Course:EOSC270/2022/Group 15 - The Effects of Bottom Trawling and Marine Floor Fishing

What is the problem? (Lochlan)

What is bottom trawling?

A side view of a bottom-trawling net. Credit: Anders Måløy.

Bottom trawling is the act of using a weighted net that is dragged across the seafloor to catch fish near the seafloor. This method requires large and specialized boats and can result in the acquisition of large amounts of fish each time the net is brought up to the surface. Bottom trawling has become more popular in recent years as overfishing has depleted surface fish stocks in multiple regions, making fishing at the surface no longer sufficient to meet demand. Globally, bottom trawling is estimated to catch 30 million tonnes of fish and invertebrates each year, making bottom trawling responsible for the largest quantity of catch today.

What problems does bottom trawling cause?

Bottom trawling has several disadvantages with regards to environmental preservation. The large nets indiscriminately catch organisms that may have little market value and play key roles in their ecosystems, such as sharks, invertebrates, and turtles, or may catch juvenile fish that cannot be sold due to local regulations. In addition, bottom trawling can kill or injure benthic prey species such as crabs and shrimps that target fish feed upon, which may contribute to declining fish stocks in trawled areas.

Weighted nets can also damage the sea floor. Deep-sea coral and sponge reefs often grow slowly and are home to numerous endemic species. Since benthic currents tend to be much weaker than near-surface currents, these reef structures also tend to be easily broken by weighted nets, allowing bottom trawling to destroy a deep-sea reef which may take decades to recover.

Insufficient data regarding the distribution of deep-sea reefs and the species that inhabit them has resulted in an incomplete picture of the resiliency or fragility of these ecosystems, and environmental protection laws often have few or insufficient restrictions on bottom trawling.

Where does bottom trawling occur?

A Landsat image showing sediment resuspension due to bottom trawling off the coast of Louisiana, USA. Credit: USGS

Six countries, namely the Cook Islands, the Faroe Islands, Japan, New Zealand, Spain, and South Korea allow fishing fleets to engage in bottom trawling in international waters. Bottom trawling is commonly employed in many countries and is particularly concentrated around highly productive locations, although many countries have regulations that prohibit trawling in certain protected areas. Hong Kong, Indonesia, Belize, Palau, and Venezuela have banned all bottom trawling. The large global area over which bottom trawling occurs means a large amount and diversity of seafloor habitats have been impacted.

How pervasive is bottom trawling?

The frequency and distribution of bottom trawling varies within and between countries. In the Adriatic sea, up to 50% of the seabed is trawled each year, and over a quarter of seabed area is trawled annually in other European seas. Some areas of the Northwest Iberian coast were recorded as being trawled 84 times per year, and worldwide it is estimated that 95% of all seamount ecosystem damage has been inflicted by bottom trawling alone. In addition, it is estimated that up to 13.67 million km² of soft-sediment areas on continental shelfs have been disturbed by trawling. The high frequency of bottom trawling in certain regions has contributed to large amounts of ecological damage in these areas.

The Impact of Trawling on Marine Ecosystems (Isabella)

How does bottom trawling impact marine ecosystems?

Bottom trawling impacts and damages all marine ecosystems in which it takes place. The mass caches removed from the environment put the systems out of balance. Unwanted bycatch is often discarded back into the ocean. Most sea life is tossed back injured, and usually dies soon after release. Any seabed that is within reach of trawling is at risk of being disrupted. The harmful results of trawling stem from its destructive tools.

Trawling results in the disruption of fine particles and structural integrity along the seafloor. Any layering of the seabed, animals living in/on the ocean floor, and structures present can be uprooted. Delicate ecosystems, such as coral reefs, are completely decimated by the heavy weights.

