FRESHWATER MEGA-FAUNA

SOME HAVE SURVIVED for hundreds of millions of years, but many of the world’s freshwater Mega-fauna—including sumo-sized stingrays, colossal catfish, giant turtles, and gargantuan salamanders—may soon find themselves on the brink of extinction.

For the first time, researchers have quantified the global decline of freshwater mega-fauna—including fish, reptiles, amphibians, and mammals—and the results paint a grim picture. In four decades since 1970, the global populations of these freshwater giants have declined by almost 90 percent—twice as much as the loss of vertebrate populations on land or in the oceans.

Large fish species, such as sturgeons, salmons, and giant catfishes, are particularly threatened, with a 94 percent population decline. Most large freshwater reptile and many mammal species are also in trouble. The baiji, a Chinese river dolphin, is likely the first dolphin species driven to extinction by humans, and the Chinese paddlefish, which can grow 20 feet long, has not been seen in over a decade. Other species may be down to their last few individuals.

UNDERWATER SPECIES

Freshwater ecosystems are generally less studied than their marine counterparts, despite being home to a third of all vertebrate species and nearly half of all fish species worldwide. While population declines have been well-documented for both terrestrial and marine mega-fauna.

For the study, a team of international researchers compiled population data on 126 out of 207 freshwater species weighing at least 30 kilograms (66 pounds) from 1970 to 2012, drawing in part on The Living Planet Index, a database managed by the Zoological Society of London in cooperation with the World Wildlife Fund. While that index shows that populations of all freshwater species declined by 83 percent during roughly the same period, the new study shows an even higher rate of decline in big freshwater animals, at 88 percent.

Public awareness of the freshwater biodiversity crisis is limited, with many people unaware that the giant creatures even exist. “They are not like tigers, pandas, lions, or whales—species that receive a lot of attention in the media and school education.

Among the biggest threats facing large freshwater species are overexploitation and habitat degradation.  Many of the these animals are targeted for meat, skin, and eggs. Mega-fish in particular tend to be more vulnerable than other fish to dams that block their migratory routes and limit access to spawning grounds. Large animals also tend to be slow to mature and have low reproduction rates, making them particularly vulnerable.

RIVER MEKONG

The bio-geographic zones that have seen the greatest declines in freshwater mega-fauna are Indo-Malaya (99 percent) and the Palearctic, which encompasses Europe, northern Africa, and northern Asia (97 percent). The most critical region may be Southeast Asia, and in particular the Mekong River, which runs through China, Myanmar, Thailand, Laos, Cambodia and Vietnam. More than a thousand species of freshwater fish live in the Mekong, including many of the world’s largest. The Mekong giant catfish, for example, is the current record-holder for the world’s largest freshwater fish ever caught, at 646 pounds. Existing and planned dams on the river may drive the species to extinction and it could threaten the food security and livelihoods of millions of people living along the river.

Amid the grim overall findings, the study did suggest that 13 freshwater mega-fauna species have seen their populations stabilize or even grow. Among them are the green sturgeon and the American beaver, both in the United States. In Europe, the Eurasian beaver has returned to many regions from where it had once disappeared, and in Cambodia, the population of Irrawaddy river dolphins has increased for the first time in 20 years.