Fishery resources, the harvest of the oceans, are concentrated in marine areas where the environmental conditions support a high productivity. Such areas are found in coastal waters as well as in deeper waters on the continental shelves and around seamounts (Roberts et al., 2006; Garcia et al., 2007).
The severe decline of stocks in many traditional coastal fishing grounds has given rise to an increase in regulations. This, in turn, has intensified the search for new and less controlled fish stocks and fishing grounds. Modern technology, such as remote sensing, sonar and Global Positioning Systems, together with incentives and subsidies, has brought deep-water and high sea areas and habitats with high production, such as continental slopes, seamounts, cold-water coral reefs, deep-sea sponge fields, into the reach of fishing fleets trying to exploit the last refuges for commercial fish species. Fishing vessels are now operating at depths greater than 400 metres, sometimes as great as 1,500 to 2,000 metres (Morato et al., 2006a). New species are being targeted, often with great success and large catches in the first 2–3 years.
However, this success is in most cases only short-lived, and followed quickly by a complete collapse of stocks (‘boom and bust’ cycle). Especially seamounts with their unique and often endemic fauna are particularly vulnerable to trawling (Koslow et al., 2001; Morato et al., 2006b). The reason for this is the special life history of many deep-water organisms, including fish species of commercial interest. Unlike their counterparts which are adapted to live in the much more variable and dynamic shallow waters systems, deep sea fish species are characterised by low reproduction and fecundity, long life, and reach maturity at a late stage. Orange roughy, one of the species often targeted by deep-water and seamount fisheries, matures from 20 to 30 years of age. Individuals can live to more than 200 years of age, which means that a fish ending up on a dinner plate could have hatched at the time of Napoleon Bonaparte. These traits render deep-water fish stocks highly vulnerable to overfishing with little resilience to over-exploitation (Morato et al., 2006b; Cheung et al, 2007). With very few exceptions, and especially without proper control and management, deep-sea fisheries cannot be considered as a replacement for declining resources in shallower waters (Morato et al., 2006a).
Among the most destructive fishing methods in the World is bottom trawling (Thrush and Dayton, 2002; Pusceddu et al., 2005; Tillin et al., 2006; de Juan et al., 2007, Hixon et al., 2007). Large nets, kept open and weighted down by heavy ‘doors’ and metal rollers, are dragged by a trawler across the sea bed. This virtually plows and levels the seafloor, picking up fish and shrimps but also catching, crushing and destroying other marine life.
The North Sea and Grand Banks have been major sites of bottom trawling, with some traditional and easily accessible areas being trawled multiple times per year. Indeed, landings data collated for round- and flatfish caught in the northern, central and southern North Sea from 1906 to 2000 as proxies for total otter and beam trawl effort, respectively, indicate that the southern and much of the central North Sea were fished intensively throughout the 20th century, whilst the northern North Sea was less exploited, especially in earlier decades. The fisheries efforts intensified markedly from the 1960s onwards. Biogeographical changes from the beginning to the end of the century occurred in 27 of 48 taxa. In 14 taxa, spatial presence was reduced by 50% or more, most notably in the southern and central North Sea; often these were long-lived, slow-growing species with vulnerable shells or tests. By contrast, 12 taxa doubled their spatial presence throughout the North Sea. Most biogeographical changes had happened by the 1980s. Given that other important environmental changes, including eutrophication and climate change, have gained importance mainly from the 1980s onwards, the study concluded that the changes in epibenthos observed since the beginning of the 20th century have resulted primarily from intensified fisheries (Callaway et al., 2007). Whereas trawling in shallow coastal waters is often carried out by smaller vessels, deep-water and high sea bottom trawling requires large and powerful ships. Such fleets are mostly based in industrialised countries, but fish intensively and for months at a time across the World’s oceans. Often these distant water fishing fleets are fuelled and kept afloat (literally) by subsidies and incentives, without which their operation would hardly be economically viable.
