Pelagic fish

Pelagic fish are animals that live in the open water of oceans or lakes.

Unlike demersal fish that stay near the bottom, or reef fish that live in coral, pelagic fish swim in the water column. The marine pelagic environment is the largest aquatic habitat on Earth. It covers about 1,370 million cubic kilometres. This massive space holds 11% of all known fish species. These fish must adapt to living in a vast, often featureless space. They range from tiny forage fish to massive apex predators.

Many pelagic fish live in the epipelagic zone.

This is the uppermost layer of water, extending from the surface down to about 200 metres. It is often called the sunlit zone or the photic zone. In this zone, sunlight is strong enough for phytoplankton to perform photosynthesis. This light allows visual predators to use their eyesight to hunt. The surface waters are also well mixed and oxygenated by waves. However, this zone lacks many physical structures. Because it is so open, it supports less than 2% of the world's fish species.

To find food, these fish often gather where upwelling brings nutrients to the surface.

To survive in open water, many fish use a specialized form of camouflage called countershading.

Many species have silvery colors that scatter incoming light. This is achieved through reflective scales that act like tiny, vertical mirrors. When light comes from above, these vertical mirrors make the fish appear transparent from the side. In shallower waters, the fish have crystal stacks with different spacings to reflect various wavelengths. This helps them hide from predators in an environment with nowhere to tuck away. This silvering effect is a vital tool for survival in the vast blue.

Pelagic fish can be divided into coastal and oceanic groups.

Coastal fish, or neritic fish, live in the relatively shallow waters above the continental shelf. These areas are often very productive due to land runoff and nutrient upwelling. Oceanic fish live in the deep, open waters beyond the continental shelf. Some oceanic species are true residents, meaning they spend their entire lives in the open ocean. Examples include tuna, billfish, and the ocean sunfish. Others are partial residents, such as salmon or whale sharks, which move between zones. Some fish are even accidental residents, carried into the open ocean by currents.

In the open ocean, floating objects are very important.

Because the epipelagic zone can be a visual void, objects like drifting seaweed or jellyfish act as landmarks. Many juvenile fish use these objects for refuge from predators. For example, the Sargassum seaweed provides a unique habitat for species like the sargassum fish. Fishermen even use man-made fish aggregating devices, or FADs, to catch fish. These anchored rafts attract large groups of tuna. Studies show that different sizes of fish gather at different distances from these devices.

In the deep, dark layers of the ocean, life relies on a different energy source.

Sunlight cannot reach these depths, so plants cannot grow there. Instead, organisms depend on marine snow. Marine snow is a continuous shower of organic material falling from the upper layers. It includes dead plankton, fecal matter, sand, and even soot. These particles can grow to several centimetres in diameter. While microbes consume much of this snow in the upper 1,000 metres, the remains provide essential energy for deep-sea ecosystems.

Pelagic fish play a massive role in the global economy and food web. Forage fish, like sardines and anchovies, exist in enormous numbers. They often school together, sometimes weighing hundreds of tonnes. These small fish are eaten by larger predators like bluefin tuna and sharks. Because of this, epipelagic fishes form the most valuable fisheries in the world. The entire system, from the tiny plankton to the massive predators, moves energy through the vast reaches of the planet's waters.