There are a cpl of things you can do without using poisons, that I like, depending upon the severity of the problem.
1 is to put a flat pan, like a pie pan, at the edge of the plants, ground level. Put flat beer in it at night. Pick it up during the day.
The other is to make a little ring around them on the ground, of food grade diamateous earth. Be careful with it's use though, as beneficial bugs can be affected.
If those are trails on the leaves then it has to be snails or slugs. Both are heavy feeders, and are not welcome in my garden. There is the chance that it could be cabbage moth caterpillars as well. Or both
My biggest problem with any of my cabbage family plants are the darn cabbage moth. Those pesky critters will fly around and around the garden, till they zero in on one of the cabbage plant family veggies. Its like they have radar or something. They lay the eggs which form a caterpillar, that is not always easy to spot. The first sign that you have these, next to the leaves being eaten, is the droppings they leave. If you see the droppings, start looking very carefully, and you will see the green caterpillars. I pick them off with long tweezers, and crush them.
If it is slugs or snails, a little slug bait sprinkled here and there will get rid of them pretty fast. A flashlight excursion at night, with a pair of tongs and a tin, with some salt in it to drop them into, is a more earth friendly method then using the poison though.