I'm not sure that I would even suggest threadfins for a northern pond even if they did survive cooler water. I am not saying that SMB won't do well with threadfins because big smallies are produced in some waters with threadfins. But on the other side big smallies are also produced without threadfins as noted by others and more recently by BobbyRice, a member here.
http://forums.pondboss.com/ubbthreads.php?ubb=showflat&Main=25055&Number=317191#Post317191Also see "Talking Points: Smallmouth Bass" by Dave Willis and Bill Cody in Jan-Feb 2013 Pond Boss Magazine.
Threadfins are filter plankton feeders and work well with lots of plankton which means one would likely have to fertilize to get good results or truly worthwhile benefits from threadfins. With a limited amount of plankton the threadfins may actually end competing heaavily with the smallie fry and other desired fish in the pond by eating too many zooplankton needed by the more desirable fish. Fertilizing northern ponds or any pond with SMB is risky and dangerous due to possible plankton crashes and associated DO sags. Risky and dangerous expecially if one is not well versed in fertilizing and being able to properly monitor the blooms.
Smallmouth are IMO more sensitive to DO sags compared to largemouth bass. So in trying to properly maintain threadfins with good plankton blooms to enhance smallmouths, one might end up killing the smallmouths due periodic to lower annual water quality events; which is sort of like 'throwing the baby out with the bathwater'. The chances of periodic DO sags happening would depend on many interacting variables of the pond ecosystem and ambient conditions. It is too risky a plan IMO if you really value the smallies once you have raised them to 16+".
