Conservation Gear Engineering—Finding Solutions

With all types of gear there have been many suggestions on how to reduce bycatch. These solutions fall into three main categories. The first is how to keep fish out of the gear, basically how to not catch the animal. The second is how to help fish escape the gear once the encounter it. The third is how to minimize damage to the animal when it encounters the gear.

Gear specific bycatch issues

There are specific bycatch issues associated with each type of gear and fishery. A general list of commercial gear consists of trawls, gillnets, pots, dredges and longlines. The main recreational gear is hook and line.

Trawl

Trawls are mobile gear. They are dragged or towed through the water or over the seabed. They are also considered to be active gear because fish don't voluntarily enter the gear but are caught by the moving gear. Most trawl fisheries are considered multispecies meaning that the catch is a mixture of different species. In general, trawls are relatively non-selective to species; however, they are comparatively selective in terms of size of animals captured. This selection occurs mainly in the codend and is primarily affected by mesh size and secondarily by other factors such as mesh shape, twine characteristics, and net construction.

There are a variety of technological innovations that have been utilized to reduce the catch from a trawl. The most commonly used method to limit the capture of sublegal size fish is the regulation of minimum codend mesh size. In addition to changes in mesh size, size selectivity of trawls has been improved by altering the shape of the mesh, i.e., square mesh. Excluder devices are another method for separating out non-target species. These devices consist of a system of large windows, funnels, and grids Excluder devices have been developed to reduce the catch of sea turtles in the tropical shrimp fishery as well as reduce the capture of fish in the northern shrimp fisheries. Fish behavior has been utilized to develop methods for eliminating catch. Large mesh panels have been placed in the bottom of the net and separator panels have also been used to separate different species. The raised footrope has been used to reduce the capture of fish based on their habitat preference and swimming behavior.

Dredges

Dredges are metal framed mobile gear that are towed on the seabed. Dredges target shellfish and other benthic or bottom dwelling creatures. Bycatch in dredges include finfish such as monkfish and flounders, as well as, lobsters and other benthic organisms. The selectivity of dredges is controlled by ring diameter, metal bar spacing, and mesh size on the top of the dredge. For example, large mesh twine tops are being explored as a means of reducing finfish bycatch in the scallop fishery. Even though the width of dredges is limited, the impact they have on the bottom is another key issue.

Gillnets

Gillnets are passive gear, meaning that the fish has to voluntarily swim into the gear to be captured. In general, gillnets are considered to have a low impact on the seabed. The means of capture is that the fish are gilled, entangled, or enmeshed in the netting. Gillnets, are, in general, considered as having a high degree of selectivity, in terms of fish species, as well as size of the fish which directly depends on the size of the mesh. Gillnets are considered very size selective, capturing fish of uniform size. Gillnet selectivity is influenced by a variety of properties such as mesh size and shape, as well as, the hanging ratio which influences the slack in the netting. Mesh size is probably the most important property affecting capture. The material used for the netting also influences its catch. Thickness and color help to either increase or reduce the catch of certain animals, depending on the objective.

Marine mammals have been one of the major bycatch concerns for gillnets. One solution that was adopted was the use of acoustic pingers which make the net visible to harbor porpoise. The use of weak links at the buoy in some gillnet fisheries is another method of reducing the incidental capture of marine mammals, specifically whales.
Illustration of a gillnet

Pots

Pots are considered one of the most environmentally friendly gears. In addition, the quality of the catch is good and the survivability of discards is considered better than other gear. However, they too have bycatch associated with them. Bycatch includes undersized animals and non-target species as well as marine mammals that get caught in the lines. To reduce the bycatch within the pots a variety of techniques have been utilized. These include escape vents that help reduce the number of undersized animals. The mesh size of the pot itself also helps smaller animals to escape. To aid in minimizing the incidental capture of marine mammals, specifically right whales, a variety of techniques have been suggested including break-away lines and weak links at the buoy line and float lines.

Hooks

Hooks are used in both commercial and recreational fisheries. Commercially there are longline, and hook and line fisheries, and rod and reel in the recreational fishery. Bycatch occurs in all hook fisheries. The longline is a passive gear that has minimal impact on the seabed and lands a high quality catch. In the longline fisheries, bycatch includes undersized and non-target species including marine mammals and birds. Bycatch of marine organisms is not limited to just commercial fishing operations. In recreational fisheries, like in commercial fisheries, there are regulatory discards, fish that are below the minimum size or over the bag limit as well as unwanted species or sizes that are discarded. The use of circle hooks in these hook fisheries help to minimize bycatch mortality because circle hooks typically secure the fish by the mouth instead of the gullet.

Home | Bycatch | Solutions | Workshops | Experts References | Results | Events | Links Northeast Sea Grant Fisheries