Monday, January 13, 2014

Oceans, Marine Life, and Sound Pollution

Rose George has produced an interesting book on the shipping industry: Ninety Percent of Everything. Globalized trade has resulted in great increases in the number and size of ships transporting goods across the oceans. One of the topics she covers involves pollution. The issue of oil spills is well known. Somewhat less familiar is the amount of air pollution generated: large amounts of carbon dioxide, and, even more disturbing, large amounts of soot. It is obvious that ships will generate significant amounts of noise in the air as they pass by, and it is easy to view this ship noise as a minor inconvenience. George alerts us to the fact that ships also generate enormous amounts of noise underwater, and there the issue of sound pollution is more serious.

Light cannot penetrate very far in sea water; consequently many marine species rely on sound to transact their business: finding food, avoiding predators, searching for a mate, or tending to offspring. The range of human-generated sounds is much greater under water than one would expect from a lifetime of dealing with noise in the air.

"Sounds underwater can travel astonishing distances: a right whale [a species of whale] can hear another from ten miles away. The ocean has natural sound channels, like noise byways: if noise falls into a channel, it can travel hundreds of miles and perhaps thousands. It can travel an ocean. When a foghornlike signal was transmitted from Australia in 1991, it was heard off Oregon three hours later."

Most sound created by transport ships arises from the cavitation induced by the propellers. The stress created by the propeller blades plowing through the water generates voids (cavities) in the water that then implode. This implosion causes noise in a manner similar to that from a tiny explosion. The result is a lot of continuous underwater noise.

"The cavitation of a freighter leaving Cape Cod Canal can be heard all over the bay. A supertanker can be heard in the sea a day before its arrival."

That means the supertanker can be heard underwater about 400 miles away. George tells us that there are about 100,000 transport ships on the move at any time. That can add up to a lot of background noise for marine life to deal with.

"Researchers had been looking at daily, weekly, or monthly noise rates. When they compared levels over a longer time scale, the results were shocking. Ambient noise in the deep ocean was increasing by three decibels every decade. Every ten years, noise from commercial shipping had doubled."

Whales are particularly sensitive to noise. Surprisingly, whale-ship collisions are quite common. For some reason, perhaps confusion caused by the noise, whales have difficulty avoiding a passing ship, and have been observed to swim directly into its path.

The effects of noise on whales have been well documented. The background noise from shipping limits the range at which a whale can discern the signals it requires for doing what whales do.

"Without global regulations, acoustic pollution will grow because shipping is growing (by 2 to 6 percent annually). Already the acoustic habitat of the right whale—the range it needs to hear around—has been reduced by 90 percent. Humpback whales now have 10 percent of the acoustic range they used to have, so that their chances of finding a mate, food, and probably surviving have all been decimated."

Since some species of whale are in danger of extinction, researchers have been studying them to discover what might be limiting their viability. George describes the work of Roz Roland who had been studying right whales for many years. She had been measuring stress levels by collecting scat (whale excrement) and analyzing it for levels of stress-related compounds. Noise levels had not been her concern in 2001 when the attack on the World Trade Center occurred. The result of that attack was that for several days essentially all transport came to a halt and the source of man-made ocean noise disappeared. Subsequently, she thought to go back and evaluate the whale data accumulated during that period.

"For the whales it was….remarkable—they were swimming in a preindustrial ocean….an ocean quieter than it had been for over a century. It stayed that way for most of the week."

"But the message of the quiet ocean data was unmistakable. During the week that ships were stilled, underwater noise was lower by 6 decibels, and the levels of whales’ glucocorticoids (stress-related fecal hormone metabolites) were lower too."

The implication is that whales, and perhaps many other species, live in a heightened state of stress due to elevated and constant noise. It is well-known that high levels of stress are harmful to humans. Should it not be a cause of concern that marine life, which we depend upon to continue to function normally, might be endangered by our practices?

Noise from shipping is the most ubiquitous source, but it is far from being the most dangerous. Navy sonar signals and the underwater seismic testing used in searching for fossil fuels are dangerous episodic events for many species. Linda S. Weilgart, a marine biologist, has compiled a short summary of what is known about the effects of sound: The Impact of Ocean Noise Pollution on Marine Biodiversity. It makes interesting reading. Here are a few highlights.

"The U.S. Navy’s Low Frequency Active Sonar, used to detect submarines, could affect marine life over an area of about3.9 million square kilometers (Johnson 2003). This figure is based on levels shown to produce avoidance in fish and whales. Seismic airgun noise from oil and gas exploration measured 3,000 km away was the loudest part of the background noise heard underwater (Nieukirk et al. 2004). In some areas, underwater background noise levels have doubled every decade for the last several decades, most likely due to commercial shipping (Andrew et al. 2002; McDonald et al. 2006)."

"Noise in the form of naval sonar or seismic surveys can be deadly to cetaceans in at least some cases. Whales have been found to die within hours, by stranding or deaths at sea, from even a transient and relatively brief exposure to moderate levels of mid-frequency military sonar (Fern├índez et al. 2005; NOAA and U.S. Department of the Navy 2001). Since 1960, when more powerful sonars emerged, more than 40 mass strandings of Cuvier’s beaked whale have been reported world-wide. About 28 of these occurred together with naval maneuvers involving sonar or near naval bases, or with seismic surveys. In contrast, from 1914 to 1960, there was only one mass stranding reported of this species. Whales appear to die from hemorrhaging in their brain and heart, perhaps as a result of decompression sickness from an altered dive pattern induced by a panic response to the noise."

"Many marine species have been shown to be impacted by ocean noise to some degree. A cursory review of the literature on noise impacts revealed that at least 55 marine species have exhibited impacts from underwater noise in scientific studies. These impacts are not from nearby explosions, but more moderate noise….At least 3 seal species have reacted negatively to underwater noise: elephant seal, gray seal, and harbor seal. In fish, the following 21 species have shown impacts to noise: pink snapper, goldfish, cod, haddock, rockfish, herring, sand eel, blue whiting, catfish, sea bass, thicklip mullet, horse mackerel, bluefin tuna, fathead minnow, toadfish, carp, gudgeon, perch, silver bream, trumpeter, and trevally. At least 5 invertebrate species (squid, giant squid, snow crab, lobster, and brown shrimp) and 2 marine turtle species (loggerhead turtle and green turtle) have shown negative responses to noise. Thus, it is hardly plausible that underwater noise would not affect marine biodiversity to some degree."

More information on the issue of ocean noise can be obtained from the International Ocean Noise Coalition.

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