Oxygen-free seas, ghost nets, and demolished reefs. How fisheries and aquaculture destroy ocean ecosystems

    17 Oct 2021

    Fish and other seafood have always played an important role in the life of humanity. According to the Food and Agriculture Organization of the United Nations, fish accounts for approximately 16% of the animal protein consumed by the inhabitants of the planet. However, the appetites of civilization are growing. Like an older woman from a fairy tale, we run the risk of being left behind because of greed.

    Vsevolod Rudy, the author of the telegram channel “Crazy Naturalist Bar”, talks about how fishing leads to the depletion of the ocean’s biological resources. Let’s get acquainted with his story.


    How industrial fishing destroys ecosystems

    The oceans are inhabited by dozens of commercial fish and invertebrate species, from flounders and crabs at the bottom to tuna and mackerels in the water column. Fishing boats go out to sea every day to bring all this wealth to our table. However, along the way, they inflict colossal damage to everything that comes their way. Two fishing methods are incredibly destructive for nature: bottom trawling and drifter fishing.

    The trawl is a net in the shape of a giant sack. To open the mouth of the trawl and keep it in this position, special spacers are used on the sides, the so-called trawl doors. During bottom trawling, a heavy structure is dragged along the bottom, gathering its tasty inhabitants…

    …and destroys everything else at the same time. Dragging along the bottom, the net destroys fragile corals and tears up algae forests. Both are so-called edificators – species that create habitats for other members of the ecosystem. Reefs and underwater thickets are home to thousands of different species of creatures. After their destruction, the territory swept by the trawl automatically becomes a lifeless desert.

    The damage is especially significant in the places where the above-mentioned trawl doors pass, because the weight of such a structure is several tons. As if this is not enough, clouds of sediment rise from the bottom. Upon returning to the bottom, clouds of turbidity often bury entire coral reefs under them.

    Imagine how a huge forest is being bulldozed to catch a few squirrels on a fur coat, and get a comprehensive idea of ​​trawling.

    Thus, studies have shown that on the Norwegian shelf, such fishery has destroyed 30-50% of the reefs of the unique deep-sea coral Lophelia pertusa. These pale “bushes” are one of the few corals that do not need sunlight to grow. Surviving at depths unsuitable for their colorful cousins, lofelias create unique reefs – hotspots for deep-sea biodiversity. A wide variety of animals live here – from starfish and lobsters to sharks. These oases are also crucial for commercial fish such as seabass (Sebastes marinus) and lemongrass (Brosme brosme), the density of which is much higher on the reefs than in the surrounding waters. The devastation of such areas threatens the very existence of local fish populations. At the same time, the lost bottom ecosystems are being restored at a truly snail’s pace. The same long-suffering lofelias have an increase of only 1 millimeter per year! In other ecosystems, the process is faster, but also not at the speed of light. Scientists call numbers from two to six and a half years – depending on the frequency of trawling and the type of biome studied.

    But let’s leave the trawls aside and look at the drift nets. As the name suggests, they drift in the upper and middle layers of the water. These fishing gear are not anchored and are kept upright by floats on the surface. This fishing method is highly effective – not only due to the enormous amount of fish caught but also due to the shallow fuel consumption when setting up nets. So what’s the problem?

    The pelagial (that is, the water column) is inhabited not only by commercial fish but also by other animals. Sea turtles, dolphins, seals, fish-eating birds, for example gannets, all of these creatures do not have gills. They need to emerge periodically to breathe in the atmospheric air. And they often get entangled in drift nets that are virtually invisible in the water. Fishing gear can stay in the water for several days, condemning the marine life caught in them to a slow, painful death from suffocation.

    But the worst thing happens when the drift net breaks off from the ship – during a storm or, for example, due to currents. Such ghost nets set off on long-term voyages across the ocean – and the marine fauna continues to get entangled in them.

    Synthetic nets do not rot and do not disappear anywhere, only for a short time they sink to a depth under the weight of dozens of decomposing corpses.

    But when the animals finally decay, the ghosts once again rise from the depths to continue their endless deadly journey through the waves.

    Why bycatch really matters

    “Unwanted” animals that are caught by fishermen during the fishery are called by-catch. These are not only the turtles, cetaceans, seals and birds mentioned above, but also, for example, rays, sharks, as well as species of Osteichthyes, fish not used in the food industry. The scale of this phenomenon is truly catastrophic.

