It isn’t, on the face of it, a hard case to make. In fact, it would seem obvious: dragging bottom trawls, complete with giant steel doors and rollers, across the sea bed is bound to be immensely damaging to the deep sea habitat and the species that live there. As far back as 1376, fishermen from the Thames Estuary petitioned king Edward III of England to ban primitive trawl nets that they feared could cause “great damage of the common’s realm and the destruction of the fisheries.”
Unfortunately for the Thames, the king didn’t heed their request. And unfortunately for the global ocean, so far the nations of the world have not responded to repeated entreaties for a moratorium on high seas bottom trawling (HSBT). In February last year, more than 1,000 scientists signed a consensus statement that “urge[d] the United Nations and appropriate international bodies to establish a moratorium on bottom trawling on the High Seas.” UN Secretary General Kofi Annan has acknowledged that, “What we need is high-level political commitment for marine conservation and protected areas.” And yet, the most important commitment the UN could make for marine conservation—a moratorium on high seas bottom trawling—remains elusive. The irony is that, as a new paper (1) shows, the scientific case against HSBT is about as strong as such a case could be, as deep sea habitats and wildlife are uniquely vulnerable to human activities, and bottom trawling is a uniquely destructive activity. That trawling should be particularly damaging is hardly surprising, given the nature of the fishery. As the paper’s authors explain: “Weighted with massive bobbins, rollers, or rockhoppers, the trawl nets stretch up to 40 m in width and are held open by pairs of steel trawl doors weighing as much as 7 tons apiece. Trawler footropes can roll seafloor rocks weighing as much as 18 tons, the weight of a city bus, and both rolled boulders and trawl doors can plow deep gouges in soft sediments. A trawler towing at 3 to 4 knots for a period of 4 hours directly impacts an area of 2.5 km2. Trawling trips can be as long as 4 to 6 weeks, during which fishing occurs around the clock. During this time, trawlers sweep a vast area of seafloor, crushing corals, sponges, and most any other living things they hit. The estimated total area swept annually by trawl nets (the same area is often trawled many times a year) is equivalent to about 50 percent of the world’s continental shelf area, or approximately 150 times the area of forest that is clearcut worldwide every year.” That kind of impact would have a deleterious effect on any environment, but deep sea ecosystems are especially vulnerable. The deep sea is cold, often just above freezing. Low food availability and cold temperatures mean deep sea fish reproduce infrequently, grow slowly, and take a long time to mature. For example, many of the rockfishes that live on continental slopes and seamounts in the North Pacific live to be as old as 200 years, and take 10 to 39 years to reach maturity. As a result of their slow growth and low reproductive rates, deep-sea fishes are the most vulnerable of all fishes to overfishing. Because of the generally low food supply in the deep ocean, fish populations are generally dispersed, and come together only for spawning. It is then – when a single trawl can, for example, net 60 tons of orange roughy in 20 minutes – that the trawlers strike. In the words of the paper’s authors: “Targeting spawning aggregations is … the most effective way to rapidly deplete fisheries, but that is exactly what some HSBT operations do … Exploiting their spawning aggregations is more like mining than fishing, because it so severely reduces the chance of recovery.” Nor are target species the only ones affected. Bottom trawls are completely indiscriminate, flattening, churning up, entrapping, and destroying everything in their path. For example, after ten years of the orange roughy fishery on the Chatham Rise, off New Zealand, 13 out of 17 bycatch species showed lower biomasses; populations of Plunket’s shark and black cardinal fish decreased to only 6 percent of their original biomass. Atlantic wolfish may be on the path to becoming an endangered species in the northwest Atlantic, largely due to mortality relating to bycatch. Trawling doesn’t just affect deep sea wildlife; it devastates the habitat on which that wildlife depends. Off Tasmania, some trawled seamounts are 95 percent bare rock. One comparison of trawled and untrawled seamounts on the Chatham Rise showed that coral habitat covered 52 percent of the seafloor on undamaged seamounts versus 2 percent on seamounts that had been trawled. Deep sea corals and sponges which flourish on seamounts are hotspots of biodiversity – deep sea life that is particularly vulnerable because of the high levels of endemism on seamounts, and also because habitat-forming animals, like deep sea fishes, are extremely long-lived. Individual gold corals from seamounts have been estimated to be 1,800 years old, while deep sea, cold water Lophelia reefs are estimated to be an astonishing 8,000 years old. Because these organisms are so slow-growing, the paper notes, a single trawl can cause damage that can not be repaired for decades or even centuries. If it were not enough that high seas bottom trawling should involve a uniquely destructive fishery in a uniquely vulnerable ecosystem, the paper’s authors assert that scientific understanding of the deep sea is far too incomplete to allow for the development of any kind of management plans. As they point out: “It is much easier to kill huge numbers of deep-sea fish in trawls than it is to study these fish as living animals”; as a result, “most deep-sea food webs are still a scientific mystery.” And even if scientists could develop management plans, there is virtually no mechanism available by which such plans could be implemented. Existing mechanisms for protecting, recovering, and ensuring the sustainability of high-seas deepwater resources are, they argue, “extremely poor. Unfortunately, the stark reality is that access to high-seas living resources is virtually unimpeded and unregulated.” For all those reasons, the paper’s authors can come to only one conclusion: “Deep-sea bottom trawling is the most destructive form of fishing and one of the most significant human impacts on the globe. Life-history characteristics of deep-sea fishes and benthic invertebrates and the high species endemism found on seamounts make these species and ecosystems exceptionally vulnerable to overfishing and disturbance by bottom trawling. Bottom trawling on the high seas is not sustainable given the inadequacy of current management and may very well be unsustainable at even greatly reduced levels of fishing. That is why 1,136 scientists have called for a moratorium on high-seas bottom trawling until the nations of the world can establish strong management measures for deep-sea fisheries and biodiversity on the high seas. They should be heeded.”
Notes: (1) Why the World Needs a Time Out on High Seas Bottom Trawling L.E. Morgan, E.A. Norse, A.D. Rogers, R.L. Haedrich, S.M. Maxwell. 2005.
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