[adamanteus]

I agree with Udo, if she is indeed gravid let her eat all she wants, and whole fish are better than lean pieces. Thamnophis melanogaster is much more aquatic than most Garter species (it fills the Mexican niche normally occupied by Nerodia in the US) and as such the diet is predominantly fish. You could still try offering her scented pinkies though, it has to be worth a try.