Bottom Trawling

Introduction

When people think of fishing, they often picture a single rod and line, or maybe a small boat pulling in a few fish at a time. However, there is a much more industrial method used around the world called bottom trawling. Bottom trawling is a fishing technique where large, heavy boats drag enormous nets across the ocean floor (FoodSpan, 2023). These nets are held open by heavy metal doors and weighted with rubber rollers or chains. The goal is to catch fish that live near the bottom, such as shrimp, cod, flounder, and rockfish (Marine Conservation Biology Institute [MCBI], 2005). However, this method causes major ecological damage. This paper will explain how bottom trawling works, why it is so destructive, and what effects it has on the ocean environment, including damage to seafloor habitats, reductions in biodiversity, and negative impacts on fish feeding and condition.

How Bottom Trawling Works

Bottom trawls are very large nets that are dragged on the seafloor to catch groundfish and other species. The most common type in the United States is the otter trawl, in which the net is held open vertically by the force of water moving against two massive otter doors or boards. A large, fully rigged net weighs well over a ton and requires a large, powerful boat to drag it (MCBI, 2005).

The opening of a bottom trawl net is lined with floats on the headrope and with weighted ground gear on the footrope. The ground gear ranges from small steel or rubber rollers, used on flat sand and mud, to very large molded rubber rollers and rockhoppers, used to gain access to structurally complex habitat such as boulder fields, deep sea coral reefs, or rocky pinnacles. Rollers and rockhoppers allow the net to maneuver over structural seafloor obstacles and help ensure that the footrope maintains constant contact with the seafloor (MCBI, 2005).

Destruction of Habitats and Reduction in Biodiversity

One of the most serious problems caused by bottom trawling is the destruction of habitats on the ocean floor. Fishing gear like bottom trawls act like bulldozers on the ocean floor, destroying centuries worth of coral in only a few seconds (Oceana, 2021). Deep-sea corals can be hundreds of years old and provide habitat for fish and other marine animals (Oceana, 2021). The National Academy of Sciences found in its 2002 report that U.S. trawlers have damaged over 230,000 square nautical miles of seafloor, an area greater than California (MCBI, 2005).

Research on deep-sea sediments in the Mediterranean Sea shows that chronic and intensive bottom trawling causes severe damage to seafloor biodiversity. A study by Pusceddu and colleagues (2014) found that compared to untrawled areas, trawled sediments had up to 52% less organic matter content, 37% slower organic carbon turnover, and 80% lower abundance of meiofauna (tiny animals that live in the sediment). The same study also reported a 50% reduction in biodiversity and a 25% reduction in nematode species richness (Pusceddu et al., 2014). The researchers concluded that intensive bottom trawling transforms large portions of the deep continental slope into "faunal deserts" and highly degraded seascapes (Pusceddu et al., 2014, p. 8864).

Habitats that contain fragile, long lived, or slow growing species such as corals and seapens are particularly vulnerable and should not be trawled as they may be unable to recover (Marine Stewardship Council [MSC], n.d.).

Sediment Clouds and Their Effects

Another problem is the stirring up of sediment. As the heavy net drags across the bottom, it kicks up massive clouds of sand, mud, and dust. The impact to the seafloor from trawling is a compound of the weight of the net and the forward momentum of the gear as it is dragged forward (MCBI, 2005). On relatively smooth seafloor, tickler chains may be added to the footrope to disturb the sea floor and to frighten fish up from the shelter of the bottom and into the net (MCBI, 2005). This disturbance creates sediment clouds that can block sunlight and clog the gills of fish and other animals (Stockholm University Baltic Sea Centre, 2021). Pusceddu et al. (2014) estimated that the organic carbon removed daily by trawling in their study region represented as much as 60 to 100 percent of the natural input flux, meaning trawling removes more food from the seafloor than naturally arrives. Research also shows that sediment plumes from trawling can extend tens of meters above the bottom, remain suspended for days, and be transported several kilometers by currents, affecting areas far beyond the actual trawling site (Stockholm University Baltic Sea Centre, 2021).

Bycatch: Catching the Wrong Animals

Bottom trawling is known for catching many non-targeted species, also called bycatch. Because the net is so large and drags across everything, it does not select only the intended fish. Turtles, sharks, rays, starfish, and other animals have been caught in bottom trawl nets. In the shrimp harvesting industry, only 5 percent of what some trawlers catch is actually shrimp, and the rest is bycatch (FoodSpan, 2023). Bycatch is often thrown back to sea, often dead or dying (Davies et al., 2009, as cited in FoodSpan, 2023).

According to a study by Davies and colleagues (2009, as cited in FoodSpan, 2023), bycatch represents 40.4 percent of global marine catches. This means that for every five pounds of fish caught worldwide, about two pounds are bycatch that is either unused or unmanaged. The study also noted that bycatch is one of the most urgent threats to the world's remaining fish stocks, especially the indiscriminate capture of non target organisms (Davies et al., 2009, as cited in FoodSpan, 2023).

Effects on Fish Foraging and Body Condition

Bottom trawling does not only harm small invertebrates and corals. It also affects the fish that fishermen actually want to catch. When trawling reduces the abundance and biomass of prey animals on the seafloor, fish that depend on those prey may struggle to find enough food or may have to eat lower quality food.

