Large submarine volcanic mountain rising at least 3,000 ft (1,000 m) above the surrounding seafloor; smaller submarine volcanoes are called sea knolls, and flat-topped seamounts are called guyots. Seamounts are abundant and occur in all major ocean basins. By the late 1970s more than 10,000 seamounts had been reported in the Pacific Ocean basin alone. Virtually every oceanographic expedition discovers new seamounts, and it is estimated that about 20,000 exist worldwide.
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A seamount is a mountain rising from the ocean seafloor that does not reach to the water's surface (sea level), and thus is not an island. These are typically formed from extinct volcanoes, that rise abruptly and are usually found rising from a seafloor of 1,000 - 4,000 meters depth. They are defined by oceanographers as independent features that rise to at least 1,000 meters above the seafloor. The peaks are often found hundreds to thousands of meters below the surface, and are therefore considered to be within the deep sea. An estimated 30,000 seamounts occur across the globe, with only a few having been studied. However, some seamounts are also unusual. For example, while the summits of seamounts are normally hundreds of meters below sea level, the Bowie Seamount rises from a depth of about 3,000 meters to within 24 meters of the sea surface.
Isolated seamounts and those without clear volcanic origins appear to be less common. In recent years, geologists have confirmed that a number of seamounts are active undersea volcanoes: Lo‘ihi in the Hawaiian Islands and Vailulu‘u in the Manu‘a Group (Samoa) are examples.
Seamounts often project upwards into shallower zones more hospitable to sea life, providing habitats for marine species that are not found on or around the surrounding deeper ocean bottom. Because seamounts are isolated from each other they form "undersea islands" creating the same biogeographical interest. As they are formed from volcanic rock, the substrate is much harder than the surrounding sedimentary deep sea floor. This causes a different type of fauna to exist than on the seafloor, and leads to a higher degree of endemism.
In addition to simply providing physical presence in this zone, the seamount itself may deflect deep currents and create upwelling. This process can bring nutrients into the photosynthetic zone, producing an area of activity in an otherwise desert-like open ocean. Seamounts may thus be vital stopping points for some migratory animals such as whales. Some recent research indicates whales may use such features as navigational aids throughout their migration. Due to the larger populations of fish in these areas overexpoitation by the fishing industry has caused some seamount fauna populations to decrease considerably.
The primary productivity of the epipelagic waters above the submerged peak can often be enhanced by the hydrographic conditions of the seamount. This increases the densities of the zooplankton and leads to the high concentrations of fish in these areas. Another theory for this is that the fish are sustained on the diurnal migration of zooplankton being interrupted by the presence of the seamount, and causing the zooplankton to stay in the area. It is also possible that the high densities of fishes has more to do with the fish life histories and interaction with the benthic fauna of the seamount. The benthic fauna of the seamounts is dominated by suspension feeders, including sponges and true corals. For some seamounts that peaks at 200-300 metres below the surface benthic macroalgae is common. The sedimentary infauna is dominated by polychaete worms.
For a long time it has been surmised that many pelagic animals visit seamounts to gather food, but proof this of this aggregating effect has been lacking. The first demonstration of this conjecture has recently been published
In 2005 a Census of Marine Life project "CenSeam" (a global census of marine life on seamounts) was formed. CenSeam is intended to provide the framework needed to prioritise, integrate, expand and facilitate seamount research efforts in order to significantly reduce the unknown and build towards a global understanding of seamount ecosystems, and the roles they have in the biogeography, biodiversity, productivity and evolution of marine organisms. CenSeam researchers have identified two core research themes – (1): What factors drive community composition and diversity on seamounts, including any differences between seamounts and other habitat types? And (2): What are the impacts of human activities on seamount community structure and function?
The main cause for the recent interest in seamounts is the discovery that they maintain large stocks of commercially important fishes and invertebrates. This began during the 1960s when Russia, Australia and New Zealand started to look for new stocks of fish and began to trawl the seamounts. The majority of the invertebrates brought up are corals, and are mainly used for the jewelry trade. The two major fish species were the orange roughy (Hoplostethus atlanticus) and pelagic armourhead (Pseudopentaceros wheeleri), which were quickly overexploited due to lack of knowledge of the longevity of the fish, late maturity, low fecundity, small geographic range and recruitment to the fishery. As well as the fishes being overexploited the benthic communities were destroyed by the trawling gear.
One of the core research themes of CenSeam (a Census of Marine Life project - see link below) is the impact of human activities (e.g. fishing) on seamount community structure and function. CenSeam's Data Analysis Working Group recently assessed the vulnerability of deep-sea corals to fishing on seamounts beyond areas of national jurisdiction (Clark et al. 2006 - see external links).