    For example, studies conducted between 1990 and 1999 showed that in the United States alone, about 6,125 cetaceans and seals die in nets annually. Given that populations of large marine vertebrates are already going through hard times, this can be fatal for many species. This happened with the vakita, or California porpoise (Phocoena sinus), a small dolphin from the Gulf of California.

    The word vaquita literally translates from Spanish as “little cow”. These tiny mammals, slightly smaller than humans, live in a small corner of the bay. Most of their diet consists of totoaba fish (Totoaba macdonaldi), which was once an essential fish in these waters. Interestingly, the fish was hunted not only by hunters, but also by poachers: its swim bladders are still in great demand in China, where they are used in folk medicine. Due to the huge volumes of catch in the 1970s, the number of totoaba fell sharply – as did the number of vakit, which not only perished in huge numbers in the nets, but also lost their main source of food. Today the population of “small cows” is estimated at less than 19 individuals. Most likely, this species cannot be saved. Another prime example is the moonfish (Mola mola). These incredibly strange fish, like a shovel blade with two elongated fins, are found in temperate and tropical waters throughout the oceans. Peaceful, slow giants weighing up to 1000 kilograms feed on various plankton.

    Scientists suggest that moons play a critical role in marine ecosystems – after all, they hold back the excessive reproduction of jellyfish, eating them in huge quantities. The meat of the moonfish is not very edible: it is tasteless and flabby. Yet every year, thousands of these unique fish are killed in nets worldwide – again, as a bycatch. So, when fishing for swordfish in the Mediterranean Sea, 71-90% of the total catch that ends up in the nets is made up of moonfish.

    The moon also gets it in the case of another fishery – the so-called almadraba, the traditional Mediterranean way of catching tuna. Giant labyrinths of nets are established in places through which pelagic fish migrate annually to their spawning grounds. And hundreds of young moonfish end up in these nets every year. Being very poor swimmers (the maximum speed is 3.28 kilometers per hour), they are trapped by the currents and cannot get out. In the process of removing the nets, most fish either die or are thrown overboard, but receive injuries incompatible with life. Almost no type of commercial fishing is complete without bycatch. Even longline fishing gear, where hooks are used instead of nets, are deadly: albatrosses and other seabirds are caught by the bait. However, the champions in this sense are the already familiar bottom trawls. Bottom animals for the most part swim slowly and poorly and in the end inevitably end up in nets.

    Shrimp fishing stands out here: for every kilogram of target crustaceans, there is an average of 5.7 kilograms of bycatch that will be sent overboard to die.

    We still have little idea of ​​the real extent of the bycatch. Even in developed countries it is not possible to equip observers on all ships. What can we say about the coasts of Africa and Asia, where, on top of that, poaching flourishes? This phenomenon requires close study, but one thing is clear: bycatch must be reduced by any available means.

    We are a karakul shark – a brick: why sharks are dying out and what it is fraught with

    A recently published three-year study found that 19% of coral reefs around the world are no longer sharks. Take your time to rejoice: this does not mean calm swimming in peaceful waters, but huge problems for marine ecosystems.

    Sharks’ reputation for killers and cannibals is greatly exaggerated. Even off the coast of Australia, where these top predators still thrive, an average of no more than 16 attacks a year is recorded. Moreover, not all of them are fatal. Much more interesting is another question: how many sharks die from human hands?

    And it’s not just about the bycatch issue. Although, for example, in the 1990s, fishermen incidentally caught 12 million sharks and rays – and this is only in international waters. Another problem that threatens ancient creatures is the extraction of fins.

    Right now, as I write this, 265 Chinese ships are located near the Galapagos Islands. Using the corridor of international waters adjacent to the border of the Galapagos marine reserves, they catch tons of marine life every day – primarily sharks. Sharks of all kinds – from medium-sized reefs to gigantic, peaceful whales – end up on decks where fishers chop off their fins; and then humans throw the still-living but helpless fish into the ocean to die.

    The authorities of Ecuador, which owns the Galapagos, are only making a helpless gesture: technically, the courts do not violate anything if they do not invade the territory of reserves. But, as you might guess, sharks do not know where the protected area ends, and swim out beyond safe borders to find themselves in the bottomless mouth of a Chinese trawler. However, cases of outright poaching were also recorded – for example, in 2017, a ship with 300 tons of prey on board was detained in the protected area.

