Also need to know how long the wire is between the battery location and the trolling motor. And the size of the wiring makes a difference in the amount of current that can flow though the wires. Larger wiring will carry more current longer distances. There is a voltage drop as well with a longer run that has greater resisntance.
If you are getting interference with the trolling motor try putting just the trolling motor on the new battery and then hook up the depth finder to the other starting battery of your boat.
Use the new trolling motor battery (Optima Blue TOP is what I recommend) to power only the trolling motor and nothing else.
I use a 50 amp circuit breaker on the wiring for my trolling motor. I have the AT 55 # Minkota Trolling motor.
Check you
r manual to see what size circuit breaker they recommend for your unit. (Manuals for current models available online).
Maybe if you state what model TM you have and the size of the battery you currently have, someone may be able to help you.