Pelagic fish

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.

Filter feeding fish usually use long fine gill rakers to strain small organisms from the water column.

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.

[13] Mobile oceanic species such as tuna can be captured by travelling long distances in large fishing vessels.

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.

Forage fish thrive in those inshore waters where high productivity results from the upwelling and shoreline run off of nutrients.

Marine snow includes dead or dying plankton, protists (diatoms), fecal matter, sand, soot, and other inorganic dust.

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.

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.

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.

These fish have muscular bodies, ossified bones, scales, well developed gills and central nervous systems, and large hearts and kidneys.

[26] 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.

[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 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.

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.

What little energy is available in the bathypelagic zone filters from above in the form of detritus, faecal material, and the occasional invertebrate or mesopelagic fish.

[46] Despite their ferocious appearance, these beasts of the deep are mostly miniature fish with weak muscles, and are too small to represent any threat to humans.

Approximately 40% of the ocean floor consists of abyssal plains, but these flat, featureless regions are covered with sediment and largely devoid of benthic life (benthos).

Deep sea benthic fishes are more likely to associate with canyons or rock outcroppings among the plains, where invertebrate communities are established.

Undersea mountains (seamounts) can intercept deep sea currents and cause productive upwellings that support benthic fish.

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.

The anchoveta population was greatly reduced during the 1972 El Niño event, when warm water drifted over the cold Humboldt Current, as part of a 50-year cycle, lowering the depth of the thermocline.

[14] Epipelagic fish generally move long distances between feeding and spawning areas, or as a response to changes in the ocean.

These are high trophic level species that undertake migrations of significant, but variable distances across oceans for feeding, often on forage fish, or reproduction, and also have wide geographic distributions.

[84] According to the Food and Agriculture Organization (FAO), the world harvest in 2005 consisted of 93.2 million tonnes captured by commercial fishing in wild fisheries.

[86] In 2009, the International Union for Conservation of Nature (IUCN) produced the first red list for threatened oceanic sharks and rays.

A school of large pelagic predator fish ( bluefin trevally ) sizing up a school of small pelagic prey fish ( anchovies )
Herring reflectors are nearly vertical for camouflage from the side.
Schooling threadfin , a coastal species
Oceanic fish inhabit the oceanic zone , which is the deep open water which lies beyond the continental shelves.
Scale diagram of the layers of the pelagic zone
Most mesopelagic fishes are small filter feeders that ascend at night to feed in the nutrient rich waters of the epipelagic zone. During the day, they return to the dark, cold, oxygen-deficient waters of the mesopelagic where they are relatively safe from predators. Lanternfish account for as much as 65% of all deep sea fish biomass and are largely responsible for the deep scattering layer of the world's oceans.
Most of the rest of the mesopelagic fishes are ambush predators, such as this sabertooth fish . The sabertooth uses its telescopic, upward-pointing eyes to pick out prey silhouetted against the gloom above. Their recurved teeth prevent a captured fish from backing out.
The humpback anglerfish is a bathypelagic ambush predator, which attracts prey with a bioluminescent lure. It can ingest prey larger than itself, which it swallows with an inrush of water when it opens its mouth. [ 40 ]
Many bristlemouth species, such as the "spark anglemouth" above, [ 41 ] are also bathypelagic ambush predators that can swallow prey larger than themselves. They are among the most abundant of all vertebrate families. [ 42 ]
Young, red flabby whalefish make nightly vertical migrations into the lower mesopelagic zone to feed on copepods . When males mature into adults, they develop a massive liver and then their jaws fuse shut. They no longer eat, but continue to metabolise the energy stored in their liver. [ 43 ]
Giant grenadier , an elongate benthic fish with large eyes and well-developed lateral lines
Orange roughy
Patagonian toothfish
Cross-section of an ocean basin, note significant vertical exaggeration
Major ocean surface currents
Areas of upwelling in red
Pacific decadal anomalies – April 2008
Shortfin mako shark make long seasonal migrations. They appear to follow temperature gradients, and have been recorded travelling more than 4,500 km in one year. [ 80 ]