The follow list of Safe and Unsafe fish is paraphrased from:
Nutrient Requirements of Mink and Foxes, Second Revised Edition, 1982([url]http://books.nap.edu/openbook.php?record_id=1114&page=64)
This should not be considered a complete list!
This is a list of fish containing and not containing thiaminase.
It does not reflect whether a specific fish that doesn't contain thiaminase is a appropriate or even safe fish to feed to garter snakes as there are other issues to be concerned about.
Please research any fish from this list to determine if it is safe to feed your snakes.


Unsafe fish (Species reported to contain thiaminase)

Alewife (Alosa pseudoharengus)Anchovies (Anchoa hepsetus)
Anchovies (Engraulis mordax)
Bass (white) (Lepibema chrysops)
Black quahog (Artica islandica)
Bowfin (dogfish) (Amia calva)
Bream (Abramis brama)
Buckeye shiner (Notropus atherionoides)
Buffalofish (Ictiobus cyprinellus)
Bull Head (Ameirurus m. melas)
Burbot (Lota lota maculosa)
Burbot (Lota lota)
Butterfish (Poronotus triacanthus)
Carp (Cyprinus carpio)
Catfish (channel) (Ictalurus laccustris punctatus)
Clams (chowder, steamer, cherrystone)
Fathead minnow (Primephales p. promelas)
Garfish (garpike)
Goldfish (Carassius auratus)
Herring (Baltic) (Clupea harengus var. membranus)
Herring (Clupea harengus)
Lamprey (adult) (Petromyzon marintus)
Mackerel (Scomber japonicas) (Pacific)
Menhaden (Brecoortia tyrannus)
Menhaden (large scale) (Brecoortia patronus)
Moray ell (Gymnothorax ocellatus)
Mussel (bigtoe) (Pluerobema cordatum)
Razor belly (scaled sardine) (Harengula pensacolae)
Sauger (Stizostedion c. canadense)
Scallop (Placopecten grandis)
Sculpin (Myooxocephalus quadricornis thompsonii)
Shad (gizzard) (Dorosoma cepedianum)
Shiner (spottail) (Notropis hudsonius)
Smelt (freshwater) (Osmerus mordax)
Stoneroller (central) (Campostoma anomalum pullum)
Sucker (common white) (Catostomus c. commersonii)
White bass (Lepimbema chrysops)
Whitefish (Prosopium cylindraceum quadriaterale)
Whitefish (Coregonus clupeaformis)


Safe fish (Species reported not to contain thiaminase)

Ayu (Plecoglossus altivelis)
Bass (largemouth) (Huro salmoides)
Bass (rock) (Ambloplites r. rupestris)
Bass (smallmouth) (Micropterus d. dolomieu)
Black backs (pseudopleuronectes americanus)
Bluegill (Lepomis m. macrochirus)
Chub (bloater) (Coregonus hoyi)
Cod (Gadus morrhua)
Crappie (Pomoxis nigro-maculatus)
Croaker (Micropogon undulatus)
Cunner (Tautogolabrus adspersus)
Cusk (Brosme brosme)
Cutlassfish (silver eel) (Trichiurus lepturus)
Dogfish (squalus acanthias)
Eel (anguilla rostrata)
Gar (northern longnose) (Lepisosteus osseus oxyurus)
Haddack (Melanogrammus aeglefinus)
Halibut (Hippoglossus hippoglossus)
Hake (Urophycis spp.)
Herring (Leucichthys artedi areturus)
King whiting (ground whiting) (Menticirrhus americanus)
Lemon sole (Psuedopleuronectes americanus dignabilis)
Lizard Fish (Synodus foetens)
Mackerel (Scomber scombrus)
Mullet (Mugil spp.)
Perch (yellow) (Perca flavescens)
Pike (northern) (Esox lucius)
Pike (wall-eyed) (stizostedion vitreum)
Plaice (Canadian) (Hippoglossoides platessoides)
Pollock (Pollachitus virens)
Porgy (scup) (Stenotomus aculeatus)
Porgy (scup) (Stenotomus chrysops)
Pumpkinseed (Lepomis gibbosus)
Redfish (Sebastes marinus)
Salmon (Salmo salar)
Salmon (Coho) (Oncorhynchus kisutch)
Seabass (c entropristis striatas)
Sea catfish (galeichthys felis)
Sea robin (Prionotus spp.)
Smelt (pond) (Hypomesus olidus)
Spot (Leiostomus xanthurus)
Squid (Loligo brevis)
Tautog (blackfish) (Tautoga onitis)
Trout (brown) (Salmo trutta fario)
Trout (lake) (Christiconer n. namaycush)
Trout (rainbow) (Salmo gairdnerii irideus)
White trout (Cynoscion nothus)
White trout (Cynoscion avenarius)
Whiting (Merluccius bilinearis)
Yellow tails