A weir may be used to trap marine fish in the intertidal zone as the tide recedes, fish such as salmon as they attempt to swim upstream to breed in a river, or eels as they migrate downstream. Alternatively, fish weirs can be used to channel fish to a particular location, such as to a fish ladder. Weirs were traditionally built from wood or stones. The use of fishing weirs as fish traps probably dates back prior to the emergence of modern humans, and have since been used by many societies across the world.
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 weirs were frequently the cause of disputes between various classes of river users and tenants of neighbouring land. Basket weir fish traps are shown in medieval illustrations and surviving examples have been found. Basket weirs are about two metres long and comprise two wicker cones, one inside the other — easy for fish to get into but difficult to escape.
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