Unique characteristics of coral reef forests that make them vulnerable to trawling

Coral reefs are at extreme risk of habitat destruction. They're uncommon and susceptible to minor changes. Coral itself is easily broken and grows incredibly slowly, around 0.3-2 cm per year. Small coral reef forests could take tens of thousands of years to form. Even if trawling occurs close to a reef, currents created by their wake, the displacement of the boat and nets being rapidly dragged resulting in powerful outward force in the water, could still damage the forests.

Deep-sea reefs are at notable risk due to their location. The deep sea itself is poorly studied and monitored, which poses a challenge for environmental protection and regulation. Trawling can decimate a deep-sea reef without knowledge, and the protections put in place may be easily cast aside by trawlers due to difficult proper regulation of the reefs.

Coral reefs currently support a massive amount of biodiversity and different marine communities.

What organisms does bottom trawling impact? How and why?

Any marine dwelling organism caught within the nets or wake of bottom trawling are at risk of being harmed. Large quantities of ocean animals can be swept up in the nets and extracted from the ocean, affecting the populations and food webs of sea life.

Coral reefs are one of the most biologically diverse ecosystems on the planet, of which over a million species are native to, such as the mandarin fish, clownfish, and zooxanthellae. These native species would be unlikely  to survive elsewhere, should a reef be destroyed, and are unlikely to migrate to another reef. The native species may become extinct or endangered with further habitat destruction.

The marine life that resides in coral reefs depend on them for food and shelter. Without the coral productivity that these environments and organisms thrive off of, they will struggle to survive and likely die off.

Zooxanthellae, the single-celled algae microorganism found in almost all corals. They provide a symbiotic relationship, assisting the corals with feeding and growing.

Unique characteristics of coral reef organisms that make them vulnerable

Many coral reefs are densely populated with ideally large masses of coral. These obstacles make it difficult for coral reef inhabitants to see and escape trawling nets. Many of the organisms living here move relatively slowly and are unable to move quickly enough to evade the nets.

Most deep-sea fish and invertebrates have poor eyesight, meaning that they would likely be unable to avoid deep sea trawling nets. They also tend to reproduce slowly, making them unable to adapt to the increasing pace of bottom trawling and their populations would rapidly decline. Trawling’s impacts are felt in ecosystems across the globe.

What is the extent of the problem? (Jasjit)

What are the measurable ecosystem changes that have occurred?

Sediment resuspension occurs in areas where bottom trawling is common practice. As an example, the Iberian Peninsula is the site of high intensity trawling. Sediment resuspension has been studied and measured here and extrapolated to a world scale and estimated to be 21.87 Gt/yr. In conjunction with habitat destruction, sediment resuspension and its detrimental effects on photosynthesis would have a negative effect on localized species that rely on the biomass generated in the disturbed areas.

Graph depicting frequency of trawling within borders, outlined in black.

Seafood stock status varies in different parts of the world, in most areas stock is sustainable, however in certain areas such as Russia, Japan, and South America, stocks are below sustainable levels. Even though seafood stock may not reflect the damage caused by bottom trawling, the carrying capacity of their environments will. Destruction caused by bottom trawling activity causes depletion of nutrients and habitat removal for targeted and non-targeted species.

Bottom trawling can be measured via the frequency of sweeps and area trawled. European nations tend to have greater bottom trawling footprints than African, American or Australasian nations. It may be possible to observe the environmental impacts of bottom trawling in these areas to record data on species population, turbidity, and habitat destruction.

What is the present status compared to the past?

Seabed before and after bottom trawling activity.

Bottom Trawling has existed for hundreds of years in archaic forms across Europe and in Japan. Similar to the present, concerns about the destruction of habitat, economic competition, species destruction and overfishing existed then as now. Advancements in technology have contributed to the growth and sophistication of bottom trawling and has exponentially grown the potential of catch. The advent of steam power and ultimately diesel power helped to spread bottom trawling globally.

What is the prognosis for the future if we continue our current trajectory?

