Never tried but would bet they ball up with the cold water temps.
Exactly right… they’re actually very easy to keep alive as long as you don’t freeze them but they go into a near dormant state when fished in cold water. Basically you get a little black ball of leechy goodness on your hooks and no movement to entice biters.
The good news is it sounds like you’re already set for spring.
A local baitshop has a couple hundred left from the summer and those things are just little black balls now inside where its warm! I haven’t seen one in that tank move for a couple months, yet they are fresh looking leeches.
I’ve used the Gulp Alive Leeches, they work well at times. Like any bait, they have their time and place. I don’t think real leeches would work well in this cold water.