What is bottom trawling

Fishing methods: systematic down to the last fish

The more sophisticated the fishing methods, the closer-meshed the nets and the heavier the devices that are dragged across the bottom to scare the fish, the greater the pressure on the fish stocks in the oceans and the greater the damage caused by fishing.

The price for this is high: The Fish stocks are being depleted, the The marine environment is being damaged and other Marine animals are unintentionally caught or injured. Due to the current intensive and high-tech fishing only a few fishing methods are ecologically harmless. The main fishing methods in alphabetical order:

Fishing rods are used to catch species of fish that form schools. The fish are lured to the surface of the water with (live) bait. Then they are brought into the boat with the fishing rods.

Target species: various schooling fish species, especially tuna

Effects: very selective, no impact on the seabed

Ankerwade (also: Danish seine)

When fishing with an anchor seine, an anchor with a fishing line is attached first. From the anchor, the net and the second fishing line are laid out in a circle. Back at the anchor, the fishing lines are pulled in, which drive fish lying on the bottom into the net. Anchor seine are suitable for coastal flatfish fishing. This technique is particularly widespread in Denmark (“snurrevaad”).

Target species: Flatfish such as plaice or sole

Effects: The impact on the seabed is small compared to bottom trawls in particular. The fish are of high quality.

Beam trawls are a form of bottom trawl. The net is pulled across the seabed by the cutter on skid-like shoes. A rigid Kurrbaum keeps the opening of the net open under water. Due to their high dead weight, beam trawls are real energy guzzlers. They put a lot of stress on the seabed and its inhabitants through the rollers and runners.

Target species: Flatfish, prawns.

Effects: Bycatch of flatfish, juvenile fish, bottom-living organisms and vegetation, corals, etc., damage to the seabed.

A dredge consists of a mesh bag or metal basket mounted on a frame that is pulled across the seabed. The frame is provided with a ploughshare-like, sometimes toothed steel edge that literally plows the seabed.

Target species:Mussels, mollusks, sea urchins.

Effects: Bycatch of fish, juvenile fish, vegetation, corals, etc. Damage to the sea floor

Hook and line

Hook and line is used to describe various fishing methods that use short fishing lines with a hook attached to the end (as opposed to long lines). These include, for example, hand lines, fishing rods, trolling or so-called jigging. For the most part, these methods are very selective, do not harm the marine environment and the fish are of high quality.

Hand lines are short fishing lines, at the end of which a hook equipped with a bait is attached. They are operated from a moving or stationary boat.

Target species: various types of fish, for example, cod, mackerel, tuna, grouper, snapper, squid

Effects: very selective, no impact on the seabed.

Harpoons are used by skilled fishermen to catch large, valuable predatory fish. The harpooner plunges his harpoon into the fish and pulls it on board. This method is very selective because the harpooner recognizes the type and size of the fish beforehand.

Target species: Tuna, swordfish

Effects: very selective, no impact on the seabed.

This method is used in particular to catch squid. A line is moved up and down in the water. A light is often used at night to attract the squid, which are then caught by the moving hook.

Target species: especially squids

Effects: low environmental impact

Gill nets form a "net wall" by hanging them vertically in the water. They can also be anchored to the ground and be up to 30 meters high. The nets are designed so that fish of a certain size can fit their heads through the mesh but not the rest of their bodies. When they want to swim out of the net, they get caught with their gill covers.

Target species: Different types of fish such as herring, plaice, cod

Effects:By-catch of juvenile fish, marine mammals, sea birds, sharks, turtles, seals, etc.

These are nets that are baited and attached to the seabed. The animals are kept alive in the traps until they are overtaken. The target species and the selectivity of the traps depend on the mesh size. Animals that are too small or fish that cannot be used can be thrown back alive.

Target species: different types of fish, for example cod, salmon, herring

Effects: Low impact on the sea floor. Relatively selective, but bycatch of non-target species and marine mammals possible.

Wooden or wire baskets that are deployed on the seabed and are equipped with bait. The animals are kept alive in the baskets until the fisherman catches up with them.

Target species: Lobsters, crabs, shellfish, octopus

Effects: selective, but bycatch of marine mammals possible. Low impact on the sea floor.

The longline fishery works with fishing lines up to 100 kilometers long, to which several thousand baited hooks are attached. A distinction is made between pelagic longlines and bottom longlines.

Pelagic longline

Pelagic longlines are hung vertically in the body of water. With this technique, sea turtles, seabirds, shark species and juvenile fish in particular are by-catch.

Target species: Various types of fish, especially tuna and swordfish

Effects: By-catch of juvenile fish, turtles, sea birds, sharks, marine mammals.

Floor longline

Bottom longlines are deployed horizontally along the seabed. Which species is caught depends on the location, type of hook and bait.