A study by Johnson and colleagues (2015) investigated how bottom trawling affects the feeding and condition of two flatfish species, plaice (Pleuronectes platessa) and dab (Limanda limanda), in the Irish Sea. The researchers found that as trawling frequency increased, both fish biomass and prey biomass declined. However, the ratio of fish to prey remained unchanged, meaning that both fish species maintained constant levels of stomach fullness even at heavily trawled sites (Johnson et al., 2015). This suggests that the fish were still finding enough to eat in terms of sheer quantity.

However, when the researchers looked more closely at the diet of plaice, they discovered a problem. Plaice are specialized feeders that prefer infaunal prey like polychaetes and bivalves. As trawling increased, plaice shifted their diet toward energy poor prey items (Johnson et al., 2015). This dietary shift, combined with potentially lower foraging efficiency due to low prey densities, was identified as the most likely cause of reduced body condition in plaice from heavily trawled areas. In contrast, dab are opportunistic feeders that eat a wider variety of prey, including mobile crustaceans. Dab did not show the same reduction in body condition because their feeding strategy was more adaptable to changes in prey availability (Johnson et al., 2015). The researchers concluded that understanding the relationship between trawling, benthic impacts, and fish foraging is an important step in designing successful management strategies for bottom trawl fisheries (Johnson et al., 2015).

Possible Solutions

There are several ways to reduce the harm caused by bottom trawling. One solution is to create marine protected areas where bottom trawling is completely banned. In 2021, NOAA Fisheries issued a final rule to protect over 25,000 square miles of deep sea coral habitat in Georges Bank and the Gulf of Maine from destructive fishing gear like bottom trawls (Oceana, 2021). This action protects an area roughly equivalent to the size of Vermont, New Hampshire, Connecticut, and Rhode Island combined (Oceana, 2021).

Another solution is to modify fishing gear to reduce bycatch. Many fisheries modify their gear using bycatch reduction devices, which are an effective way to allow large or unwanted species to escape trawl nets. For example, the use of exclusion devices has virtually eliminated turtle bycatch in the Australian Northern Prawn fishery (MSC, n.d.). Fisheries can also avoid bottom trawling in areas with more sensitive habitats by using seabed mapping and habitat surveys (MSC, n.d.).

Additionally, managing trawling intensity is important. The research by Johnson et al. (2015) suggests that reducing trawling frequency in certain areas could help maintain higher quality prey populations for specialized fish like plaice. Pusceddu et al. (2014) argue that their findings support the need for immediate actions toward sustainable management of deep sea fisheries.

Conclusion

Bottom trawling is a fishing method that causes major ecological damage. It crushes coral and sponge habitats, stirs up sediment clouds, and catches many non-targetnon target species like turtles and sharks (MCBI, 2005; Oceana, 2021; FoodSpan, 2023). Scientific research shows that chronic bottom trawling reduces biodiversity and organic matter in sediments by over 50%, slows carbon cycling, and transforms deep sea ecosystems into degraded seascapes (Pusceddu et al., 2014). It also negatively affects fish condition by forcing specialized feeders like plaice to consume lower energy prey (Johnson et al., 2015). While trawling may be an efficient way to catch large quantities of fish in the short term, it destroys the ocean floor and reduces fish populations for the future (MSC, n.d.). To protect the ocean, we must support laws that limit bottom trawling, create protected areas for marine life, reduce bycatch, and choose sustainable seafood. The ocean is a fragile environment, and every part of it, including the bottom, is worth protecting.

References

Davies, R. W. D., Cripps, S. J., Nickson, A., & Porter, G. (2009). Defining and estimating global marine fisheries bycatch. Marine Policy, 33(4), 661-672. https://doi.org/10.1016/j.marpol.2009.01.003 

FoodSpan. (2023). Production Bycatch [Slide]. Johns Hopkins University. https://foodspan.org/sites/default/files/2023-09/lesson3-seafood-slides.pdf 

Johnson, A. F., Gorelli, G., Jenkins, S. R., Hiddink, J. G., & Hinz, H. (2015). Effects of bottom trawling on fish foraging and feeding. Proceedings of the Royal Society B: Biological Sciences, 282(1799), 20142336. https://doi.org/10.1098/rspb.2014.2336  

Marine Conservation Biology Institute (MCBI). (2005, May 17). What is a Bottom Trawl? [PDF]. https://marine-conservation.org/archive/mcbi/What_%20is_a_Bottom_Trawl.pdf 

Marine Stewardship Council (MSC). (n.d.). Demersal or bottom trawls. https://www.msc.org/what-we-are-doing/our-approach/fishing-methods-and-gear-types/demersal-or-bottom-trawls  

Oceana. (2021, June 21). Over 25,000 Square Miles of New England Deep Sea Corals Officially Protected from Destructive Fishing. https://usa.oceana.org/press-releases/over-25000-square-miles-new-england-deep-sea-corals-officially-protected-destructive/  

Pusceddu, A., Bianchelli, S., Martín, J., Puig, P., Palanques, A., Masqué, P., & Danovaro, R. (2014). Chronic and intensive bottom trawling impairs deep-sea biodiversity and ecosystem functioning. Proceedings of the National Academy of Sciences, 111(24), 8861-8866. https://doi.org/10.1073/pnas.1405454111 

Stockholm University Baltic Sea Centre. (2021, May). Bottom trawling threatens European marine ecosystems [Policy brief]. https://www.su.se/english/divisions/stockholm-university-baltic-sea-centre/policy-analysis/policy-briefs-and-fact-sheets/bottom-trawling-threatens-european-marine-ecosystems