Pelagic fish

The marine pelagic environment is the largest aquatic habitat on Earth, occupying 1,370 million cubic kilometres (330 million cubic miles), and is the habitat for 11% of known fish species. The oceans have a mean depth of 4,000 metres (2.5 miles). About 98% of the total water volume is below 100 metres (330 ft), and 75% is below 1,000 metres (3,300 ft).^[2]

Marine pelagic fish can be divided into coastal (inshore) fish and oceanic (offshore) 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 (even though they also may swim inshore).^[3][4]

Pelagic fish range in size from small coastal

predator fish, such as this Atlantic bluefin tuna, have a deeply forked tail and a smooth body shaped like a spindle tapered at both ends and countershaded with silvery colours.

Small epipelagic forage fish, such as this Atlantic herring

, share similar body features as the predator fish described above.

Most epipelagic predator fish and their smaller prey fish are countershaded with silvery colours that reduce visibility by scattering incoming light.^[6] 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.^[7]

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.^[7]

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

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.^[8]

Ocean waters that are exceptionally clear contain little food. Areas of high productivity tend to be somewhat turbid from

Drifting Sargassum seaweed provides food and shelter for small epipelagic fish. The small round spheres are floats filled with carbon dioxide which provide buoyancy to the algae.

Great barracuda accompanied by a school of jacks

Lines of Sargassum can stretch for miles along the ocean surface.

The camouflaged sargassum fish has evolved to live among drifting Sargassum seaweed.

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".^[10] 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.^[11]

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.^[8] One study, off Florida, found 54 species from 23 families living in flotsam from Sargassum mats.^[12] Jellyfish also are used by juvenile fish for shelter and food, even though jellyfish can prey on small fish.^[13]

Mobile oceanic species such as

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.[8]

Oceanic fish

Oceanic fish (also called open ocean or offshore 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.^[8]

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

Partial residents occur in three groups: species that live in the zone only when they are juveniles (drifting with jellyfish and seaweeds); species that live in the zone only when they are adults (salmon, flying fish, dolphin, and whale sharks); and deep water species that make nightly migrations up into the surface waters (such as the lanternfish).^[8] Accidental residents occur occasionally when adults and juveniles of species from other environments are carried accidentally into the zone by currents.^[8]

• 
  The huge ocean sunfish, a true resident of the ocean epipelagic zone, sometimes drifts with the current, eating jellyfish.
• 
  The giant whale shark, another resident of the ocean epipelagic zone, filter feeds on plankton, and periodically dives deep into the mesopelagic zone.
• 
  Lanternfish are partial residents of the ocean epipelagic zone During the day they hide in deep waters, but at night they migrate up to surface waters to feed.

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.^[2]

In deep water,

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.^[18] 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.^[19]

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.

Mesopelagic fish

Below the epipelagic zone, conditions change rapidly. Between 200 metres and approximately 1000 metres, light continues to fade until darkness is nearly complete. Temperatures fall through a

Mesopelagic organisms migrate into shallower water at dusk to feed on plankton. The layer is deeper when the moon is out, and may move higher when the sky is dark. This phenomenon has come to be known as the deep scattering layer.^[22]

Most mesopelagic fish make daily vertical migrations, moving each night into the epipelagic zone, often following similar migrations of zooplankton, and returning to the depths for safety during the day.^[21][2][23] These vertical migrations occur over hundreds of meters.

These fish have muscular bodies, ossified bones, scales, well developed gills and central nervous systems, and large hearts and kidneys. Mesopelagic plankton feeders have small mouths with fine gill rakers, while the piscivores have larger mouths and coarser gill rakers.^[21][2]

Vertically migratory fish have

Mesopelagic fish are adapted for an active life under low light conditions. Most of them are visual predators with large eyes. Some of the deeper water fish such as the Telescopefish have tubular eyes with big lenses and only rod cells that look upward. These give binocular vision and great sensitivity to small light signals.^[2] This adaptation gives improved terminal vision at the expense of lateral vision, and allows the predator to pick out squid, cuttlefish, and smaller fish that are silhouetted above them.^[21]

