A fishing weir, or fish weir, is an obstruction placed in tidal waters or wholly or partially across a river, which is designed to hinder the passage of fish. Traditionally they were built from wood or stones. They can be used to trap fish. For example, salmon and other fish can be trapped when they attempt to swim upstream, or eels can be trapped when they attempt to migrate downstream.
Alternatively, fish weirs can be used to redirect fish elsewhere, such as to a fish ladder.
As fish traps, fishing weirs date back to the Bronze Age in Sweden and to Roman times in the UK. They were used by native North Americans and early settlers to catch fish for trade and to feed their communities.
In medieval Europe, large fishing weir structures were constructed from wood posts and wattle fences. 'V' shaped structures in rivers could be as long as 60 m and worked by directing fish towards fish traps or nets. Such fish traps were evidently controversial in medieval England. The Magna Carta includes a clause requiring that they be removed:
All fish-weirs shall be removed from the Thames, the Medway, and throughout the whole of England, except on the sea coast.
Basket weir fish traps were widely used in ancient times. They are shown in medieval illustrations and surviving examples have been found. Basket weirs are about 2 m long and comprise two wicker cones, one inside the other—easy to get into and hard to get out.
In the UK the traditional form was one or more rock weirs constructed in tidal races with a small gap that could be blocked by wattle fences when the tide turned to flow out again. Surviving examples, but no longer in use, can be seen in the Menai Strait. Because they were so effective they reduced inshore fish stocks and in 1861 Parliament banned their use except where they could be shown to have been in use prior to the Magna Carta. An example of such a fishing weir was at Rhos Fynach in North Wales, which survived in use until World War I.
An enormous series of fish weirs, canals and artificial islands was built by an unknown pre-Columbian culture in the Baures region of Bolivia, part of the Amazonian savannah. These earthworks cover over 500 square kilometres (190 sq mi), and appear to have supported a large and dense population around 3000 BCE. See also Dr. Erickson's research website. An approximate location for these sites is 63.4128° W, 13.4812° S.
In North America, fishing weirs are constructed using wooden stakes woven together to create a barrier that traps fish while letting water pass through. The pattern of wooden stakes depends on the location and nature of the waters being fished.
In the Back Bay area of Boston, Massachusetts, wooden stake remains of the Boylston Street Fishweir have been documented during excavations for subway tunnels and building foundations. The Boylston Street Fishweir was actually a series of fish weirs built and maintained near the tidal shoreline between 3,700 and 5,200 years ago.
Natives in Nova Scotia use weirs that stretch across the entire river to retain shad during their seasonal runs up the Shubenacadie, Nine Mile, and Stewiacke rivers, and use nets to scoop the trapped fish. Various weir patterns were used on tidal waters to retain a variety of different species, which are still used today. V-shaped weirs with circular formations to hold the fish during high tides are used on the Bay of Fundy to fish herring, which follow the flow of water. Similar V-shaped weirs are also used in British Columbia to corral salmon to the end of the "V" during the changing of the tides.
The Martinsville Fish Dam, Virginia, an historic Native American Indian fishing weir built with rocks
Remains of an ancient stone fishing weir in the tidal Menai Strait in Wales.
Fishing weir on the rapidly flowing Mogami River in Japan
Fishing weirs using baskets at a river waterfall, Democratic Republic of the Congo.
See also 
- The Text of Magna Carta, see paragraph 33.
- Shooting and Fishing the Trent, ancient fish traps.
- Reid, Ian (2001): "Rhos-on-Sea Heritage Trail". BBC Wales North West website retrieved 7 August 2007.
- Erickson, Clark (2000): "An artificial landscape-scale fishery in the Bolivian Amazon". Nature, 408(6809):190-193
- Erickson, Clark (2000b): "AN ARTIFICIAL LANDSCAPE-SCALE FISHERY IN THE BOLIVIAN AMAZON" University of Pennsylvania website retrieved 12 Oct. 2007
- Herring Weirs in the Gulf of Maine (informational website on the herring fishery in the state of Maine, U.S.A.)