Otoliths Of Amazon River Fishes Reveal Life Cycles

By on June 29, 2016
amazon river fishes

Despite its status as the world’s largest river system, scientists know relatively little about the fishes that call the Amazon River home. These include fish species like the Arapaima, the largest fish in the river, and giant catfishes that boast some of the longest migrations of freshwater fishes in the world.

This is so for a few reasons, not least of which is the vastness of the Amazon River system, which drains nearly 4.3 million square miles. That means that most locations are remote enough to make tagging fish impractical and expensive for many species.

Some species move extensively throughout their lives, making direct tracking difficult. And tagging and tracking devices typically cannot be used on larvae or small juveniles, leaving a fish’s early life unknown. The costly satellite tags can also be lost if a fish is caught by a fisherman.

To get around those issues and to gain a better understanding of how Amazon River fishes travel through the waterway, scientists at the State University of New York have dissected the otoliths, or earbones, of five fish species from the river. These included the Arapaima and two long-distance migratory catfishes.

The study is part of an emerging body of knowledge that lays critical groundwork for the conservation and management of threatened Amazon River fish species. The goal is to provide fisheries managers and conservationists with better information about how to protect the fishes from threats like deforestation, mining and construction of dams for hydroelectric power.

amazon river fishes

Colors on the otolith indicate strontium to calcium ratios over the fish’s life time. (Credit: Hermann, et al.)

Importantly, scientists wanted to pin down the fishes’ life histories, from hatching through larval, juvenile and adult stages as well as spawning cycles. Otoliths can also reveal if a fish lived in more acidic or more sediment-heavy rivers. The Amazon estuary also has its own chemical markers related to the mix of fresh and salt water that indicate a fish spent some of its life in that sprawling, unique ecosystem.

Made of calcium carbonate, otoliths grow as the fish grows, forming rings each year that can be read much the same way as a tree’s rings. Their growth incorporates traces of other elements that reflect the inherent chemistry of the water in which the fish lived.

Laser ablation was used to cut into the otoliths, which were then assessed for barium content. That element is useful as a tracking agent to determine where the fish have been. Two-dimensional chemical maps of the otoliths were also made using X-Ray fluorescence, keying on elements like selenium and calcium.

Analysis of the earbones showed that Arapaima in the river don’t have a life history involving long-distance migrations. As for the catfish, results revealed that the catfish spend their first one and a half or two years of life in the Amazon River estuary. This means the fish may have two life stages, one that lasts till around age 2 and another that follows.

Full results of the work are published under open-access license in the journal Royal Society Open Science.

Top image: Arapaima. (Credit: Flickr User Ralta Futo via Creative Commons 2.0)

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