Is Oreochromis niloticus invading the Samborombón Bay , Río de la Plata , Argentina ?

The Nile tilapia (Oreochromis niloticus) is a species widely cultivated worldwide. In recent decades it was an increasing development of fish farming of this species and the red variety in Argentina and Uruguay. From January to March 2010, four specimens of O. niloticus were captured in the south boundary of Samborombón Bay (S 36° 17 ́W 56° 46 ́), which is the external sector of the Río de la Plata. Probably the collected specimens were released accidentally from hatcheries placed on the banks of aquatic environments in communication with the Samborombón Bay. The patterns and mechanisms of species dispersal are of significant interest, while the interactions among factors determining invasion success often remain poorly understood. Invasion success is influenced by the ability of invading specie to withstand, interactions with native species and oceanographic characteristics of the new habitat. Of four specimens obtained two were females, one in spawning stage. This is the first record of Nile tilapia from a natural environment in Argentina and could indicate the beginning of a new invasion by a non native species.


INTRODUCTION
The tilapiine fishes are originally an exclusively African group of species (Trewavas, 1983;Pullin, 1988).They have been introduced in different parts of the world for biocontrol of aquatic weeds and insects for improve or to develop aquaculture fisheries.A huge amount of information is available on this group, which has received as much attention in the tropics as salmonids in temperate areas.Today, the tilapiine fishes are likely the world most distributed exotic fish species, having established in every tropical and subtropical environment to which they have gained access (Costa-Pierce, 2003).
The African Nile tilapia, Oreochromis niloticus (Linnaeus, 1758) is a cichlid fish native to the large lakes and rivers of Africa.This species is the best for aquaculture, due to its wide resistance to environmental factors and their well-known ma-nagement (Welcomme, 1988).It is an omnivorous species with parental care and territorial habits.After the common carp (Cyprinus carpio), the tilapia is the freshwater fish species with a high production by world aquaculture, with excellent markets and sustained demand.Its cultivation is widespread in Asia and Latin America (Dirección Nacional de Pesca y Acuicultura, DNPA, 2003;De Silva et al., 2004).Particularly in Argentina, the Nile tilapia was introduced in the North of the country (Misiones province) at the end of 1930, and later other imports were made in 1950, 1980 and 1990.All these introductions and transfers were without legal authorization of the DNPA, in certain cases because there was no yet official control and in others because they were illegal introductions.More recently, the DNPA authorized the introduction of the red hybrid nilotica in the Misiones province and O. niloticus in the Corrientes province.There are other projects in the provinces of Formosa, Chaco, Santa Fe, Entre Ríos and Buenos Aires.Farming has been also developed at Lavalleja and Salto departments in Uruguay.All these places are located in the Paranoplatense Basin (DNPA, 2003;DINARA-FAO, 2008).
In this paper, we report for the first time the presence of O. niloticus in the Samborombón Bay, a main mixohaline environment in the same basin where cultivation of this species is carried on.

MATERIAL AND METHODS
From January to March 2010 four specimens of O. niloticus were captured with a 80 to 110 mm stretched mesh size bottom-fixed gill-net positioned for up to 12 hs of fishing in the south boundary of Samborombón Bay (S 36° 17´ -W 56° 46´) at the external sector of the Río de la Plata (Fig. 1).Measurements (mm), body weights (grams) are included and sex and maturity stage were determined.Three specimens were deposited in the Ichthyological Collection of the Museo de La Plata, MLP 9770.

