The caveat to stocking crayfish before getting any plants established in the pond is that it will be very difficult to get plants established in the pond once the crayfish are in there. My advice is to stock the number of them that Bill recommended, BUT do whatever you can do to get underwater plants established this late Spring, and stock the crays in the early Fall.
I have this exact problem with my pond. Stocked crays before plants were established, coupled with a decent amount of rocky cover, now I can't get plants to grow.
