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[APD] Aquatic Snail removal challenge

I would like some help with ideas on how to rid some ponds of a tough local
species of snail.    Glyptophysa badia is a small NT, Australian pond snail
that lays a little patch of eggs in jelly like many other species of pond
snail.   It does not eat plants and would be left in place if I did not sell
plants.   However the NT Dept of Business Industry and Resource Development,
Fisheries Division, Aquatic Pest Managers (If you work in or near a
Government and like acronyms then perhaps that is the DBIRD FD APM) stop me
selling plants every time I get ramshorn or any other non native species
occuring in the shops where I sell my plants.   The Aquatic Pest managers
are actually very nice and helpful but have not come up with a snail remedy
that won't kill plants.  No snails from the start is the only answer,
however I started years ago when snails did not matter to me or anybody else
at the time. 

The ponds at present are 14 X 4,000 litre ponds linked to a common sump.
I have put a 50 micron sieve on the intake of the pump to stop snails from
the sump being sent back out to the other ponds but most of the ponds have
snails already.

We have tried copper in 2002 which killed most of the snails but the
Glyptophysa and ramshorns came back.    In August 2004 I tried trichlorofon
up to 2 ppm which killed off most of the ramshors but not all, it did not
seem to affect Glyptophysa.  In October 2004 I tried a funguscide called
chlorothalonil because it had been used successfully in rice growing areas
of NSW Australia to rid snails from the paddys but did not hurt the rice at
2 parts per million.    I put two parts per million in with the aquatic
plants and it defoliated the lot killing of Ludwegia,  Water Sprite and
Ottelia completely.   The rice must have been up out of the water.
Hindsight is wonderful.

I have tried native fishes such as Obbes Catfish that eat snails as I am not
permitted to put clown loaches in the ponds.  I practise hours of physical
removal each week but the system is too big to get them all out.

Any ideas would be appreciated.


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