That’s an intended mechanic for less micromanaging. Whenever you place a crayfish spot, gatherers will go there to collect crayfish until it’s empty and then automatically return to their permanent workplace. Same with hunting spots pulling hunters from a campfire or cabin. This way you don’t have to assign workers manually to those temporary spots anymore.
I have used this “behaviour” in several games; combining crayfishing in winter with wild food in summer and it has worked very well but in my present game it doesn’t work anymore. It’s some kind of bug and I’m sure it’s nothing you can do to fix it, Tom but I’ll tell you anyway.
When you have several gather’s huts (I first noticed it with 4 huts and 12 gatherers) and set several crayfish spots (about 10) it looks like the game can’t assign a certain gatherer to a certain crayfish spot anymore. After I saw that the spots wasn’t harvested after the winter, I watched a few gatherers a bit closer and they went from one spot to another doing one work and back to a hut to harvest one firewood then back to one crayfish spot, often far away. Not much was harvested.
I guess the game got confused by those many spots. Actually it shares workers of same profession over all workplaces equalizing the ratio of assigned workers to max workers. So a gatherer shelter with 3/3 should send out 2 gatherers to a new crayfish spot because 1/4 would be still less than 1/3, or 1 gatherer to each spot if you placed two. Maybe you created a case where things are so even that it constantly switches workers.