So far, all of the crimes have taken place near water, albeit both fresh and salt water. This could just be taking advantage of the natural strengths of any shark, or it could indicate that the creature is amphibious, unable to stay out of water for very long. That would certainly narrow the area you need to search. If this other sharkman is like you, however, he could be anywhere.

The scent of the dead man isn't as strong as you'd think here, but as the medical examiner said the bridge is very busy. It's unlikely the victim was washed up here for very long before someone noticed him. While weak, the scent is strong enough to track upriver if you want, for a little ways at least. The constant flow of water is going to wash away any sort of trail before you get too far.

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...Water flow? You're in a river, the water is probably flowing in the direction the water is flowing. Not really something you need to roll for, unless I'm misunderstanding what you're after?