When I was a kid (and that was a while ago), there was a stream a couple of gullies away from our place with freshwater crays (koura) living there. They were not big but they did taste good.
On a camping holiday at Taupo there was the opportunity to head off to the lake outlet and dangle tempting tit bits on string and pull up a few koura, and they were slightly bigger than the ones at home.
So it was with interest that recently I read about a South Island forestry company who decided to stock their fire-fighting ponds with fresh water crayfish. What a smart idea. They now have 400 ponds.
Takes a while for the little koura to get big enough to provide a good meal, and I don’t know how they would co-habit with ducks, but for those with a good sized pond/lake it just might be a new idea.
It seems their distribution is shrinking through loss of habitat. So this could be an interesting venture or hobby it you have the right sized water area. The Department of Conservation regard them as a threatened species, so if you lean more toward conservation than a good meal, this could be an opportunity to do a bit for their survival.
Koura may possibly prefer free flowing water so a quiet pond might not be their home of choice.
But don’t be in a rush to eat them – it takes a while for them to reach eatable size. Feeding them might help. Oh - and you do need a permit if you consider farming them as a commercial venture.