the swimming creatures away and 

 kill clams and other sedentary spe- 

 cies on the spot. The Gulf's sea- 

 sonal dead zone now encompasses 

 more than 8,000 square miles, an 

 area the size of New Jersey, every 

 spring and summer. Much smaller 

 dead zones occur on the Mekong, 

 too. Worldwide, there are 146, ev- 

 ery one increasing in size, intensity, 

 and often duration. 



Besides agricultural runoff, 

 pollution from industry and mu- 

 nicipalities is also a big problem 

 for freshwater systems. In addition 

 to contributing extra nutrients 

 that promote algal overgrowth, 

 municipal wastewater also carries 

 thousands of chemicals from prod- 

 ucts used in daily life: cosmetics, 

 soaps, pharmaceuticals, cleaning 

 supplies, and more. Most ot it 

 winds up in aquatic systems. 



The long-term consequences 

 of dumping so many chemicals 

 in the water are just coming to 

 light. More than 200 species are 

 thought to have adverse reactions 

 to endocrine disruptors — such as estrogen and its chemical 

 mimics — that get into the environment via human and 

 veterinary pharmaceuticals in wastewater and farm run- 

 off. Sightings of frogs with deformities, such as extra legs, 

 mushroomed in the Midwest about a decade ago. Ecolo- 

 gists think chemicals or an interaction between chemicals 

 and parasites could be causing the deformities. Indeed, 

 chemicals in freshwater may be a factor in the alarmingly 

 sharp worldwide decline of amphibians. 



Biological introductions to waterways, like chemical 

 introductions, are extremely problematic. In their own 

 communities, most species are held in check by natural 

 predators or other environmental constraints. But organisms 

 from afar can crowd, devour, or outcompete native species 

 in their new neighborhoods, and can even change entire 

 ecosystems. Most biological introductions by people are 

 accidental, but some, such as fishes stocked for anglers or 

 plants brought in to stabilize soils, are intentional. 



Mimosa pigra, a spiny shrub native to the Americas and 

 planted abroad as an ornamental or to control erosion, is 

 now one of the world's worst aquatic invasive species. Once 

 established, it quickly forms dense stands and outcompetes 

 native plants. First spotted on the Mekong in 1979, it 

 spreads in floodwaters and in truckloads of construction 

 sand, and is now devastating parts of the watershed. The 

 mimosa has taken over several irreplaceable wetlands. 



doubling its area almost every 

 year in some places. Several 

 endangered water birds that 

 depend on native grasses for 

 food and shelter are undergoing 

 population declines as mimosa 

 stands replace their habitat. 



Controlling freshwater in- 

 vaders and mitigating the dam- 

 age they cause costs some 9 

 billion dollars each year in the 

 U.S. alone. Yet the rate of inva- 

 sions everywhere is on the rise 

 as global commerce, trade, and 

 travel increase. 



s 



Laotian fishmonger offers Mekong giant catfish 

 for sale. The giant catfish, which can grow to nine feet 

 long and 600 pounds, is now critically endangered. 



o much for the organisms 

 people add to freshwater 

 systems. What about the 

 ones — too many— that we take 

 out? Overexploitation for food, 

 medicine, and recreation poses a 

 major threat to freshwater birds, 

 crocodiles, fishes, frogs, and 

 turtles, as well as some inver- 

 tebrates. More than 40 million 

 people rely on the waters of the 

 Mekong River Basin for their 

 protein and income, and they are overfishing numer- 

 ous species — indeed entire fish assemblages in certain 

 areas — as a result. 



The Mekong giant catfish, Pangasionodon gigas, is just 

 one of the region's struggling, overfished residents. Reach- 

 ing nine feet in length and more than 600 pounds, it is 

 the world's largest catfish [see photograph above]. With 

 such grand proportions, a jackpot of succulent flesh that 

 once sold at a premium to urban restaurants, the giant 

 catfish was a fisherman's prize catch. In the mid-twenti- 

 eth century, hundreds of giant catfish — a naturally rare 

 species — were caught each year, but recently the annual 

 catch has declined to fewer than ten. Overfishing is the 

 main cause of the decline, but habitat fragmentation and 

 alteration of spawning grounds by dams and navigation 

 projects also contribute. Today, the giant catfish is criti- 

 cally endangered, its range is greatly restricted, and the 

 average size of individuals is declining. In recent years, 

 Cambodia, Laos, and Thailand have outlawed catching 

 the giant catfish. But the species is migratory, so a regional 

 agreement may be necessary to prevent its demise. 



Fish aren't the only victims of overexploitation. As 

 many as 10,000 water snakes are fished from Tonle Sap 

 Lake each day. The water snakes mainly go to feed hungry 

 crocodiles raised for commercial export; they substitute for 

 fish, whose populations have declined. People are fishing 

 down the food chain m the Mekong River Basin, as in 



44 



natural history November 2007 



