9. Bottom Trawling
For over 25 years I commuted to work three days a week in my kayak. Rain or shine, all seasons. This is the perfect way to start the day. If it was a calm day I’d just ease into the kayak and push offshore. Slowly the susurration of the sea would tell me what currents lurk beneath, and smells wafting in the air would indicate what seabirds and other creatures are nearby. This information would drift in without any conscious thought. I would often be mesmerized by the feel of the water pushing quietly against the soft skin of my kayak. A very gentle awakening. On stormy days I would have to be instantly mindful of the waves buffeting me, and the wind pushing against me one way or another. In both cases I would feel energized and alive by the time I reached the narrow inlet that separated the open bay from False Creek. But often, as I was approaching the bridge, a fishing boat would come towards me and disrupt my reverie. It wasn’t the rough, unpainted ugliness of the boat that upset me, nor the black smoke funneling out of its never-tuned diesel engine, which made me choke, nor even that it would often come at me on the wrong side of the channel, forcing me against the pilings. No, it was what was on the deck: a large pole running diagonally from above the wheelhouse down over the transom. It was a bottom trawling fish boat and it was off to do a day’s serious bottom-dragging.
A bottom trawl is a large, funnel shaped net held open at the large end by a beam or rectangular shaped “otter boards” that are dragged along the sea bed. The trawl digs up to 15 cm of the seabed, creating a turbid cloud, which attracts fish. The fish tire at the front of the trawl and end up at the small, or “cod end” of the net. The trawl scars and damages the seabed. 90% of the fish caught may be “by-catch” and are thrown away. Trawling along the sea bed destroys the complex environment that nourishes marine plants and fauna, leaving the bottom smooth and lifeless. This doesn’t just damage the bottom, benthic layer of the sea. The whole water column up to the surface is affected. Studies have shown that after a few years of a moratorium on bottom trawling in a defined area, the overall catch there increases substantially as the sea bed starts to recover. (Callum Roberts, The Ocean of Life, pg. 295) That is not always the case. A deep sea study off the west coast of U.S. found almost no recovery after several years because the sea floor had been permanently damaged. (“Ploughing the deep sea floor”, Nature, September 5, 2012) The UN estimates that up to 95% of global ocean damage is due to bottom trawling while landing just 27% of the world’s fish catch.
The following is a quote about bottom trawling:
During the reign of Edward 111…a petition was presented to Parliament in 1376 calling for the prohibition of a “subtlety contrived instrument called the wondyrchoum”. This was an early beam trawl with a wooden beam, and consisted of a net 6 m (18 ft.) long and 3 m (10 ft.) wide,”of so small a mesh, no manner of fish, however small, entering within it can pass out and is compelled to remain therein and be taken…by means of which instrument the fishermen aforesaid take so great abundance of small fish aforesaid, that they know not what to do with them, but feed and fatten the pigs with them, to the great damage of the whole commons of the kingdom, and the destruction of the fisheries in like places, for which they pray remedy.”
Footnote: Collins, J.W. (1887) “The Beam Trawl Fishery of Great Britain with notes on Beam-Trawling in other Countries”.
Even in medieval times, the effects of trawling (as well as small net size, and feeding fish to livestock) were understood by people connected to the sea. Yet, industrial-scale trawling has been expanded worldwide. Callum Roberts considers industrial fishing to be the most destructive activity done by man, and bottom trawling to be the worst of the lot.
Progress is being made in some countries. In Norway, where trawling has been estimated to have damaged or destroyed 30% to 50% of the lophelia coral, the Sula and Rost reefs are now closed to trawling. In the U.S., the National Oceanic and Atmospheric Administration (NOAA) has banned bottom trawling off most of its Pacific coast as well as restricting it off its other coasts. (In New England the Trump government has since reversed some of these closures.) There are restrictions in sensitive areas in Scotland, Australia and New Zealand, and in Palau all trawling is banned. Kiribati announced in 2006 the formation of the world’s first deep sea marine reserve area, but the country only has one patrol vessel. Chile recently announced a ban on trawling over seamounts. Hong Kong and Belize recently enacted a ban. Although some progress has been made in these and other countries, a proposal by Palau and other South Pacific nations for a UN ban on all trawling in 2006 failed to gain sufficient support and was blocked by nations with large industrial fleets, including Canada.West Africa’s coast has long been one of the world’s richest fisheries. Traditionally, fishing has produced up to a quarter of the jobs and two-thirds of all animal protein in the region. But these days giant Asian and Russian trawlers routinely catch 250 tonnes of fish a day while ripping up the sea bed. This is more than 50 local pirogues would catch in a year. Local leaders are easily corrupted by the fishing companies and traditional fishing is dying out. It is contributing to the mass exodus and migration of people from this area and to the refugee crisis in the Mediterranean. Trawlers routinely under-report their catch by 50%.
