I can't bait them, my darn cat catches the things and eats them (good for it catching them but poison is bad for cats!)
I've been using peanut butter with a combination of a live trap and a whole bunch of snap traps. But they often eat the peanut butter off the traps without it snapping... smart things!
I don't like killing them necessarily, because I used to have a few mice as pets. they are really nice pets as long as they are social and you keep the cage cleaned, and just have females...
I guess maybe I gotta suck it up and buy some ducts for them...
Oops yes the cat would get quite ill. I use the bait at my weekend cabin and leave the cat at home. My dog doesn't eat what she kills- she offers her victims to me whole in exchange for a dry biscuit.
I also use the electronic repellents, $30 at the big box stores. These emit a high frequency sound that doesn't effect pets but drives the pests crazy. I had "roof rats" in the peak of a cathedral ceiling. One problem was that the noise does not pass through the ceiling, so its not a simple matter of leaving the unit inside the living space. I had to remove some ceiling boards and aim the speakers straight down the peak of the roof, and opening in the framing about 3" wide and 12" tall. I dismantled the electronic unit, attached the speakers to a light gauge steel strap, fastened the strap to a rafter and bent the strap to aim the speakers straight down the opening. I then extended the speaker wires, which of course are low voltage, through a small hole in the ceiling then to a surface mounted box at the top of the wall. I then put the electronics in the box and ran line voltage to the box through a surface raceway to a point near the floor. I used an extension cord from a nearby outlet to a strain relief the bottom of the raceway, then stripped off the cord outer jacket and let the insulated wires run up the raceway to the box.
I had been using the bait for weeks and noticed a gradual decline in activity (stools falling though the ceiling boards onto the floor below) but after I installed the electronics that way the mice left for good.
I suspect that there are openings in your joist-ducts, like at knots or framing joints, where the critters can get in. With an all metal system you should have much less of a problem.