A decade ago, there was still much debate on the impacts on bottom trawling, as summarized in several reviews including those by the FAO. Today, there is a much larger growing body of empirical evidence, along with improved models, that document severe impact of trawling worldwide (Hiddink et al., 2006a, b, c; Hiddink et al., 2006; 2007; Callaway et al., 2007; Davies et al., 2007; Gray et al., 2006; Tillin et al., 2006). This includes, but is not limited to, China (Yu et al., 2007); the North Atlantic region (Tillin et al., 2006; Callaway et al., 2007; Eastwood et al., 2007; Kensington et al., 2007; Liwuete et al., 2007; Waller et al., 2007); the Wadden Sea (Buhs and Reise, 1997; Lotze, 2005); the Mediterranean (Coll et al., 2007); the Caribbean (Garcia et al., 2007); the East and Western Pacific (Pitcher et al., 2000; Hixon and Tissot, 2007; Fergusson et al., 2008); and the South Atlantic (Keunecke et al., 2007). Several of these studies have reported reductions in taxa and/or abundance in the range of 20–80% following years of intensive trawling (compared to pristine and/or historic data). This is especially so for demersals and benthic fauna, with reductions reported up to 80% on fishing grounds. The damage exceeds over half of the sea bed area of many fishing grounds, and is worst in inner and middle parts of the continental shelves, severly affecting in particular small-scale coastal fishing communites (Dcruz et al., 1994; Liquete et al., 2007). Unlike their shallow water counterparts, deep sea communities recover slowly, over decades. Indeed, the impact varies with type of trawl, habitat and frequency and intensity of trawling (Kaiser et al., 2006; Quieros et al., 2006). Trawling at the scales frequently observed today accounts for a major or even the most damaging practice in the fisheries industry. Studies have suggested that the impacts of trawling on the seabed equals or exceeds the impact of all other types of fishing combined (Eastwood et al., 2007).
Bycatch is also a major problem associated with trawling (Kumar and Deepthi, 2006). For many coastal populations, large-scale, industrial bottom trawling of their tradional fishing grounds (often carried out unregulated illegally and unreported by distant fishing fleets) ruins local fisheries with devastating effects on local fishermen, industry and livelihoods. Many of the larger ships process the fish directly onboard in enormous quantities. Most likely over one-third of the World catch is simply discarded due to inappropriate fish sizes, or simply due to unintended bycatch, particularly as a result of bottom trawling (Kumar and Deepthi, 2006).
Bottom trawling physically impacts the seabed and thereby some of the most productive marine habitat. Moreover, the intensity of the fisheries is a critical factor as it may take place simultaneously with other pressures, including land-related or climate change threats. Over 65% of the World’s seagrass communities have been lost by land reclamation, eutrophication, disease and unsustainable fishing practices (Lotze et al., 2006), and nearly all cold-water coral reefs observed in the North East Atlantic show scars and impacts from bottom trawling.
It is important, however, to realize that many types of fishing gear other than trawling may be severely damaging as well. A major challenge is the fact that very modest levels of trawling may increase productivity of certain genera, and localized small-scale trawling practices will likely have limited impact. Much debate has taken place on fisheries and particularly bottom trawling, and many reviews have pointed to the effect that the practice sometimes may be sustainable in some regions. However, given the capacity of most of the world’s fishing fleet, of growing pollution, climate change and coastal development, little doubt now remains that trawling practices in very many places are quite unsustainable (Callaway et al., 2007; Davies et al., 2007).
In the light of the impact which bottom trawling has on the marine fauna, ecosystems and biodiversity, more than 1,400 scientists and marine experts have signed a petition. International policy and decision makers started to address this issue in 2003/4, and the 58th session of the United Nations General Assembly considered proposals for a moratorium on bottom trawling and called for urgent consideration of ways to integrate and improve, on a scientific basis, the management of risks to the marine biodiversity of seamounts, cold water coral reefs and certain other underwater features.
However, without marine protected areas and appropriate enforcement, especially in the deeper waters and the high seas, these damaging practices are continuing. Without increased regulation, governance, enforcement and surveillance on the high seas and on the continental shelves in many regions, unsustainable and damaging fishing practices will continue. Currently, there is virtually no protection of the vulnerable marine ecosystems and biodiversity occurring on continental shelves. Indeed, in most regions, marine protected areas (MPAs) are non-existent, in others they only amount to less than 1% of the marine area. Targets have been set for setting up MPA networks and systems, however, it is apparent that under the current rate of establishment, the CBD’s target and the WPC (World Park Congress) target will not be met (Wood et al. in press).
Several countries have started some restrictions on bottom trawling in their national waters, but bottom trawling in areas beyond national jurisdiction is mostly unregulated. A few regional fisheries management organisations, such as the North East Atlantic Fisheries Commission (NEAFC), have (temporarily) closed some high risk areas beyond national jurisdiction to bottom fishing in order to protect vulnerable ecosystems. However, these measures apply only to member states (i.e. not to foreign fishing fleets) and cannot be properly controlled and enforced, which seriously weakens their effectiveness. There are now discussions ongoing with several bodies including the FAO on developing better international guidelines for the management of deep-sea fisheries in the high seas, but urgent action is needed.