a good thing.....pressure on fish stocks (crustacean stocks in the sea) has the same effect, numbers become reduced.

I think it is just a question of numbers - to seriously deplete the lobster population doesn't actually take that much effort, one boat with several strings of pots can make a bay devoid of lobsters in one season. For a heavy infestation of Signal crayfish, the fishing pressure may need to be higher, but it will certainly have an effect - you just need to put sufficient pots down and empty them regularly. My line of pots I empty every few hours, including at sunset and then again 2 hours after dark and again first thing in the morning - it has had a noticable effect and we have been able to move the pots both upstream and downstream. We started in the middle and are now slowly working outwards. As an experiment, I put one pot in the middle pool again for an entire day, with fresh bait - didn't catch a single crayfish. The other traps were catching between 20 and 30 crayfish per day each. The same sort of numbers we were catching in the middle section when we first started trapping.

I am no illusion that we will have solved the problem, merely seriously reduced the problem for a short time. In the autumn we will have to intercept the migrantion too, to ensure the effort already made doesn't go to waste. I expect the chubb population to drop as their food source is reduced - will the average size reduce as well though? Maybe they will have to move in order to find sufficient food to maintain their weight? I don't know. We shall see, hopefully if we get better weed growth, because the crayfish don't eat everything green as soon as it shoves it's head out of the gravel, and the insects have somewhere to live because the weed can flourish a bit more, and the minnows can establish....etc, the trout will once again thrive and then the grayling.

Only time will tell. In the meantime, we shall have some more salads and strifries with these invaders as the basis. :D
 

Bourach

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Substitute your lobster analogy for a faster growing and reproducing decapod like Nephrops, and then look at the effect cropping the population has?
 

linkledger

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Why the negativity? Seems to me the OP and his people are doing an admirable job.

Perhaps they would come down to London and sort parts of the River Lea out:thumbs:
 

johnclayton

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Apparently when you set traps for signal crayfish it mostly the larger ones that you will catch and as they are cannibalistic the smaller ones won't venture into the trap as they would probably be eaten.
To combat this you actually have to set different size traps so the small and medium size crayfish can also be caught.

JohnClayton
 
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