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Week 7: Seamounts in the southern seas

Seamounts are mountains that are found under the sea. They are generally volcanic and can be found along the mid-ocean ridges, at tectonic plate boundaries or over hotspots in the Earth’s crust. There are thought to be more than 100,000 seamounts in the world’s oceans.

Seamounts can be very deep – up to several kilometres under the sea surface. In the last 20–30 years, technology to study seamounts has developed dramatically. Researchers are now using manned submersibles and remotely operated vehicles. On this survey, we used special video cameras to see what lives there and a specially designed sled that can be pulled over the surface of the seamount to collect animals.

Marine biologists often find new species of animals on these mountains under the sea. Water currents can keep the seamounts clear of sediments, leaving bare rock for animals like corals and sponges to grow on, forming reefs like the ones you see in shallow waters such as the Great Barrier Reef in Australia. These reefs can be thousands of years old and can support lots of other animals including sea stars, brittle stars and fish.

The animals that live on seamounts can be very different to those on the surrounding seafloor. Some of the animals found on seamounts can be very rare and, in certain cases, only found on that particular seamount. Scientists have even found animals on seamounts that we thought became extinct millions of years ago.

IPY blogs week 7

The seamount environment
 Seamounts are underwater mountains that can rise several thousand metres from the deep seafloor. Usually formed by volcanic activity, they can consist of hard rock and lava flows. There are believed to be more than 100,000 large seamounts in the world’s oceans. Seamounts enable animals who could not survive the 4,000–5,000 metre depths of most oceans to live on them. They have steep slopes and hard surfaces that animals such as large corals and sponges can attach themselves to, in comparison to the soft sediment that covers much of the seafloor. Seamounts can accelerate currents and create eddies that retain eggs, larvae and food over the seamount rather than dispersing into the wider ocean environment, thus providing for diverse and productive ecosystems.

Written by Malcolm Clark

Seamount mapping
 NIWA scientists map the seamounts in order to decide where it is best to take samples. For this, they use a multibeam echo-sounder, mounted on the hull of the research ship Tangaroa. A multibeam echo-sounder uses sound waves to measure the depth of the water. A transceiver sends out a narrow beam of sound waves that are reflected off the seafloor, back to a receiver on the ship. The signal is divided into 135 separate beams, 5 kilometres wide on the seafloor. The multibeam can calculate the depth of the water from the time it takes the sound to reach the seafloor and back to the ship. As the ship maps the seabed, multiple parallel stripes gradually build up a picture of how the seafloor looks.

Written by Arne Pallentin

Get video: Mapping Seamounts

The King of the seamounts
 King crabs are the only crab-like crustaceans known to occur as far south as the Antarctic Ocean. They have adapted to the cold temperatures that limit muscle function in other crabs (making them too sluggish to survive). Three species are currently known from the Ross Sea. Not much is known about these species, including their abundance. During the 1 hour Deep Towed Imaging System (DTIS) video camera transect over the rough terrain surrounding Scott Island, we observed up to 22 large king crabs going about their business, and we collected the 2 endemic Ross Sea species. The combination of direct seafloor observations and the addition of valuable specimens to our collection will significantly add to our knowledge of an important aspect of this seamount megafauna.

Written by Kareen Schnabel

Seamount biodiversity
 During the past week, we have been finding new and exciting invertebrates while sampling on the Scott seamount group. Seamounts provide a unique habitat for different groups of sea creatures. The volcanic rocks of the seamount peaks make a suitable home for brightly coloured branching gorgonian corals, zoanthids, hydroids, soft pink corals, white hard corals and towering black corals that attach onto hard surfaces. On the gravelly slopes of the seamount, we discovered large fields of sea pens and anemones, anchored down against the strong currents. We found many different species of starfish and brittle stars at every site we sampled on the seamount – some clambering over rocks, some sitting on top of softer sediments and some of the brittle stars clinging onto corals.

Written by Sadie Mills

Get video: Biodiversity on seamounts

Seamount fisheries and conservation
 Seamounts can be sites of high plankton abundance and attract aggregations of fish, seabirds and marine mammals that feed on the abundant prey. These aggregations form the basis of commercial fisheries around the world. New Zealand fish like orange roughy, cardinalfish and alfonsino are often fished from seamounts. In Antarctic waters, we find icefish and toothfish. To protect fish communities, there is a limit of catch that can be taken and the type of fishing gear used (for example, bottom trawling is banned in some areas to prevent any damage to the benthos on the seafloor). In New Zealand, there are 19 seamounts closed to fishing, and strict regulations and monitoring programmes aim to reduce the effects of fishing on the environment to ensure the fragile seamount ecosystem is conserved.

Written by Malcolm Clark

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