Mesopelagic fish usually lack defensive spines, and use colour for camouflage.^[21] Ambush predators are dark, black or red. Since the longer, red, wavelengths of light do not reach the deep sea, red effectively functions the same as black. Migratory forms use countershaded silvery colours. On their bellies, they often display photophores producing low grade light. For a predator from below, looking upward, this bioluminescence camouflages the silhouette of the fish. However, some of these predators have yellow lenses that filter the (red deficient) ambient light, leaving the bioluminescence visible.^[26]

• 
  The Antarctic toothfish have large, upward looking eyes, adapted to detecting the silhouettes of prey fish.^[27]
• 
  The Barreleye has barrel-shaped, tubular eyes that generally are directed upward, but may be swivelled forward.^[28]
• 
  The

  lower jaw one-quarter as long as its body. The jaw has no floor and is attached only by a hinge and a modified tongue bone. Large fang-like teeth in the front are followed by many small barbed teeth.^[29][30]
• 
  The stoplight loosejaw is also one of the few fishes that produce red bioluminescence. As most of their prey cannot perceive red light, this allows it to hunt with an essentially invisible beam of light.^[29]

The brownsnout spookfish is a species of barreleye and is the only vertebrate known to employ a mirror, as opposed to a lens, to focus an image in its eyes.^[31][32]

Sampling via deep trawling indicates that lanternfish account for as much as 65% of all deep sea fish biomass.^[33] Indeed, lanternfish are among the most widely distributed, populous, and diverse of all vertebrates, playing an important ecological role as prey for larger organisms. The estimated global biomass of lanternfish is 550–660 million tonnes, several times the entire world fisheries catch. Lanternfish also account for much of the biomass responsible for the deep scattering layer of the world's oceans. Sonar reflects off the millions of lanternfish swim bladders, giving the appearance of a false bottom.^[34]

The 2010 Malaspina Circumnavigation Expedition traveled 60,000 km, undertaking acoustic observations. It reported that mesopelagic biomass was 10 billion tonnes or more (10x prior estimates), comprising about 90 percent of all ocean fish biomass.^[35] Estimates of how much carbon these fish sequester remained highly uncertain as of 2024.^[36]

Mesopelagic fish do not constitute a major fishery as of 2024. Initial efforts in Iceland, Norway, and the Soviet Union did not create a commercial industry. The European Union funded the MEESO project to study abundance and fishing technologies for key mesopelagic species. To date, fish that appeal to the human palate have not been identified, leading harvesters to focus on animal feed markets instead.^[36]

Bigeye tuna are an epipelagic/mesopelagic species that is carnivorous, eating other fish. Satellite tagging has shown that bigeye tuna often spend prolonged periods cruising deep below the surface during the daytime, sometimes making dives as deep as 500 metres (1,600 feet). These movements are thought to be in response to the vertical migrations of prey organisms in the deep scattering layer.

• 
  Longnose lancetfish. Lancetfish are ambush predators that frequent the mesopelagic. They are among the largest mesopelagic fishes (up to 2 metres).^[37]
• 
  The telescopefish has large, forward-pointing telescoping eyes with large lenses.^[38]
• 
  The

  daggertooth slashes other mesopelagic fish when it bites them with its dagger-like teeth.^[39]
• 
  Bigeye tuna cruise the epipelagic zone at night and the mesopelagic zone during the day.
• 
  A collection of mesopelagic forage fishes trawled from the Gulf of Mexico that includes

  bristlemouths, and a barracudina

Conditions are somewhat uniform throughout these zones, the darkness is complete, the pressure is crushing, and temperatures, nutrients, and dissolved oxygen levels are all low.[2]

Bathypelagic fish have special adaptations to cope with these conditions – they have slow metabolisms and unspecialized diets, being willing to eat anything that comes along. They prefer to sit and wait for food rather than waste energy searching for it. The behaviour of bathypelagic fish can be contrasted with the behaviour of mesopelagic fish. Mesopelagic are often highly mobile, whereas bathypelagic fish are almost all lie-in-wait predators, normally expending little energy in movement.^[44]

