Pelagic fish

Pelagic fish live in the pelagic zone of ocean or lake waters—being neither close to the bottom nor near the shore—in contrast with demersal fish that live on or near the bottom, and reef fish that are associated with coral reefs.
The marine pelagic environment is the largest aquatic habitat on Earth, occupying 1,370 million cubic kilometres, and is the habitat for 11% of known fish species. The oceans have a mean depth of. About 98% of the total water volume is below, and 75% is below.
Marine pelagic fish can be divided into coastal fish and oceanic fish. Coastal pelagic fish inhabit the relatively shallow and sunlit waters above the continental shelf, while [|oceanic pelagic fish] inhabit the vast and deep waters beyond the continental shelf.
Pelagic fish range in size from small coastal forage fish, such as herrings and sardines, to large apex predator oceanic fishes, such as bluefin tuna and oceanic sharks. They are usually agile swimmers with streamlined bodies, capable of sustained cruising on long-distance migrations. Many pelagic fish swim in schools weighing hundreds of tonnes. Others, such as the large ocean sunfish, are solitary. There are also freshwater pelagic fish in some of the larger lakes, such as the Lake Tanganyika sardine.

Epipelagic fish

Epipelagic fish inhabit the epipelagic zone, the uppermost layer of the water column, ranging from sea level down to. It is also referred to as the surface waters or the sunlit zone, and includes the photic zone. The photic zone is defined as the surface waters down to the depth where the sunlight is attenuated to 1% of the surface value. This depth depends on how turbid the water is, but can extend to in clear water, coinciding with the epipelagic zone. The photic zone allows sufficient light for phytoplankton to photosynthesize.
A vast habitat for most pelagic fish, the epipelagic zone is well lit so visual predators can use their eyesight, is usually well mixed and oxygenated from wave action, and can be a good habitat for algae to grow. However, it is an almost featureless habitat. This lack of habitat variation results in a lack of species diversity, so the zone supports less than 2% of the world's known fish species. Much of the zone lacks nutrients for supporting fish, so epipelagic fish tend to be found in coastal water above the continental shelves, where land runoff can provide nutrients, or in those parts of the ocean where upwelling moves nutrients into the area.
Epipelagic fish can be divided broadly into small forage fish and larger predator fish that feed on them. Forage fish school and filter feed on plankton. Most epipelagic fish have streamlined bodies capable of sustained cruising on migrations. In general, predatory and forage fish share the same morphological features. Predator fish are usually fusiform with large mouths, smooth bodies, and deeply forked tails. Many use vision to prey on zooplankton or smaller fish, while others filter feed on plankton.
Most epipelagic predator fish and their smaller prey fish are countershaded with silvery colours that reduce visibility by scattering incoming light. The silvering is achieved with reflective fish scales that function as small mirrors. This may give an effect of transparency. At medium depths at sea, light comes from above, so a mirror that is oriented vertically makes animals such as fish invisible from the side.
In the shallower epipelagic waters, the mirrors must reflect a mixture of wavelengths, and the fish accordingly, has crystal stacks with a range of different spacings. A further complication for fish with bodies that are rounded in cross-section is that the mirrors would be ineffective if laid flat on the skin, as they would fail to reflect horizontally. The overall mirror effect is achieved with many small reflectors, all oriented vertically.
Although the number of species is limited, epipelagic fishes are abundant. What they lack in diversity they make up for in numbers. Forage fish occur in huge numbers, and large fish that prey on them often are sought after as premier food fish. As a group, epipelagic fishes form the most valuable fisheries in the world.
Many forage fish are facultative predators that can pick individual copepods or fish larvae out of the water column, and then change to filter feeding on phytoplankton when that gives better results energetically. Filter feeding fish usually use long fine gill rakers to strain small organisms from the water column. Some of the largest epipelagic fishes, such as the basking shark and whale shark, are filter feeders, and so are some of the smallest, such as adult sprats and anchovies.
Ocean waters that are exceptionally clear contain little food. Areas of high productivity tend to be somewhat turbid from plankton blooms. These attract the filter feeding plankton eaters, which in turn attract the higher predators. Tuna fishing tends to be optimum when water turbidity, measured by the maximum depth a secchi disc can be seen during a sunny day, is 15 to 35 metres.

Floating objects

Epipelagic fish are fascinated by floating objects. They aggregate in considerable numbers around objects such as drifting flotsam, rafts, jellyfish, and floating seaweed. The objects appear to provide a "visual stimulus in an optical void". Floating objects may offer refuge for juvenile fish from predators. An abundance of drifting seaweed or jellyfish can result in significant increases in the survival rates of some juvenile species.
Many coastal juveniles use seaweed for the shelter and the food that is available from invertebrates and other fish associated with it. Drifting seaweed, particularly the pelagic Sargassum, provide a niche habitat with its own shelter and food, and even supports its own unique fauna, such as the sargassum fish. One study, off Florida, found 54 species from 23 families living in flotsam from Sargassum mats. Jellyfish also are used by juvenile fish for shelter and food, even though jellyfish can prey on small fish.
Mobile oceanic species such as tuna can be captured by travelling long distances in large fishing vessels. A simpler alternative is to leverage off the fascination fish have with floating objects. When fishermen use such objects, they are called fish aggregating devices. FADs are anchored rafts or objects of any type, floating on the surface or just below it. Fishermen in the Pacific and Indian oceans set up floating FADs, assembled from all sorts of debris, around tropical islands, and then use purse seines to capture the fish attracted to them.
A study using sonar in French Polynesia, found large shoals of juvenile bigeye tuna and yellowfin tuna aggregated closest to the devices, 10 to 50 m. Farther out, 50 to 150 m, was a less dense group of larger yellowfin and albacore tuna. Yet farther out, to 500 m, was a dispersed group of various large adult tuna. The distribution and density of these groups was variable and overlapped. The FADs also were used by other fish, and the aggregations dispersed when it was dark.
Larger fish, even predator fish such as the great barracuda, often attract a retinue of small fish that accompany them in a strategically safe way. Skindivers who remain for long periods in the water also often attract a retinue of fish, with smaller fishes coming in close and larger fishes observing from a greater distance. Marine turtles, functioning as a mobile shelter for small fish, can be impaled accidentally by a swordfish trying to catch the fish.

