Fishermen use a wide range of gear to land their catch. Every type has its own effects on the ocean. By selecting the right gear for the right job, the fishing industry can help minimize its impact on the environment.
A bottom trawl is a type of fishing net that's pulled along the seafloor. Fishermen commonly use bottom trawls to catch shrimp and bottom-dwelling fish like halibut and sole. However, in addition to these target fish, the nets also catch a variety of ocean life that's usually thrown back dead or dying. Dragging heavy gear across the seabed can also damage sensitive seafloor habitat. The harmful effects of bottom trawling on bottom-dwelling organisms and their habitat can be reduced by modifying the fishing gear or limiting trawling areas.
Dredges are large, metal-framed baskets that are dragged across the seafloor to collect shellfish like oysters, clams and scallops. In order to lift the catch into the basket, metal teeth dig into the seafloor, which can significantly impact seafloor habitat and bottom-dwelling species. Dredging also results in high levels of bycatch. By restricting dredging areas, bycatch and damage to seafloor life can be reduced.
Gillnetting uses curtains of netting that are suspended by a system of floats and weights; they can be anchored to the seafloor or allowed to float at the surface. The netting is almost invisible to fish, so they swim right into it. Gill nets are often used to catch sardines, salmon and cod, but can accidentally entangle and kill other animals, including sharks and sea turtles.
Harpooning is a traditional method for catching large fish—and it's still used today by skilled fishermen. When a harpooner spots a fish, he thrusts or shoots a long aluminum or wooden harpoon into the animal and hauls it aboard. Harpooners catch large, pelagic predators such as bluefin tuna and swordfish. Harpooning is an environmentally responsible fishing method. Bycatch of unwanted marine life is not a concern because harpoon fishermen visually identify the species and size of the targeted fish before killing it.
Jigging is another way of hand-catching fish. A jig is a type of grapnel (or grappling hook) attached to a line, which is manually or mechanically jerked in the water to snag the fish. Jigging often occurs at night, aided by light to attract the fish. This targeted method of fishing has low levels of bycatch, making it an environmentally responsible fishing method.
Longlining employs a central fishing line that can range from one to 50 miles long; this line is strung with smaller lines of baited hooks, dangling at evenly spaced intervals. Longlines can be set near the surface to catch pelagic fish like tuna and swordfish, or laid on the seafloor to catch deep-dwelling fish like cod and halibut. Many lines, however, can hook sea turtles, sharks and seabirds that are also attracted to the bait. By sinking longlines deeper or using different hooks, fishermen can reduce the bycatch problem.
Midwater trawlers vary in size—from small ships to large factory vessels. Large industrial ships pull gigantic nets through the open ocean and can catch an entire school of fish—spanning the size of five football fields—at once. These trawls don't impact the seafloor when used in the midwater zone. Setting these trawls on schooling fish using streamer lines to scare away seabirds and avoiding areas with an abundance of marine mammals can help ensure low levels of bycatch in midwater trawl fisheries.
Pole-and-line fishermen use a fishing pole and bait to target a variety of fish ranging from open-ocean swimmers, like tuna and mahi mahi, to bottom-dwellers, like cod. Pole/troll fishing methods have very low bycatch rates because fishermen catch one fish at a time and can release unwanted species when they're caught.
Purse seining establishes a large wall of netting to encircle schools of fish. Fishermen pull the bottom of the netting closed—like a drawstring purse—to herd fish into the center. This method is used to catch schooling fish, such as sardines, or species that gather to spawn, such as squid. There are several types of purse seines and, depending on which is used, some can catch other animals (such as when tuna seines are intentionally set on schools of dolphins).
A seine is a small-meshed net, suspended vertically in the water with floats and weights to enclose and concentrate fish. Beach (or haul) seines are dragged over the bottom into shallow water or onto the beach, either by hand or with power winches. Danish, Scottish and Japanese seines are typically larger gears pulled by vessels. They're deployed over soft sediment, like sand or mud, as the cloud of sediment helps herd the fish into the net.
Traps and Pots
Traps and pots are submerged wire or wood cages that attract fish with bait and hold them alive until fishermen return to haul in the catch. Traps and pots are usually placed on the ocean bottom, often to catch lobsters, crabs, shrimp, sablefish and Pacific cod. They generally have lower unintended catch and less seafloor impact than mobile gear like trawls.
