I would pretty much agree with the others about it not being the best food source. But I would also never feed a pet dog or cat commercial processed pet food. In my opinion it is always best to feed animals a biologically appropriate raw food diet.

That being said I don't see any problems using it occasionally if/when other food items run out as long as it is not a large part of their diet. There shouldn't be anything in there that a garter snake can not digest even if the nutrient ratios are not ideal. I would basicly treat it as a junk food item for snakes. A person wouldn't want to make mcdonalds meals a dietary staple but the occasional cheeseburger won't do much damage. A visual inspection of fecal matter after a trout chow meals should give you clues as to whether there are any serious digestion problems going on.