Eutrophication of coastal waters in the Gulf of California (Mexico) and the spatial development of shrimp farming: the case of the Rio Fuerte delta
By Loïc MENANTEAU and Diana C. ESCOBEDO URIAS
L’Atlas Bleu / Exploiting
Eutrophication, shrimp farming, spatial spread, environment, Río Fuerte delta, Mexico
The article focuses on two major phenomena that combine on the coastal zone of the Gulf of California (Sonora, Sinaloa, Mexico) especially in the Río Fuerte delta. On the maritime side, pollution of coastal waters is due to intensive cultivation, while on the terrestrial side, the spatial extension of shrimp farming causes health and biological problems. The result is a strong degradation of coastal ecosystems.
Eutrophication of coastal waters in the Gulf of California (Mexico)
Phytoplankton blooms (in shades of green) in the Gulf of California off the Sinaloa and Sonora coasts.
Partial reproduction of the true-colour image acquired by the Terra/MODIS sensor on 26-04-2001. Sources: J. Descloitres, MODIS Rapid Response Team, NASA/GSFC.
Irrigation discrit 075
Quantities of fertiliser used per agricultural year in units of nitrogen (N) and phosphorus (P) in the irrigation district (Distrito de Riego) 075 Río Fuerte, and trophic index (TRIX) as an indicator of the environmental quality of the Northern Sinaloa coastal systems (Topolobampoand Navachistlagoon systems) during the 1987-2007 period.
The Gulf of California, off northwestern Mexico, is subject to severe pollution (land and air) due to the massive use of nitrogen fertilisers and insecticides on irrigated and intensive crops on the coastal plain (6,000 km2) in the states of Sonora and Sinaloa (Mexico). The plain, which extends between the Sierra Madre Occidental and the eastern shore of the gulf, is one of Mexico’s main agricultural areas (one-third of total production, of which almost 50% is exported). The consequences are major phytoplankton blooms up to the centre of the gulf (Beman et al., 2005) and eutrophication of the coastal lagoons on the coast (see the graph and image above). These consequences are compounded by the impacts of the rapid development of semi-intensive and intensive shrimp farming (camaronicultura) in ponds. The release of large amounts of organic matter (feed waste, excreta, urea used as fertiliser, etc.) and various antibiotics alters the ecological conditions of coastal ecosystems (Escobedo-Urías, 2010).
Mexican annual production of large shrimp (camarones), 1984-2012
The states of Sinaloa and Sonora are the largest producers of large shrimp (camarones) in Mexico (85% of production in 2001, see the graph), that are primarily exported to the United States. In 2008, Sinaloa counted 300 marine farms covering an area of 280 km2. The farmed species, Litopenaeus vannamei, whiteleg shrimp or Pacific white shrimp, can adjust to salinity levels from 9 to 55‰. Water temperature plays an essential role: the shrimp can only be harvested between May-June and October (sometimes longer if an exemption is authorised), because the temperature of the water during the rest of the year, often lower than 20°C, is too cold (a minimum of 29°C is required).
Shrimp farming is exposed to serious biological and health risks. The observed declines in production are due to infectious animal diseases (zoonoses) caused by different viruses (60%), bacteria and other pathogens. As a result, from 1995 to 1997, yields were affected by the Taura syndrome virus (TSV). Then, the state of Sonora (which became the leading producer in 2009 with an output of 81,422 tonnes), lost more than 50% of its production in the years that followed (35,305 tonnes in 2012) due to the white spot syndrome virus (WSSV). In 2013, another virus, from Asia, once again caused a decline in production, resulting in the loss of 80% to 100% of shrimp larvae.
The first Mexican shrimp farm (Ejido las Grullas, by Acuacultores de Sinaloa) was built in 1983-1984 in the Río Fuerte delta, to the north of Sinaloa. As part of an international collaborative research project, we carried out a study of the development of shrimp farming in the delta. A sequence of satellite images (Landsat and Spot) were processed and analysed to accurately monitor its spatial coverage over the first 25 years of shrimp farming activity (1983-2008). North of Boca las Piedras (see interactive map), the rapid extension of these farms has resulted in the creation of a kind of barrier that is more or less continuous, with changes to or loss of the natural drainage. To the south, they are expanding more slowly, but pose higher risks for the environment as the farming is done directly on the edge of fragile ecosystems consisting of mangroves and lagoons.
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