The dominant bathypelagic fishes are small

The swimbladders of deep sea fish are either absent or scarcely operational, and bathypelagic fish do not normally undertake vertical migrations. Filling bladders at such great pressures incurs huge energy costs. Some deep sea fishes have swimbladders that function while they are young and inhabit the upper epipelagic zone, but they wither or fill with fat when the fish move down to their adult habitat.[47]

The most important sensory systems are usually the

It is not easy finding a mate in this zone. Some species depend on

• 
  The gulper eel uses its mouth like a net by opening its large mouth and swimming at its prey. It has a luminescent organ at the tip of its tail to attract prey.
• 
  The

  bony fishes ten times its mass.^[50][51]
• 
  Female

  gonads

  , for use when the female is ready to spawn.
• 
  The widespread fangtooth has the largest teeth of any fish, proportionate to body size.^[52] Despite their ferocious appearance, bathypelagic fish are usually weakly muscled and too small to represent any threat to humans.
• 
  The Sloane's viperfish can make nightly migrations from bathypelagic depths to near surface waters.^[53]

Demersal fish

Giant grenadier, an elongate benthic fish with large eyes and well-developed lateral lines

See also: Demersal fish

hadopelagic depths or on the abyssal plain. They occupy a range of seafloors consisting of mud, sand, gravel, or rocks.^[54]

In deep waters, the fishes of the demersal zone are active and relatively abundant, compared to fishes of the bathypelagic zone.^[44]

lumpfishes.^[45]

The bodies of deep water

swimbladders

range from absent to well developed. They vary in size, with larger species greater than one metre not uncommon.

Deep sea benthic fish are usually long and narrow. Many are eels or shaped like eels. This may be because long bodies have long lateral lines. Lateral lines detect low-frequency sounds, and some benthic fishes appear to have muscles that drum such sounds to attract mates.^[18] Smell is also important, as indicated by the rapidity with which benthic fish find traps baited with bait fish.

The main diet of deep sea benthic fish is invertebrates of the deep sea benthos and carrion. Smell, touch, and lateral line sensitivities seem to be the main sensory devices for locating these.^[55]

Deep sea benthic fish can be divided into strictly benthic fish and benthopelagic fish. Usually, strictly benthic fish are negatively buoyant, while benthopelagic fish are neutrally buoyant. Strictly benthic fish stay in constant contact with the bottom. They either lie in wait as ambush predators or move actively over the bottom in search for food.^[55]

Benthopelagic fish

See also:

Benthopelagic fish

Benthopelagic fish inhabit the water just above the bottom, feeding on benthos and benthopelagic zooplankton.^[56] Most dermersal fish are benthopelagic.^[54]

They can be divided into flabby or robust body types. Flabby benthopelagic fishes are like bathopelagic fishes, they have a reduced body mass, and low metabolic rates, expending minimal energy as they lie and wait to

Robust benthopelagic fish are muscular swimmers that actively cruise the bottom searching for prey. They may live around features, such as seamounts, which have strong currents.^[59] Examples are the orange roughy and Patagonian toothfish. Because these fish were once abundant, and because their robust bodies are good to eat, these fish have been harvested commercially.^[60][61]

Benthic fish

Benthic fish are not pelagic fish, but they are discussed here briefly, by way of completeness and contrast.

Some fishes do not fit into the above classification. For example, the family of nearly blind

The deepest-living fish known, the strictly benthic Abyssobrotula galatheae, eel-like and blind, feeds on benthic invertebrates.^[63][64]

• 
  Pacific hagfish resting on bottom. Hagfish coat themselves and any dead fish they find with noxious slime, making them inedible to other species.
• 
  The tripodfish (Bathypterois grallator), a species of spiderfish, uses its fin extensions to "stand" on the bottom.^[65]
• 
  The

  blotched fantail ray feeds on bottom-dwelling fish, bivalves, crabs, and shrimps.^[66]

At great depths, food scarcity and extreme pressure works to limit the survivability of fish. The deepest point of the ocean is about 11,000 metres (6.8 miles). Bathypelagic fishes are not normally found below 3,000 metres (1.9 miles). The greatest depth recorded for a benthic fish is 8,370 m (5.20 mi).^[67] It may be that extreme pressures interfere with essential enzyme functions.^[40]

