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The sea trees of Trondheim

Appeared in DIVER - April 1998.
Part one

Text by Rudolf Svensen
Pictures by Erling Svensen and Rudolf Svensen.

Unique conditions in the chilly waters of Norway's Trondheimsfjord (above) provide a habitat for coral sea bushes and sea trees normally found only at great depths. There's even a coral reef there, albeit no bigger than a Volkswagen van! Rudolf Svensen recalls an autumn expedition not far south of the Arctic Circle.

My 10-litre deco-gas cylinder thumps rhythmically against my hip as I battle the current's efforts to knock me off the wall. Below is the abyss - should I slip and fall I would travel 100m before hitting the bottom.
Ahead I see it, a sea tree, perhaps the first ever to be observed by a diver. Broadside on to the current, it looks like a big fan. It is eating!

Sponge on Lophelia coral reef

Gorgonian fan with Gorgon's head

On the opposite side of the sea tree Frank Emil has obviously managed to settle on firm ground, because I can see his camera flashing continually.
Sea trees (Paragorgia arborea) normally grow to 1-2m, but can reach as much as 6m. These horn corals usually live between 200 and 1300m deep, but we have found the exception, in this shallow habitat in Norway's Trondheimsfjord. Here unique marine conditions allow such corals to grow within reach of divers.
Sea trees are filter-eaters, picking out their food from the water that passes through their polyps. The one we have found has all its polyps out, and thered, flower-like eating organs make a beautiful spectacle.
We would have liked to track down some of the numerous marine creatures that live on such corals, but we have been at 55m and all too soon have to return to the surface.
On our way up we pass orange sea bushes (Paramuricea placomus), their flat sides set against the current. These are smaller relatives of the sea tree, corals that form colonies up to 1m in height as far up as 25m from the surface.
Colonies are shaped like fans and are known to have many lodgers, particularly amphipods and isopods. Six thousand five hundred such small crustaceans have been found on a single sea bush just 35cm high.

Gorgonian fan with a isopod

Gorgonian fan with a isopod

We stop at 30m and take pictures of the basket stars sitting on top of a big sea bush. At 9m we exchange breathing gas and begin a 30-minute decompression. A week of diving in Skarnsundet at the head of Trondheimsfjord is about to end.
Eight days earlier we had driven 1200km to arrive, loaded down with equipment, in the town of Steinkjer, We had spent more than six months planning this autumn expedition, and besides our photographic and diving equipment we had a compressor, GPS, portable echo-sounder, laptop computer with software for dive planning and tide tables, and 15,000 litres of nitrox 80 for decompression.
We planned to use a computer program to set up deco schedules and plan bottom times. Alternative dives would be simulated on the computer before we hit the water, and each diver brought his own collection of diving profiles.
As we had six days of diving (one day off for every three days of diving), we would allow twice as much deco time as necessary on every dive. Even on shallow night dives down to 20m the plan was to breathe nitrox 80 before ascending.
We hoped not simply to take pictures of sea bushes - which in this part of the world can be found all over the place - not only of the sea tree, but of the ratfish and of no less a phenomenon than a coral reef in Norwegian waters.


Read part two



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