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Regional Overview & Fact Sheet
The Middle American Pacific Ecoregion—largely free of the southernmost winter influence of the California Current and therefore described as a year-round tropical sea—supports important fisheries such as yellowfin and skipjack tuna, as well as shrimp. Bycatch, however, is of great concern—the region’s bycatch ratio is the highest in Mexico. Although relatively small, the region’s bathymetry is quite diverse and includes a relatively wide continental shelf that drops off to the continental slope and Mesoamerican Trench, and rises to the Guatemala Basin, and the Tehuantepec Ridge. Waters off the Mexican states of Oaxaca and Chiapas are included in this region.
Fact Sheet
Largely free of the southernmost winter influence of the California Current and therefore described as a year-round tropical sea.
avg. 26–27°C (winter), 29.5 °C (summer)
North Equatorial Countercurrent/Costa Rica Coastal Current, California Current.
shelf (roughly 0–200 m): 16%; slope (roughly 200–2,500/3,000 m): 30%; abyssal plain (roughly 3,000+ m): 54%
silt, mud, sand
Coastal lagoons, mangroves, sandy shore and benthic communities.
High productivity ecosystem (>300 gCm-2yr-1). Due to the equatorial upwelling, open ocean and coastal upwellings, and nutrient inputs coming from river run-off along the tropical areas.
graceful herring, Mexican clingfish, green alga Codium oaxacensis
loggerhead turtle, East Pacific green turtle, leatherback turtle, olive ridley turtle, purpura conch, hammerhead shark, Scalloped hammerhead shark, silky shark, smooth hammerhead shark
brown alga Sargassum muticum
Coastal lagoons, mangroves, and coral reefs.
Pollution from agricultural runoff and urban activities, and overfishing of commercial species. A highly economically important artisanal shark fishery in the Gulf of Tehuantepec, and particularly in Chiapas, concentrates on the silky shark (representing 60 percent of the catch) and the hammerhead sharks (with 36 percent of the catch) (Del Prado Vera et al., in Tapia and Gutierrez 1998).
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Physical and Oceanographic Setting
The Middle American Pacific remains essentially free from the southernmost winter influence of the California Current year round and thus is a tropical sea. The Mexican portion of the Middle American Pacific, encompassing the Gulf of Tehuantepec and adjacent waters, is influenced by the Costa Rica Current (derived from the North Equatorial Counter Current). It is also a region that experiences high seasonal variability due to upwelling, and is strongly influenced by freshwater discharge from coastal lagoons and river systems present in coastal areas in Chiapas, as well as winds from the Gulf of Mexico. Because the Gulf of Tehuantepec lies south of a major gap in the Central American sierra chain, transmontane winds (called “Tehuanos”) from the Gulf of Mexico have easy passage. The northern winds force the surface airflow from the Gulf of Mexico towards the Gulf of Tehuantepec. Interaction between this and the northward flowing Costa Rica Coastal Current generates a meridional thermocline (pycnocline and nutricline) ridge. During the winter months, the layer above this feature may be completely mixed by wind stress, and surface temperature, salinity, and nutrient levels resemble values in the pycnocline. After extreme transmontane wind events, a plume having such characteristics may stretch several hundred kilometers to the southwest from the Gulf. Surface productivity is high.
The Middle American Pacific includes a moderate to narrow continental shelf that widens towards the southeast; a continental slope with varied grades from gentle to steep; part of the Mesoamerican Trench—a subduction zone with steep slopes and profound depths (6,000 m); the eastern portion of the Guatemala Basin that is undulated with trenches that reach depths of 4,600 to 4,900 m; as well as the Tehuantepec Ridge—a submarine mountain chain of volcanic origin. The region’s bottom type varies from mixtures of mud, sand and gravel, and the zone is characterized by shallow waters with minimal oxygen.
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Biological Setting
Available data suggest that at least during the northerly windy season, the Gulf of Tehuantepec acts as a nutrient and phytoplankton-carbon pump, enriching adjacent offshore waters. When the Gulf behaves as a tropical ecosystem, there is low phytoplankton biomass and low productivity rates. The most evident interannual variation is the effect of ENSO events, associated with a deep thermocline in the whole region, even during winter, as well as very low chlorophyll concentrations.
Much of the region’s communities are characteristic of those found in upwellings. At least 153 species of marine algae have been found on the seafloor of the Gulf of Tehuantepec. At least 123 families with 172 genera and 239 species constitute the benthic invertebrate community of the Gulf. At least 178 species in 103 genera and 52 families constitute the demersal fish community. The highest diversity is found offshore of the estuarine systems during the rainy season. Mangrove communities are found in the region, and are more developed in Chiapas than in Oaxaca. The Oaxacan coast presents limited coral reef structures (in Bahia de Huatulco, La Entrega and Puerto Angel) in relatively good condition.
The loggerhead, East Pacific green, leatherback and olive ridley sea turtles, as well as other species at risk such as the purpura conch and several shark species use the waters of this region as their home. Endemic species are also found in the region, and include fishes such as the graceful herring and the Mexican clingfish, as well as the alga Codium oaxacensis.
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Human Activities and Impacts
Fishing and coastal industrial development play important roles in the economy of the Middle American Pacific. This region supports important fisheries, such as that of yellowfin and skipjack tunas, as well as shrimp. Intense artisanal fisheries are found in the coastal lagoons, particularly in Chiapas and more recently in Oaxaca. Over the continental shelf, industrial shrimp trawlers sweep the sea bottom, killing juveniles of countless untargeted species (i.e., bycatch). Shrimp yields have been sharply declining over the last decades—a phenomenon that is creating conflicts between fishers in the region. The region’s bycatch ratio is also the highest in Mexico (1:16 to 1:41, versus 1:12 in the Gulf of Mexico, and usually 1:10 in the Mexican Pacific) (Tapia-Garcia and Garcia-Abad 1998). Moreover, coastal industries and activities based on oil, sugar and transportation are also placing pressures on the Middle American Pacific. Several areas within the region are showing signs of critical pollution problems: the Salina Cruz port and surrounding areas contain oil and heavy metal pollution; in the Ventosa Bay and Estuary, oil and heavy metal pollution is compounded by the presence of fecal coliforms; and in Laguna Superior, pollution from pesticides, herbicides, oil and domestic discharges is prevalent (Tapia-Garcia et al.1998).
Level II and Level III Descriptions
Level II captures the break between neritic (near shore) and oceanic areas and is determined by large-scale physiography (continental shelf, slope, and abyssal plain, as well as areas of oceanic islands and major trenches, ridges and straits). This level reflects the importance of depth as a major determinant of benthic marine communities as well as the importance of major physiographic features in determining current flows and upwelling. Like Level I, it extends from the coasts to the EEZ.
Level III captures differences within the neritic realm and is based on more locally significant variables (local characteristics of the water mass, regional landforms, as well as biological community type). Level III is limited to the continental shelf, since only this area has sufficient information for finer-scale delineation.
Level II and Level III Ecoregions
Other Marine Ecoregions
Marine ecoregions are areas of general similarity in terms of physiographic, oceanographic and biological characteristics, which fall within the Exclusive Economic Zones of the North American countries. There are 24 marine ecoregions in North America.
