I thought this thread was about th eeffects of cast nets we do seem to have altered our focus a bit here.
I use a cast net. Quite infrequently - probably 2 or three times a month when my target species are about (ie barra, fingermark and spanish mackerel - I am not a gun fisho but I can still bring a fish home for the table). I may get off about 20 shots before I am stuffed and usually all for what? 2 or 3 decent sized mullet for bait and up to 1/2 a kilo of prawn.
The river bed where I usually cast is silt over mud lying next to sand over mud. It has very little grass or weed but a healthy green algal growth which is exposed at low tide. This area is also has a good yabbie population and appears to be a good nursery for all inshore species as I have caught all manner of juvenile fish in my net - very few are meshed and swim away easily when tipped out of the pockets undamaged.
I would offer however, that the river bed causes more damage to my net than I cause to it due to natural littering of it by branches and other debris that has travelled there or been exposed with tidal action. I accept that as fair change.
I understand the comparitive argument re: trawl netting in estuaries and inshore nurseries but I did read the thread title as concerning cast net usage.