There are plenty of hungry animals that would be quite happy to eat a young salmon. You'll also have to avoid fishing nets and toxic waste that's been dumped in the rivers you're swimming in.
EEL: Eels slither quickly through the water. They like taking bites out of baby salmon.
TROUT: Trout often eat newly hatched alevins for lunch.
NET: Fishnets, designed to catch fish, are layed across the stream in certain areas. You're small enough to slip through them, but you'll be slowed down.
MERGANSER: This duck swims along the surface of the water. If you jump out, it may take a nip out of you.
DOGFISH: A large and nasty shark. Will cause a fair bit of damage to your hapless self.
NET: Nets are not so easy to hide from anymore. If you run into one, you'll be trapped and pulled up to be somebody's dinner.
MACKERAL: A nasty fish whose favorite dish is unfortunate smolt.
HAKE: Another food-happy smolt-eater.
SEAGULL: these large predators fly above the water. If an unsuspecting smolt gets too close to the surface, one may swoop down and snatch you away.
TOXIC WASTE: barrels of poison waste have somehow ended up in the river. If you get too close, the fumes might hurt you.