Seamount Research in the Western and Central Tropical Pacific
V. Darnitsky
In the 1970s and 1980s, the Pacific Research Fisheries Center (TINRO) carried out
research on the productivity of ichthyofauna during surveys of a number of underwater
mountains in the tropical and subtropical Pacific. The surveys were incidental to other research,
and conducted by the expedition on the way to another area. The areas surveyed included the
Marcus-Nekker Ridge and Magellan Seamounts in the West and Central Pacific, to the southeast
of the island of Taiwan. This area is largely independent of large-scale patterns of circulation,
and only peripherally connected to the continental slope (for example, in the Gulf of Alaska and
Cobb Seamount off British Columbia).(picture…).
The Marcus-Nekker Ridge is positioned at 18–26°N and 146-–165°W, between the
subtropical convergence area near 25°N and the northern periphery of the North Passat Current
in the southwestern part of the North Subtropical Anticyclonic Circulation. The ridge includes
more than 350 underwater mountains, which as a rule are guyots, with flat tops overlain by less
than 1500–2000 m of water. Reconnaissance of 22 seamounts in this group was carried
out.Tropical and strongly stratified waters, among the least productive in the ocean, are
characteristic of the area (picture…). Biogenic salts were almost absent from the 0–250 m layer.
At 300 m depth, the concentrations of phosphates and silicic acid were 0.01 mkg-atm /l and 1.0
mkg-atm/l, respectively. In February 1985, low productivity was found in the area. The average
biomass of zooplankton was 60–100 mg/m3 at 0–100 m, 30–50 mg/m3 at 100–200 m, 25–40
mg/m3 at 200–300 m, and 18–35 mg/m3 at 300–500 m. At depths below 500 m, zooplankton
biomass averaged less than 15 mg/m3. On some seamounts the maximum concentration of
plankton was found at 50–100 m depth.
The 1984 survey by the scientific research vessel Khronometr showed that widely spread tropical
species of plankton prevailed, including Eucheta marina, Eucheta wolfendeni, Candacia
aetiopica, Undinula darwini, Stilocherion afime, Sagitta enflata, Neocalanus gracilis, and
Euphausia sp. The trawl catches of the RV Khronometr included the following fishes: sharks
(Squalidae), Gonostomatidae, Gempylidae, Mictophidae, hatchetfishes (Sternophychidae), eels
(Synaphobranchidae and Serrivomeridae), Nomeidae, Brotulidae, Chauliodontidae, and
grenadiers (Macrouridae).
A 1985 survey by the RV Odyssey found glass sponges, thorny corals, and sea lilies on
the seamount tops. In some areas sea lily density reached 2–3 plants per cubic meter, but on
average, the density of benthic species was significantly lower.
In April and May 1984, the RV Khronometr surveyed the Magellan chain of seamounts,
the productive character and ichthyofauna of which radically differ from those of the Hawaiian
and Emperor Ridges in the northern hemisphere, Lord Howe and Norfolk Seamounts in the
southern hemisphere, and other ridges of the temperate latitudes.
The Magellan Seamounts are a vast underwater mountain system in the central part of the
eastern Mariana Trench. The mountains are massive and isolated from one another, and take the
form of guyots with vast flat tops, 15 of which are at depths of less than 1500 m. The mountains
in the western part of the area have tops at 5000 m depth. The surface of the plateaus is fine
sand; parental rock outcrops are very rare.
During trawling, 51 species were caught, but it was impossible to identify some of them. It was
supposed that some of them are endemic (Kodolov 1985). The most characteristic ichthyofauna
were 7 species of smooth-heads (Alepocephalidae), 7 species of one species of Brotulidae, 2
Sinaphobrancidae, one type of duckbill eel (Nemichthidae) and Aldrovavidia sp. Bathypelagic
fishes and cephalopods were very rare in this area, compared to the other mountains. Typically,
the trawl net located not more than 5 species of interzone macroplankton (Darnitsky and
Kodolov 1997).
The benthos, however, was well developed; in all catches, 3 types of shrimp, 2 types of
lobster, sea urchins, and sea stars were found. Deepwater jellyfishes constituted up to one-third
of every trawl catch.
Because fish species were represented in catches by a range of sizes, it appeared that they
may spend their entire life cycle on the same mountain. Taking into consideration the weak
seasonal character of the
, it may be supposed that individual Magellan Seamounts harbor
isolated biocenoses, because of organic sediments and benthos.
