Movements Of The Amazon's Giant Fish Revealed By Looking In Their Ears

The arapaima is the largest freshwater fish in the world, and an important commercial fish. Jeff Kubina/Flickr CC BY-SA 2.0

The movements of some of the Amazon River's giant inhabitants have been tracked by looking in their ears. Researchers have been able to use what are called “ear-stones” to work out where the fish travel to during their life cycle, and thus reveal their migrations. The research is published in Royal Society Open Science.

Despite being home to over 5,000 species of fish, very little is actually known about them, and particularly their movements throughout the river basin. With some of the biggest residents, such as the Arapaima – coming in at up to 3 meters (10 feet) long, and thus a target for fishers – the fact that so little is known about them is a bit of a worry. This is because the fish are commercially caught in fisheries in multiple countries right along the river, with little knowledge about how this might be affecting the species.


To try and fill in some of the blanks about the fish's movements, the researchers decided to see if they could glean anything from the animals' ear bones. This may sound like a bizarre way to track animals, but in fish, the ear stones – or otoliths – grow in size as the fish does, meaning that they contain growth rings in a similar way to trees. As a new layer of calcium carbonate is laid down, they also trap other trace elements that can give details about the type of water the fish was living in at the time. By using C-ray fluorescence and mass spectrometry on the otoliths, the researchers were able to unlock these secrets.

The otolith showing the rings that allow the researchers to work out how the giant catfish migrates over 3,200 kilometers (2,000 miles). Theodore W. Hermann, Karin E. Limburg, Donald J. Stewart, R. Barringa

By doing this, the researchers were able to reveal the life histories of five of the species of fish that live in the Amazon, including some of the largest: the Arapaima, Plagioscion, Prochilodus nigricans, and two species of goliath catfish. They found that they were able to track whether the fish were moving between black water and white water rivers, the two main types of river in the Amazon basin, as well as whether or not the fish were moving between the estuary and headwaters. This gives them vital clues about how, and when, the fish migrate.

They discovered that one of the two species of catfish, for example, spent the first two years of their lives in the Amazon estuary, before moving 3,200 kilometers (2,000 miles) upstream to mature. They also found that a species thought to be sedentary actually migrated great distances within the river. These discoveries could have profound implications on how the river system, and the commercial fisheries they support, should be managed.


The researchers want their study to contribute to the conservation and management needed to ensure the survival of these threatened species. Despite being heavily exploited, mainly due to their massive size, very little is currently known about these giants. If more information can be given, hopefully conservationists can better understand how the growing mining, deforestation and building of hydroelectric dams will impact these commercially important fish.

Main image: Jeff Kubina/Flickr CC BY-SA 2.0