|
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, which do live on or near the bottom, and reef fish, which are associated with coral reefs. 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 4000 metres. About 98% of the total water volume is below , and 75% is below .〔Moyle and Cech, p. 585〕 Marine pelagic fish can be divided into pelagic coastal fish and oceanic pelagic fish.〔McLintock, A H (ed.) ("Fish, Marine" ). ''Te Ara – The Encyclopaedia of New Zealand''. Updated 18 September 2007.〕 Coastal fish inhabit the relatively shallow and sunlit waters above the continental shelf, while oceanic fish (which may well also swim inshore) inhabit the vast and deep waters beyond the continental shelf.〔Walrond, Carl. ("Oceanic fish" ). ''Encyclopedia of New Zealand''. Updated 21 September 2007.〕 Pelagic fish range in size from small coastal forage fish, such as herrings and sardines, to large apex predator oceanic fishes, such as the Southern bluefin tuna and oceanic sharks.〔 They are usually agile swimmers with streamlined bodies, capable of sustained cruising on long-distance migrations. The Indo-Pacific sailfish, an oceanic pelagic fish, can sprint at over 110 kilometres per hour. Some tuna species cruise across the Pacific Ocean. Many pelagic fish swim in schools weighing hundreds of tonnes. Others are solitary, like the large ocean sunfish weighing over 500 kilograms, which sometimes drift passively with ocean currents, eating jellyfish.〔 ==Epipelagic fish== Epipelagic fish inhabit the epipelagic zone. The epipelagic zone is the water from the surface of the sea down to 200 metres. 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 point where the sunlight has attenuated to 1% of the surface value. This depth depends on how turbid the water is, but in clear water can extend to 200 metres, coinciding with the epipelagic zone. The photic zone has sufficient light for phytoplankton to photosynthese.〔 The epipelagic zone is vast, and is the home for most pelagic fish. The 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 diversity 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.〔Moyle and Cech, p. 571〕 Epipelagic fish can be broadly divided into small forage fish and larger predator fish which 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 predate zooplankton or smaller fish, while others filter feed on plankton. Most epipelagic predator fish and their smaller prey fish are countershaded with silvery colours which reduce visibility by scattering incoming light.〔 The silvering is achieved with reflective fish scales that function as small mirrors. This can give an effect of transparency. At medium depths at sea, light comes from above, so a mirror oriented vertically makes animals such as fish invisible from the side.〔Herring, Peter (2002) ''The Biology of the Deep Ocean'', pp. 192–195, Oxford University Press. ISBN 978-0-19-854956-7.〕 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.〔 Though the number of species is limited, epipelagic fishes are abundant. What they lack in diversity they make up in numbers. Forage fish occur in huge numbers, and large fish that prey on them are often 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 energetically that gives better results. 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.〔Moyle and Cech, p. 572〕 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.〔Blackburn (1965) "Oceanography and the ecology of tunas" ''Oceanography and Marine Biology: An Annual Review'' Volume 3, 299–322〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Pelagic fish」の詳細全文を読む スポンサード リンク
|