As bottom trawling targets species en masse and may not allow repopulation of targeted or slowly regenerating species, the intensity of bottom trawling may increase as demand for fish grows. Catch has grown by 3% per year from 1950 to the present and will increase into the future. Environmental destruction due to bottom trawling will also displace communities of fish, which can expand the range and intensity of trawling to new environments. Bycatch from bottom trawling makes up nearly a quarter of the global fish oil and fishmeal industry. As the mesopelagic zone is investigated for its viability to quell the growing demand for fish and fish oil, bycatch use in these industries may reduce. However, organisms that are caught as bycatch may be released injured and unable to survive back into their habitats. The exploration of the mesopelagic zone for its species viability in fishmeal may reduce the necessity for bottom trawling, as mesopelagic fish may provide cheaper fishmeal and optimize aquaculture. Bottom trawling may be difficult to phase out in communities that have already had traditional fishing practices disturbed and replaced by bottom trawling. In addition to the impacts of sea biomass, the potential effects in disturbing the seafloor as a carbon sink and releasing more carbon into the ocean can lead to ocean acidification and the introduction of more CO₂ into the atmosphere, accelerating climate change.

Given the impact, what are the solutions? (Mehr)

There are many different solutions for bottom trawling and they exist on a spectrum that ranges from making modifications to trawling and to adopting entirely new and innovative solutions. There's a key decision to be made in terms of the tradeoff between sustaining the local and global food economy and preserving marine environments. Local context and history will also matter when determining which solutions will work best for certain areas, since there's no one-size-fits-all approach.

What are the local solutions, if any?

This diagram shows modification spots for trawling gear, such as the footrope, where disks or truck tires can be added to decrease the impact on benthic communities.

On a local scale, trawling gear can be modified to be less destructive for invertebrates on the seabed, and increase the effectiveness of capturing the intended species. Decreasing the amount of rough contact by adding disks or “tires” along the footrope has reduced unwanted bycatch by 40% in the North Sea.

Smaller-scale fishing can also be encouraged with clearly defined areas. For essential local fishing industries, this should be preferred instead of larger companies having access, since local fisheries have more traditional equipment and don’t have the technology required for trawling. This will allow for local fisheries to protect their livelihoods and prevent being wedged out by corporations. As well, local fishing stocks can be replenished. However, the challenges include not being able to meet global demand and potentially higher prices of products.

Another solution that can be implemented locally is replacing deep-sea bottom trawling with longlining. Although longlining disturbs more physically complex species, it’s significantly less impactful since 296-1719 longlines are equivalent to the effects of a single trawl. However, while much more sustainable than trawling, longlining has been shown to cost 30% more for catching Atlantic cod in Norwegian fisheries, highlighting an important tradeoff to consider.

The image shows a scientist in a lab holding a Petri dish with cell lines for producing cell-based seafood.

What are the global solutions, if any?

59% of marine protected areas still permit trawling, highlighting the unprecedented ineffectiveness of these measures. However, more stringent no-take areas can be established to ban bottom trawling. This would prevent diversity and abundance loss, as well as allow the ocean to continue mitigating climate change. Observations of the Hong Kong seabed 2.5 years after a ban have shown that the ecosystem recovered in many ways. The richness and diversity of species increased while the sediment became more nutrient rich and turbidity decreased. Similar efforts can be initiated by global organizations like the United Nations, and more specifically the UN Convention on the Law of the Sea.

Lab-grown seafood has exciting prospects that can change the way we get our protein. This involves cell-cultured seafood where cells are extracted from organisms and propagated in labs in bioreactors. The challenges with this technology include encouraging the switch from traditional seafood, increasing funding for research and development, making the products cost-effective, creating diversity of choice parallel to traditional seafood options, and ensuring this transition truly leads to a decrease in fishing. However, this provides the opportunity to regrow fish stocks while meeting the increasing demands for seafood globally since global demand is predicted to increase by 30% from 2010 to 2030. Investing in this technology might also prove to be beneficial for humans, because of a decreased threat of being exposed to mercury and micro-plastics through the fish we consume.