Have a friend who used to be able to watch a river. He could tell when the hatchery Chum ( and others ) out migrated as a substantial numbers of birds noticed also. He didn't think much of it from a conservation point of view until they stopped hatchery production. Dawned on him that now birds had only wild Chum ( and others ) smolt but same birds. Not a good scenario.

Living on the river myself one can tell when smolt show by the birds and more smolt more birds.