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Changing Oceans May Confuse Critically Endangered European Eels

Note: This article was originally published in February 2019.
 Original Article: Borges FO, Santos CP, Sampaio E, Figueiredo C, Paula JR, Antunes C, Rosa R, Grilo TF. 2019 Ocean warming and acidification may challenge the riverward migration of glass eels. Biol. Lett. 15: 20180627.


Young “glass eels” migrating to freshwater from the sea (Credit: Uwe Kils, commons.wikimedia.org/wiki/File:Glasseelskils.jpg)

As humanity’s carbon emissions continue to heat and acidify the world’s oceans, many marine species and ecosystems face daunting new challenges for survival.

European eels (also called glass eels, yellow eels, or silver eels depending on life stage,) might be a species that deserves optimism in the face of rapidly changing seas. They already survive astonishing body transformations and changes in water conditions throughout their lives, and even have what a recent study calls an “intrinsic resilience towards extreme conditions.” But that same study exposes exactly how climate change may bring challenges too great for the species to overcome.

A Great Migration

European eels are long distance migration champions among fish. To breed and spawn, they journey from rivers, lakes, and estuaries across Europe and North Africa to the Sargasso Sea in the western Atlantic Ocean. The transparent, jelly-like larvae born there then complete the same migration in reverse over two years, navigating thousands of miles across the Atlantic Ocean and Mediterranean Sea. As the larvae approach land for the first time, they must decide where they will settle and grow into four-foot long adults. The answer can range from Portugal to Syria to Sweden.

Migration Imitation: Studying Glass Eels in a Lab

 To imitate the young eels’ river-bound journey from the open ocean, researchers set up tanks with flowing water ocean that became gradually fresher over time. The juvenile eels were split into four random groups. One batch went into tanks with water that was warmer and more acidic than normal, mimicking how ocean conditions may look in 2100 with business-as-usual carbon emissions. Another batch was placed in more acidic, normal temperature water, and a third group went into warm water with a normal pH (acidity). The final group of eels lived in tanks with regular ocean water conditions.

After more than three months in these conditions, the scientists put the eels’ migration instincts to the test. They had already reduced the salinity in all the tanks to trigger the eels’ instincts for find a stream to live in as adults. The researchers began directing two water flows with different cues for the eels to sense through each of the tanks. The eels would normally sense these cues to find an estuary, stream, or river, and the young eels that followed these scented waterflows found themselves in an eel trap. At the end of the experiment, the eels that had swum ‘upstream’ and into the traps were considered those that would have successfully completed their migration in the wild.

How Warm, Acidic Water Waylays the Eels

Adult European eel, Anguilla anguilla (Credit: GerardM, static.digischool.nl/bi/onderwaterbiologie)

The results showed a surprising mix of consequences to changing ocean waters. By the end of the experiment, the eels that survived the most were actually the ones from tanks with acidified water, not regular ocean water. However, those eels were also much worse at accurately sensing cues for rivers and estuaries. Fewer eels survived in the warm-water tanks. but those that survived through the end of the experiment showed stronger impulses for migration. The authors speculate that in the wild, the surviving eels might migrate up rivers earlier.

The eels exposed to both of the two future ocean conditions survived almost as well as the eels in regular ocean water, but were worse at sensing water cues. As the ocean continues to warm and acidify, the repercussions for European Eels will be complex and even contradictory. Managers trying to preserve this endangered species for future generations would be wise to anticipate changes in when, how, and even where the eels populate Europe’s streams and rivers—because as surely as the ocean is changing, strategies to save species like the European Eel have to change as well.


Additional Sources

Jacoby, D. & Gollock, M. 2014. Anguilla anguilla. The IUCN Red List of Threatened Species 2014: e.T60344A45833138. dx.doi.org/10.2305/IUCN.UK.2014-1.RLTS.T60344A45833138.en


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