addict,

I'd tell your prof to stick with biology. Manta rays feed mostly on plankton, and some small bony fish. I doubt salmon compose any of their diet. The scars you most often see on fish are unmistakably made by the claws of a pinniped that took a swipe at the fish.

Fish on........
_________________________
She was standin' alone over by the juke box, like she'd something to sell.
I said "baby, what's the goin' price?" She told me to go to hell.

Bon Scott - Shot Down in Flames