The Dying Fish "reveals": the Congo River or the deepest river on Earth

The mysterious phenomenon of a fish dying in the lower Congo River has helped scientists discover that the river is the deepest river on Earth.

According to the American Fun Science website reported on January 12, in this river, turbulent turbulence, strong water flow and even underwater "waterfalls" separate the waters, just like mountains can separate the living areas on land.

According to a study presented at the American Geophysical Union Annual Meeting on December 12, 2019, these insurmountable river barriers isolate fish stocks, resulting in hundreds of species of fish in the river.

Scientists' earliest observations of the depths of the Congo River began more than 10 years ago, when they discovered pale colorblind fish that appear only when they are nearing death or have died. The fish proved to have died of decompression sickness. When the disease occurs, bubbles form in the blood and body tissues.

In an effort to understand the cause of this phenomenon, the researchers found that part of the bottom of the Congo River lies hundreds of feet below the river surface (1 foot equals about 0.3 meters) — deeper than any river on Earth.

Melanie Stiasni, director of the Fish Museum at the American Museum of Natural History, specializes in biodiversity and fish evolution in the lower Congo River, the last 322 kilometers before it flows into the Atlantic Ocean. Steasni led the study into the mystery of why the fish were on the verge of death.

Stiasni said at the annual meeting of the American Geophysical Union that more than 300 species of fish have been found in the lower Congo River alone. The turbulence there is so strong that they physically separate fish populations, prompting new species of fish to appear even in areas not far from their close relatives.

But even with so many fish populations, there is one eye-catching fish that stands out.

Stiassney said: "In one place, we found this very strange fish. It's a cichlidine fish with no eyes and no pigmentation — it looks a lot like a cave fish, but there are no caves in the river. Steasni and her colleagues wondered why they couldn't find an individual alive in the fish until she found important clues in a dying fish.

"When the fish died in my hands, bubbles formed under its skin and on its gills," Stiasni said, a definitive symptom of decompression sickness. As it rises rapidly from extremely deep waters to shallower depths, the pressure drops dramatically, causing gases dissolved inside the body to form bubbles in the body. If left untreated, the disease can be fatal.

This raises a question that the researchers have not considered before: Could there be deep waters in the lower Congo River?

To find out, the scientists sent brave kayak enthusiasts into the rapids in 2008 and 2009 and equipped the kayaks with equipment to measure the depth of the river. The researchers also used an instrument called an acoustic flow profiler to measure the direction and velocity of water flow across the entire column of water.

Stiassney said: "The results we got were quite amazing: the water was very deep, very deep. ”

According to a survey released by the U.S. Geological Survey in 2009, the bottom of the lower Congo River is located more than 200 meters below the river surface.

The U.S. Geological Survey data also shows that strong water flows through the water, forming a strong jet stream that shoots from the bottom of the river to the surface. Stiassney explained that a small fish that lived near the bottom of a river and swam into one of the jet streams would quickly rise to hundreds of feet above the surface of the river and then die of decompression sickness.

While the mystery surrounding these near-death fish may have been solved, much remains to be discovered about this unique section of the river and the animals that live in it. Interestingly, some isolated fish populations have evolved similar traits, a process known as convergent evolution. Stiassney told the Fun Science website that as to why this phenomenon occurs in this unique and extreme environment, this is the next big question she and her colleagues are working on. (Compilation/Du Yuanjiang)