Most fish written in Japanese are only one Kanji character, and the character consists of two elements. One of them is a radical “魚” (meaning fish) on the left and then something specific to that fish is on the right.
For example, Kanji character for Mackerel is 鯖 which is a combination of “魚” and “靑”. The radical on the left “魚” means fish, and “靑” means blue. Since the skin of Mackerel is silver-blue, the Kanji character for this fish is 鯖 directly translating to “Blue Fish”.
There are more than 150 different Kanji characters expressing all the different varieties of fish. Very few people can read all of them – most Japanese people can read only about a dozen of them. However, they can identify that the character is for a type of fish and can usually guess what fish it might be based on the description.