Is there any possibility that there has been an episode of Agricultural pesticide ie Sheep dip (SP's) leaked into the system.
if this has occurred then Crays will be wiped out without a doubt
Is there a water quality issue that doesn't effect the resident fish ???
Just a thought!

Seem to have seen something similar before, HERE perhaps 

its the same outbreak

I know quite a few anglers around here that if they see lots of dead reggies would be jumping up and down with joy !!

surely not if they were the native ones.

they wouldnt know the difference !

Im having a blonde moment