I am a crayfish catching contest winner!

Posted on 16 February 2021 by Jeroen Verhoeff
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    Gosh!
    For the first time in my life I participate in a competition and…
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    In short:
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    The best ideas to do something about the abundance of exotic crayfish in the Netherlands would win nice cash prizes at the “Delfland crayfish challenge”. Once there was  only one species of native crayfish in the Netherlands, but it was quite sensitive to water pollution, brook destruction and the (imported) crayfish plague, so now it only occurs in one little pond. Now we have something like seven foreign freshwater crayfish species that have changed the water considerably, up to three individuals per square meter locally!
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    I came up with the “crayfishcrate” where both small and large lobsters could easily climb on and fall into. Since there is a lot of time, energy and expense involved in controlling lobster traps, I figured we should take advantage of the fact that more and more native animals are learning to eat crayfish. So the crayfishcrate immediately became a public ‘swimming’ buffet for grebes, coots, rats, otters etc etc.
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    So the second prize is for me! Yippie! The “crayfishbox” took first prize and was largely of the same principle, except for openness to native predators. I am curious what will be achieved with the results!
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    Please call or email me with questions or suggestions; I would love to help resolve this underestimated problem!
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    Extensive:
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    Exotic crayfish are becoming a huge and growing ecological problem in the Netherlands. Control appears to be very labor-intensive and capture is not really the solution, according to experts. Sometimes it makes things worse, because the small lobsters, which are not caught by fish traps, etc., are no longer eaten by the large crayfish. The ‘Delflandse rivierkreeftenchallenge’ therefore calls for innovative traps to better combat the different types of exotic crayfish. These improved traps must be as little labor-intensive as possible, cause minimal by-catch and, above all, catch the young, small crayfish, so that fishing interests and waterboard interests are separated.
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    Possible solution:
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    More and more native animals are learning to eat the exotic crayfish. These are herons, cormorants, grebes, coots, pike, bass, eels, otters, brown rats and muskrats. However, crayfish mainly forage at night and hide in burrows and under objects during the day when most predators hunt. In addition, crayfish can shoot backwards very fast and far in the event of danger, whereby pike often even turn out to be too slow (own observation during underwater filming).
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    How can these ideal natural crayfish predators be used?
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    For this I designed a trap in which crayfish (especially) at night let themselves be lured to the bottom of a large tank, where the natural predators can pick them out from above later during the day, without the lobsters being able to escape.
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    The traps are made of plastic crates, preferably with sloping walls, but perpendicular walls may also work. In the top of the crate there is a large hole, such that a wide overlapping horizontal edge remains (overhanging edge must be at least 10 cm?). The inside of the crate should be smooth (and black) so that lobsters cannot climb up the walls. The outside of the crate should be coarse, so that lobsters can easily climb up the walls there.
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    A crayfish are lured in by carrion secured to the bottom of the crate.
    B Upon arrival, the crayfish crawl in through the one-way trap gates.
    C Or they climb up.
    D Arrived at the top, they fall down through the hole.
    E After eating the bait at night, they find no hiding place in the crate during the day.
    F In case of panic, they will occasionally try to shoot backwards, but since lobsters almost never do so vertically, in most cases they will bump into the side walls or against the overhanging top edge of the crate and return to the bottom.
    G Ultimately, the crayfish will likely settle around the edges of the bottom of the crate.
    H The trick now is not to collect the caught crayfish, but to leave them there. On the light bottom of the crate, the lobsters are then in full view of cormorants, grebes, coots, pike, bass, eels, otters, brown rats and muskrats swimming over during the day. These can freely swim in and out of the tank through the large opening, to easily catch the crayfish, because the chance of escape for the crayfish is very small in the crate. In addition, the trapped larger crayfish are expected to eat the trapped smaller crayfish so that the problem will partly resolve itself.
    I Due to the stick protruding above the water with the characteristic flag, the crate above water is very recognizable for humans and animals. My expectation is that the smarter predators will quickly learn to recognize the crates as a food source (“Those sticks give easy to get food!”), which increases the effectiveness even more.
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    Please note:
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    1: The risk for the predators to get caught in the tank themselves, as often happens with fish traps and other traps, is very small because the opening, where all the light comes through, contrasts strongly with the dark, overhanging edge of the crate top. In addition, almost all predators are much more intelligent than the simple lobsters. Pike and grebe may be less likely to swim into the crate, but that doesn’t seem to be a problem for perch and eel, as they are used to hunting and hiding in cavities. I wouldn’t be surprised if the ubiquitous coot eventually turns out to be the prime “lobster crate profiteer.”
    2: In this way these bins can remain in place for a long time without having to be checked often. Slightly clear water may be quite important.
    3: It will have to be tested whether the bycatch will indeed be minimal, that straight walls will possibly work as well as sloping walls (this saves production costs, because then the crayfish crates can be made from old laundry baskets and old PVC bins), how often bait will have to be refreshed (due to the pole system, applying new bait is less labor-intensive) and to what extent the trap holes on the side contribute to the catches (the hole at the top may be sufficient).
    4: Many small crayfish will also be caught through the large opening at the top. Due to the lack of openings on the sides, these little ones will not be able to escape, as is the case with many traps and baskets.
    5: Humans can also be counted among the predators that can eat from the crayfish crate. Poachers or people who want to get hold of a meal of lobster are not a problem in that sense, but part of the solution.
    6: As a possible bycatch I expect eel, but this one eats a lot of crayfish and hopefully it will swim out of the hole in due course; if not, an upside-down trap opening can be made in each top corner of the tank to allow the eel to escape again. In addition, possibly exotic gobies and locally perhaps bullheads might get caught. This possible minimal by-catch is unlikely to be a problem for conservation or fisheries.
    7: The crates must be properly weighted, so that current, storm and theft will not move them.
    8: It may be more effective to use two flag poles to make it easier to change the bait (just drop it between the poles) and to make it easier to lift the crates out of the water. For the latter, handles can of course also be made on both sides of the top.
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