Gillnet

From Wikipedia, the free encyclopedia

Jump to: navigation, search
Oil painting of gillnetting, The salmon fisher by Eilif Peterssen.

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.

External images
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.

Contents

[edit] History

See also: History of fishing

Gillnetting began with aboriginal 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 Northwest, 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 maintenance than natural fibres. In addition, multifilament nylon, monofilament or multimonofilament fibres 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 and degradation, hence the netting has the potential to last for many years if it is not recovered. This ghost fishing is of environmental concern. Attaching the gillnet floats with biodegradable material can reduce the problem.[1] However it is difficult to generalize 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[1], while others have found lost nets to be destroyed by wave action within one month[2] 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[3].

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[4]. 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

Selectivity properties of a gillnet on a hypothetical population.

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[5].

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

The FAO classifies gillnet gear types as follows:

[edit] Set gillnets

Set gillnets consist of a single netting wall kept vertical by a floatline and a weighted groundline. Small solid floats, usually plastic and cylindrical or egg-shaped, are attached to the floatline (upper line/headrope) while lead weights are evenly distributed along the groundline (lower line/footrope). The lower line can also be made of lead cored rope which does not need additional weight. The net is set on the bottom, or at a distance above it and held in place with anchors or weights on both ends. By adjusting the design these nets can fish in surface layers, in mid water or at the bottom, targeting pelagic, demersal or benthic species. On small boats gillnets are handled by hand. Larger boats use hydraulic net haulers or net drums. Set gillnets are widely used all over the world, and are employed both in inland and sea waters. They are popular with artisanal fisheries because no specialized gear is needed, and it is low cost based on the relationship of fuel/fish.[1]

[edit] Encircling gillnets

Encircling gillnets are gillnets which are set vertically in shallow waters with the floatline remaining at the surface so they encircle fish. Small open boats or canoes can be used to set the net around the fish. Once the fish are encircled, the fishers shout and splash the water to panic the fish so they gill or entangle themselves. There is little negative impact on the environment. As soon as the gear is set the scaring takes place and the net is hauled back in. The fish are alive and discards can be returned to the sea. Encircling gillnets are commonly used by groups of small-scale fishers, and does not require other equipment.[2]

[edit] Combined gillnets-trammel nets

This bottom-set gear has two parts

  • the upper part is a standard gillnet where semi-demersal or pelagic fish can be gilled
  • the lower part is a trammel net where bottom fish can entangle.

The combined nets are maintained more or less vertically in the usual way by floats on the floatline and weights on the groundline. They are set on the bottom. After a time depending on the target species, they are hauled on board. Traditional combined nets were hauled by hand, especially on smaller boats. Recent hydraulic driven net haulers are now common. The gilled, entangled and enmeshed fish are removed from the net by hand. Of some concern with this method is ghost fishing by lost nets and bycatch of diving seabirds. Nets combined in this way were first used in the Mediterranean.[3]

[edit] Driftnets

Driftnets consist of a string of gillnets, kept more or less vertical by floats on the floatline and weights on the groundline, which drift with the current usually near the surface or not far below it. These nets drift freely with the current while connected to the operating vessel, the driftnetter. They are especially employed in marine waters, and are used to catch schooling pelagic species like herring, mackerel and sardines (forage fish, but also salmon, tuna and pelagic squid. Net haulers are usually used to set and haul driftnets, with a drifter capstan on the forepart of the vessel. In developing countries most nets are hauled by hand. Gillnets have a high degree of size selectivity for fish, efficiently regulated by the mesh size. It is also a gear with low fuel/fish energy consumption. The issue of concern with this type of net is the bycatch of non-target species like marine mammals, seabirds and to a minor extent turtles. In 1991, the United Nations banned the use of large scale high seas driftnets longer than 2.5 kilometers. Prior to this ban, these nets were reaching lengths of 60 kilometres. However, there are serious concerns exist regarding ongoing violations.[4]

[edit] Gillnets and entangling nets

[5]

[edit] Gillnets and entangling nets (not specified)

[6]

[edit] Fixed gillnets (on stakes)

[7]

[edit] Notes

  1. ^ a b FAO: Fishing Gear Types: Gillnets
  2. ^ FAO: Fishing Gear Types: Gillnets
  3. ^ FAO: Fishing Gear Types: Combined gillnets-trammel nets
  4. ^ FAO: Fishing Gear Types: Driftnets
  5. ^ FAO: Fishing Gear Types: Gillnets and entangling nets
  6. ^ FAO: Fishing Gear Types: Gillnets and entangling nets (not specified)
  7. ^ FAO: Fishing Gear Types: Fixed gillnets (on stakes)

[edit] References

  1. ^  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.
  2. ^  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.
  3. ^  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.
  4. ^  Potter, E.C.E. & Pawson, M.G. (1991) Gill Netting. MAFF Fisheries Leaflet 69. [6]
  5. ^  Puente, E. (1997) Incidental impacts of gill nets. Report to the European Commission, No. 94/095,152.

[edit] External links

v • d • e
Fishing techniques
Gathering
Spears
Lines
Nets
Traps
Other
v • d • e
Fisheries and fishing topic areas
Fisheries
Fishing
Industry
Recreational
Techniques
Tackle
Locations
List of articles by topic areas · Alphabetical list of articles · Fisheries glossary
Retrieved from "http://en.wikipedia.org/wiki/Gillnet"
Personal tools