Welcome Mike. Glad to have you posting.
FWIW the recent AFS mag has an article on crawfish. It contains a 4 pg grid of craw species - there must be 150 in the US . This confirms what Bill Cody has said about craws - lots of types so " it all depends".
Here is the major reason they disappear in ponds with LMB.
Abstract.�We attempted to control a population of papershell crayfish (Orconectes immunis) in an 11-hectare fish-rearing impoundment in Jackson County, Wisconsin, by using traps and by stocking largemouth bass (Micropterus salmoides). Crayfish were harvested with traps during the summer of 1985, and the pond was stocked with 386 largemouth bass (mean weight, 1.1 kg) in spring 1986. The pond was drained in the fall of 1985 and 1986, and crayfish burrow counts were made to estimate the population. In 1985, we trapped more than 18,000 crayfish, of which 72% were adult males. Trapping had minor effect on the young-of-the-year crayfish. In 1986, the crayfish population was reduced by 98%, predation by largemouth bass being the probable major cause of the reduction.
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Here is a very good thread on crawfish as forage.
http://www.pondboss.com/forums/ubbthreads.php?ubb=showflat&Number=25597&fpart=1 
