You might want to get a positive ID on what type of garter you have. I don't think there are any ribbon snakes native to California. The website California Reptiles and Amphibians may help you determine what you have (it has range maps and photos).

I've had a very easy time converting fish-eating snakes to rodents using the methods others have descibed in this post.

Though I've often wondered about wild snake species that are primarily fish-eaters. I know that they have access to frogs, toads and other prey items - but it seems to me there are some types of wild snakes that mostly (and perhaps exclusively) eat fish, and this would imply their wild diet is not nutritionally deficient.

This is a great topic of discussion and I think it shows that there are still unanswered questions about both wild and captive snakes.

Santa Cruz Garter that I found in April: