Norm is right, as usual. There's a good thread on this topic at:

http://www.pondboss.com/ubb/ultimatebb.php?ubb=get_topic;f=6;t=000037;p=

Dave Willis said exactly what I've seen...they (the grass shrimp) hang out around submerged weeds along the shoreline...since my submerged weeds are threatened by the grass carp and Tilapia I may loose these guys over time.