This is actually a set-piece in problem solving for young vet students (and your vets have done a reasonable thing). Has nothing to do with dogs
having a strong stomach - just what gets "hooked up" along the way. It is surprising the hardware
that will pass through unattended. Open safety
pins, razor blades, glass etc all seem to make it through on occasion. Not unreasonable to wait and see but monitor
carefully - Pain, vomiting, not eating. At least the vets will know what they are chasing. Long pieces of fishing
line (linear foreign bodies) are possible worse than the fish
By the bye, there is a technique for removing a fish
hook - basically using a loop of line over the hook to pull away from the shank. Difficult in the mouth though and even more difficult in an active dog. You might get away with it using a pair of long-nosed pliers.
Possibly best option initially would have been to give it a pull to thoroughly imbed it in the tongue - at least you would know where it was and cheaper than the stomach!
Hope it turns out OK.