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Toxic to Fish and Shellfish
Many people think that natural gas would just bubble up to the surface and quickly evaporate off but in fact a significant portion dissolves in the water and is highly toxic to marine life. The gas can rapidly penetrate the bodies of fish, doing direct damage to gills, skin, chemoreceptors and eyes, and filling up the gas bladder, making the fish unable to control its buoyancy.
At concentrations of 0.02 – 0.05 mg/l, gas will be sensed by fish and they will move away. If however, fish are exposed to concentrations above 1 mg/l they become excited within seconds of contact, then disoriented and unable to flee. Within 15 – 20 minutes fish exposed to such concentrations show signs of acute poisoning, and they die within 1-2 days of exposure. Shellfish are also killed by exposure to gas. Zooplankton and phytoplankton can tolerate higher concentrations of gas than fish or shellfish can (i.e. they die at 2 – 5 mg/l).
Accidental gas releases on a migratory route of fish such as salmon, either in the sea or from a pipeline close to a river, can block a spawning migration. A localized release can thus have a regional impact.
In the Gulf of Mexico, high concentrations of methane were found around offshore drilling rigs during a 1975 study by Sackett and Brooks. In the North Sea and in California, bottom ecosystems are disturbed and their species compositions altered where there is gas seepage. Biomass of living things declines and shellfish disappear.
Some fish, such as flounders, are more sensitive to gas than other species. Juveniles are more sensitive than older fish. Fish also become more sensitive if repeatedly exposed to low concentrations of gas. Fish are more vulnerable when water temperatures are high or when oxygen concentrations are low (as in an eutrophic estuary in summer). Under conditions of cold temperature and high pressure, gas may react with water to form hydrates. These can be trapped and accumulate under ice in winter and be converted to methane as water temperature rises in spring, with serious environmental consequences. If the gas is “sour,” or contains sulfides, it is much more highly toxic to marine life.''
Impact of Natural Gas in the Marine Environment | Watershed Sentinel