Fish are being threatened by rising levels of man-made noise pollution. So say scientists who have reviewed the impact on fish species around the world of noises made by oil and gas rigs, ships, boats and sonar. Rather than live in a silent world, most fish hear well and sound plays an active part in their lives, they say. Increasing noise levels may therefore severely affect the distribution of fish, and their ability to reproduce, communicate and avoid predators. "People always just assumed that the fish world was a silent one," says biologist Dr Hans Slabbekoorn of Leiden University, The Netherlands. But in the journal Trends in Ecology and Evolution, Dr Slabbekoorn and colleagues in The Netherlands, Germany and US report how the underwater environment is anything but quiet. So far, all fish studied to date are able to hear sounds, either by an inner ear or a lateral line that runs along a fish's side. Different fish vary in the sensitivity of their hearing. For example, Atlantic cod have "average" hearing abilities, say the authors, while freshwater goldfish can hear at higher frequencies. Generally fish hear best within 30-1000Hz, though species with special adaptations can detect sounds up to 3000-5000Hz.80% of global freight transport takes place by motorised shippingThe global shipping fleet comprises around 1.2 million vesselsUnderwater sounds are produced by navies, fisheries, the oil and gas industry and scientistsFish-finding echo sounders have been used by fishing boats since the 1950s Some exceptional species are sensitive to ultrasound, while others such as the European eel, a freshwater species that spawns at sea, are sensitive to infrasound. That means human-generated underwater noise has the potential to affect fish just as traffic noise affects terrestrial animals such as birds, say the researchers. "The level and distribution of underwater noise is growing at a global scale but receives very little attention," says Dr Slabbekoorn. To date, most research has focused on the impact sound might have on marine mammals, such as whales and dolphins. But noise pollution might severely affect the distribution of fish, and their ability to reproduce, communicate and avoid predators. For example, some studies have reported that Atlantic herring, cod and blue-fin tuna flee sounds and school less coherently in noisy environments. That could mean that fish distributions are being affected, as fish avoid places polluted by man-made noise. Noise pollution could significantly impact communication between fish: so far over 800 species of fish from 109 families are known to produce sounds, generally broadband signals at less than 500Hz.Fish make sounds when fighting over territories, competing for food, within spawning aggregations and when under attack from predators. Earlier this year, Dr Slabbekoorn published a report in the journal Behavioral Ecology that suggested that cichlid fish in Lake Victoria, East Africa produce species-specific sounds that also correlate with the size of the fish. The sounds play an essential role in mating and sexual selection among cichlids in the lake, they say. So as well as affecting the distribution of fish, that means noise pollution could interrupt their reproduction, by causing stress or restricting their ability to find a mate or keep them from preferred spawning sites. It could also prevent fish from hearing each other and communicating effectively, and affect their ability to detect noisy prey, or hear oncoming predators. Visit Trends in Ecology and Evolution to read more about the review into underwater noise pollution Noise pollution may not be as big a threat to fish as other environmental pressures, the report's authors concede. "Fisheries for example are likely to be much more devastating," says Dr Slabbekoorn. "However, none of the threats can be considered on their own: any negative consequence of anthropogenic noise will come on top of the fisheries impact, and together they may lead to more critical situations for some species. "The phenomenon is concealed by the fact that underwater sounds are difficult to hear by people living in air."