Aquaculture workers usually divide fish into pelagic, middle and lower fish, and bottom fish according to the water layer and depth. The color and morphological characteristics of fish at each layer are closely related to the natural environment they inhabit.

Pelagic fish:

According to the plane, it can be divided into an intertidal zone, shallow sea zone, and pelagic zone.

The typical pelagic Negatives have a fusiform body with tapered ends and low swimming resistance in seawater. Most of them are high-speed swimming fish.

Pelagic fish in the pelagic area, such as blue-spotted mackerel, tuna, oriental sailfish, white marlin, swordfish, shark, etc. are fish that swim fast in the ocean, and their backs have the same color as the seawater in the pelagic area. Blue-black, with a lighter abdomen.

For the pelagic fish in the shallow sea area, the color of the back is gray-black like the seawater in the shallow sea area, and the abdomen is silver-white. The typical representative is the jellyfish, and the flying swallow rays are also often active in shallow sea areas.

Middle and lower fish:

This refers to the middle and lower fish with a water depth of fewer than 200 meters. Shallow seabeds are often divided into rocky reefs and sandy bottoms. Black snappers and red snappers often live in rocky areas.

Sturgeon, wrinkled-lipped sharks, flat-headed Hana sharks, flat-headed sharks, etc., with their mouths on the lower part of their heads, often forage on the sandy seabed.

Lower-middle-water fish with a water depth of more than 200 meters are often referred to as deep-sea fish. People call a water depth of 200-3000 meters a semi-deep sea, a water depth of 300-6000 meters a deep sea, and a trench below 6000 meters an ultra-deep sea. Strange, the colors are generally silver, black, and purple-black.

Bottom fish:

The typical bottom fish has a flat body and a gray-black back. It is often attached to the bottom of the sea, making it difficult to find predators.

The body types of marine fish can generally be divided into:

①Torpedo type, this type of fish inhabits the middle waters and is best at swimming, such as mackerel, mullet, tuna, etc.

② Arrow type. Similar to the torpedo type, but the body is more elongated, the odd fins are moved back, and they inhabit the surface water. They are good at swimming, such as pikes and needlefish.

③ side flat. This type of fish has an increased height of the dorsal-ventral axis, and the left and right sides are extremely flat. It can be further divided into the spotted catfish type, the sunfish type, and the flounder type, which inhabit the near bottom and the bottom respectively.

④ snake type. This type of fish has a long and slender body, and its cross-section is almost circular.

⑤ Belt type. The body height is elongated into a lateral flat type, and it is not good at swimming, such as hairtail and oarfish.

⑥ Ball type. The body of this type of fish is almost spherical, and the caudal fin is generally underdeveloped, such as box pufferfish, some round-finned fish, etc.

⑦ vertical flat type. The height of the dorsal and ventral axis of this type of fish is reduced, and the body is flat, such as various rays, anglerfish, etc.