The first words long-distance swimmer Ben Lecomte uttered after becoming the first person to swim 3,716 miles across the Atlantic Ocean in 1998 were, "Never again." That feat took him 73 days. In comparison, it takes dorado catfish hatchlings about a month to cover 3,595 miles (5,786 km) as they travel from the Andes mountains to the mouth of the Amazon river. And then they make the return journey just as effortlessly a few years later when they are ready to breed. For the dorado catfish, life is one epic endurance test. It's hardly any wonder then that a new study has just crowned it the world record holder for the longest freshwater fish migration, beating out the chinook salmon (3,000km or 1,900 miles) and European eel (5,000 km or 3,106 miles).

Part of the goliath catfish genus, this silvery gold specimen is an apex predator that can reach lengths of two meters (6.56 ft) long. The countdown to its epic journey begins when new adults bid goodbye to their nurseries in the Amazon estuary and travel thousands of kilometers upriver to spawning grounds in or near the Andes. Once they are done breeding, they then travel downstream to their new home in the western Amazon, only returning to the Andes to spawn again the next year.

When the hatchlings emerge from the eggs, they instinctively head for the nurseries in the estuary where they will spend the next two to three years leading a carefree existence maturing into adults before embarking on the journey that their parents embarked upon before them. All in all, researchers estimate that the dorado catfish has an average life-cycle migration of approximately 11,600 km (more than 7,200 miles).

Scientists have long had a hunch about its migratory prowess, but the size and scale of the Amazon river made it difficult to track and reconstruct the animals' movements. In this study, an international team of researchers adopted a three-prong approach by studying the distribution of mature size classes, downstream migration of larvae and juveniles, as well as otolith signatures of four goliath catfish species including the dorado.

They mapped their movements and distribution in river channels across the Amazon using two kinds of data: year-round data from the Madeira Basin, one of the river's largest Amazon-Andes sub-basins and home to the catfish's breeding grounds in Peru's Madre de Dios; as well as available data from multiple years of the entire Amazon basin.

The route taken by the Dorado catfish during its migration through the Amazon River basin(Credit: WCS)

Their findings confirmed that the long-distance migratory journey of the adult dorado catfish, traveling upstream from the Amazon estuary to reach their spawning grounds in or near the Andes, can take up to two years. In addition, the scientists found that the area is also a breeding hotspot for at least two other goliath catfish species.

"This is the first time that scientific research has linked the full range of these fish species, some of which stretch from the Andes to the Amazon river estuary," says lead researcher Ronaldo Barthem at the Museu Paraense Emílio Goeldi in Brazil.

The dorado is a crucial food source for the local community(Credit: Michael Goulding/WCS)

Despite the fact that the dorado is a prized commercial species in the Amazon, surprisingly little is known about it. Given the vital role that the goliath catfish plays in the local commercial fishing industry, understanding its migratory patterns is key to maintaining the health of the goliath catfish population so that it doesn't suffer from the same problems that are driving another behemoth – the Mekong giant catfish in Southeast Asia – to extinction.

In this regard, the evidence of long-distance migratory movements in the river channels highlights the potential dangers posed by encroaching development, such as dam construction and mining operations, to the catfish population and overall health of the river ecosystem. As the authors note in their study:

Of special relevance is the expected infrastructure development in the Andes, especially the combination of dams, headwater deforestation and mining activity, which could present major threats to important spawning areas ranging from Colombia in the north to Bolivia in the south. Even if high-wall dams [in the Andes] are located upstream of spawning sites, they would greatly alter sediment and nutrient cycles downriver where spawning occurs. The long-distance migratory goliath catfishes provide a profound biological indicator of ecosystem health from the Andes to the freshwater Amazon River plume in the Atlantic, and the impacts on them should be considered in all major infrastructure development.

The dorado faces the same problems plaguing other migratory fish: overfishing, pollution and dam construction(Credit: Michael Goulding/WCS)

For now, while this study might have shed light on one mystery, it's also shown the researchers how little they know about this Amazon native. "Many questions remain about these incredible fish, such as why they travel so far to reproduce and do they return to their place of birth to spawn?" muses aquatic scientist Michael Goulding, a co-author on the study. "Now we have a baseline that will help direct future research and conservation efforts."

The study was published in the journal Scientific Reports.

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