Where finding fish is easy

Researchers have found more than 450 species of fish in a pocket of northern South America just half the size of Belgium. By comparison, they say, the Mississippi River Basin is 200 times its size but has only around 200 species.

Scientists from the Field Museum, US, spent a dry season in the Rupununi region of central Guyana looking for fish and were overwhelmed with what they found.

“We were taking fish out of rocks and caves, fish that live in woody debris in the water, sticking our hands in holes and hollow logs and finding fish,” says conservation ecologist Lesley de Souza. “Anywhere there might be a fish, we checked.”

They collected fish that looked like little silver knives, others camouflaged as dead leaves, and the three-metre Arapaima, which gulps air at the water’s surface as well as breathing underwater with gills. 

But the amount of diversity in this one area is not the most important point, de Souza says.

The region is a “portal”, because when river levels rise in the rainy season, two river systems that remain separate during the rest of the year are connected by floodwaters and fish can travel from the Guiana Shield to the heart of the Amazon. 

As the water recedes, the savannas and wetlands re-emerge, and the fish separate into their respective river systems again until the next rainy season.

During their recent studies, which are reported in a paper in the journal Frontiers in Forests and Global Change, de Souza and colleagues found a previously unknown portal farther to the south.