Commercial Fishing Methods
The way that a fish is caught is essential to understanding its sustainability.
Most Common Methods
A fishery is partially defined by the way that it is caught. Gear selection plays a major role in determining the cost, efficiency, and bycatch of a fishery. In this post, we explain all the various methods to get fish out of the water and onto your plate.
The figure below shows the proportion of wild-caught fisheries by fishing method.
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Fishing with Nets
When you think of commercial fishing you probably think of big, giant nets swooping up a school of fish. You’re not wrong! Over 80% of fish are caught via nets. There are several different kinds, though:
Purse Seine
In purse seine fishing, the most common way fish are caught, a boat locates a school of fish, then, using either a crane or small boat, takes one end of a net around the school and back to the fishing vessel. The ends of the net are synched together like a drawstring bag and pulled aboard with the fish inside.
Because purse seining targets a particular school of fish after it has been located, bycatch is extremely low. Sometimes fishing boats will deploy floating objects like big rafts or floating barrels to attract fish (fish love structure); these fish aggregating devices (FADs) make seining much easier by reducing fuel use and time spent looking for schools of fish, however FADs also attract other marine life and bycatch is higher. Over half of all tuna is caught using purse seines; when using FADs bycatch ranges between 1-8%, whereas without FADs bycatch is less than 1%.
Trawling
Trawling is dragging a net through the water behind a boat. There are two different kinds: bottom trawls and midwater trawls.
Bottom trawl
Bottom trawls involve weighing a net down to the seafloor then dragging it across the bottom to scoop up fish.
Bycatch is not overly concerning with bottom trawls, but habitat damage is. Sandy bottoms and rocky environments regenerate fairly quickly after a bottom trawl net comes through, but bottom trawls can significantly impact sensitive habitat like deepwater coral or sponge gardens. Good fishery management ensures that bottom trawling is done in sustainable areas and not in places with irreplaceable habitat.
Midwater trawl
Midwater trawls pull a net through the water off the bottom. Bycatch is low.
Gillnet
Gillnets are set up to be a wall with holes in it. Fish unknowingly swim into it and get stuck. Gillnets don’t require a boat with a big engine so they are often used in less developed areas of the world.
Gillnets have the most bycatch of any kind of fishing net, but their use has been declining. Gillnets can be sustainable in some cases, for example salmon congregate at choke points that can be walled off without affecting other species. Gill nets that are not set at a particular location and drift with the current are called drift nets. These also have serious issues with bycatch.
Fishing with line
Longlines
Longlines are very long fishing lines that have a hook every few feet. They can be many miles long.
Bycatch in longline fisheries is highly variable depending on the fishery. Halibut longlines in Alaska have very little bycatch while longlines meant to harvest tuna catch about 20% bycatch. Issues in longlining are more common in fisheries close to the surface where seabirds, sharks, and turtles get caught eating baited hooks. Regulations are better in higher capacity countries where fishery managers can require specialized hooks and weights that reduce bycatch.
Pole and line
A fishing pole and line catches fish individually. There is no concern over bycatch.
Harvesting Shellfish
Dredge
Dredging is similar to bottom trawling, but instead of a net, a metal rake of sorts is dragged across the bottom to collect shellfish and bivalves buried in the substrate, e.g. scallops, clams, or mussels.
Bycatch is low but the same considerations apply for bottom trawling: good fishery management ensures that dredging is done in sustainable sandy areas and not in places with irreplaceable habitat
Traps and Pots
Traps and pots are mainly used to catch invertebrates like crab and lobster. Traps or pots are dropped to the bottom with bait to attract crab and lobster. Once they crawl inside, they can’t escape and are pulled back to the surface when the fishers return.
Bycatch is not a problem, but sometimes gear gets swept away or fishers forget where they set their traps. This has led to some whale entanglements, particularly on the West Coast of the U.S. Some states have started offering reward to people who collect escaped traps and pots.
Diving
Some commercial fisheries, like sea urchin, geoduck, and sea cucumber are harvested by divers.
This post is part of Sustainable Seafood 101.
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