Seamounts and Continental Shelves - The Ocean's Unprotected Treasure Vaults

Continental shelves are the gently sloping areas of the ocean floor, contiguous to the continent, that extend from the coastline to the shelf-break. The shelf break, which is located around 150–200 meters depth, is the area of the continental margin where there is an abrupt change between the shelf and the steeper continental slope.

Figure 4. The continental shelves and under-water mountain ranges, so called seamounts (light blue shaded areas), are of immense importance to fisheries. Indeed, over half of the World’s marine landings are associated with ca 7.5% of the oceans, concentrated on the continental shelves.

Primary production in the oceans, i.e. the production of organic compounds from dissolved carbon dioxide and nutrients through photosynthesis, is often associated with upwellings (Botsford et al., 2006). Upwelling occurs when winds blowing across the ocean surface push water away from an area and subsurface water rises up from beneath the surface to replace the diverging surface water. These subsurface waters are typically colder, rich in nutrients, and biologically productive. The relation between primary production and coastal upwelling, caused by the divergence of coastal water by land or along-shore blowing winds, is clearly shown in ocean primary production maps. Therefore, good fishing grounds typically are found where upwelling is common. For example, the ecosystems supporting the rich fishing grounds along the west coasts of South America and Africa are maintained by year-round coastal upwelling. However, these systems are affected by changing oceanographic conditions and how they – and the dependent fisheries – will respond to sea temperature change as a consequence of climate change is highly uncertain. These upwelling fishing grounds, especially in South America provide the raw materials for feeds used in intensive animal production and so any decreases in production will have effects on the price of farmed fish, chicken and port.
 
The far largest share of all life in the oceans is in direct contact with or dwells just above the sea floor. Continental shelves and seamounts host – in addition to petroleum and mineral reserves – by far the largest share of the World’s most productive fishing grounds (Ingole and Koslow, 2005; Roberts et al., 2006; Garcia et al., 2007; Mossop, 2007). Technological advances have made continental shelves and shallow seamounts easily accessible to the World’s fishing fleet and to coastal communities all across the planet. However, they are also critically placed in relation to threats from (land-based) pollution, sea bed and habitat destruction from dredging and trawling, and climate change. With traditional fishing grounds depleted and/or heavily regulated, fisheries are increasingly targeting productive areas and new stocks in deeper waters further offshore, including on and around seamounts. 

Seamounts are common under-water features, numbering perhaps as many as 100,000, that rise 1000 m or more from the seabed without breaking the ocean’s surface (Koslow et al., 2001; Johnston and Santillo, 2004). The rugged and varied topography of the seamounts, and their interaction with nutrient-rich currents, creates ideal conditions and numerous niches for marine life. Compared to the surrounding deep-sea plains and plateaus, they are some of the primary biodiversity hotspots in the oceans.

Seamounts can be home to cold-water corals, sponge beds and even hydrothermal vents communities. They provide shelter, feeding, spawning and nursery grounds for thousands of species, including commercial fish and migratory species, such as whales (Roberts and Hirschfield, 2004; Roberts et al., 2006; UNEP, 2006). Separated from each other, seamounts act like marine oases, often with distinct species and communities. Some, like the Coral Sea and Tasman seamounts, have endemism rates of 29–34%.

Figure 5. Primary production in the World’s oceans provide a quite similar pattern to the World’s fisheries (see Figure 6), concentrated along the continental shelves.

These unique features make seamounts a lucrative target for fisheries in search of new stocks of deep-water fish and shellfish, including crabs, cod, shrimp, snappers, sharks, Pacific cod, orange roughy, jacks, Patagonian toothfish, porgies, groupers, rockfish, Atka mackerel and sablefish. Our knowledge of seamounts and their fauna is still very limited, with only a tiny fraction of them sampled and virtually no data available for seamounts in large areas of the world such as the Indian Ocean (Ingole and Koslow, 2005). Often, fishermen arrive before the scientists. For a short time period, sometimes less than 3 years, the catches around seamounts can be plentiful. However, without proper control and monitoring, especially in areas beyond national jurisdiction, stocks are exploited unsustainably and collapse rapidly. The reason for this ‘boom and bust’ are the characteristics of many deep-water organisms: unlike their counterparts in traditional, shallow-water fishing grounds, the deep-sea fish targeted around seamounts are long-lived, slow to mature and have only a few offspring (Glover and Smith, 2003; Johnston and Santillo, 2004). This makes them highly vulnerable to over-fishing by industrial fishing practices (Cheung et al., 2007). In addition, the benthic communities, which support these fish stocks and their recovery, are seriously damaged or completely destroyed by the impact of heavy bottom trawling and other fishing gear (Johnston and Santillo, 2004; Morato et al., 2006b). Once depleted and devastated, often for decades to centuries, fishermen move on to the next seamount to start the next cycle. However, with many known seamounts already (over)exploited, recovery of fish stocks on seamounts varies with each species. Stocks of orange rough on the Chatham Rise in New Zealand, for example, show possible improvements after 5 years, whereas the grenadier stocks in the Northwest Atlantic show no signs after a number of years of reduced quotas.

Figure 6:The World’s most productive fishing grounds are confined to major hotspots, less than 10% of the World oceans. The maps shows annual catch (tonnes per km2) for the World’s oceans. Notice the strong geographic concurrence of continental shelves, upwelling and primary productivity (see Figures 4 and 5) and the amount of fish caught by fisheries.

The depletion of seamount populations indicates that the current focus and levels of fishing on seamounts is not sustainable. More depletion, extirpations, and even species extinctions may follow if fishing on seamounts is not reduced (Morato et al., 2006). Very common however, rather than fishing until near extinction, is that the fishing vessels will move on to the next location as soon as the first is exhausted. With the large capacity of the fleet, the result is that more and more locations become impacted and damaged.

When primary production and bathymetric maps (showing the distribution of continental shelves) are compared to the intensity of fisheries (catch), a clear pattern erupts, reflecting the productivity and accessibility of these ocean hotspots.

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