A fish stringer goes through the gill and mouth of a live fish, providing a way to keep them captive and alive in the lake or river they are caught from.
I would imagine the key to survival is how much the stringer impacts the gills ability to uptake oxygen. This question is limited to fresh water fishing, using a stringer in salt water is likely to get your catch eaten.
Assuming the fish was not significantly harmed when caught, how long can it live on a stringer?