What gauge wire do you guys recommend for jumpers between batteries wired in paralell?
Thanks for your help!

The same gauge as is being used. The jumper needs to be able to carry the same amount of current.

never jump batteries with smaller than 8 ga. (do not use 10 or 12 etc.) regular battery cables are the best. get ground cables without the clamp style connectors.
trolling motors pull a lot of amps, the bigger the wire the better they will perform. if using non battery cable hookups remember the smaller the number the larger the wire.

I use welding cable 4/0. I also solder on the lugs at each end. Remember this from high school. Electric does not travel in wire, instead it travels on the outside of each strand of wire. So the more strands the higher the amperage it will allow to travel. That is why I use welding calble. It has like a b-zillion wires. I even use it as battery cable when we relocate the cranking battery away from the motor and the orginal battery cables become too short. I also use it in every bass boat that I have owned to run the trolling motor. My trolling motor does not have to look for electric to run, but instead, it has to duck. (little joke there)