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Mobile nurseries

Fish reproduce and raise the young in specific parts of mangrove swamps

Marcos André / Opção Brasil ImagensThere are good reasons to dislike mangrove swamps: they are ugly, muddy, full of mosquitoes and generally smell bad. However, there are also good – and new – reasons to value more highly these areas that combine seawater with river water in the midst of trees with exposed roots. Studying in depth the old explanation that mangrove swamps are the nurseries of marine animals, a team from the Federal University of Pernambuco (UFPE) found that several fish species require different havens within the mangrove swamps, with greater or lower salt content, in order to spawn and raise their young until they are able to swim to sea.

“The fish mate in one place, spawn in another, and use a third as a nursery; sometimes these are tens of meters away from each other, all within the same estuary,” says Mario Barletta, who, along with his group, is travelling through the estuaries of South America. Another conclusion is that these reproduction, spawning, growing, protection and feeding grounds for fish vary over the course of the year, depending on the phases of the Moon and the amount of rainfall , with different levels of turbidity, salinity and concentration of oxygen dissolved in the water. “These havens, often located in the very narrow channels of the mangrove swamps, may be further away from the coast, when it rains a lot, or closer, when it rains little,” says Barletta.

Found along the entire Brazilian coastline other than in the state of Rio Grande do Sul, the mangrove swamps are protected by federal law, but are nonetheless being taken over by roads, residential condominiums and industries, and are becoming polluted. Without their safe havens, growing fish and sea turtles change their diet and even eat plastic. In the estuary of the Goiana river, a 475 thousand sq. m area of mangrove swamp between the states of Pernambuco and Paraiba, Barletta and his team analyzed 60 madamango sea catfish (Cathorops spixii), 60 white catfish (C. agassizii) and 62 pemecou sea catfish (Sciades herzbergii), species that the riparian population eat a fair amount of. They opened the belly of each and in at least 20% of the samples of each species they found nylon line from the ropes of boats or from fishing lines. Fernanda Possatto, Barletta and other researchers from UFPE say it is impossible to quantify the reach of this phenomenon or the consequences of this type of pollution, but they recommend greater care to keep it from damaging the lives of fish and of people even further.

Moreover, this is not taking place only at this site. In the Paranaguá estuary, a 600 square kilometer area covered with woodlands and mangrove in the South of Brazil, researchers from UFPE, from the Federal University of Paraná and from the Cananeia Research Institute, in Campinas, collected 80 young green turtles captured in fishing nets from June 2004 to July 2007. In the stomachs and intestines of 76 of them, besides the algae, mangrove plants and and shells that they normally eat, the researchers found remains of plastic shopping bags, nylon string and polystyrene pieces, as shown in detail in the February issue of the journal Endangered Species Research. According to Barletta, this indicates that the spatial separations of the estuary – divided into conservation and non-industrial fishing areas, the port and urban or tourist development areas – are not working. Another of his group’s studies identified traces of heavy metals, particularly of mercury, and of other types of waste in fish that spend at least part of their life in the mangrove swamps.

Mangrove swamps are shrinking and becoming more polluted throughout the South American coast. Experts from Brazil, Argentina, Venezuela and Colombia have concluded that, in addition to over-fishing, the loss of these natural environments caused by the discharging of sewage and of the residues of mining and of other industrial activities underscore South America’s nine main hydrographic basins and estuaries: the basin of the Madeira river, in Colombia; that of the Orinoco river, in Venezuela; the Amazon basin, including land in Brazil, Peru and Bolivia; the estuaries of the Goiana River, between Pernambuco and Paraiba, as well as of the Paranaguá River, in Paraná state. The Lagoa dos Patos lagoon, in Rio Grande do Sul; and the bay and the estuary of the Plate and Paraná rivers, on the border between Argentina, Paraguay and Brazil.

According to this survey, which resulted in a 59-page study published in 2010 in Journal of Fish Biology, the construction of dams fragmented the course of rivers such as the Paraná and the Uruguay and caused severe changes in mangrove swamps and other coastal environment where the fish live. The dredging of the estuaries, such as was conducted in Paranaguá to build the port in the innermost portion of the estuary, led to a reduction in the number of species. In the basin of the Urucu river, a tributary of the Amazonas, the chief problem identified was pollution caused by the overflowing of oil during prospection.

Clemente Coelho JuniorMaracaípe River, in Pernambuco: estuaries undergoing transformationsClemente Coelho Junior

Barletta recommends conserving, recovering and acknowledging the value of these environments. For him, mangrove swamps, despite all these pressures, do not have to remain untouched. “We can exploit them, of course, but with criteria,” he suggests. Half of the worldwide area of mangrove swamps has disappeared over the last 50 years due to the growth of coastal cities, according to a study by Daniel Donato, from the United States Forestry Service, published in April in Nature Geoscience. The conclusions of this study might help to conserve these undeniably fetid areas: mangrove swamps are among the most carbon-rich forests in the world. Losing them could generate the equivalent of 10% of the total carbon dioxide released by deforestation, even though mangrove swamps account for only 0.7% of all the tropical forests in the world.

“Mangrove swamps are highly fragile environments and they are sensitive to contamination,” observes Itamar Soares de Melo, a researcher from Embrapa, who coordinated a survey of microorganisms in São Paulo mangrove swamps. In the town of Bertioga, he found bacteria of the Pseudomonas genus, which produce compounds that can degrade hydrocarbons such as those in petroleum.

Henrique Santos, Raquel Peixoto and other researchers from the Federal University of Rio de Janeiro found that the populations of microorganisms before and after the arrival of contaminants change substantially. The populations of the genera Mirinobacterium, Marinobacter, Clostridium and Fusibacter expand, whereas those of the genera Haliea and Chromatiales drop a lot. The researchers believe that these changes might help to forecast the possible environmental impact in regions such as the Bay of Sepetiba, in Rio de Janeiro state, which is home to the port of Itaguai, currently being dredged in order to be used by larger vessels.

Melo and his team identified new species of microorganisms adapted to the high salinity of the mangrove swamps, a selective environment for plants as well, as only a few species of trees live in such areas, some with exposed roots, which make it easier for them to fix themselves in the mud. In pollution-free mangrove swamps, Melo also found non-pathogenic species of bacteria of the genus Vibrio living within trees. “The bacteria might be furnishing the trees with nitrogen and phosphorus, which are important nutrients for growth but are scarce in that environment,” he comments.

Scientific articles
GUEBERT-BARTHOLO, F.M. et al. Using gut contents to assess foraging patterns of juvenile green turtles Chelonia mydas in the Paranaguá Estuary, Brazil. Endangered Species Research. v. 13, p. 131-43. 2011.
BARLETTA, M. et. al. Fish and aquatic habitat conservation in South America:
a continental overview with emphasis on neotropical systems
. Journal of Fish Biology. v. 76, p. 2.118-76. 2010.