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Climate Change

Changing Oceans May Confuse Critically Endangered European Eels

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. http://dx.doi.org/10.1098/rsbl.2018.0627

 

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,) may be a particularly good species to keep an eye on in the face of rapidly changing seas. They thrive in a wide range of environments, and even have what a recent study calls an “intrinsic resilience towards extreme conditions.” But the same study exposes exactly how climate change may bring serious challenges for this endangered and economically important species.

 

Full grown European Eels, also called silver eels, migrate from rivers and other freshwater bodies across Europe and North Africa to breed in the western Atlantic Ocean. (Freshwater and Marine Image Bank, Felice Supino – Supino, Felice (1916) Pesci d’Acqua Dolce d’Italia, Milan: Ulrico Hoepli, Editore Libraio della Real Casa)

A Great Migration

European eels are long distance migration champions among fish. They are catadromous, meaning they spend most of their lives in freshwater but breed in the ocean—the reverse life cycle of fish like salmon and steelhead. The eels journey seaward from rivers, lakes, and estuaries across Europe and North Africa to breed and spawn in 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 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.

Tiny and almost transparent, glass eels are the lifestage of the European Eel that completes the long journey across the Atlantic and populates European rivers and estuaries. They are also the lifestage that researchers used to test how future ocean conditions might impact the European Eel population. (Uwe Kils, https://commons.wikimedia.org/wiki/File:Glasseelskils.jpg)

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 to find a stream to live in as adults. The researchers began directing two different water flows through each of the tanks. One flow was freshwater, and the other was freshwater with geosmin, a compound that gives off an earthy smell and that the European Eels are very sensitive to when searching for suitable habitat to live in. The young eels that followed either the regular freshwater or geosmin-scented waterflows ‘upstream’ in the tanks found themselves in eel traps, harmless corrals at the front of the tank. 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. The scientists recorded how many eels in each type of tank survived their 14 weeks in the mock-ocean, and how many successfully followed clues in the water towards healthy eel habitat.

 

How Warm and Acidic Water Waylays the Eels

The results revealed a surprising mix of consequences for the eels’ migration ability. By the end of the experiment, the eels that survived the most were actually the ones from tanks with acidified water and regular temperature water. However, those eels were also significantly worse at sensing cues for rivers and estuaries.

The eels in regular seawater conditions had the next-best survival rates, with fewest eels surviving in the two types of warm-water tanks (regular and acidified). Eels that did survive through the end of the experiment in warmer water showed stronger impulses for migration, both with and without higher acidity added as well. The authors speculate that in the wild, surviving eels in a warm ocean might migrate up rivers earlier.

The eels exposed to water that was both warm and acidic survived almost as well as the eels raised in regular ocean water conditions. Yet when their migratory instincts were put to the test, they performed worse than the eels raised in regular conditions at sensing where to find good habitats. These results suggest that as the ocean continues to warm and acidify, the repercussions for European Eels could be complicated and contradictory. Scientists and managers in European Eel conservation must 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.

 

Sources

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

“Science for Environment Policy”: European Commission DG Environment News Alert Service, edited by SCU, The University of the West of England, Bristol.

Tosi, L. and Sola, C. (1993), Role of Geosmin, a Typical Inland Water Odour, in Guiding Glass Eel Anguilla anguilla (L.) Migration. Ethology, 95: 177-185. doi:10.1111/j.1439-0310.1993.tb00468.x

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