Here's why:
* Proper nouns name specific people, places, or things (e.g., John, London, Eiffel Tower). They are capitalized.
* Common nouns refer to general categories of people, places, or things (e.g., man, city, building). They are not capitalized.
"Chum salmon" is a common noun that refers to a type of salmon. It's not a specific individual or place.