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Tables turned

Contamination of rivers by oil threatens above all fish with aerial respiration, an ancient evolutionary advantage

A little more than 400 million years ago, some tropical fish started to develop a respiratory strategy that turned into an evolutionary advantage for occupying waters with a low natural concentration of oxygen, like the rivers of the Amazon basin. Instead of trying to capture the small quantity of this gas available in the liquid medium, they became capable of breathing in oxygen directly from the air, a characteristic that, to a greater or lesser degree, has perpetuated itself up to the present day in many species typical of the tropical regions. Studies carried out by researchers from the National Institute for Amazonian Research (Inpa) show that if the rivers of the northern region are contaminated by oil, this resource for seeking oxygen out of the water can, ironically, magnify the risk of poisoning and death amongst the fish.

The danger is greater amongst the species with obligatory aerial respiration, like the arapaima (Araipama gigas ), one of the great freshwater fishes in the world, which can reach 2 meters in length and weigh over 100 kilos. In the course of its evolution, this kind of fish has totally lost its capacity for withdrawing oxygen dissolved in water, and boasts a swim bladder, an organ that can do the work of the lungs. When it needs to stock up on oxygen, the arapaima has, out of necessity, to rise to the surface from time to time, in a movement for survival, which leads it to breath in air inside its swim bladder.

As spills form a film of oil on the surface of rivers and lakes, where some fish find oxygen, the species with obligatory aerial respiration and those of facultative aerial respiration end up swallowing high quantities of oil, become intoxicated, and die rapidly. “In the case of contamination by oil, the evolutionary strategy conspires against these species of tropical fish”, explains Adalberto Luis Val, one of the researchers from Inpa’s Ecophysiology and Molecular Evolution Laboratory, who, together with his team, is researching into the impact of oil spills on the fish in the Amazon basin.

Without oxygen
In a study into three species that show different standards of respiration, Val raised evidence that the contamination of the waters by petroleum represents an extra danger for fish that breath in the area of contact between the air and the water, or, like the arapaima, get their oxygen straight from the air. The team from Inpa simulated oil spills in aquariums and analyzed the impact of this aggression against the environment on specimens of arapaima and of two kinds of aquatic breathers with differentiated adaptations to the lack of oxygen ( hypoxia ), the boari ( Mesonauta insignis ) and the tambaqui or black pacu( Colossoma macropomum ).

As soon as they come into contact with the contaminated environment, the three types of fish started to adapt their physiological parameters so as to expand as much as possible the oxygen intake and the transport of this gas to their tissues. But the one that was most sensitive to oil was the arapaima, precisely the species that has obligatory aerial respiration. “It doesn ‘t tolerate more than 24 hours in an environment contaminated with oil petroleum, even in low doses”, Val comments.

The one that felt least the effects of contamination was the boari , which takes all its oxygen from the water and has a large capacity for suppressing its metabolism and resisting hypoxia. The tambaqui suffered a little more that the boari, from the impact of the adverse situation created by the experiment. Albeit susceptible to the presence of petroleum, the boari and the tambaqui managed to survive in waters contaminated with petroleum for up to 66 hours, three times as much as the arapaima.

The risk of there being contamination from petroleum in rivers of the northern region is not just theoretical. It is real. Petrobras ( the Brazilian state oil company) extracts 40,000 barrels a day, besides hundreds of tons of natural gas, from an area close to the Urucu River, an affluent of the Amazon, some 700 kilometers away from Manaus. From the spot where it is extracted, the oil is pumped through an oil pipeline to the Solimões terminal, near to the town of Coari.

The oil is then carried in barges to the capital of the state of Amazonas, where it is refined. “There may be a spill both at the moment when it is extracted and during the transport of the oil”, says Val. “But Petrobras seems to be concerned with the environment and has supported research into the possible environmental impact of petroleum in the Amazonian region.” Up until now, there has been no record of any major accident in extracting or transporting oil in the region of the Urucu, although small oil leaks are now affecting, in a way that is still little known, the diversity of fish in the tropical ecosystems. Hence Inpa’s interest in seeking ways to assess the dimension of this problem.