In terms of scarring on the Spino, I imagine they are reticent to use a profusion of scars in such a way as to end up duplicating a feature present in other pieces already - the T.rex and Allos. Not that we shouldn't have some scarring, and we very well may, but not as great an amount in the other theropods. A thrashing fish could cause some damage, but not nearly as much as the prey items which the other two aforementioned theropods would have taken. Fish are naturally streamlined for efficiency in an aquatic environ, so as long as an animal as large as the spino pins the fish's head, damage should be fairly minimal.