This is a jukugo word, which usually means on'yomi readings from the kanji. If you know the readings of your kanji you'll know how to read this as well.
Is salmon a saltwater fish? Or is it a freshwater fish?
In the sea, there were a lot of colorful ocean fish swimming around.
There is a wide variety of saltwater fish at the aquarium, but the staff said we’re not allowed to eat any of them.