The lower, slowly migrating ЗРС-? (Fish aggregations zone - ???) was located in the
400–700 m layer. Hydroacoustic apparatus showed the presence of a certain exchange between
this layer and ЗРС. ЗРС was impossible to catch. The ichthyo-planctonic net returned from
the 500 m depth with several representatives of small invertebrates (Copepodae and Sagittae).
The descent of ЗРС to underwater mountains was not traced. Bycatch of jellyfishes is
presumably in the ЗРС.
Eddy systems in the vicinity of mountains help to enrich nearby waters with biogenic
elements, creating local semi-closed biocenoses. Exchange of marine organisms between
adjacent mountains occurs as a result of the pulsing of weak antidromous currents in the tropical
North Pacific (Nelezin 1994). When larger circulation patterns prevail, the local ecosystems are
restricted to local resources. Therefore, the productivity of the Magellan Seamount ecosystems
cannot reach the levels of productivity typical of shelf ecosystems, or of seamount ecosystems in
temperate waters located in the way of strong currents such as the Kurosio, North Pacific, and
Antarctic Circumpolar Currents. Biomass in high-productivity seamount areas sometimes can
reach commercially important levels.
Two oceanographic surveys were made in the area of the Marcus-Nekker Ridge, by the
RV Chronometr in 1984 and the RV Academic Vinogradov in 1988; also, a magnetometry
survey was conducted by the RV Vulcanolog (Belyaev et al. 1993).
An oceanographic survey over the Lamont guyot at 21°20'N, 159°30'E in the MarcusNekker Ridge showed a distinct eddy structure in waters over the top (1250 m depth), localized
in the upper barocline layer (1000 m) over the main pycnocline layer (picture …). The whirl
field consisted of four eddies of quasi-chess structure, oriented along the guyot slope perimeter,
but there was no contact with the top. Apparently, the eddies were influenced by the pycnocline
at the mountain (picture…). The peculiarity of the topographic eddies was their quasi-stationary
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position in space. New eddies periodically separated themselves from the eddy system, which as
a rule consisted of several eddies. Thus, the system looked like an oscillator in the general
circulation system of the World Ocean, in which intensity, size, and other parameters change,
but whirling in current fields is preserved in time and space, depending on the position of
underwater mountains (Darnitsky and Staritsyn 1978, Darnitsky 1991). (picture…).
It is worth noting also that underwater mountains greatly influence the ocean’s magnetic field.
The magnetic anomaly over one of the mountains (23°45'N, 148°40'E) of the Marcus Wake
Ridge (top depth, 1090–1100 m) measured by the RV Vulkanolog was 960 nanotesla (nT). The
central magnetic nucleus was the main contributor to the anomaly, while the lower parts of the
slopes (4–4.5 km depth) turned to be much less magnetized. Presumably, strong magnetic fields
near underwater mountains, conditioned by the mountains’ inner magnetic nucleus, interact with
weaker fields, conditioned by the whirl fields of currents in the vicinity.
Overall, analysis of the samples taken showed that productivity in the vicinity of
underwater mountains depends on rotating or whirling currents, including Tailor eddies, over the
mountain tops, where epi- and meso-pelagic plankton concentrate. The majority of larval species
there spend several years in pelagic stages of development. Both large and small concentrations
of fish are associated with the currents. Benthic fauna are very poorly represented on the
seamount tops.
These fragmentary observations suggest that overall biological productivity falls as one
travels from northern temperate waters to the tropical latitudes. In the area of the underwater
mountains studied, total biomass was concentrated on the mountain tops, due to eddies and other
hydrophysical processes. These seamount hydrophysical processes are of lower intensity than
those in the temperate latitudes of the ocean (Darnitsky and Boldyrev 1977).
Belyaev et al. 1993
Darnitsky 1991
Darnitsky and Boldyrev 1977
Darnitsky and Kodolov 1997
Darnitsky and Staritsyn 1978
Kodolov 1985
Kulikov, M. Yu., and V. B. Darnitsky 1992. Dynamics of halassobathyal fish abundance in the Northwest
Pacific and its possible causes. In: Oceanological Basis of Biological Productivity in the
Northwest Pacific . TINRO, Vladivostok, p. 4–19 (in Russian).
Nelezin 1994
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