The only reason you have any chicks left is because you caught the fisher cat in the act.  You need to predator proof your cages/coops now.  Fisher cats are not evil, but they are THE PERFECT PREDATOR.  They are tenacious, wily, fast, difficult to shoot, and even harder to trap.  They are weasels on steroids.  (They are also my worst nightmare and the reason we can't have a cat.)  Hopefully, the fisher cat you have is a juvenile and hasn't acquired a lot of experience, but if you have an oldster, you have a big problem.  Predator proof everything because chances are you will not see the fisher again, just a trail of blood and feathers.