This is part of a previous posting I did on the same topic.
The Garter Snakes, Evolution and Ecology by Rossman, Ford and Seigal report that they can be difficult captives due to diet restriction. They seem to specialize on slugs and worms. Amphibians are rarely eaten. Garters fed on worms need to be fed often and Calcium supplements are important. You might want to try fish and/or scented pinkies. If you are going to try and keep this species please keep us posted on what does and does not work. I hope to keep them myself if I can ever acquire any. I love Garters and the Northwesterns are of great interest to me.
EdK's advice is excellent but it will not work on all Garter species. It may also require patience especially with a new captive. I would add that the pink should also be scented when put in a bowl with fish as it gives better results (it should be a very small shallow bowl). My advice is to get the snake settled on its natural diet and then try to convert it. Try to feed it fish before trying fish scented pinks. Make sure you will have a steady supply of worms for the winter if it will not take feeder fish.
Keep us posted. I am looking forward to keeping this species one of these days. AND WE ALL WANT TO SEE A PIC OF IT LOL.
Randy
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Firehouse Herps