Under close scrutiny the seafloor around New Zealand is as broken and mountainous as the bush-covered lands above. Plunging to depths of 10,000 metres in the extreme, but averaging around 3,000 metres in general, the topography of the seafloor surrounding New Zealand reflects the turbulent origin of this tiny country country, and its continuing evolution.

Most of us have access to a boat to doddle out for a day’s fishing or diving, or cruise to a sheltered bay for lunch. But rarely do we leave the relative shallows of the continental shelf and plunder out over water that drops incredible depths into the blackness below. But recently a strong interest has been taken in these deep unsurveyed waters as the importance of Seamounts has been uncovered.

The first inklings of something unknown came about when deep sea trawlers began to bring in fantastic Species of previously unseen specimens that had been caught as by-catch when bottom-trawling for orange roughy and other commercial deep sea fish. Newspapers bristled with articles and photos of the giant octopus caught as by-catch in a research trawl net back in 2001, because until then, only commercial fishermen had tested the fertile waters of the 800 seamounts found off New Zealand shores.

Seamounts are technically described in the McGraw Hill Dictionary of Geology and Mineralogy as ‘an elevation of the seafloor that is either flat-topped or peaked, rising to about 900 – 300 metres or more [from the seafloor] with the summit approximately 300 – 1800 metres below sea level’. Basically a high mountain deep under the sea, they are definitely not something that can be explored on snorkel or with a scuba tank strapped to your back. Volcanic in origin, many are still active, either volcanically or as hot water vents.

Following the tantalising discoveries brought up by commercial fishing trawlers, a number of exploratory research trips were taken by groups such as the New Zealand-based National Institute of Water and Atmospheric Research (NIWA) and the Commonwealth Scientific and Industrial Research Organisation (CSIRO) from Australia, to ascertain the extent and importance of the biodiversity found in these mountainous marine ecosystems. Not only are they home to well-known deepwater fish such as orange roughy, alfonsino and the black and smooth oreo, but previously unknown species such as a deep-water mussel that grows up to 30 centimetres long, strange rock sponges, a rare stalked barnacle, and various unknown tropical crabs, corals, starfish and worms have been dredged up. Seamounts are also known to be home to at least 29 different shark species at various times in their life cycles.

Following this initial research, the New Zealand Ministry of Fisheries announced they were closing 17 key seamounts to bottom trawling ‘to safeguard the marine life and habitats they support’. These particular seamounts had been identified by NIWA for the Ministry as being either representative of others in the area, in their geology and biology, or were considered to be unique in New Zealand’s waters. Most had never been trawled, but several had been, providing a control for the comparisons between a modified and an unmodified Environment. They are still heavily protected from dredging and over-trawling.

Although the 17 closed seamounts are well spread around New Zealand’s offshore waters, several are in an area along the Chatham Rise called the Graveyard Complex, about 275 kilometres off the lower North Island east coast. This region is of great interest as it contains at least 21 seamounts over an area of just 140 square kilometres. With suitably morbid names such as Mt Gloom, Crypt, Ghoul and Mummy, they are easily accessible by research vessel from the coast and have been the subject of a number of serious research trips over the years.

What researchers found is that seamounts are an incredibly rich underwater environment where a wide variety of species are found. In particular, isolated seamounts, or those that are volcanically venting, attract a higher number of rare or unusual species than those that are in groups or arranged along ridges. Researchers have also found that few species live in both the rocky reef seamount environment and the neighbouring soft sandy seafloor from where the seamount rises, but tend to live in either one or the other.

Many of the species found are large and slow-growing. Orange roughy can easily live for 150 years and some of the corals are dated between 300 and 500 years old. Deep-water trawling over these areas wreaks a double havoc in that it not only destroys a unique environment, but the recovery time of that environment must be measured in centuries, not years.

Research by the various agencies is building a solid inventory of seamount environments – in physical, biological and hydrographic terms. As new species and features are found they are being added to this knowledge base. The research also aims to determine the interaction between seamount habitats and fisheries to evaluate how commercial fishing will be accommodated within that.

There is a fine balance between these rugged, volcanic undersea mountains and the fragile life that goes on around them. But like other marine areas of significant value they must be evaluated and suitably protected before it is too late to appreciate their real value in this world. Hopefully the high-powered spotlights on the underwater research vessels will shine some more light on this mysterious, hidden world.

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(C)  Sue Farley 2018

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