Signal crayfish are an invasive species in the UK.  They are also really tasty and catching them can be a fun picnic activity for adults and children.  A recent study has found that they, along with all decapod crustaceans, appear to be sentient and should be protected from unnecessary suffering:  There is also substantial evidence of sentience in astacid lobsters/crayfish (infraorder Astacidea). We have either high or very high confidence that these animals satisfy criteria 1, 2 and 4.  They have some recommendations, that appear aimed at professional catering establishments:  Slaughter (decapods). We recommend that the following slaughter methods are banned in all cases in which a more humane slaughter method is available, unless preceded by effective electrical stunning: boiling alive, slowly raising the temperature of water, tailing (separation of the abdomen from the thorax, or separation of the head from the thorax), any other form of live dismemberment, and freshwater immersion (osmotic shock). On current evidence, the most reasonable slaughter methods are double spiking (crabs), whole-body splitting (lobsters), and electrocution using a specialist device on a setting that is designed and validated to kill the animal quickly after initially stunning it.  I have searched for whole-body splitting (lobsters), but only get instructions relating to cooked lobsters.  It also seems that doing so prior to boiling exposes the flesh to much more of the water as well as releasing intestinal contents, which would be expected to impair their flavor.  I am somewhat familiar with working with electricity, and I could probably rig up something to expose them to mains AC, but it sounds tremendously dangerous to attempt.  For reference the ones we tend to get are in the range 2 - 4 inches long. What is the best way to kill and cook signal crayfish in a way that is consistent with humane animal welfare? I note this question, but it is specifically about lobster, the greater size of which would make splitting easier and also the answers recommend methods that are advised against in the recent paper such as freezing (which in a domestic freezer could well be worse than boiling). 
Food tech. guru Dave Arnold experimented with crustacean killing...mainly lobster, and mainly in search of quality.  He wound up with the use of anesthesia as the best method...and probably most humane.  He uses clove oil to do the job.  It was done years ago, but you can read his entire post here.  He adds 10ml of clove oil to 90ml of ethanol.  He places lobsters in salt water (sea water is best, otherwise approximate ocean salinity).  Then add about 2ml of anesthesia per gallon.  Read the post.  It is quite detailed and offers other options for fish and seafood slaughter. 