I think that's so informative it ought to be copied into here:
"At low tide you can see the remains of the foundations of the medieval RHOS FYNACH FISHING WEIR (2), one of many once found round the Welsh coastline. There used to be two fishing weirs at Rhos. They consisted of a large V-shaped enclosure made out of wicker fencing. At high tide the fish swam into the structure, and were then trapped in a pool as the tide went out. Weirs such as these were so effective that by Victorian times they were a danger to fishing stocks, particularly salmon. Therefore in 1861 Parliament passed a law ordering their destruction. As a result the other weir in Rhos was demolished.
However the new law granted an exemption where the owner could prove his weir had existed before the time of Magna Carta (1215 AD). The owner of this Rhos Fynach weir was able to prove its medieval origins to the satisfaction of the Commissioners, and so it escaped destruction - further evidence of the antiquity of the settlement at Rhos.
The weir was amazingly effective and enormous quantities of fish could be caught. For example, in 1850 there was a record catch of 35,000 herrings in a single night! The weir even trapped an 8-foot shark in 1865, which was then put on display in Llandudno market. On another occasion (in 1907) 10 tons of mackerel were caught on a single tide.
The last owner of the weir, one John Parry Evans, trained terrier dogs to retrieve salmon from the weir. The first dog he trained, Jack, was so admired that he was awarded with a silver collar and became a popular tourist attraction. Unfortunately he died in 1873 after receiving fatal injuries from another shark trapped in the weir! The weir fell into disuse during World War I, and regrettably its stakes were later removed as a danger to boats."
Courtesy of the WSF forum