December 8, 2018
Day 16: Malcolm Clark, NIWA, New Zealand
As we saw from earlier blogs, seamounts occur in all oceans of the world from the tropics to the poles, and cover depth ranges from near the surface to the abyss. They provide a wide variety of habitat types for a huge range of animals, and often feature high levels of biodiversity and abundance. This can make them important components of oceanic ecosystems, yet also the target of commercial exploitation.
Seamounts have been known for centuries to be good fishing sites, and have long supported fisheries for pelagic tunas, mackerels, and smaller-scale line fisheries such as black scabbardfish. In the 1970s, however, there was a major change in the nature and scale of fishing on seamounts. Offshore ‘distant water’ trawlers that could range over large ocean distances began extensive trawling on seamounts as fleets discovered large aggregations associated with them. There are now major seamount fisheries for alfonsino, pelagic armourhead, black cardinalfish, orange roughy, roundnose grenadier, oreos and toothfish.
At least two million metric tons of deep-sea species are known to have been trawled from seamounts since the late 1970s, mainly pelagic armourhead in the Pacific Ocean and alfonsino on the Hawaiian and Emperor seamount chains. Orange roughy have been targeted on seamounts worldwide, and with oreos and alfonsino continue to support locally important fisheries in the SW Pacific. Roundnose grenadier are a major seamount fishery in the North Atlantic. Smaller fisheries for alfonsino, mackerel, and cardinalfish have occurred in the Mid-Atlantic, SE Pacific, and off the west coast of North Africa. In the Southern Ocean, fisheries for toothfish, notothenids, and icefish can occur on seamounts.
Orange roughy has been an important fishery for Australia. Aggregations were found off western Tasmania in 1986, but when spawning fish were discovered on St Helen’s Seamount in 1989 along with non-spawning aggregations on seamounts south of Tasmania, catches rapidly increased to almost 60,000 tonnes (at that time the most valuable fishery in Australia) before controls were introduced and catches were reduced. In common with most fisheries worldwide, overfishing occurred. Stocks were estimated in the 2000s to have been reduced to levels below 20% of their unfished size, and all target fishing was stopped. A small fishery (465 t) recently restarted off Tasmania.
Like orange roughy, few large-scale seamount trawl fisheries have proved sustainable. Many deep-sea commercial species have characteristics that generally make them more vulnerable to fishing pressure than shallower shelf species. They can form large and stable aggregations over seamounts for spawning or feeding, which enables very large catches and rapid depletion of stock size. Biological factors such as high longevity, low reproductive output and slow growth rates make their recovery slow. Successful seamount fisheries today are typically low-volume and based on high-value species.
Finding the balance
The rapid rise and subsequent fall of many fisheries on seamounts challenges effective management. Fishing affects more than just the target fish species and other bycatch fish species – bottom trawling operations in particular can have serious physical and biological impacts on seamount habitats and communities. Deep-sea, coral-dominated, communities are highly vulnerable to physical disturbance, with the coral matrix being fragile to impact. As well as direct removal of species and structural habitat, there can be indirect effects caused by sediment resuspension, changes in community structure can occur with selective removal of species, and the interactions between pelagic and benthic components of seamount ecosystems may change.
There has been limited research on deep-sea fishing impacts, but studies on seamounts off Australia and New Zealand have demonstrated differences in the structural complexity of benthic habitats, species numbers and abundance, and overall community composition and structure between fished and unfished/lightly fished seamounts. Sessile fauna, such as deep-sea corals, sponges, echinoids, and other invertebrates we are finding on this survey, are particularly vulnerable to damage because they can be large, fragile and long lived.
Although fishing has historically been the main industry exploiting seamounts, the deep sea is of interest for oil and gas exploration (to sequester carbon dioxide), and for mining manganese nodules, cobalt-rich crusts, and polymetallic sulfides for their minerals which can be used in “green technology” (such as electric cars and wind turbines). Seamounts can have thick deposits of cobalt-rich ferromanganese crusts, especially in the northern Pacific Ocean, and can also be a source of precious metals deposited at sites of hydrothermal activity on volcanically active seamounts. Deep-sea mining is not yet happening, and will be difficult to develop to cope with the often rugged seamount conditions, but exploration is underway in several ocean basins. The effects of mining are likely to be broadly similar to fishing.
Natural changes are also occurring, and long-term ocean acidification will have important consequences for seamount coral species which are affected by the chemical composition of the water at depth. In the short-term though, direct human impacts are of most concern and careful management of seamount resources is required to balance exploitation and conservation of the habitat and minimize conflict in decision-making.
Seamount conservation strategies may be apportioned into activity-specific measures (short-term) and site-specific measures (long-term). Most countries have a variety of legislative options (fisheries management, minerals, transport and navigation, protected areas) that can be used to manage seamounts through tools such as marine protected areas, closed-areas, site-based effort control, licensing, and gear restrictions. Australia and New Zealand were among the first to specifically protect seamounts, with the Tasmanian Seamounts Reserve created in 1997 and Seamount Closure Areas in New Zealand in 2001.
Ecological research such as being carried out on this voyage is crucial to support the design and evaluation of the effectiveness of marine reserves and networks. The data being gathered on the benthic communities on a range of seamounts, and their changes over time, will inform managers (and other stakeholders) about the resilience of fauna to impact and their recovery potential, to evaluate the effectiveness of closed areas as well as improve future reserve design.