The other day I got the opportunity to join a friend who is a professional diver and marine biologist on one of his assignments. The task was to dive a local limestone quarry to assess the crayfish population. In Sweden there are two species of crayfish, the naturally occurring species Noble Crayfish (Astacus astacus, flodkräfta in Swedish, left image) and the signal crayfish (Pacifastacus leniusculus, right image) which has been imported.
Since the imported signal crayfish is a carrier of the crayfish plague to which the noble crayfish is highly sensitive, efforts are made to eradicate the signal crayfish from private fishing waters, as in the case of the quarry in question. The purpose of our dive therefore was to assess the crayfish population to see how many of the unwanted signal crayfish remained in the quarry.
Before we got in, I got a short briefing on the dive plan, where to look for the crayfish and how to tell the two species apart.
Basically, the claws of the Noble crayfish are longer and more slender, whereas the claws of the Signal crayfish are comparatively broader. The claw of the Signal crayfish also has a blueish spot at the base of the claw, where the “thumb” and the other “claw finger” are joined (see the picture to the right).
The dive itself was nice, as the water temperature was around 20°C and our depth didn’t exceed 10m, we had no problem covering the needed bottom area in a single dive of about 70 minutes. Being my first dive in a quarry, it was an interesting environment, with everything covered in calcium deposits. As usual we found a lot of evidence of human activity, from crayfish cages, old pipes, and even pieces of a grenade. Supposedly a leftover from previous military activity…
But what about the crayfish spotting? Well, the best part was although we saw a lot of crayfish during our dive, we didn’t find a single individual of the unwanted signal crayfish! This means, at least in this location, the removal of a foreign and unnatural species has been successful. And that the owners of the quarry can look forward to a great crayfish party this fall. 🙂