Calcium is important for a yabby to form a new shell and to molt successfully.
A yearly application of Agricultural lime (NOT HYDRATED OR BRICKIES LIME) on the banks around your dam will ensure an available source of calcium is always available.
Applying Ag lime on the banks of your dam allows it to be gradually washed in when it rains.
An indication that there may be a lack of available calcium in your water is that a great many of your yabbies will have soft shells.
One or two soft shelled yabbies in a harvest is acceptable as they may have recently molted, however more than that may indicate a lack of calcium.
Happy Yabby Farming
Importance of calcium (alkalinity) in your
Pond, Dam or Aquarium