Bottom trawler dragging nets and weights along ocean floor, Baja California, Mexico. Photo by Brain Skerry / National Geographic

Ending Bottom Trawling

Bottom trawling of the ocean must end.

This hugely common form of fishing is devastating for a huge array of marine populations and for habitats. Bottom trawlers scour the bottom of the ocean and catch everything in their path, willy nilly. Bottom trawling is also implicated in slavery and indentured labour, and often persists only because of harmful fishing subsidies.

Seahorses are caught primarily by non-selective and destructive fishing gears, and particularly by bottom trawls.

A mark of the intensity of such bottom trawling globally is that catches of only one or two seahorses per vessel per night amounted to a total extraction of more than 11 million seahorses annually from just the first 21 countries we surveyed.

Bottom trawling, more aptly referred to as dragging, is one of the most common and most destructive types of fishing gear used in the oceans today.

  • Hundreds of thousands of bottom trawlers operate all over the world.
  • More than 80% of the seabed is trawled in some regions,1 most of it repeatedly.
  • Trawling generates 25% of global marine catches2 and as much as 50% in Asia.3

Bottom trawlers drop wide-mouth nets with heavy weights that gouge and scrape.4

  • When pulled behind a single boat, the mouth of the net is most commonly held open by two large “doors” – made of heavy wood or steel – attached to either side of the net (otter trawl).
  • Less commonly, the net is held open by a steel beam (beam trawl).
  • Sometimes two boats act together to pull a single net which keeps the net mouth open (pair trawling).

Catch landed by bottom trawlers is often sold, unsorted, for mere pennies, as feed for farmed fish or other animals.

  • In India, feed fish sold for ~USD 0.10 per kg.
  • In Viet Nam, the selling price of feed fish varied from ~USD 0.22 – 0.53 per kg.5
  • In China, the selling price varied from ~USD 0.10 to 0.6 per kg.6

Bottom trawlers catch everything in their path willy nilly, including thousands of species from seahorses to sharks.

  • Tropical trawl fisheries in Asia catch approximately 800 species of elasmobranchs, teleosts, crustaceans, molluscs and echinoderms.7
  • Trawls catch tens of millions of seahorses each year. The catch is unmanaged and unreported.
  • Trawls were responsible for 63% and 82% of shark and ray catches in the Gulf of Thailand and 92% and 64% of shark and ray catches in Thai waters of the Andaman Sea.8

Bottom trawlers mow down seagrasses, corals, sponges and other habitats.

  • Bottom trawl fisheries have negative impacts on benthic habitats which include: the “alteration of physical structure, sediment suspension, changes in chemistry, and changes to the benthic community… resulting in changes to the ecosystem”.9

Bottom trawling reduces carbon sequestration

  • Bottom trawl gear physically disturbs the sediment, disrupting carbon sequestration and re-suspending stored carbon into the water where it can more easily break-down.10 One report claimed that bottom trawl nets release as much carbon dioxide as the entire aviation industry.11

Bottom trawlers destroy opportunities for artisanal & small-scale fishers.

  • Inshore waters have, historically, provided for artisanal and small-scale fishers that operate using passive gear. However, there is social conflict with the demersal trawlers—whose gear in this comparison is additionally more ecologically devastating – as the latter compete for resources and can harm the small-scale fishermen’s gear.12
  • Overfishing in the Gulf of Thailand means demersal stocks in the 1990s were already one-tenth of the mid-1960s when trawling began; this depletion has come at socio-economic cost to small-scale fishers.13

Many bottom trawlers rely on crews of slaves or other forced labour.

  • News outlets, including The Guardian, Associated Press, and The New York Times, have documented multiple instances of trafficking and labor violations on trawl vessels. Examples include trafficked victims being forced onto fishing boats that catch ‘trash fish’ — juvenile or inedible fish — to be processed into feed for shrimp.14

Bottom trawls remove tonnes of juvenile fish that should grow into valuable human food.

  • Half of the total catch by smaller sized Thai trawlers was juveniles of food fish.15
  • 30-40% of trawled catch landed in one Indian port was diverted to fish food, including species traditionally destined for human consumption.16
  •  Only 20% of the catch landed by Vietnamese trawlers was used for direct human consumption and raw materials for human food products. The other 80% was destined for use as fish and animal feed. Two thirds of the catch was juvenile fish.17
  • Half of China’s trawler catch, 35% of the total catch in national waters, is comprised of low‐valued “feed‐grade fish”. It included 218 fish species – half of which were food species and the vast majority were individuals in their juvenile size ranges.