    The main market for fins is in China and Hong Kong, where they are used for making soup, as well as in traditional medicine. Nowadays, the bloody commodity comes mainly from developing countries, for example, from Indonesia. This is reminiscent of the famous situation with rhino horns and elephant tusks: poor poachers kill highly prized animals on the black market to get big money for them. Thus, 73 million slaughtered sharks end up on Asian shelves every year.

    However, the primary absurdity of this situation is different. While fins are used as medicine, they are actually dangerous.

    Toxic chemicals accumulate in the bodies of many marine organisms due to the pollution of the oceans. In particular, mercury plays a special role here. In the meat of large predatory fish, its concentration becomes simply prohibitive and can exceed the safe norms by several times. Testing of the meat of European anglerfish (Lophius piscatorius) from the Adriatic Sea has shown that it contains 2.2 milligrams of mercury per 1 kilogram of meat – with an MPC of 1 milligram per kilogram.

    As you might guess, the meat of the sea’s top predators also contains off-scale concentrations of mercury.

    So, scientists studied three species of fish-eating sharks caught off the coast of Brazil – the Cuban night shark (Carcharhinus signatus), the short-nosed katran (Squalus megalops) and the Mitsukuri katran (Squalus mitsukurii). Mercury concentrations in each species have exceeded the Brazilian norm of 0.5 milligrams per kilogram. They were 1.77, 1.9 and 2.22 milligrams per kilogram, respectively.

    Mercury poisoning causes neurological disorders – ataxia (erratic muscle movement), partial loss of sensory sensations, problems with hearing, speech and vision, and in severe cases, paralysis, coma and death. It is sometimes called Minamata disease, after the Japanese city where 2,265 people fell ill after eating poisoned seafood in 1956. 1,784 patients died. It turns out that the adherents of barbaric medicine, on the contrary, expose themselves to danger – and nevertheless, the carnage at sea continues.

    Sharks really need the ocean. As apex predators, they play an essential role in the ecosystem, helping to get rid of weak and sick individuals among prey. Sharks are especially important for coral reefs. If removed from there, smaller predatory fish such as grouper will increase in numbers and begin to eat herbivores uncontrollably. The decline in the herbivore population, in turn, leads to the fact that the reef is invaded by algae. Corals often cannot compete with plants, and soon the reef full of life turns into a desert overgrown with mud.

    The first sharks appeared 450,000,000 years ago. They reigned in the seas all these years, but they may not survive the neighborhood with humanity. However, there is hope for salvation. This is ecological tourism. In many countries, living sharks, which thousands of tourists come to admire, have long brought more money to the economy than dead ones bleeding on the seabed. Perhaps the inhuman trade will be forgotten one day, and the beautiful ancient fish will only swim in the sea, not in soup bowls.

    Cultural substitution: how do aquaculture and capture fisheries relate to each other?

    It’s not just wild seafood that makes it to dining tables these days. Salmon, sea bass, shrimps, mussels – these are just a small part of the organisms artificially grown by mankind for food. It would seem that here it is – a panacea for the oceans: if you grow, and do not catch marine life, then all problems disappear. In fact, aquaculture, like any human activity, is not without negative consequences.

    First, wild fish are still critically important for aquaculture. It is processed into fat and flour, and then used in feed for some “domestic” species, such as salmon.

    A study published in 2000 in the journal Nature found that up to 3 kilograms of processed wild can be used to raise 1 kilogram of cage fish.

    Thus, aquaculture in many cases simply cannot exist without industrial fishing. For food, so-called forage fish are usually used – small pelagic schooling fish like herring, anchovy and anchovy. These species are one of the foundations of marine food chains. They are caught and eaten by everyone: from tuna and swordfish to humpback whales, dead ends and seals. That is why you need to be especially careful with their populations: as you can easily guess, overfishing here can lead to an ecological catastrophe.

    Another significant problem is the pollution of the waters surrounding the farm. No one is watching to see if there is enough food for each individual fish, and excess portions of food are thrown into the garden, the uneaten remains of which fall to the bottom. There is eutrophication – pollution of water bodies with nutrients such as phosphorus and nitrogen. This leads to a decrease in the oxygen content in the water, as well as to the rapid multiplication of harmful microorganisms.

    Moreover, along with food waste from the farm, antibiotics, pesticides and other potentially dangerous substances end up in the sea. Such water cannot be used for bathing, drinking and other human needs. Living organisms also have a hard time – most of them simply cannot adapt to a poisoned, oxygen-deprived habitat.