This hook-and-line method of fishing tows lines behind or alongside a boat, catching species, such as salmon, mahi mahi and albacore tuna, that follow a moving lure or bait. Fishermen can quickly release unwanted catch from their hooks since lines are reeled in soon after a fish takes the bait. This selective method of fishing results in very low bycatch levels.
Fish Farming (Aquaculture)
In the next decade, the majority of fish we eat will be farm-raised, not wild. Global aquaculture includes over 100 species, farmed in everything from traditional earthen ponds to high-tech tank systems. Each farming system has its own distinct environmental footprint. By choosing seafood from better farms and production systems, consumers can play a positive role in reducing aquaculture's potential negative impacts.
In bag-and-rack shellfish culture, juveniles are cultivated in bags on racks above the seabeds. The juveniles (or spat) come from hatcheries, so wild populations aren't depleted and habitats aren't damaged by dredging for starter organisms. Since most cultivated shellfish filter-feed, there's no addition of wild fish feed, and therefore no impact on wild fish stocks. While nonnative species may be grown in these systems and can possibly escape—typically due to the contained shellfish spawning—overall, this is an environmentally responsible method of aquaculture.
Hatchery fish are bred and reared in nurseries, either for use in aquaculture or to be released and caught in wild-capture fisheries. When released in large numbers, hatchery-raised fish can compete with wild populations for food. There are also concerns about genetically weaker hatchery fish interbreeding with and threatening the viability of wild populations.
Open Net Pens or Cages
Open net pens or cages enclose fish such as salmon in offshore coastal areas or in freshwater lakes. Net pens are considered a high-impact aquaculture method because waste from the fish passes freely into the surrounding environment, polluting wild habitat. Farmed fish can also escape and compete with wild fish for natural resources or interbreed with wild fish of the same species, compromising the wild population. Diseases and parasites can also spread to wild fish living near or swimming past net pens.
Ponds enclose fish in a coastal or inland body of fresh or salt water. Shrimp, catfish and tilapia are commonly raised in this manner. Wastewater can be contained and treated. However, the discharge of untreated wastewater from the ponds can pollute the surrounding environment and contaminate groundwater. Moreover, the construction of shrimp ponds in mangrove forests has destroyed more than 3.7 million acres of coastal habitat important to fish, birds and humans.
Raceways allow farmers to divert water from a waterway, like a stream or well, so that it flows through channels containing fish. In the U.S., farmers use raceways to raise rainbow trout. If untreated, wastewater from the raceways can contaminate waterways and spread disease. Many governments require strict regulation and monitoring of water quality; by treating wastewater these risks can be greatly minimized. Escaped fish are another potential problem—they can compete with wild fish for natural resources and also interbreed with wild fish of the same species, potentially weakening the species genetically.
Recirculating systems raise fish in tanks in which water is treated and recycled through the system. Almost any finfish species such as striped bass, salmon and sturgeon can be raised in recirculating systems. Recirculating systems address many environmental concerns associated with fish farming—fish cannot escape, and wastewater is treated—but they are costly to operate and rely on electricity or other power sources.
Shellfish culture means that farmers grow shellfish on beaches or suspend them in water by ropes, plastic trays or mesh bags. The shellfish farmed using these methods—oysters, mussels and clams—are filter feeders and require only clean water to thrive. Filter feeders can actually filter excess nutrients out of the water, but farming shellfish in high densities in areas with little current or tidal flow can lead to the accumulation of waste. Also, historically, some shellfish culture has been responsible for the introduction of exotic species that can sometimes out-compete native species for natural resources.
Submersible Net Pens
Submersible net pens (or cages) are designed for use in offshore aquaculture and can be used for a variety of fish species. This method can be damaging to the local environment because water flows freely through the cages, carrying fish waste, uneaten food and chemicals into the surrounding water. Also, since the pens are open, there's the potential for farm diseases to transfer to wild fish populations.
Shellfish culture can be very sustainable. Species such as oysters, mussels and clams are filter feeders that don't require additional feed and therefore don't impact wild fish stocks. Farmers grow shellfish on beaches or suspend them in water by ropes, nets, trays or mesh bags. Habitat damage and impacts from effluent are generally minimal.
Ranching is the process of capturing valuable juvenile species, usually carnivorous top predators, then growing them to harvest size on farms. Highly profitable species such as bluefin tuna are caught at a relatively large size and "fattened" before harvest. In addition to concerns about the high use of feed and waste from such farming operations, ranched species can't contribute to their natural ecosystems, threatening the overall health of those environments.