Bottom trawl fisheries are unprofitable so our governments use our public money to keep them fishing.

  • Declining catch rates combined with rising overhead costs (mainly fuel) and falling shrimp prices (due to world-wide competition with lower-cost farmed shrimp), have led to low profitability for most of the worlds commercial shrimp fishing operations.18 In many cases government-funded subsidies have been required for continued operation. Indeed, half of all shrimp landings came from countries with subsidies on fuel.19
  • Without fuel subsidies, more than 90% of China’s fishing vessels would lose money.20
  • The money made from low value/trash fishing is now also a main reason why many vessels continue to be economically viable and remain in fisheries.21

[Updated 10 June 2021]
  1. Amoroso, R.O., C.R. Pitcher, A.D. Rijnsdorp et al. 2018. Bottom trawl fishing footprints on the world’s continental shelves. Proceedings of the National Academy of Sciences 115(43):E10275-E10282
  2. Amoroso, R.O. et al. 2018.
  3. Sumaila, R. and W.W.L. Cheung. 2015. Boom or Bust: The Future of Fish in the South China Sea. Vancouver, Canada: Fisheries Centre, The University of British Columbia.
  4. FAO. 2021. Fishing gear types: Bottom Trawls
  5. Sadovy de Mitcheson,Y., D. Leadbitter and C. Law. 2018. History, profiles and implications of feed fish and fishmeal supply from domestic trawlers in the East and South China Seas. Final Report to ADMCF pp.131
  6. Sadovy de Mitcheson, Y. et al. 2018.
  7. Food and Agriculture Organization of the United Nations (FAO). 2014. Regional guidelines for the management of tropical trawl fisheries in Asia. Asia-Pacific Fishery Commission (APFIC)/FAO Regional Expert Workshop. Phuket, Thailand, 30 September–4 October 2013.  pp.91
  8. Keong 1996 as cited in Leadbitter, D. 2019. Driving change in South East Asian trawl fisheries, fishmeal supply, and aquafeed. Report to IFFO, The Marine Ingredients Organisation and the Global Aquaculture Alliance (GAA). pp.150
  9. Gillett, R. 2008. Global study of shrimp fisheries. FAO Document Technique Sur Les Pêches No. 475. Rome, FAO. pp.331
  10. Parker, R.W.R., J.L. Blanchard, C. Gardner, et al. 2018. Fuel use and greenhouse gas emissions of world fisheries. Nature Climate Change 8:333-337
  11. Sala, E., J. Mayorga, D. Bradley, et al. 2021. Protecting the global ocean for biodiversity, food and climate. Nature 592:397-409.
  12. Pauly, D. 1996. Fleet-operational, economic, and cultural determinants of bycatch uses in Southeast Asia. Solving bycatch: consideration for today and tomorrow. Alaska Sea Grant College Program, Seattle, Washington. p.285-288.
  13. Teh, L. and D. Pauly. 2018. Who brings in the fish? Relative contribution of SS and Industrial fisheries to food security in Southeast Asia. Frontiers in Marine Science 5:44.
  14. The Guardian, 2018; The Associated Press, 2016; The New York Times, 2015
  15. Noranarttragoon 2014 as cited in Sadovy de Mitcheson, Y. et al. 2018.
  16. Changing Markets. 2019. Fishing for Catastrophe. Changing Markets Foundation, Netherlands. pp46.
  17. Sadovy de Mitcheson, Y. et al. 2018.
  18. Gillett, R. 2008.
  19. Sumaila, U.R., L. Teh, R. Watson, P. Tyedmers and D. Pauly. 2008. Fuel price increase, subsidies, overcapacity, and resource sustainability. ICES Journal of Marine Science 65:832-840.
  20. Guo 2015 as cited in Zhang, W., M. Liu, Y. Sadovy de  Mitcheson, et al. 2020. Fishing for feed in China: Facts, impacts and implications. Fish and Fisheries 21(1):47-62.
  21. Funge-Smith, S., M. Briggs and W. Miao. 2012. Regional overview of fisheries and aquaculture in Asia and the Pacific 2012. FAO RAP Publication, Bangkok. pp.154

[Updated 3 May 2021]