Terry,
It sounds like you've stumbled upon a long-standing tradition of feeding frozen/thawed fish chunks to garter snakes. It's a common practice, and if you leave the bones within the chunks, they get an added bit of calcium and phosphate. Their preference for eating fish is instinctive, and garters are known to "go fishing" both above and below the water's surface to catch a meal. Trout and chub are both known NOT to contain thiaminase, so either would be a good food choice. Here's a link to a page that will give you an idea of some other common food items (
http://www.thamnophis.com/caresheets..._Sheet#Feeding). Cheers,
Rick