再Artificial reefs, December 6th 2014 PTQ4 (人工珊瑚礁)
Artificial reefs：New ways to construct underwater environments are encouraging marine life and boosting fish stocks Toilets, shopping carts, washing machines and other assorted junk have been dumped into the sea to create habitats for marine organisms and fish that feed upon them. But making reefs from refuse is frowned upon. Alabama, for example, banned fishermen from sinking vehicles in the Gulf of Mexico in 1996, even when drained of potentially harmful fluids. Now more bespoken artificial reefs are taking shape. Reefs improvised from junk often do not work well. Corals struggle to colonise some metals and domestic appliances mostly disintegrate in less than a decade. Some organisms do not take to paints, enamels, plastics or rubber. Precious little sea life has attached itself to the 2m or so tyres sunk in the early 1970s to create a reef off Fort Lauderdale, Florida. Tyres occasionally break free, smash into coral on natural reefs and wash ashore. Yet building artificial reefs that are attractive to marine life can pay dividends. Some reefs built in Japanese waters support a biomass of fish that is 20 times greater than similarly sized natural reefs, says Shinya Otake, a marine biologist at Fukui Prefectural University. He expects further gains from a decision by the Japanese government to build new reefs in deep water where they will be bathed in nutrients carried in plankton-rich seawater welling up from below. The potential bounty was confirmed in a recent study by Occidental College in Los Angeles. Over five to 15 years researchers surveyed marine life in the vicinity of 16 oil and gas rigs off the California coast. These were compared with seven natural rocky reefs. The researchers found that the weight of fish supported by each square metre of sea floor was 27 times higher for the rigs. Although much of this increase comes from the rigs providing fish with the equivalent of skyscraper-style living, it suggests that leaving some rigs in place when production ceases might benefit the environment. Making reefs with hollow concrete modules has been especially successful. Called reef balls, these structures are pierced with holes and range in height up to 2.5 metres. The design is promoted by the Reef Ball Foundation, a non-profit organisation based in Athens, Georgia. Reef balls can be positioned to make the most of photosynthesis and for plankton to drift slowly across their curved inner surface. This improves the nourishment of plants and creatures setting up home within. A hole in the top reduces the chance of them being moved about by storm currents. Concrete used to make a reef ball is mixed with microsilica, silicon-dioxide powder, to strengthen the material and lower its acidity level to be more organism-friendly. The balls are cast from fibreglass moulds, which are typically sprayed with a sugary solution before the concrete is poured. This creates tiny hollows which provide a foothold for larval corals. Over 500,000 reef balls have been placed in the waters of more than 60 countries, and each one should last for some 500 years, says the foundation. The value of artificial reefs has been boosted by the spread of GSP devices and sophisticated sonars on boats. This allows fishermen to locate the subsea structures precisely. It is necessary to be directly above the reef to reel in more fish, says David Walter of Walter Marine, an Alabama company that used to sink vehicles for fishermen but now places pyramid-shaped, hurricane-resistant steel, concrete and limestone structures to create artificial reefs. These constructions can cost nearly $2,000, but many fishermen consider them to a good investment, especially to catch red snapper. Using underwater drones for long-term studies of reefs and their associated marine life is also helping improve designs. Sensors can be installed on reefs to monitor boat traffic and activities such as fishing and scuba diving. Perhaps the most innovative way to build a reef involves anchoring a frame made with steel reinforcing bars to the sea floor and zapping it continuously it with electricity. This causes minerals dissolved in seawater to crystallise on the metal, thickening the structure by several centimetres a year. Biorock, as the resulting material has been trademarked, becomes stronger than concrete but costs less to make. More than 400 “electrified” reefs, many the size of a small garage, have been built this way. Three-quarters of them are in the ocean around Indonesia. The electricity needed to power Biorock can be supplied by cable. But it can also be generated at sea in various ways. A floating buoy can produce power from wave motion, or raft can be tethered nearby fitted with solar panels or a small wind turbine. Only a dribble of electricity is needed, so marine life and people swimming in the area are not harmed, says Thomas Goreau, the boss of Global Coral Reef Alliance, a Massachusetts-based non-government agency which is behind the technology. Artificial reefs are also used for other purposes, such as to improve surfing or to help prevent beach erosion. Less wattage than it takes to power a domestic air conditioner is presently being used to build a 50-meter reef designed to slow erosion along part of the coastline in the Maldives. The electric current also stimulates coral growth. Once the mineral substrate has formed, divers use plastic cable-ties to attach bits of dying coral that have snapped off nearby reefs. Some pieces will recover their colour and start to grow again within an hour, says Mr Goreau. With a rapidly expanding world of population, artificial reefs appear to be a promising way to improve fish catches.