Extra nitrogen in the environment makes its way into oceans as run-off from fields through water channels. This causes havoc in the aquatic ecosystem. Earlier, the extra nitrogen helped fish because it resulted in good growth of marine plants fish ate. But due to commercial fisheries, the number of fish has gone down considerably and only the plants remain.

Until 40 years ago, commercial fishing removed almost 60 per cent of the nitrogen from oceans. Today, the figure has dropped to 20 per cent, says a study published in the February issue of Nature Geoscience (Vol 1, No 2). Researchers studied 58 coastal regions across the world and concluded that commercial fishing played an important role in removing manmade nitrogen from coastal waters.

This, they say, has led to several problems, eutrophication for example. Eutrophication leads to dense growth of plants, which depletes the supply of oxygen in water, affecting marine life.