Bottom trawling is a method of commercial fishing in which a large-net weighted with chains, rollers or rock hoppers (large rubber tires that allow the net to be dragged over an uneven surface) is towed across the seafloor. Though they catch desired species, bottom trawls also catch millions of pounds of unintended species. Bottom trawling in North Pacific fisheries accounts for only 18% of the retained ground fish catch but 82% of the discarded fish (Fisheries Information Services 2006). Research shows that bottom trawling also damages marine habitats by removing, crushing, or overturning corals, sponges, and other species that form the living seafloor.
Recognizing the importance of some habitats, fishery managers have protected some areas from bottom trawling; however, many sensitive areas are still at risk from the impacts of this destructive fishing practice.
Bottom Trawl Gear
Bottom trawl gear includes two steel doors (otter boards) attached to the mouth of the funnel-shaped net to keep it open as it is towed through the water and dragged along the seafloor behind the vessel. The foot rope at the front of the net is outfitted with chains (to scare fish into the net) or airplane tires, metal discs or bobbins (to keep the net from snagging on rocks or other high relief habitat features).
In the Bering Sea and Aleutian Islands, trawl vessels range from 107-295 feet (an average city block is about 300 feet). In the Gulf of Alaska, trawl vessels range from 58-125 feet. Worldwide, an area twice as large as the lower 48 states is trawled every year (Eilprin 2004).
The ocean floor supports a rich diversity of life. Whether deep or shallow, soft mud (like much of the Bering Sea shelf) or complex, rough, high relief structures such as rocky outcroppings and coral (like parts of the Gulf of Alaska, Bering Sea canyons along the continental slope and Aleutian Islands), the seafloor provides the shelter and food on which marine animals depend (Heifetz 2000). Like clear-cutting a forest, bottom trawling flattens some habitat structures. Some can take years, decades or even longer to recover (Watling and Norse 1998). Protecting sensitive habitats from the damaging effects of bottom trawling fosters a healthier marine ecosystem, greater fish production in the long run, and a richer diversity of species.
Epifauna are animals that anchor themselves to the seafloor. They live on the bottom substrate and filter food carried by currents. Infauna are animals that live within the substrate of the seafloor. Some examples of infauna include burrowing mollusks and polychaetes (marine worms). These animals stabilize the sediments and are a food source for crab and fish.
Photos from top to bottom: 1. Sea whips (Halipteris willemoesi) off Kodiak Island. A basket star uses a sea whip as a feeding platform. 2. Basket star with gorgonian coral (Thouarella superba). 3. Bubblegum coral (Paragorgea arborea sp.) covered in shrimp (Heptacarpus sp.) and basket stars (Asteronyx sp.). 4. Deepsea sole (Embassichthys bathybius) amid an aggregation of brittle stars (Class: Ophiuridea) with a solitary white seastar (Class: Asteroidea, true seastars). All photos couresy of NOAA.
Impacts on Habitat
From slow growing corals to marine worm tunnels, bottom trawling crushes and overturns marine life attached to the seafloor, boulders and other physical structures. This reduces habitat complexity and associated shelter for fish and other species (National Research Council 2002). Areas with little natural disturbance, such as deep-water habitats, are more affected by trawling than are sand and softer bottom types that are exposed to regular natural disturbances such as waves, wind or tide-induced currents (Ibid).
Coral and Sponges
Corals and sponges are refuges for fish and crab from currents and predators, as well as habitat for spawning and feeding. At least 70 coral species have been identified in Alaska waters. The Aleutian Islands may harbor the highest abundance and diversity of cold water corals in the world (Heifetz, et al. 2003). In Alaska, over 1 million pounds of corals and sponges are removed from the seafloor every year; bottom trawls are responsible 90% of the damage (National Marine Fisheries Service 2004).
Impacts on Ecosystems
Bottom trawling contributed to the collapse of the Bristol Bay red king crab population in the early 1980s. Since then continued trawling in this area has contributed to low population levels (Dew and McConnaughey 2005). There are 33% more juvenile crab and increased abundance of other species in protected groves of sea whips around Kodiak Island than in similar habitat in adjacent areas open to bottom trawling (Stone et al. 2005). Comparing trawled and non-trawled areas in the Bering Sea, researchers found that after trawling the level of species diversity decreased and some rare species groups were absent entirely (Brown et al. 2005).
