Cetacean Monitoring Systems
Fishing gears are classified in several ways and the classifications overlap causing much confusion. The following list of terms and descriptions therefore has some repetition.
The nets are of heavy material.
Also know as pelagic trawls. These are towed to catch shoaling fish. In some fisheries they may reach the sea bed or sea surface.
These come in various types:
They consist of a lightweight curtain of net hanging from floats at the surface and moving with the water. They are generally accompanied by the boat that set them.
In gill netting, fish are caught when they enter the meshes, but are unable to go right through. In practice, many fish are caught without actually being 'gilled' in that way. Gill nets are usually made of nylon, mono or multifilament and have floats on the headrope to hold them up. Fish caught are often species that swim actively and nets are typically hauled every day to keep the fish in good condition and prevent scavengers from eating them.
These are similar looking nets, but with larger meshes and little or no flotation. These nets sit on or close to the sea bed and large species such as flatfish or monkfish become entangled. Some of these fish stay alive without swimming and nets are typically hauled every 3 - 5 days.
These are a more complicated nylon nets that catch fish in pockets of a light, small mesh net that is hung between very large mesh nets so that a fish that has created a pocket of the small mesh net cannot get out of it.
This is the collective name given to any form of gill, tangle or trammel net, i.e. they are not drift nets.
Hake, whitefish, ray, monk nets etc
So named to identify specific mesh sizes, net heights, hanging ratios, twine weights, footrope weights, and floatation. The hanging ratio is how long a piece of net is fastened to how much headrope, and affects how loose the net is and whether it is more tangling than gilling.
A term used for boats using set or drift nets.
A term used for boats using trawls.
Boats fishing with lines and hooks.
Many small boats switch between gear types on a seasonal basis.
Any of the lightweight nets made using mono-filament nylon.
The same as monofilament but consisting of multifilament strands. This net is more flexible and packs down more compactly on board. Sometimes more efficient but also more liable to catch unwanted species.
Bycatch and other impacts of fisheries
Bycatch is the capture of non-target species, mostly of no commercial value to the fishery.
These capture many bottom living species of fish and other animals. They often take large numbers of undersized fish. Trawls flatten out the habitat of juvenile fish and destroy shellfish beds, with knock-on effects on fish recruitment, and water quality. They have high energy use.
Mid-water trawls and purse seines
Bycatches are generally lower and vary with location. Capture of mixed species shoals can be a big problem. Dolphins are attracted to these trawls in many places around the world and exploit the feeding opportunities they present, but often die in the net.
These can be very good at avoiding the capture of undersized fish and are much lower in energy costs. They catch porpoises, where this species occurs, and other cetaceans when they are bottom feeding. This can be prevented by deployment of pingers. The nets sweep areas of sea bed as they move with the tide and catch up sea fans, which can be greatly reduced in numbers as a reult. All forms of set net, and especially tangle nets, catch crabs that can be too entangled to be removed alive. Some nets have ‘legs’ on the bottom to reduce this problem.
These catch all cetacean species in the area. This can be prevented by deployment of pingers.
These may catch unwanted crustacean species and may continue fishing if lost, as successive captures become bait for further captures.
Please let us know if you need any specific information on fisheries.