    The third most important problem of aquaculture is the physical destruction of existing ecosystems. Here it is worth noting the shrimp industry. Today, shrimp is one of the most popular aquaculture products in the world. In 2005, farm revenues from these crustaceans amounted to $10.6 billion. Shrimp production is growing by about 10% each year, one of the highest growth rates of all aquaculture.

    Most shrimp are grown in the tropics, in shallow salt ponds. To create these artificial reservoirs, tens of square kilometers of mangrove forests are destroyed every year – a unique, very important and critically vulnerable ecosystem.

    Mangroves are 110 species of shrubs and trees adapted to life in salt and brackish water. They are found in the tidal zone of tropical seas – at the mouths of rivers and along the coast. These plants have a set of unique adaptations that allow them to survive in such unusual conditions – for example, the roots-supports, which do not allow the waves to pull the tree out of the silt, and the glands on the leaves, which remove excess salt.

    The variety of animals that inhabit mangrove forests can compete with that of coral reefs. Numerous waterfowl nest and winter here, including “our” migratory waders and herons. The abundance of small fish and other food attracts the so-called megafauna – large marine vertebrates like whales and turtles. 174 species of marine megafauna have been recorded in mangroves around the world.

    In addition, for some animals, this habitat serves as a kind of “manger”. Thus, many sharks spend their childhood here, in particular lemon (Negaprion brevirostris). An impenetrable labyrinth of roots and branches reliably protects young from large predators, allowing them to live to maturity.

    Finally, mangroves help people living on the coast. A strong root system strengthens the shores and resists erosion, and the interweaving of trunks and branches plays the role of a natural breakwater, weakening hurricanes and tsunamis. In areas like the Bay of Bengal, where cyclones occur every year, this is especially important.

    In 1999, a major hurricane struck the Indian mangrove forest of Sundarban, killing 15,000 people. The study showed that in places where there was a green layer between settlements and the sea, the number of deaths was about ⅔ lower than where mangroves were cut down.

    However, nowadays the reduction of mangrove forests to shrimp farms continues. It is estimated that 30% of all mangroves cut down in the world are destroyed for the needs of aquaculture. In the place of forests, endless small ponds appear, from which harmful substances, antibiotics and pathogenic microorganisms enter the sea, affecting the populations of many wild species. Used reservoirs are thrown away – it is cheaper to dig a new pond than to clean the old one from pollution.

    Medicines and pesticides in shrimp are also dangerous to human health. A Consumer Reports study found that of the 205 shrimp samples imported from Vietnam, Thailand and Bangladesh, 11 were contaminated with antibiotic residues. Potentially, this can lead to the emergence of new resistant strains of bacteria that will be very difficult to cure. Despite the development of new methods of growing shrimp, allowing to integrate farms directly into mangrove forests, the industry still remains destructive to wildlife.

    To sum up, although some types of sustainable aquaculture help reduce the pressure on natural marine populations, this industry often does more harm than good. Mankind is not yet ready to completely give up fishing for wild fish, and hence the associated environmental problems.


    Fortunately, the situation with fishing and seafood is gradually improving. Many countries ban drift fishing, nets are equipped with special devices freeing turtles and dolphins from them, and locals who until recently sold shark meat to Chinese traders are becoming guides to ecotourism companies.

    Moreover, everyone can contribute to the fate of our common oceans. It is not enough to eat destructive species such as flounder caught by bottom trawls or endangered species such as bluefin tuna. If you have a choice, it’s best not to support the ocean-killing industry. May the waters of our planet preserve the amazing, diverse, colorful and majestic life.


    Ghost nets are fishing nets that have been left or lost in the ocean by fishermen. These nets, often nearly invisible in the dim light, can be left tangled on a rocky reef or drifting in the open sea. They can entangle fish, dolphins, sea turtles, sharks, dugongs, crocodiles, seabirds, crabs, and other creatures, including the occasional human diver. Acting as designed, the nets restrict movement, causing starvation, laceration and infection, and suffocation in those that need to return to the surface to breathe.

    Ghost nets were one of the topics highlighted in our exclusive interview with General Secretary of Azraq NGO Kimberley Monges. This volunteer organization has upcoming projects to clean up ghost nets drifting at sea. Azraq aims to protect coral reefs, which Dubai can also be interested in for the purposes of eco-tourism development. Check the interview here.

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