I always thought that it was an eco consious thing to do, but after doing some research for the second book it Seems that it only sustainable and benefical if you continuously do it.
Here is an extract from the environment agency.
The Crayfish Code: How Can You Help?
You Catching Crayfish will not help the environment
Unlike the message from many TV programs, catching crayfish on an ad hoc basis can actually make the situation worse and not better. You will be catching the bigger crayfish in any stretch of water. These will eat their smaller siblings which helps keep the population in balance. By taking out the bigger dominant crayfish you will inadvertently destabilise the population which could cause it to expand in area.
here is the rest