Target species: various types of fish, including cod or halibut

Effects: Less problems with seabirds than pelagic longlines because bottom longlines are deeper. Low impact on the sea floor.

Fish traps are nets that stand on the seabed. The fish are guided to the entrance of the fish traps with one or more control nets. They are basically ecologically harmless.

Target species: Fish, especially eels

Effects:Hardly any environmental impact

The 120 to 250 meter high and up to two kilometers long nets are put around a school of fish by boats as a ring. The lower net is pulled together, the fish are caught like in a big bag. So-called fish aggregation devices (FADs) are used to maximize the success of the catch. FADs are floating objects that take advantage of the natural instinct of many species of fish to group around such objects. First small fish are attracted, which are then followed by the larger fish. The use of FADs results in high bycatch amounts.

Target species:Many species of fish, especially tuna.

Effects: By-catch of juvenile tuna, sharks, turtles, marine mammals.

Traditional method of harvesting shellfish or clams in particular. Mostly pliers or rakes are used as auxiliary tools.

Target species:Mussels, lobsters.

Effects:Very selective, no impact on the seabed.

With this method (usually several) fishing lines with bait are pulled through the water. They can be used at different heights.

Target species: for example tuna, marlin, barracuda, salmon.

Effects: very selective, no impact on the seabed.

Trawls are cone-shaped nets, often kilometers long, that are either pulled through open water (pelagic trawls) or across the sea floor (bottom trawls). Bottom trawls produce high levels of bycatch and can cause significant damage to the marine environment.

The WWF regards bottom trawling as an obsolete technique that should be ended in the coming years.

Target species:Fish, shrimp.

Effects: By-catch of turtles, marine mammals, juvenile fish, non-marketable fish species, invertebrates, corals (up to 60 percent!). Damage to the sea floor.

Spears are used by fishermen while diving. This method is banned in Germany, but is used as a commercial fishing method in many countries. Like spearfishing, spearfishing is very selective as the type and size of the fish are identified beforehand.

Target species: Fish and crabs.

Effects: very selective, no impact on the seabed.

Mirror net / trammel net

Mirror nets (also called trammel nets or fetlock nets) are similar to gill nets, but consist of three nets vertically attached to the ground. The outer nets have larger mesh openings, in the middle is a fine-meshed net in which the fish can get caught.

Target species: bottom fish species.

Effects: By-catch of small fish, undesirable species, marine mammals.

The net floats like a wall in the water and is held open with buoys on the upper edge and weights on the lower edge. Driftnets cause high levels of bycatch, such as marine mammals, turtles and sharks. Lost nets do not rot and continue to catch up as “ghost nets”. These networks are now largely banned by a UN resolution.

Target species: Many species of fish, such as salmon and tuna.

Effects: Bycatch of marine mammals, turtles, sharks etc.

There are several methods of farming fish. One can roughly distinguish between open and closed systems. In open systems there is an exchange between the breeding facility and the environment, while closed systems are cut off from the natural environment.


Flow systems

Water is drained from flowing water into channels where the fish are kept. After treatment, the wastewater is returned to the natural water.

Cultivars: especially trout.

Effects: If the water is poorly treated, it can pollute the natural body of water and lead to the transmission of diseases. High energy expenditure.


Mussel cultures

Mussels grow in hanging cultures on nets or ropes. In soil cultures, small mussels are brought to a previously prepared area and can grow there.

Cultivars: Mussels, scallops.

Effects: Since mussels filter their food from the surrounding water, they do not need any additional feed. Hanging cultures have little impact on the sea floor. In the case of soil cultures, the seabed is plowed up with dredges when the mussels are harvested, but these are mostly areas that have been used for a long time, have been ecologically modified and are poor in species.


Open net enclosures

Large net structures are hung up in the sea or in inland waters and the fish are fattened there.

Cultivars: various types of fish, for example tuna or salmon.

Effects:Wastewater, faeces, leftover feed, chemicals and antibiotics used get into the surrounding water. The floor under the net pens is often heavily polluted with organic material and residues. Farmed fish can escape into the environment, mix with wild populations, and transmit diseases.


Pond management

In pond systems, the species is kept inland or in coastal areas in artificial or closed natural ponds. Wastewater can be retained and treated in such a system. In extensive farms the density of fish in the ponds is low, in intensive farms it is high.

Cultivars: for example tilapia, shrimp, carp.

Effects: The construction of ponds for shrimp farming in particular has led to the destruction of sensitive areas such as mangroves in many coastal areas. Untreated sewage can pollute the environment. Residues from abandoned and dried-out systems can spread into the environment.


Circulation systems

The fish are kept in basins or tanks. The water is treated and circulated through the system so that hardly any fresh water is required. The water quality can be precisely monitored and adjusted. Almost all types of fish can be bred this way. Such systems address many of the environmental concerns of other systems, but are expensive to operate.

Cultivars: various types of fish

Effects: high energy consumption

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