Active or mobile gears are those that are actively moved in order to catch fish by trapping or encirclement. They may conveniently be divided into those that are towed on the seabed (demersal gear) and those that remain clear of the seabed (pelagic gear).

 

Otter trawl

Otter trawls are cone-shaped nets which are towed along the seabed to catch groundfish and prawns. They take their name from the rectangular 'doors' or 'otterboards' that serve to keep the mouth of the net horizontally open while the net is being towed. A vertical opening is maintained by weights on the bottom and floats on the top. The net traps fish in the end of the bag-like section known as the cod-end. The cod-end has a mesh size that permits only the smaller fish to escape. The net is dragged along the sea bed with the aid of bobbins, chains, rollers and "rockhoppers" which can roll across or dig into the bottom or bounce over obstacles. The movement of the net over the seabed can crush and kill seabed organisms. Depending on when and where the net is used, this method can have a very high bycatch, most of which will probably be discarded.

 

Very similar to otter trawl, but in this method of fishing the net is towed by two vessels working together. As the separation between the boats maintains the horizontal opening of the net, no doors are required.

 

Danish seines, also referred to as demersal seines, are used to catch species of groundfish, generally in shallower waters than otter trawls. A net and ropes are spread out in a pear shape along the ocean floor. The vessel remains in a fixed position while the gear is hauled back inboard along the bottom. The rope movement herds the fish into the path of the net. While these fisheries have broadly the same effects as otter trawls, the area damaged will be less and penetration into the seabed will be less as the gear is not so heavy.

 

Beam trawls are used principally in waters shallower than 50m in depth to catch flatfish over relatively even sandy seabed. A beam is used to hold the net up and open. Chains are dragged under the base of the nets to disturb fish from the sediments. This gear, being heavier, penetrates further into sediments and thus causes greater mortality than lighter gears.

 

A scallop dredge consists of a metal frame with spring-loaded teeth to which a chain-mesh bag is attached. The dredge is towed over suitable seabed habitats, where the teeth rake the seabed disturbing the scallops. This gear is extremely robust and can be used over much harder grounds than traditional trawls. As a consequence, scallop dredges can severely damage other seabed organisms and habitats.

 

Suction or hydraulic dredging is used to harvest shellfish buried in the sea bed. All work on the principle of shooting water into sediments and then collecting the displaced shellfish. In the hydraulic dredge, they are collected in a mesh bag while in the suction dredge, they are sucked to the surface through a pipe. The size and type of the suction heads vary and can be either self-propelled or hand-operated. This gear is used in shallow water, often in estuaries and can result in significant scarring of the sediment which may affect sensitive species and habitats. It can be a very efficient method for collecting shellfish and regulation is usually needed to prevent over-harvesting.

 

 

Mid-water trawls resemble otter trawls as they are cone-shaped. Unlike otter trawls, they do not come into contact with the sea bed and can be adjusted for towing at various depths. This adjustment is made by increasing the vessel's speed or by increasing or decreasing the length of the cable between the vessel and the net. Since the fish targeted by this method are often in single species shoals, the bycatch of fish may be low. However, in some areas and fisheries, dolphins appear to be particularly susceptible to being caught and killed. Measures such as the inclusion of a grid to enable dolphins to escape upwards out of the net are being tested to alleviate this problem. Mid water trawls are usually very large, and are often towed by two vessels working together, known as "pelagic pair trawling".

 

A purse seine is a wall of net used to encircle shoals of mid-water fish. The purse seine has floats on the top and weights on the bottom to keep it vertical in the water. A wire rope passing through rings on the bottom of the net enables the net to be drawn together to form a purse and trap the fish. As with mid-water trawls, the fish targeted are usually in single species shoals. Tuna fishermen using purse seines may encircle dolphins also. However, a technique, known as the backdown procedure, has been developed to allow the dolphins to escape and relatively few cetaceans are now killed in these fisheries.