Benthic fishes are more diverse and are likely to be found on the

Small pelagic fish are usually forage fish that are hunted by larger pelagic fish and other predators. Forage fish filter feed on plankton and are usually less than 10 centimetres (3.9 inches) long. They often stay together in schools and may migrate large distances between spawning grounds and feeding grounds. They are found particularly in upwelling regions around the northeast Atlantic, off the coast of Japan, and off the west coasts of Africa and the Americas. Forage fish are generally short-lived, and their stocks fluctuate markedly over the years.^[68]

Traditional fisheries for anchovies and sardines also have operated in the Pacific, the Mediterranean, and the southeast Atlantic.^[72] The world annual catch of forage fish in recent years has been approximately 22 million tonnes, or one quarter of the world's total catch.

• 
  These

  Pacific sardines are forage fish

  .
• 
  Herrings ram feeding on copepods
• 
  Capelin
• 
  Anchovies
• 
  Peruvian anchoveta

Predator fish

Medium size pelagic fishes include

Around Europe there are three populations of coastal mackerel. One population migrates to the North Sea, another stays in the waters of the Irish Sea, and the third population migrates southward along the west coast of Scotland and Ireland. The cruise speed of the mackerel is an impressive 10 kilometres per hour.^[69][74]

Many large pelagic fish are oceanic nomadic species that undertake long offshore migrations. They feed on small pelagic forage fish, as well as medium-sized pelagic fish. At times, they follow their schooling prey, and many species form schools themselves.

Examples of larger pelagic fish are

Tuna in particular are of major importance to commercial fisheries. Although tuna migrate across oceans, trying to find them there is not the usual approach. Tuna tend to congregate in areas where food is abundant, along the boundaries of currents, around islands, near seamounts, and in some areas of upwelling along continental slopes. Tuna are captured by several methods:

Other large pelagic fish are premier game fish, particularly marlin and swordfish.

• 
  Yellowfin tuna are being fished as a replacement for the now largely depleted Southern bluefin tuna.
• 
  Atlantic pomfret
• 
  Swordfish
• 
  King mackerels cruise on long migrations at 10 kilometres per hour.^[69][74]

Productivity

Upwelling occurs both along coastlines and in midocean when a collision of deep ocean currents brings cold water that is rich in nutrients to the surface. These upwellings support blooms of phytoplankton, which in turn, produce zooplankton and support many of the world's main fisheries. If the upwelling fails, then fisheries in the area fail.^[14]

In the 1960s the

Off Japan, the collision of the Oyashio Current with the Kuroshio Current produces nutrient-rich upwellings. Cyclic changes in these currents resulted in a decline in the sardine sardinops melanosticta populations. Fisheries catches fell from 5 million tonnes in 1988 to 280 thousand tonnes in 1998. As a further consequence, Pacific bluefin tuna stopped moving into the region to feed.^[76][77]

Ocean currents can shape how fish are distributed, both concentrating and dispersing them. Adjacent ocean currents can define distinct, if shifting, boundaries. These boundaries can even be visible, but usually their presence is marked by rapid changes in salinity, temperature, and turbidity.^[14]

For example, in the Asian northern Pacific, albacore are confined between two current systems. The northern boundary is determined by the cold North Pacific Current and the southern boundary is determined by the North Equatorial Current. To complicate things, their distribution is further modified within the area defined by the two current systems by another current, the Kuroshio Current, whose flows fluctuate seasonally.^[78]

Epipelagic fish often spawn in an area where the eggs and larvae drift downstream into suitable feeding areas, and eventually, drift into adult feeding areas.^[14]

Islands and

Epipelagic fish generally move long distances between feeding and spawning areas, or as a response to changes in the ocean. Large ocean predators, such as salmon and tuna, can migrate thousands of kilometres, crossing oceans.^[81]

