Fry,
Fresh water dips are said to kill the bacterial/parasite that causes pinched mantle disease on clams - since it is not 100% known if it is a bacteria or a parasite, more research is being done on this and Barry Neigut, one of the leaders in clam collection and husbandry is starting. You can also go directly to the www.clamsdirect.com forum to see the discussions on the topic.