The impact of human activity along the 336 miles of the San Pedro Mezquital River is breaking the balance that the entire Marismas ecosystem needs to survive—and it helps explain the drop in oyster numbers.
One of the most severe problems is water sequestration— the removal of water from a natural system, like a river or lake, for purposes ranging from agriculture to aquaculture to human consumption. Another is the illegal construction of shrimp farms in the lower parts of the basin. In the biosphere reserve alone, Moran explains, there are 7,413 acres (3,000 hectares) of shrimp farms. And, while there are regulations in place to make their operations more sustainable, new production units continue to open illegally.
These unregulated, unsustainable farms siphon massive amounts of water from the river. Once the shrimp are harvested, the dirty water is deposited back into the river, dragging with it an excess of organic matter that ends up taking oxygen away from the mangroves, fish species, and mollusks that live in the lower river basin.
Since 2010, the National Commission for Natural Protected Areas (CONANP) and other federal agencies have been working to make shrimp farming more sustainable. In addition, the reserve’s management is constantly monitoring Marismas to ensure that no new production units are established that have not complied with the law.
It’s essential to connect the dots between upstream water impacts and coastal ecosystems. The interdependence from zone to zone is high, says Ricardo Domínguez, water reserves officer for WWF-Mexico. “What happens in the upper and middle basin always has repercussions downstream.”