RESULTS AND DISCUSSION
The specimens caught measured 600, 580, 450 and 290 mm in total length and 4,900, 4,010, 2,350 and 490 g respectively.The last two specimens were females, one in spawning stage.
The analysis of the average monthly ∆SST time series shows a warm period (positive ∆SST) in the study area from November to April with a maximum of 6.07° C in February and a cold period (negative ∆SST) from May to October, with a lowest value off -5.89° C (Fig. 2).The most elevated monthly average ∆SST (8.37° C) was observed in February 2010, which is consistent with the occurrence of the species in the study area.
In spite of introductions in Argentina, intensive production performed by Brazil for several decades and culture by Paraguay, and considering the wide connections among watersheds, apparently the tilapia did not established in the Paranoplatense basin.This suggests that in these environments could be limiting physical and che- the introduction of O. mossambicus, O. niloticus and O. spp in Venezuela has been the main cause of the extinction of the autochthonous atherínid Atherinella venezuelae (Infante, 1985, Solórzano et al., 2001).This country also lost more than 50% of its diversity (from 23 to 10 species) after 12 years of the introduction of tilapia (Carvajal, 1965).Tilapia also surpassed the lagoon and populated some rivers and colonizes nearby marine environments.In some rivers of Colombia the tilapia displaced Prochilodus magdalenae (Aguilera & Carvajal, 1976).
The FAO (1997) noted that in aquatic environment fishes easily exceed the limits of installations devoted to its cultivation.Therefore, the introduction of organisms for aquaculture activities should be considered a deliberate introduction in the natural space, even if the quarantine centre or the farm may be a closed system.It is necessary to emphasize that invading exotic species that do not find predators and natural parasites in the new environment may widely expanse their population.For these reasons and the risks involved in the cultivation of tilapia due to the possibility of some casual escapes from the sites of cultivation to the natural environment, organizations such as the International Council for the Exploration of the seas, the Environmental Protection Agency and the Fish and Wildlife Service have proposed several codes of procedure management to mitigate the devastating effects of these introductions (TED, 2001).
This record for the Río de la Plata is the first of this species in a natural brackish (estuarial) environment in Argentina.The perfect conditions of the specimens indicate that we would face possible disruption by an invasive species that has already caused the extinction of several autochthonous species in many countries.It would be necessary to investigate the trophic ecology of O. niloticus to determine the interaction with native species in this environment and the changes that it may cause in the structure of the ecosystem.The DNPA (2003) has already indicated that farming should also be carried on with selected reverse sex lines.mical parameters for settlement and sustainability of tilapia (DNPA, 2003).
However, it is proved that tilapia is a very resistant species and is able to survive in extreme environmental conditions, such as high temperatures (up to 43° C; Bezault et al., 2007), high salinity (up to 57 ppm; Mateo et al., 2004), and elevated concentrations of toxicants (El-Shafai et al., 2003;García-Santo et al., 2006).All of these factors explain its success, sometimes invasive establishment in critical or polluted environmental conditions (Khallaf et al., 2003), although it is sensitive to cold waters.In the subtropical region temperature would be a limiting factor to the development of the species, because the lowest lethal thermal limit is between 9.6 and 13.6° C, depending on the environmental conditions and genetic origin of the fish stock (Charo-Karisa et al., 2005).However, the species shows an adaptive selection to low temperatures (Peterson et al., 2005).Tilapia also can adapt to live, play and have a viable offspring in the marine environment, condition that has been exploited to cultivate at sea (Watanabe, 1991).Tilapia hybrids also resist high salinity (Mateo et al., 2004).These traits probably facilitate the colonization and development of the species in areas as the Samborombón Bay, where ranges of water temperature and salinity are between 11.3 and 21.7°C and 15.09 and 21.3, respectively (Guerrero et al., 2004).
Often introductions have not been preceded by scientific studies of the biotic and abiotic conditions, so expected objectives were not achieved and at the same time, escape of fishes to other environments have caused a dangerous decrease in numerous aquatic species (Arratia, 1978).This situation occurred with the introduction of tilapia in several countries of the world.For example,

ACKNOWLEDGEMENT
Special thanks to Dr. R. Menni for the critical reading of the manuscript and the local fishermen for their field work support and friendship.BIBLIOGRAPHY Aguilera, L. & J.Carvajal.1976.La ictiofauna del complejo hidrográfico Río Manzanares, Estado Sucre, Venezuela.Lagena 37-38: 23-25.