Seamounts – hotspots of marine life

What are seamounts?
Seamounts are undersea mountains (usually of volcanic origin) rising from the seafloor and peaking below sea level. Underwater mountains of heights above 1000 m are considered to be seamounts, those between 500-1000 m as knolls, and those below 500 m as hills. A seamount tall enough to break the sea surface is called an oceanic island, e.g., the islands of Hawaii, the Azores and Bermuda were all underwater seamounts at some point in the past.

Though most people may be unaware of it, underwater seamounts are fairly common. Estimates vary, but recent studies suggest that there may be about 30,000 seamounts of heights over 1000 m in the Pacific Ocean, about 1,000 in the Atlantic Ocean and an indeterminate number in the Indian Ocean.

Why are seamounts so important?
Seamounts are hotspots of marine life in the vast realms of the open ocean. As they stand proud of the surrounding seabed they tend to concentrate water currents and they can have their own localised tides, eddies and upwellings (where cold, nutrient-rich, deepwater moves up along the steep sides of the seamount).

Due to the strong localised currents and upwellings, the plankton biomass is often high over seamounts and this combined with the constant influx of prey organisms, means that they can attract large numbers of fish.

Marine mammals, sharks, tuna, and cephalopods all congregate over seamounts to feed on the rich booty and even seabirds have been shown to be more abundant in the vicinity of shallow seamounts.

Down on the seabed, volcanic rocks provide lots of space for suspension feeders such as corals that capitalise on all the food that is swept around them in the strong currents. This is in sharp contrast to the typical deep-sea habitat where deposit-feeding animals rely on food that they eat on or in the sediment.

Marine mammals, sharks, tuna and cephalopods (e.g., squid) and even seabirds all congregate over seamounts to feed on the rich booty of marine life. Photo by Brandon Cole, Nature Picture Library

Though the diversity and exceptionally localized distribution of species living in these unique communities are acknowledged, their biology and life history remain poorly studied, except for indications that some of these species may be extremely long-lived; some are thought to live for more than 100 years.

Due to the large stocks of top predators, species diversity and uniqueness compared to the surrounding ocean, seamounts are often called "oceanic oases".

Threats they face – overfishing and why are seamounts so vulnerable?
Global fish catches have declined steadily since the 1980s, and it is now widely acknowledged that the world’s fisheries resources are in serious danger of depletion, undoubtedly due to poor management practices and increased fishing pressure. Unsustainable fishing practices along with an excessive level of investment in fishing capacity have resulted in serious stock depletion on most continental shelves, thus creating new pressures on alternative fishing grounds. In particular, seamounts are among those "newly" targeted ecosystems that, since the second half of the 20th century, have been intensively fished.

Deepwater fisheries in general and seamounts fisheries in particular, exhibit a boom and bust sequence, crashing within about ten years of their initial development. This was the case with the orange roughy (Hoplostethus atlanticus) fisheries off New Zealand, Australia, and even in the North Atlantic, the pelagic armourhead (Pseudopentaceros wheeleri) fisheries over seamounts in international waters off Hawaii, and the blue ling (Molva dipterygia) fisheries in the North Atlantic. As seamounts are rapidly depleted, the continued existence of the fisheries on them depends upon the discovery of unexploited seamounts with large fish aggregations.

Orange roughy are one of the stocks that are frequently overfished at seamounts. Scientists estimate that it could take decades before these local stocks are able to recover. Photo by John D. M. Gordon www.sams.co.uk

The species targeted by fisheries at seamounts have a very low overall abundance, but they aggregate at seamounts as part of their life cycle strategy, e.g. for spawning. They are often long-lived, slow growing, late maturing (at about 30 years), and have low reproductive potential.

Therefore if they are fished out, it is thought that it could be decades before these localised stocks recover; particularly as they are thought to have limited exchange with other seamounts. This makes these fish communities very vulnerable to overfishing and the problem is even more pronounced in seamounts located in international waters where effective management strategies and agreements are frequently absent.

Depletion of fish stocks is not the only concern. Extensive trawling activities on seamounts are damaging benthic (bottom living) communities, particularly dominant communities of corals and other suspension feeders. The impact of trawling on complex seamount reefs appears to be dramatic, with the result of the complete removal of coral substrate and associated community from the most heavily fished seamounts. Such massive removal of natural and structural components of the ecosystem has negative consequences on seamount biodiversity.

Evidence of such negative impacts has been recorded by declining fish stocks associated with seamount fishing in Australia. There are also concerns that damage by deepwater trawling to coldwater coral reefs in the NE Atlantic, could extend to seamount areas. Unregulated small-scale fishing can also disturb these sensitive environments, e.g., the decline in important fish stocks exploited by small-scale fisheries in the Azores.

Outlook for the future
There is a rising concern about the threats to seamount ecosystems in the Exclusive Economic Zones of coastal states and in the high seas and consequently, Canada, Australia and New Zealand have begun to take the first steps towards protecting seamounts. In the Atlantic, no such protective measures have been established but the OSPAR Commission has placed seamounts on their initial list of threatened and declining species and habitats that require conservation action. Mechanisms to provide this protection will now be considered. The developing OSPAR Marine Protected Areas programme should provide one possible mechanism.

In addition, seamounts dominated by hard substrata in the waters of the European Community may qualify for site protection under the European Habitats Directive (1992, Natura 2000 code 1170 "reefs" in the Interpretation Manual of European Union Habitats EUR 15/2). Portugal (Azores Region) may nominate one of the seamounts in their EEZ as a candidate for a Marine Protected Area (MPA) under the OSPAR convention.

The new OASIS Project
Today, little is known about seamount ecosystems in the NE Atlantic, or the impact of human activities upon these unique oceanic ecosystems, mainly due to lack of funding for research into these systems. A possible sign of change is that the European Commission has agreed to fund the first European seamount study integrating physical, biogeochemical and biological research – “the OASIS project” (OceAnic Seamounts: an Integrated Study).

Until more is known about these fragile ecosystems and the long-term impacts of fishing and other human activities, the precautionary principle should be applied to seamounts to ensure their necessary protection and management. In some fisheries, this may require changes in fishing practice, such as switching from trawling to long-lining to minimise disturbance to seabed habitats and associated fauna. Other fisheries may simply be unsustainable and therefore unacceptable practice on seamounts.

Author
Telmo Morato
Fisheries Center
University of British Columbia
2204
Main Mall, Vancouver
British Columbia
V6T 1Z4
Canada
E-mail: t.morato@fisheries.ubc.ca

Pull quotes:
“…seamounts are often called “oceanic oases”.

For more information on the ICES advice on deep sea fisheries, please consult:
ICES (2001) Answer to Special Request on the Management of Deep-water Species. Cooperative Research Report No. 246.
www.ices.dk/committe/acfm/comwork/report/2001/oct/Deep-sea-request.pdf

For description of the OASIS project, please see:
www.biologie.uni-hamburg.de/ihf/de/projects/index.htm

Other useful websites
WWF North-East Atlantic Programme
www.ngo.grida.no/wwfneap/overview/overfset.htm

EU Habitats Directive
www.europa.eu.int/comm/environment/nature/habdir.htm

Tasmanian Seamounts Marine Reserve
www.ea.gov.au/coasts/mpa/seamounts/

Map showing the distribution of seamounts in the Northeast Atlantic

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