Bottom trawling is a method of commercial fishing whereby a large-net weighted with chains, rollers or rockhopper gear is towed across the seafloor. In the process of catching desired species, bottom trawls also catch millions of pounds of unintended species. Bottom trawling in the North Pacific fisheries accounts for only 18% of the retained groundfihs 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.

Recgonizing the importance of some habitats, fishery managers have proteced selected areas from bottom trawling. however, many sensitive areas are still at risk to the impacts of this fishing practice.

Bottom Trawl Gear

Bottom trawl gear includes two steel doors (otter boards) attached to the mouth to keep the funnel-shaped net open as it is towed through the water and dragged along the seafloor behind the vessel. A footrope is outfitted with chains (to scare fish into the net) or airplane tires, metal discs or bobbins (to keep the net from snagging on rocky 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, every year an area twice as large as the lower 48 states is trawled (Eilprin 2004).

Living Seafloor

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 outrcoppings and coral (like parts of the Gulf of Alaska, Bering Sea canyons along the continental slope and Aleutian Islands), marine animals depend on the seafloor for food and shelter (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 filter food carried by currents. Infauna are animals that live under the surface. These burrowing mollusks and polychaetes (marine worms) 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 coral and sponge beds 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 similar bottom types that are exposed to regular natural distubrances such as waves, wind or tide-induced currents (Ibid).

Coral and Sponges

All corals and sponges are refuges for fish and crab from currrents and predators, as well as habitat for spawning and feeding. At least 70 coral species have been identified in Alaska waters. The Aleutian Isladns 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, 90% by bottom trawling (National Marine Fisheries Service 2004).

Impacts on Ecosystems

Bottom trawling contributed to the collpase of the Bristol Bay red king crab population in the early 1980s. Since then continued trawling in this area has helped keep the population at low levels (Dew and McConnaughey 2005). There is 33% more juvenile crab and increased abundance of other species in protected groves of sea whips around Kodiak Island as compared to 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 number of different species decreased and some rare species groups were absent (Brown et al. 2005).

Wasteful Fishing

In addition to altering seafloor habitats, bottom trawling also results in the most bycatch - unwanted fish and marine life discarded because it is the wrong size, sex or species. High levels of bycatch can affect entire marine communities, reducing 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 

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, 800% increase in yellowtail flounder, as well as increases in the spawning biomass of cod and haddock. Abundance, biomass production and epifaunal cover have all increased (Collie et al 2004; Fogarty and Murawski 2004).

Solutions

Alaska fishery managers have closed certian areas in the Gulf of Alaska, Bering Sea and Aleutain Islands to bottom trrawling 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 ecolgoically important habitats in these frontier fishing grounds. Click here to learn more about AMCC's work to establish a northern bottom trawl boundary in the Bering Sea.

In addition to protecting especially sensitive habitat from bottom trawling, gear modification to minimize contact with the seafloor can help to reduce intensity of impacts on remaining open areas.

References:

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.