OK then, more bluntly, thats a shit way to release a trout. They're not designed to be out of the water, let alone left on a shingle bank.
There is screeds of information available on how to release fish of all types with proven successful techniques. Got visit your local angers association (as they won't try and steel all your spots) or Google "how to release trout".
You'd be surprised that you will not find anything there that suggests leaving a fish on a bank is anything other than detrimental to its successful release. It really takes very little additional effort to do a much better job.
As for the marks the photo isn't that great. Eels will certainly have a go at a trout but leave a fairly distinctive "v" shape (see above photo between dorsal and adipose). That more looks like an abrasion and as you noted the fish is not in good nick so could even be disease. Ive been advised that eels will latch onto a hens vent when spawning and literally suck out the eggs.
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