by Bruce Batt, Ph.D.
The west coast of Mexico provides vital wintering habitat for North American waterfowl. Its importance grew in the middle of the last century as the million-acre Colorado River Delta was destroyed with the almost complete diversion of its water for agricultural, recreational, and domestic uses in the western United States. The Central Valley of California is now the most critical wintering area for Pacific Flyway waterfowl. Biologists consider habitat conditions there to be better now than they have been in the past 20 years. Farmers now flood more than 300,000 acres of rice stubble that provides excellent habitat for more than 5 million waterfowl.
All is not secure, though, in the Central Valley, as current rice-farming practices are not certain to continue for the long term. Drought could also easily eliminate for one or more winters much of the water needed for flooding rice fields, refuges, and wildlife management areas in California. Thus, the wetlands of Mexico's west coast are a critical safety valve for Pacific Flyway waterfowl.
Most of the important wetland habitats are in three states: Sonora, Sinaloa, and Nyarit. These wetlands—predominantly coastal lagoons, bahías, and estuaries—provide a variety of habitats grading from brackish to freshwater. Until about 15 years ago, agriculture also provided significant food resources for waterfowl, as farmers planted 300,000 acres of irrigated rice in Sonora and Sinaloa. But the rice is gone now. World markets have lowered the price so much that growing rice is no longer economically feasible in Mexico. With the loss of those 300,000 acres of rice, the remaining wetland habitats are even more important to wintering waterfowl.
Most of these coastal wetlands are nearly pristine. It is remarkable to go out into the hundreds of thousands of acres of coastal wetlands and see the masses of birds of all types that winter there. Key duck species include northern pintails, green-winged teal, and lesser scaup. Equally remarkable is the realization that these wetlands look and function very much as they have for thousands of years. One has the sense of being in an untouched wilderness. The water is just a few inches deep for thousands of acres. Thick, silty mud lies beneath, making these shallows impassable except by airboat. As a result, beachfront properties and seaports have not been developed. And because no petroleum products are known to be under these wetlands, they have not been threatened by exploration and extraction activities.
Sophisticated irrigated agriculture dominates much of the coastal plain. Irrigation has changed when and how often fresh water flows from the Sierra Nevada Mountains to the wetlands. Agricultural chemicals and fertilizers find their way into the wetlands and inevitably affect water quality. And large volumes of other nutrients come from the expanding human population, which is served by limited sewage-treatment facilities.
Despite these impacts, the wetlands remain nearly fully functional ecosystems, as they have for centuries. Many of the nutrients and chemicals are interrupted by vast acreages of cattail marshes that have formed at the mouths of many rivers and other routes that the irrigation waters follow to the coast. The marshes are natural filters of these pollutants and play a critical role in protecting hundreds of thousands of acres of other coastal wetlands.
In the last decade, a new threat to these coastal wetlands has emerged as the shrimp-farming industry has expanded along the coast. The greatest impact occurs where shrimp farms have been placed in mangrove swamps that are in the transition zone between freshwater and brackish wetlands. These mangrove wetlands not only provide crucial habitat for waterfowl and millions of other birds but are also the nursery habitats for most of the important commercial fisheries that coastal residents depend on for food. One acre of mangrove swamp is the birthplace of what eventually becomes about 1,500 pounds of seafood harvested in the ocean. For this reason alone, countries all over the world are working to restore mangrove wetlands that have been lost to road- and levee-building and the growth of their own shrimp-farming industries.
Ducks Unlimited de Mexico (DUMAC) is conducting a profoundly important assessment of the impacts of Mexico's shrimp-farming industry. Satellite technology is being used to measure where new shrimp farms have been developed between 1992 and 2003. The results for Sinaloa are alarming. During that time period, 28,202 acres of mangroves were lost under the footprint of shrimp farms or in adjacent areas where the natural water flow was interrupted, which starved or flooded the mangroves.
The analysis continues in Sonora and Nyarit thanks to funding from the U.S. Forest Service. However, the results for Sinaloa provide a wake-up call for all who are interested in conserving this vital wetland resource for Mexico and for all the North American birds that use this habitat. DUMAC shares research results with state and federal officials and is also conducting a series of training programs for biologists and managers to help them understand and be responsive to the threats that uncontrolled loss of mangrove swamps would cause.
Mexico still has some of the most unspoiled mangrove wetlands in the world, and sustaining them is vitally important. Ducks Unlimited's priority is to protect existing mangrove wetlands. Mexico is still in an excellent position to secure most of its mangrove wetlands for the future. The critical challenges being addressed by DUMAC are to bring about a much broader awareness of this problem and then to find solutions that can be applied through regulation along with better site selection and management practices for new shrimp farms. DUMAC believes the shrimp-farming industry can be developed in a manner that is compatible with wetland protection by using ecologically sound site selection processes while ensuring the industry's sustainability and profitability. Thanks to DUMAC's strong scientific work, we have a chance to successfully conserve the mangroves and spectacular associated wetlands along the west coast of Mexico.