Gillnet
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Gillnetting is a common fishing method used by commercial fishermen of all the oceans and in some freshwater and estuary areas. Because gillnets can be so effective their use is closely monitored and regulated by fisheries management and enforcement agencies.Mesh size, twine strength, as well as net length and depth are all closely regulated to reduce bycatch of non-target species. Most salmon fisheries in particular have an extremely low incidence of catching non-target species.
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A bottom set gillnet |
Gillnet, the name of the net employed, illustrates the method used to snare target fish. They try to swim through deliberately sized mesh openings but are unable to squeeze through swimming forward. Once in this position, they are prevented from backing out due to the tendency for their gills to become caught. This effectively traps them.
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[edit] History
- See also: History of fishing
Gillnetting began with First Nations fishermen using canoes and cedar fiber nets. They would attach stones to the bottom of the nets as weights, and pieces of wood to the top, to use as floats. This allowed the net to suspend straight up and down in the water. Each net would be suspended either from shore or between two boats. Native fishers in the Pacific Nortwest, Canada, and Alaska still commonly use gillnets in their fisheries for salmon and steelhead.
By around 1864, gillnetting had expanded to European, Japanese, and other international fisheries. The boats used by these fisherman were typically around 25 feet (8 m) long and powered by oars. Many of these boats also had small sails and were called "row-sail" boats. At the beginning of the 1900s, steam powered ships would haul these smaller boats to their fishing grounds and retrieve them at the end of each day. However, at this time gas powered boats were beginning to make their appearance, and by the 1930s, the row-sail boat had virtually disappeared.
In 1931, the first powered drum was created by Laurie Jarelainen. The drum is a circular device that is set to the side of the boat and draws in the nets. The powered drum allowed the nets to be drawn in much faster and along with the faster gas powered boats, fisherman were able to fish in areas they had previously been unable to go into, thereby revolutionizing the fishing industry.
During World War II, navigation and communication devices, as well as many other forms of maritime equipment (ex. depth-sounding and radar) were improved and made more compact. These devices became much more accessible to the average fisherman, thus making their range and mobility increasingly larger. It also served to make the industry much more competitive, as the fisherman were forced to invest more into their boats and equipment in order to stay up to date with the current technology.
The introduction of fine synthetic fibres such as nylon in the construction of fishing gear during the 1960s marked an expansion in the commercial use of gillnets. The new materials were cheaper and easier to handle, lasted longer and required less mainatenance than natural fibres. In addition, fibres such as nylon monofilaments become almost invisible in water, so nets made with synthetic twines generally caught greater numbers of fish than natural fibre nets used in comparable situations.
Nylon is highly resistant to abrasion, hence the netting has the potential to last for many years if it is not recovered. This ghost fishing is of environmental concern, however it is difficult to generalise about the longevity of ghost-fishing gillnets due to the varying environments in which they are used. Some researchers have found gill-nets to be still catching fish and crustaceans for over a year after loss, while others have found lost nets to be destroyed by wave action within one month or overgrown with seaweeds, increasing their visibility and reducing their catching potential to such an extent that they became a microhabitat used by small fishes.
This type of net was heavily used by many Japanese, South Korean, and Taiwanese fishing fleets on the high seas in the 1980s to target tunas. Although highly selective with respect to size class of animals captured, gill nets are associated with high numbers of incidental captures of cetaceans, (whales and dolphins). In the Sri Lankan gill net fishery, one dolphin is caught for every 1.7-4.0 tonnes of tuna landed. This compares poorly with the rate of one dolphin per 70 tonnes of tuna landed in the eastern Pacific purse seine tuna fishery. Gillnets were banned by the United Nations in 1993 in international waters, although their use is still permitted within 200 nautical miles (400 km) of a coast.
[edit] Selectivity
Gill nets are basically a series of panels of meshes with a weighted "foot rope" along the bottom, and a "headline", to which floats are attached. They can therefore be set to fish at any height in the water column. The meshes of a gill net are uniform in size and shape, hence highly selective for a particular size of fish. Fish which are smaller than the mesh of the net are able to pass through unhindered, while those which are too large to push their heads through the meshes as far as their gills are not retained. This gives a selectivity ogive which is skewed towards medium sized fishes, unlike active gears such as trawling, in which the proportion of fish entering the net which are retained increases with length.
Commercial gillnet fisheries are still an important method of harvesting salmon in Alaska, British Columbia, Washington, and Oregon. In the Columbia River, non-Indian commercial salmon fisheries for spring chinook have developed methods of selectively harvesting adipose fin clipped hatchery salmon using small mesh gillnets. Non-fin clipped (primarily natural origin salmon are required to be released. Fishery management agencies estimate a relatively low release mortality rate on salmon and steelhead released from these small mesh gillnets.
Gillnets are sometimes a controversial gear type especially among sport fishers who sometimes argue they are inappropriate especially for salmon fisheries. Most salmon fisheries are strictly managed to minimize total impacts to specific populations and salmon fishery managers continue to allow their use.
[edit] Types of Gillnets
- Anchored Sink Gillnet -
- Drift Floating Gillnet -
- Drift Sink Gillnet -
- Anchored Floating Gillnet -
[edit] References
- ^ Erzini, K. Monteiro, C., Ribeiro, J., Santos, M., Gaspar, M., Montiero, P. & Borges, T. (1997) An experimental study of "ghost-fishing" off the Algarve (southern Portugal). Marine Ecology Progress Series 158:257-265.
- ^ Hall, M.A. (1998) An ecological view of the tuna-dolphin problem: impacts and trade-offs. Reviews in Fish Biology and Fisheries. 8:1-34.
- ^ Kaiser, M.J, Bullimore, B., Newman, P., Lock, K. & Gilbert, S. (1996) Catches in "ghost-fishing" set nets. Marine Ecology Progress Series. 145:11-16.
- ^ Potter, E.C.E. & Pawson, M.G. (1991) Gill Netting. MAFF Fisheries Leaflet 69. [6]
- ^ Puente, E. (1997) Incidental impacts of gill nets. Report ot the European Commission, No. 94/095,152.
[edit] External links
- Video of gillnets in operation
- Manual on estimation of selectivity for gillnet and longline gears in abundance surveys - report for Food and Agriculture Organisation of the United Nations, 2000.
- http://www.seawatch.org/position_papers/gillnet.php
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