Mesopelagic fish are oceanic species living at depths of between two hundred and one thousand metres, called the twilight zone. These depths are beyond the continental shelf and the slope areas surrounding each continent.
As night approaches many mesopelagic fish move upwards through the water, to the shallower depths, in search of food. Some even migrate up to the surface layers of the ocean. Because of this behaviour they have been called nyctoepipelagic. Before sunrise the fish return to the depths where they live during the daytime. However, some mesopelagic fish do not make this daily migration.
Even within a single species, the migration behaviour can vary with the season, the sex of the fish and the stage in their life cycle.
The mesopelagic zone is the habitat of approximately seven hundred species. These are spread world wide, and most of them belong to families with very descriptive names. For example:
- Scaleless dragon fish
- Lantern fish
- Hatchet fish
The distribution of mesopelagic fish
Some species are distributed worldwide, and many are circumpolar, especially in the Southern Hemisphere.
Much research on distribution and natural history of mesopelagic fish was conducted in the 1970s, when FAO (Food and Agriculture Organization) searched for new unexplored commercial resources. The total biomass was estimated at around one billion tonnes with highest abundance in the Indian Ocean (about 10 times the biomass of the world's total fish catch).
Mesopelagic fish are abundant along the continental shelf in the Atlantic, Pacific, and Indian Oceans and in deep fiords, but have lower abundance offshore and in Arctic and sub-Arctic waters. Most populations show several adaptations to a life under low light intensity: sensitive eyes, dark backs, silvery sides, ventral light organs, and reduced metabolic rates for deeper dwelling fish. The ventral light organs are species specific in some families, such as the Myctophidae. The deeper-living species have reduced skeletons, a higher water content in their muscles, lower oxygen consumption, and probably reduced swimming activity compared with species that live at shallower depths
The lantern fish, or to give them their correct name, Mycyophidae, have greater biomass than any other group of living organisms inhabiting the pesopelagic zone. Their larvae are also the greatest biomass of any vertebrate family represented in the plankton of the oceans of the world. It has been estimated that there are between eight and twelve million tonnes of lantern fish in the South-Eastern Atlantic Ocean. This represents 70% of the total biomass of the mesopelagic fish making their habitat within the region. While distribution is worldwide, production appears to be highest in tropical and sub-tropical areas.
The Benguela region is more a transitional region than a subtropical zone. The species usually confined to the cooler water make up more than half the lantern fish fauna of the Benguela System. This shows a strong intrusion of the southern species into the area. The species particularly prominent are those associated with the Subtropical Convergence, and with the Sub-Antarctica region south of this convergence. The catch rates in the Southern Benguela region of the species that are normally subtropical, but that have intruded from the Indian Ocean are low. This is similar to the pattern of decreasing densities as the latitudes increase, the Agulhas Current, off the East Coast. There is a similar situation off Namibia, at the northern end of the Benguela system, where some tropical and warm water species intrude from the Agulhas Current.
Hatchet fish are found World-wide but mostly in the western Pacific in the middle layer of deep sea from 600 - 4500 feet. There are about 45 known species. The combined biomass of these fish in the waters of the Eastern Atlantic is estimated to be approximately 750 000 tonnes. This biomass is represented by eleven species. Some species, when they are dying, glow with a blue-green light.
Lantern fish and hatchet fish are not only true mesopelagic species, they are also bathypelagic, meaning that they can be found below a depth of one thousand metres. They are also pseudo oceanic species. This species occurs over the continental shelf and slope. The distinction between the various groups is arbitrary, for example, certain oceanic species may also have populations over the continental shelf and slope.
In some areas, particularly off the coast of Namibia, two oceanic lantern fish species of the genus Symbolophorus are abundant over the outer edge of the continental shelf and over the upper regions of the slope. During the day, both species are found mainly between four hundred and five hundred metres. However, during the night, when they carry out vertical migration, they also move closer inshore.
The pseudo oceanic species offer the most potential for the development of fisheries. This is because they are found close inshore, and they migrate to a suitable fishing depth. Towards evening they often form dense shoals. The catches are processed into fishmeal and oil, but some larger lantern fish have been caught for human consumption. Because of the high content of wax organic compounds the smaller ones seem to have an adverse effect when eaten by humans.
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