Artificial reef

An artificial reef is a man-made underwater structure, typically built to promote marine life in areas with a generally featureless bottom, to control erosion, block ship passage, block the use of trawling nets, or improve surfing. An artificial reef is a man-made underwater structure, typically built to promote marine life in areas with a generally featureless bottom, to control erosion, block ship passage, block the use of trawling nets, or improve surfing. Many reefs are built using objects that were built for other purposes, for example by sinking oil rigs (through the Rigs-to-Reefs program), scuttling ships, or by deploying rubble or construction debris. Other artificial reefs are purpose built (e.g. the reef balls) from PVC or concrete. Shipwrecks may become artificial reefs when preserved on the sea floor. Regardless of construction method, artificial reefs generally provide hard surfaces where algae and invertebrates such as barnacles, corals, and oysters attach; the accumulation of attached marine life in turn provides intricate structure and food for assemblages of fish. The construction of artificial reefs began in ancient times. Persians blocked the mouth of the Tigris River to thwart Indian pirates by building an artificial reef and during the First Punic War the Romans built a reef across the mouth of the Carthaginian harbor in Sicily to trap enemy ships within and assist in driving the Carthaginians from the island. Artificial reefs to increase fish yields or for algaculture began no later than 17th-century Japan, when rubble and rocks were used to grow kelp. The earliest recorded artificial reef in the United States is from the 1830s, when logs from huts were used off the coast of South Carolina to improve fishing. Beginning before the 1840s, US fishermen used interlaced logs to build artificial reefs. More recently, refuse such as old refrigerators, shopping carts, ditched cars and out-of-service vending machines replaced the logs in ad hoc reefs. Officially sanctioned projects have incorporated decommissioned ships, subway cars, battle tanks, armored personnel carriers, oil drilling rigs and beehive-like reef balls. In 2018, the world's largest 3D-printed artificial reef was introduced in the Maldives, in an aim to protect coral reefs from the ravages of climate change and warming waters. Artificial reefs tend to develop in more or less predictable stages. First, where an ocean current encounters a vertical structure, it can create a plankton-rich upwelling that provides a reliable feeding spot for small fish such as sardines and minnows, which draw in pelagic predators such as tuna and sharks. Next come creatures seeking protection from the ocean's lethal openness—hole and crevice dwellers such as grouper, snapper, squirrelfish, eels and triggerfish. Opportunistic predators such as jack and barracuda also appear. Over months and years the reef structure becomes encrusted with algae, tunicates, hard and soft corals and sponges. Electro-mineral accretion (EMA), also known as Biorock, involves applying a low voltage current to a metallic structure to cause limestone to crystallize on the surface, to which coral planulae can attach and grow. The electric current also speeds post-attachment growth. EMA works like charging a battery with a positive pole, the cathode, and a negative pole, the anode. Applying electric current attracts dissolved minerals to either the cathode or the anode. Chemical reactions take place at both poles. On the anode, bubbles of oxygen and chlorine gas form. These bubbles float to the surface and dissolve into the air. On the cathode, bubbles of hydrogen gas and a limestone precipitate appear.