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Organic material is actively transported to the depths during daily, seasonal and ontogenetic movements of pelagic animals.

Though the daily vertical migrations of aquatic animals was first recorded over 150 years ago, it is only now that there is enough direct evidence to show that migrants climb to more productive parts of the water column in search of food. Towards nightfall, over all main regions of the ocean, myriads of planktonic and nektonic animals make their ascent, and, after spending some part of the night at higher grazing or hunting grounds, descend before daybreak to their deeper, daytime residence. In the open ocean these migrations are virtually confided to the epipelagic and mesopelagic zones. The deepest records concern daily migrations that seem to reach a depth of 1,700 metres.

Every day, then, vast quantities of planktonic and nektonic forms descend from their feeding grounds, bearing means for their maintenance and growth. Vertical migrations ‘…certainly result in far greater mobilization of biological material than any other animal migration: using an average figure for plankton biomass of 25 gm/m2 to a depth of 100 metres as a reasonable estimate for all oceans and assuming that diel migration adds 10 per cent to this at night as a minimal figure an average daytime residence depth 250 metres deeper, then one arrives at vertical translocation of 25 tons/km2/day over a distance of metres’.

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Longhurst adds that ‘such calculations are not to be taken very seriously’, but his minimal figure is likely to be well exceeded in certain groups. For instance, Baker estimates that about two-thirds by weight of adolescent and adult euphausiid populations are involved in vertical migrations between 50 and 960 metres off the Canary Islands. In taxonomic terms, Omori states that daily vertical migration seems to occur in nearly all epipelagic and mesopelagic prawns. The same is true of mesopelagic lantern-fishes.

Most kinds of deep-sea migrators are cnidarians, crustaceans arrow- worms, cephalopods and fishes. Relevant investigations on the vertical distribution of such migrators reveals that some species make extensive migration of several hundred metres, usually ending in the productive surface waters, while others are partial migrators. Members of a third group stay more or less at the same level throughout the day, but there are instances of reverse migrations. Such categories of behaviour may also occur in individual species.

At certain times some of the adults migrate while others stay still and advanced larval stages and adolescents may be partial migrators. Concerning these and other matters, there is much to be found in the results of the Discovery Send Cruise to an oceanic area off Fuerteventura, Canary Islands.

During vertical migrations animals are bound to experience changes in hydrostatic pressure. Over much of the ocean, particularly in the subtropical and tropical belts they are exposed to considerable changes in temperature and, if they live in an oxygen minimum layer, to wide ranges of oxygen tension. Moreover, in seeking biological changes, they experience others, notably the attacks of predators. But their commuting seems to be timed and regulated so as to keep the light around them at a constant intensity.

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There is diverse evidence that light has a controlling and dominant influence and vertical migration. In the open ocean, daily migrations are virtually confined to the euphotic and twilight zones, and the ascent of the migrators, at least, is closely related to nightfall. Investigations of the daily migrations of deep scattering layers that are recorded by echosounders, and the main sound scatterers are fishes with a swimbladder, particularly lantern-fishes, supports earlier conclusions that migrators move so as to keep the ambient light field at a constant level : they are ‘imprinted’ on a particular isolume.

During the daytime a given DSL will seek a higher level if the sky is darkened by clouds or a solar eclipse. Artificial lights will delay a dusk ascent or drive down a DSL by night. It would seem then, that anil will only be able to shadow an isolume if their eyes are sensitive enough to perceive relatively small changes in light intensity. Mesopelagic fishes have very sensitive eyes, and so have cephalopods and decapod crustaceans. But what of migrators with much simpler eyes? Concerning these and other aspects of light and vertical migrations, Cushing, Clarke, Kampa and Blaxter may be consulted.

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