In a 2001 study, the movements of Atlantic bluefin tuna from an area off North Carolina were studied with the help of special popup tags. When attached to a tuna, these tags monitored the movements of the tuna for about a year, then detached and floated to the surface where they transmitted their information to a satellite. The study found that the tuna had four different migration patterns. One group confined itself to the western Atlantic for a year. Another group also stayed mainly in the western Atlantic, but migrated to the Gulf of Mexico for spawning. A third group moved across the Atlantic Ocean and back again. The fourth group crossed to the eastern Atlantic and then moved into the Mediterranean Sea for spawning. The study indicates that, while there is some differentiation by spawning areas, there is essentially only one population of Atlantic bluefin tuna, intermixing groups that between them, use all of the north Atlantic Ocean, the Gulf of Mexico, and the Mediterranean Sea.^[82]

The term

In 2009, the International Union for Conservation of Nature (IUCN) produced the first red list for threatened oceanic sharks and rays. They claim that approximately one third of open ocean sharks and rays are under threat of extinction.^[87] There are 64 species of oceanic sharks and rays on the list, including hammerheads, giant devil rays, and porbeagle.^[88]

Oceanic sharks are

The northwest Atlantic Ocean shark populations are estimated to have declined by 50% since the early 1970s. Oceanic sharks are vulnerable because they do not produce many young, and the young can take decades to mature.[88]

• 
  The scalloped hammerhead is classified as endangered.
• 
  The oceanic whitetip shark has declined by 99% in the Gulf of Mexico.^[88]
• 
  The devil fish, a large ray, is threatened.
• 
  The porbeagle shark is threatened.

In parts of the world the scalloped hammerhead shark has declined by 99% since the late 1970s. Its status on the red list is that it is globally endangered, meaning it is near extinction.^[88]

See also

References

Notes

Bibliography

• Bone, Quentin; Moore, Richard (2008). Biology of Fishes. Garland Science.
  ISBN 978-0-203-88522-2
  .
• Moyle, PB & Cech, JJ (2004). Fishes, An Introduction to Ichthyology (5th ed.). Benjamin Cummings.
  ISBN 978-0-13-100847-2
  .

Further reading

• Collette, BB (2010) "Reproduction and development in epipelagic fishes" In: Kathleen S Cole, Reproduction and Sexuality in Marine Fishes: Patterns and Processes, pp. 21–64, University of California Press.
  ISBN 978-0-520-26433-5
  .
• Freon, Pierre (1998) Dynamics of Pelagic Fish Distribution and Behaviour: Effects on Fisheries and Stock Assessment, Wiley-Blackwell.
  ISBN 978-0-85238-241-7
  .
• Johnsen, S (2003). "Lifting the Cloak of Invisibility: The Effects of Changing Optical Conditions on Pelagic Crypsis1". Integrative and Comparative Biology. 43 (4): 580–590.
  PMID 21680466
  .
• Makris, N; Ratilal, P; Jagannathan, S; Gong, Z; Andrews, M; Bertsatos, I; Godo, OR; Nero, RW; Jech, JM (2009). "Critical Population Density Triggers Rapid Formation of Vast Oceanic Fish Shoals". Science. 323 (5922): 1734–1737.
  S2CID 6478019
  .
• Pepperell J (2011) Fishes of the Open Ocean: A Natural History and Illustrated Guide University of New South Wales Press,
  ISBN 978-1-74223-267-6
  .
• Salvanesa AGV and Kristoffersen JB "Mesopelagic Fishes" In: Encyclopedia of Ocean Sciences, pp. 1711–1717.
  doi:10.1006/rwos.2001.0012
• Scientists IDs genesis of animal behavior patterns
  PhysOrg.com
  , 26 March 2009.
• One fish, two fish: New MIT sensor improves fish counts
  PhysOrg.com
  , 2 February 2006.

External links

Wikimedia Commons has media related to Deep sea fish.

• Glowing life in an underwater world TED video from Edith Widder
• The Open Ocean MarineBio.org. MarineBio.org. Updated 28 August 2011. TED video from Edith Widder
• The Open Ocean MarineBio.org. MarineBio.org. Updated 28 August 2011.
• Pelagic Advisory Council of the European Commission