Bottom trawls represented 26% of the catch in BC and 28 per cent of the total landed value of fisheries in eastern Canada, about $500-million, in 2001.(April 23/18, Globe)
It began in B.C. in the 1990s, initially in fairly shallow water. But, as the fishing declined due to seabed disturbances the fishers moved farther offshore. Here is a quote from a study published by Ocean Networks Canada in 2016:
“Bottom trawling represents the most pervasive human impact on the world’s continental margins, even when compared to oil and gas exploration, waste and litter disposal, and mining. As fish stocks become depleted in coastal areas, its footprint is steadily descending into deeper waters. Direct effects of bottom trawling have been extensively reviewed in scientific literature and include scraping and ploughing of the seafloor, sediment resuspension with a smothering impact on the seafloor fauna (benthos), destruction of non-target species, and organic loading from the dumping of waste from at-sea processing. Indirect effects include post-fishing mortality and long-term, trawl-induced changes in the benthos, such as reduced diversity and biomass, and changes in ecosystem structure and habitat heterogeneity.”
In response to this and other studies, a voluntary agreement, based on European standards, was reached with the trawlers association in 2012 to limit the maximum trawling depth to 800 meters and ban it from impacting coral and sponge reefs. (Ocean Networks Canada, “The cumulative effects of bottom trawling and low oxygen on marine life”, Jan 30, 2017). This is difficult to enforce. A better strategy would be to just ban all bottom dragging.
A UBC study found that globally trawling accounts for 60% of all discarded bycatch. It also noted that bottom trawlers are expensive to operate and generally receive generous government subsidies to survive. On the other hand, small scale fisheries employing traps have a higher landed value, do not require subsidies and employ far more people.
(UBC, Science Direct, Fisheries Research
Volume 206, October 2018, Pages 57–64
“Reconstructing global marine fishing gear use: Catches and landed values by gear type and sector”)
A landmark study in 2021 concluded that bottom trawling releases as much carbon as air travel. This is an amazing and shocking finding. The heavy nets and chains dragged along the seafloor disturb marine sediments, which are the world's largest carbon sink. All the more reason to ban this destructive practice.
(Protecting the global ocean for diodiversity, food and climate, Nature, April 8, 2021.)
Dredging. A fishing dredge is similar to a bottom trawl in that it is dragged along the sea bed. Although it generally covers a smaller area than a trawl it digs deeper. It is constructed of heavy steel in the form of a scoop, often with teeth on the leading edge and is especially effective at catching clams, although also used on oysters, scallops and other bottom dwellers. In some cases hydraulic jets are used to create a slurry of sand and clams which can be scooped up with a metal mesh container or brought to the surface continuously with a type of escalator. Damage to the ocean floor can be even worse than that of trawlers.
The scientific case against bottom trawling and dredging is overwhelming. Scraping clean the ocean floor impacts the whole water column above it. This effect is hard to measure and is often conveniently ignored by supporters. Trawling is hugely destructive and fisheries would be healthier if it was banned. This one act, although difficult to achieve given corporate and national interests in short term profit, would, over time, be hugely beneficial to sea life worldwide. It is an issue that should be demanded regionally, nationally and internationally. Where is the UN on this? Or Canada?
 Callum Michael Roberts is a marine conservation biologist, oceanographer, author, research scholar at the University of York, England.
 In French West Africa, pirogues refer to handcrafted banana-shaped boats used by traditional fishermen. In Madagascar, it also includes the more elaborate Austronesian lakana outrigger canoe. Pirogues are usually propelled by paddles that have one blade (as opposed to a kayak paddle, which has two).