Independent monitoring of a giant Amazon catfish population in the Madeira River, a major tributary of the Amazon, confirms that two hydroelectric dams have virtually blocked the species’ homing migration upstream — the longest known freshwater fish migration in the world.Research completed in 2018 indicates a serious decline in catches of the gilded catfish (Brachyplatystoma rousseauxii) and other key commercial species on the Madeira, both upstream and downstream of the two dams.New monitoring techniques show that the disruption of the migration route raises the risk of extinction for this species, for which researchers have recommended the conservation status be elevated from vulnerable to critically endangered.If the gilded catfish and other migratory species are to survive, mechanisms to assist their migration past the dams must be improved, researchers say. Last year, Mongabay contributor Gustavo Faleiros and filmmaker Marcio Isensee e Sá visited the unique, biodiverse Amazon forests found on the divide between the Purus and Madeira river basins, where a plan to improve the BR-319 highway, delayed by decades, is gaining momentum, bringing environmental transformation. This story is the fourth in the series. If the Madeira River were a highway, it would certainly be a major route, a central artery, connecting important points southwest to northeast across the Amazon Basin. When its 3,380-kilometer (2,100-mile) length joins the mighty Amazon River, the two waterways link the Andes foothills, Brazilian rainforest floodplains, and the Atlantic Ocean. This connectivity isn’t just important for human commerce. Among the Amazon’s great tributaries, the Madeira contributes the most sediments coming from the high Andes. Its nutrient-rich waters, driven by annual rainy season floods, jump riverbanks and spread out through the Amazon rainforest; those sediments are essential to floodplain fertility and to the forest ecosystem. The river’s connectivity is just as vital to fish, and to large-scale commercial and small-scale subsistence fisheries, from the Madeira’s Bolivian headwaters, downstream into Brazil, and to where the river merges into the Amazon east of the city of Manaus. This extraordinary aquatic interconnectivity has generated tremendous evolutionary diversity. About 1,000 species of fish have been cataloged in the Madeira sub-basin, representing a third of all the ichthyofauna found in the entire Amazon Basin. Among the travelers along this magnificent mud-brown highway are migratory fish — capable of going the distance, drifting downstream and striving upstream for thousands of miles to mate, spawn, and live out their life cycles under harsh conditions; they push through heavy rapids and navigate submerged forests, and have thrived for many centuries. Then came the dams Or at least that’s how things stood before Brazil built the Santo Antônio and the Jirau mega dams, operational in 2012 and 2013, respectively: man-made obstacles that the great waves of migratory fish find it difficult, if not impossible, to pass, despite the addition of fish ladders. A decade after the start of construction of the two Madeira hydroelectric dams in Rondônia state, new scientific research indicates that the river’s fish have been negatively affected. Investigations published in 2018 demonstrate both a reduction in fishing catches and the dangerous isolation of populations of migratory species, such as the gilded catfish (Brachyplatystoma rousseauxii), which can grow to 1.5 meters (5 feet) long, and which is considered essential to the economy and food supply of Madeira Basin residents. The gilded catfish, or dourada, which makes the longest freshwater migration ever recorded — a round trip of some 11,000 kilometers (6,800 miles) — serves as a prime example of environmental change and species impacts. Surveys of the life cycle of individuals captured after the construction of the dams show migrations practically ceased after the closure of the floodgates — and that could ultimately have dire consequences for the catfish, including eventual extinction in some parts of the basin. The gilded catfish, or dourada (Brachyplatystoma rousseauxii) as drawn during an 1856 Amazon expedition. The species makes the longest freshwater migration ever recorded — a round trip of some 11,000 kilometers (6,800 miles). Image by Francis de Laporte de Castelnau in the public domain In a study done for her Ph.D. thesis, Marília Hauser, from the Federal University of Rondônia (UNIR), investigated what she calls the “black box” of the dourada. Hauser analyzed the otoliths — the deposits of calcium carbonate that build up in the inner ear cavities of all vertebrate species over their lifetime — gathered from 265 catfish captured upstream and downstream of the dams. When strontium isotopes in the otoliths are compared with a microchemical analysis of elements found in the Madeira and Amazon rivers, it’s possible to determine the lifecycle of an individual fish and map their annual river journeys. From such analyses, Hauser discovered that 80 percent of the catfish found in the Madeira River migrated all the way upstream to reproduce in the Andes headwaters before the hydroelectric plants were built. Now, almost all the fish analyzed are confined to the parts of the river where they’re found. There is little evidence of migration. Hauser found in particular that the otoliths in catfish populations upstream of the dams lack some of the chemical elements found in the middle and lower sections of the Madeira. Fish older than 5 years in age showed no evidence of having arrived back upstream after a downstream journey, meaning they were born and raised in the upper Madeira, never making the journey to the Amazon estuary on the Atlantic coast. “These results confirmed irrefutably the impacts of the dams, both on the contribution of adults in the upper reaches of the Madeira River basin, and on the migration downstream of eggs and larvae,” says the study, published in June 2018 by UNIR and titled “Migration of large catfish from the perspective of strontium isotopes in otoliths.” Hauser told Mongabay that her research on the otoliths showed that these catfish, when allowed to migrate freely, return to spawn in the headwaters of the river where they were born. A small part of the researcher’s sample, 16 percent, eventually reached the Amazon estuary, indicating that dourada larvae can descend through the dams’ turbines. But there’s no indication that the maturing fish ever make their way back upstream to their home territories. For Hauser, the data show conclusively that there’s been “a complete change in the migration pattern.” One key finding came from the examination of a population of catfish captured in the Santo Antônio dam reservoir. All of these upstream individuals were adults and residents, which could indicate that even if some of the fish managed to migrate upstream into the first reservoir using a fish ladder, they weren’t able to continue upstream past the second hydroelectric plant, the Jirau dam.