Ha! That is funny, I found a huge garden snake in my pond yesterday, so to catch it, we ended up draining half the pond onto the grass to net it. We put it in a bucket and hauled it to our swamp 1 1/2 miles away from my pond. They eat toads for sure, a couple of weeks ago, one had a toad half way down the hatch. I smacked him with the shovel to release the poor thing. But, I made the mistake and let that snake go. Then someone was telling me they will eat your fish! So, I myself am not taking any chances, all snakes will be banished from my pond. My 30 peacocks must be slippin, they eat snakes. And they like to stand in my waterfall, so not sure how these 2 got past them. Well, that's just my input. :)

I do not think you need to worry about Garter snakes eating your fish unless they are very small, about the size of fingerlings. But Water snakes that are really large snakes are quite capable of taking modest sized koi.
You should note that if snakes are in your yard is because they are there to find food which would imply that the habitat of origin is not rich enough to offer an abundance of prey for the animals to survive on. Returning them to what is perceived to be their natural habitat does not resolve the issue for the animals. I would suggest a feeding pool for them where feeder goldfish or minnows can be kept in a shallow kiddie pool that the snakes can take.
This may not protect your favorite fish and it may create an artificial population of snakes dependent upon your feeding but it would relieve the natural habitat of the stress that induced the migration into alien turf by the reptiles.
Also this could be a great opportunity to learn more about these creatures as they may not be fish eaters per se but after the amphibians or even small rodents. It all depends on the species.

Here is a link that might be useful: Garter Snakes

Oh I forgot to mention that this pond is in the middle of woods. Not in a residential area, so it is the snakes home. This is pond up in catskills. I am always finding snakes in wood pile and stuff just never by this pond.

I have it netted and nothing seems to have bothered the pond, this is the second year for this one.

Thanks for the link terrestrial_man 9. I will read up about them.

From my experience I think that a common garter snake could eat a relatively large fish:

The koi in the middle of this photo was approximately 10-12" long when I took this photo last year. That would make the white longfin goldfish above it probably 6-8" long at least... the same goldfish that I discovered out of the water next to my pond early one morning. Imagine how surprised I was when I noticed that half of it was in the mouth of a 16" long garter snake!
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My first thought was Oh No! Not my pretty "angel" fish!!! My next thought was to use the yardstick that I was holding, which I did. The snake took off under a rock and the fish flopped back in the water, swam down to the bottom and stayed there for a few days. This photo was taken at some point after the incident so you can tell the fish is fine. Now almost a year later, the fish still is missing a few scales that I suspect were from being dragged up on the rocks, but otherwise seems to be doing well. :-)

I usually don't mind snakes in my pond and even encourage them to help control the baby goldfish population, but I'm certainly glad I went out early that morning to measure the water depth for some water lilies I was going to pot up. I've thought about naming that goldfish "Meddy", short for Medusa, since I caught her wearing a snake on her head...

End of my snake/fish story :-)

Sherry

I have lots of Garter snakes, some of them pretty big. I saw one in my front pond but no fish in there. I've also seen them going in holes in my lawn. Here is one that ate a right big toad named Mrs. Fowler! Snakes can eat prey that might look too large as they can unhinge their jaw (or so I've read).

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Your garter snakes look different from ours! It's beautiful. One of ours was sunning herself last year on the waterfall rocks, she could hardly move with the goldfish-shaped bulge in her middle.

:) Mary