In addition to altering seafloor habitats, bottom trawling also results in huge amounts of bycatch - unwanted fish and marine life discarded because they are the wrong size, sex or species. High levels of bycatch can affect entire marine communities, reducing biomass of key species important to the food web and altering the ecological structure and diversity of the oceans (Norse, ed. 1993)
To learn more about the effects of bottom trawling in the North Pacific, download a pdf file of AMCC's Bottom Trawl Report.
Lessons From Other Seas
The North Sea
Researches studying the heavily trawled North Sea found that:
- Bottom trawling could have permanently changed benthic communities. Restricting trawling would not necessarily mean a return to a pre-altered state (McGlade 2002).
- Bottom trawling homogenized the seafloor, reduced habitat complexity, reduced some species' range, decreased populations with low reproductive rates while increasing populations with high reproductive rates, fragmented some populations, and impaired epifauna more than infauna. Trawl fleets acted as a new "super-predator" to the ecosystem, altering the dynamics of the food web of the North Sea (Ibid).
Georges Bank was once one of the most highly productive and heavily fished areas of the North Atlantic. By the 1990s, several important species were declining due to overfishing and damage to essential habitat from multiple fishing gears. In 1994, 25% of the bank was closed to bottom fishing. The results: a 14-fold increase in scallop biomass, an 800% increase in yellowtail flounder, and increases in the spawning biomass of cod and haddock. Species abundance, biomass production and epifaunal cover have all increased (Collie et al 2004; Fogarty and Murawski 2004).
Alaska fishery managers have closed certain areas in the Gulf of Alaska and the Bering Sea and Aleutian Islands to bottom trawling to partially protect depleted crab populations and coral and sponge habitats. However, significant sensitive habitats remain exposed to bottom trawling warranting focused conservation consideration. As climate change advances in the Bering Sea and fishing fleets follow fish populations northward, it is imperative that fishery managers establish precautionary measures to protect ecologically important habitats in these frontier fishing grounds.
In addition to protecting sensitive habitat from bottom trawling, gear modification to minimize contact with the seafloor can help to reduce intensity of impacts on remaining open areas.
Click here to learn about AMCC's work to establish a northern bottom trawl boundary in the Bering Sea.
Brown, E., B. Finney, S. Hills and M. Commisse. 2005. Effects of commercial otter trawling on benthic communities in the Southeastern Bering Sea. Am. Fish. Soc. Symposium 41. pp. 439-460.
Collie, J., J. Hermsen, P. Valentine and F. Almeida. 2004. Effects of fishing on gravel habitats: Assessment and recovery of benthic megafauna on Georges Bank. Am. Fish. Soc. Symposium 41. pp. 325-343.
Dew, C. B., and R. A. McConnaughey. 2005. Did trawling on the brood stock contribute to the collapse of Alaska's king crab? Ecol. Appl. 15(3):919-941.
Eilperin, J. 2004. Ocean exploitation surfaces as crisis: Widespread pollution, overfishing spur presidential panel to urge new rules. Washington Post, Oct. 9 2004.
Fisheries Information Services. 2006. Discards in the North Pacific groundfish fisheries 2004.
Fogarty, M. and S. Murawski. 2004. Do marine protected areas really work? Oceanus, 43(2).
Heifetz, J. 2000. Coral in Alaska: Distribution, abundance, and species associations. Proceedings of the Nova Scotia Institute of Science. First International Symposium on Deep Sea Corals.
Heifetz, J., R. Stone, P. Malecha, et al. 2003. Research at the Auke Bay Laboratory on benthic habitat. AFSC Quarterly Report. July-Aug-Sep.
McGlade, J. M. 2002. The North Sea large marine ecosystem. In: K Sherman and H. Skjoldal (editors), Large Marine Ecosystems of the North Atlantic. pp.339-412.
National Marine Fisheries Service. 2004. Alaska Groundfish Fisheries, Final Programmatic Supplemental Environmental Impact Statement.
Norse, E., ed. 1993. Global Marine Biological Diversity, A Strategy for Building Conservation into Decision Making. Island Press.
Stone, R., M. M. Masuda, and P. W. Malecha. 2005. Effects of bottom tralwing on soft-sediment epibenthic communities in the Gulf of Alaska. In: P.W. Barnes and J.P. Thomas (editors), Benthic Habitats and the Effects of Fishing. Am. Fish. Soc. Symposium 41. pp. 461-475
Watling, L. and E. Norse. 1998. Disturbance of the seabed by mobile fishing gear: A comparison to forest clearcutting. Conservation Biology. 12(6): 1180-1197.