European eel. (Photo: Stock File)

Eel spawning ground changes observed

To improve understanding of the decline in the European eel population, a Danish research expedition with DTU Aqua's research vessel Dana explored the spawning areas in the Sargasso Sea in March-April 2014.

The recruitment of the European eel, monitored by the amount of glass-eel returning to Europe from the Sargasso Sea has been in dramatic decline during the last 30 years, and is at a severe low of only 3-5 per cent of earlier magnitude. This change and the consequences for eel fishery in Europe have led to intensified research on the oceanic phase in the life of the European eel.

Further field studies have been proposed, and in 2014 a Danish eel expedition set out – using DTU’s DANA IV- targeting the eel spawning grounds in the Sargasso Sea and the areas east of this area. The cruise was carried out in coordination with a German expedition covering the same time period and areas.

The Danish Eel Expedition 2014 was focused on improving our understanding of the importance of oceanographic processes for the eel’s choice of spawning site, and for their early life. Will the apparent climate change in the Sargasso Sea, with a significant increase in temperature during the last 40 years, impact larval life and drift and ultimately their overall survival?

The preliminary findings from the expedition indicate that a number of changes have taken place in the spawning areas that may affect the eel larvae’s chances of survival and their journey to Europe. The area was obviously warmer than during earlier expeditions in the 1970s and 1980s. In the context of the increase in temperature, we found that the fronts,  i.e. the transition zone between the hot and cold water masses, were further north than previously found. And while the eel larvae often are concentrated at these fronts, we also found that these groups had shifted northward.

The extension of the spawning area towards the east appeared limited by the water masses of especially high surface salinity located centrally in the Atlantic Ocean. Only few earlier expeditions have systematically investigated the easterly parts of the spawning areas in the Sargasso Sea, but this expedition extended sampling to these areas and landed some of the ‘most easterly’ newly hatched larvae ever caught.

The eel larvae are adapted to a very special ecosystem; the Sargasso Sea can best be described as a ‘desert’. However, the fronts in the area, and the related processes, generate a relatively higher production, and here the eel larvae are  concentrated.

The preliminary observations indicate that the plankton population in several aspects differs from what has been observed during previous expeditions in these areas. The conditions for the larvae may therefore have changed, and this might have had an impact on their chances of survival.

Given that researchers have numerous samples to analyze, they expect to know much more about ecosystem composition and function when these samples are fully processed.

A total of 33 Danish and international researchers participated in the expedition. It was headed by DTU Aqua and funded by the Danish Centre for Marine Research and the Carlsberg Foundation.

A longer version of this article has been publised in ICES Insight 51, September 2014.

Related article:

- Danish expedition to shed light on eel decline

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