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.
Some people also use the kun'yomi readings, but it's much less common, and generally considered colloquial.
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.