Coastal fish

inhabit the waters near the coast and above the continental shelf. Since the continental shelf is usually less than 200 metres deep, it follows that coastal fish that are not demersal fish, are usually epipelagic fish, inhabiting the sunlit epipelagic zone.
Coastal epipelagic fish are among the most abundant in the world. They include forage fish as well as the predator fish that feed on them. Forage fish thrive in those inshore waters where high productivity results from the upwelling and shoreline run off of nutrients. Some are partial residents that spawn in streams, estuaries, and bays, but most complete their life cycle in the zone.

Oceanic fish

Oceanic fish live in the waters that are not above the continental shelf. Oceanic fish can be contrasted with coastal fish, who do live above the continental shelf. However, the two types are not mutually exclusive, since there are no firm boundaries between coastal and ocean regions, and many epipelagic fish move between coastal and oceanic waters, particularly in different stages in their life cycle.
Oceanic epipelagic fish can be true residents, partial residents, or accidental residents. True residents live their entire life in the open ocean. Only a few species are true residents, such as tuna, billfish, flying fish, sauries, pilotfish, remoras, dolphinfish, ocean sharks, and ocean sunfish. Most of these species migrate back and forth across open oceans, rarely venturing over continental shelves. Some true residents associate with drifting jellyfish or seaweeds.
Partial residents occur in three groups: species that live in the zone only when they are juveniles ; species that live in the zone only when they are adults ; and deep water species that make nightly migrations up into the surface waters. Accidental residents occur occasionally when adults and juveniles of species from other environments are carried accidentally into the zone by currents.

Deep water fish

In the deep ocean, the waters extend far below the epipelagic zone and support very different types of pelagic fishes adapted to living in these deeper zones.
In deep water, marine snow is a continuous shower of mostly organic detritus falling from the upper layers of the water column. Its origin lies in activities within the productive photic zone. Marine snow includes dead or dying plankton, protists, fecal matter, sand, soot, and other inorganic dust. The "snowflakes" grow over time and may reach several centimetres in diameter, travelling for weeks before reaching the ocean floor. However, most organic components of marine snow are consumed by microbes, zooplankton, and other filter feeding animals within the first 1,000 metres of their journey, that is, within the epipelagic zone. In this way marine snow can be considered the foundation of deep-sea mesopelagic and benthic ecosystems: As sunlight cannot reach them, deep-sea organisms rely heavily on marine snow as an energy source.
Some deep-sea pelagic groups, such as the lanternfish, ridgehead, marine hatchetfish, and lightfish families are sometimes termed pseudoceanic because, rather than having an even distribution in open water, they occur in significantly higher abundances around structural oases, notably seamounts, and over continental slopes. The phenomenon is explained by the likewise abundance of prey species that also are attracted to the structures.
The fish in the different pelagic and deep water benthic zones are physically structured, and behave, in ways that differ markedly from each other. Groups of coexisting species within each zone all seem to operate in similar ways, such as the small mesopelagic vertically migrating plankton-feeders, the bathypelagic anglerfishes, and the deep water benthic rattails.
Ray finned species, with spiny fins, are rare among deep sea fishes, which suggests that deep sea fish are ancient and so well adapted to their environment that invasions by more modern fishes have been unsuccessful. The few ray fins that do exist are mainly in the Beryciformes and Lampriformes, which also are ancient forms. Most deep sea pelagic fishes belong to their own orders, suggesting a long evolution in deep sea environments. In contrast, deep water benthic species are in orders that include many related shallow water fishes.
Many species move daily between zones in vertical migrations. In the following table, they are listed in the middle or deeper zone where they regularly are found.

Zone	Species and species groups include:
Epipelagic	mackerel, requiem and whale sharks clupeiforms – herring, anchovy Salmonidae – salmon atheriniforms – flyingfishes, halfbeaks, sauries perciforms – jacks, dolphinfish, pomfrets, barracudas, tunas, billfish.
Mesopelagic	Lanternfish, opah, longnose lancetfish, barreleye, ridgehead, sabretooth, stoplight loosejaw, marine hatchetfish
Bathypelagic	Principally bristlemouth and anglerfish. Also fangtooth, viperfish, black swallower, telescopefish, hammerjaw, daggertooth, barracudina, black scabbardfish, bobtail snipe eel, unicorn crestfish, gulper eel, flabby whalefish.
Benthopelagic	Rattail and brotula are particularly abundant.
Benthic	Flatfish, hagfish, eelpout, greeneye eel, stingray, lumpfish, and batfish

	Epipelagic	Mesopelagic	Bathypelagic	Deep sea benthic
muscles		muscular bodies	poorly developed, flabby
skeleton		strong, ossified bones	weak, minimal ossification
scales		yes	none
nervous systems		well developed	lateral line and olfactory only
eyes		large and sensitive	small and may not function	variable
photophores	absent	common	common	usually absent
gills		well developed
kidneys		large	small
heart		large	small
swimbladder		vertically migratory fish have swimbladders	reduced or absent	variable
size			usually under 25 cm	variable, species greater than one metre are not uncommon