Talking Traps
Written by Jimmy Elliott, Greencrab.org Science Communications Advisor
In my last post I intended to kick off a series of writings where I discuss trapping variables using a list of common questions I encounter:
Where do I find green crabs?
What bait do I use to catch green crabs?
Where could I catch the biggest green crabs?
When do I trap for green crabs?
We already talked bait, so I crossed that one off...
This time I want to talk about what traps to use. While it wasn’t in my list of common questions, this is certainly a question that anybody that wants to trap green crabs would have to ask at some point. This is actually a pretty straightforward question because with all the different traps I have tried I have always caught at least some green crabs. There are a few questions that arise, even for the novice recreational trapper of green crabs:
Is by-catch a real problem for wildlife management
Does trap shape matter?
Does a more expensive trap = more efficient trapping
First, I should note the regulations as of now. You currently do not need a permit to take, keep, or land green crabs as the Massachusetts Division of Marine Fisheries labels them an (inedible?) invasive species. It is unfortunate that it says they are “inedible”, but just take it from us that they are edible (otherwise this would be a huge waste of our time). As seen in the link above, MA state law currently requires individuals to hold an authorization from the Division of Marine Fisheries (DMF) to harvest green crabs. I was personally always under a scientific collection permit in academia, and while I haven’t received another authorization, I’ve heard they’re easy to obtain. During discussions and reading forums, I’ve heard people say this is too much oversight by the DMF. However, this is done to monitor the invasive species as well as the by-catch that could be caught during trapping. We don’t want to eliminate native species while trapping an invasive one. I never experienced significant by-catch during my trapping days, but there was the occasional Cancer crab or Asian shore crab (also invasive) that would sneak into our trap among hundreds of green crabs. They were always seriously outnumbered though! In other areas by-catch can be a big problem and the type of trap that you use can seriously affect the native species that you disrupt in trapping green crabs. But more on that later.
As for trap shape, there are a few options that were known to us when we studied this directly about 4-5 years ago. For this study, various trap designs were compared for their effectiveness. To account for seasonality, and variability in location, I did this in multiple places and times of year. Of course, you can’t try every trap that is out there, but we did our best to select the traps that could be available to somebody just getting started in the world of green crab trapping. From what I remember, the larger the trap, the more expensive if was, but make sure to double-check me on that.
Here are all the details as shown in the paper:
Traps included the Ketcham box trap (C), a trapezoidal box trap (A), a slanted-sides box trap (F) and the “Terminator” (D), another box trap designed specifically for trapping green crab. The “Blanchard” trap (B), a cylindrical-design, was the largest used at nearly 30 lbs. Two additional traps not specifically designed for crabs but often used to catch green crabs: an eel trap (E) and a small minnow trap (G) were also used in the study. Each trap was baited with herring (the “best” bait) and deployed off a dock with at least a 2-meter separation between traps. After a 24 hr soak, crabs were removed, traps were re-baited, and rearranged along the dock area to eliminate possible differences due to trap location, and allowed to soak for an additional 24 hr (for more information, see the methods in citation below).
After the study was done, I found that the large cylindrical “Blanchard” trap captured significantly more crabs than any of the other traps, except for the “trapezoidal trap”. This was our largest trap, and it was extremely heavy, especially when it was full of crabs! The small capacity of a minnow trap results in rapid saturation, thereby allowing crabs to escape and reducing further entry of crabs. Also, even though we had modified the minnow trap to create a slightly larger opening to let in larger crabs, we still caught statistically smaller crabs when compared to some of the larger traps that were tested. The good news, none of these traps caught significant by-catch in the areas we trapped (please see citations below for more information)
This hasn’t been the only study of its kind recently though. In PEI another green crab researcher has spearheaded an excellent study regarding the advantages and disadvantages of two popular green crab traps: 1) Fukui traps, and 2) Fyke nets. Fukui traps are said to be the “industry standard” for trapping green crabs, so these traps could theoretically meet that goldilocks zone of high catch per unit effort and trap size. Fyke nets are very different, because they are designed more for fish and can be a more passive way of trapping (aka without bait). It consists of a long cylindrical netting bag usually with several netting cones fitted inside the netting cylinder to make entry easy and exit difficult. This net is then mounted on rigid rings or other rigid framework and fixed on the seabed by anchors. They are commonly used in estuaries or inshore shallow water. See the figure below comparing the two gears taken from Poirier et al (2020).
This study assessed both of these fishing gear and found that fyke nets were more efficient, catching roughly three times the number of green crabs caught by individual Fukui traps. There was a downside to the increased catch though. Because they are so large and have little engineering to discriminate against non-crab species, fyke nets also caught nearly ten times more by-catch, many of which were endangered and commercially important species, such as American eel (Anguilla rostrata) and winter flounder (Pleuronectes americanus). That is a huge disadvantage when we look at the big picture.
So does size/shape matter? Yes, but it depends on how you use it...
If you were going out to catch crabs in a small 14 ft. boat, then chances are you are not interested in bringing our winner, the 30+lbs cylindrical trap or maybe not even a large Fukui trap due to how large and expensive they both are. I anticipate that you would be just fine bringing out the small minnow trap and still catching 100 green crabs. Here is my logic with that...
You would be able to fit at least 10 of the minnow traps on your small boat for every “Blanchard” trap, and with every minnow trap catching ~100 crabs, you will still outperform the catch per unit of effort when compared to the “Blanchard”. At the moment, very few people have large commercial operations that warrant the use of a trap like the “Blanchard”, but it is good to know that it is effective if we get our way at greencrab.org by establishing green crab demand to match the overly abundant supply. If you are genuinely considering starting up a trapping program, I suggest that you do much more research before doing so because the regulations can change quite frequently.
If you want more information from the perspective of monitoring efforts, the ”crab team” from out west has done a great job comparing the benefits of minnow traps and Fukui traps (but they didn’t modify their minnow traps like I did).
Pro tip - while the minnow trap was highly effective for its size, remember that modification is required to the opening of the trap. Also, if it is an area with a strong current, you should place a weight in there to prevent the trap from rolling away!
Do you have a trap that works especially well for you? Or have you ever made your own green crab trap at home? Share your expertise and please don’t hesitate to add anything that I may have missed.
Thanks for reading!
Further reading:
Poirier, L. A., Flynn, P. T., Gehrels, H., & Quijón, P. A. (2020). Fukui foldable traps versus fyke nets as options for fishing European green crabs (Carcinus maenas) in soft-bottom habitats: Exploring efficiency and limiting bycatch. Fisheries Research, 230, 105637.
Young, A. M., Elliott, J. A., Incatasciato, J. M., & Taylor, M. L. (2017). Seasonal catch, size, color, and assessment of trapping variables for the European green crab Carcinus maenas (Brachyura: Portunoidea: Carcinidae), a nonindigenous species in Massachusetts, USA. Journal of Crustacean Biology, 37(5), 556-570.