Solving eel mysteries to restock our rivers

Concern over a steep decline in the European eel population has prompted the EU to implement a recovery plan to return stocks to pristine levels. The complex and mysterious life-cycle of eels does not make this an easy task says David Righton of the EELIAD project, an initiative that will help to resolve some of the questions that have been perplexing biologists for centuries..

European eels (latin name: Anguilla anguilla) play an important socioeconomic role in many European countries, but eel catches have been declining over the last 50 years. Furthermore, the number of juvenile eels returning to Europe collapsed in the early 1980s and has failed to recover since. In consequence, the EU established the Eel Stock Recovery Plan in 2007 in an effort to reverse the decline, with Member States required to implement national management plans that will safeguard eel populations and their habitats.

However, the measures needed to help eels are a matter of debate because the underlying cause of the decline in eel populations is not fully understood. Part of the answer may lie in the natural fluctuations that all fish populations undergo, but there are undoubtedly anthropogenic impacts like habitat destruction, fishing, pollution and climate change that are having an effect. Improving our understanding of some of the critical aspects of eel biology and ecology, and how they relate to both natural and anthropogenic impacts, is the focus of the eeliad project, as the coordinator David Righton explains. “Despite centuries of study, many aspects of eel biology are still poorly understood, which is attributable to the fact that eels are both born and reproduce more than 5000km from Europe, in the Sargasso Sea, near Bermuda. So, while they’re often called freshwater eels, arguably the most important moments of their lives are spent in the marine environment. There are a lot of really crucial aspects of their life in the sea that affect how eel stocks fluctuate and are sustained, and we don’t know enough about those aspects to take them fully into account in management plans.”

Spawning area

The precise location of eel spawning areas remains one of the enduring mysteries of biological science. It is known that eels reproduce in the Sargasso Sea, an area within the north-west Atlantic Ocean, a discovery made by the Danish biologist, Johannes Schmidt in 1912.

All anguillid eels undertake an enormous and risky journey at the beginning and end of their lives, but the journey of the European eel to and from the Sargasso is the longest migration of all eel species (up to 12,000km from the easternmost part of it’s distribution). While there is evidence that the distance between Europe and the Sargasso has increased over evolutionary time, the reason why eels continue to travel such huge distances is still unknown.

The Sargasso is a fairly barren part of the ocean; very little animal or plant life is found there. While this is fine for the returning adult eels, which have no need for food, it presents something of a problem for newly hatched eel larvae, which need food to help them grow and start their migration towards Europe. “While being born into a barren environment sounds like a bad start in life, one benefit of that for the eel larvae is are that there are also very few predators that might eat them, so they can concentrate on finding food rather than avoiding being eaten,” Righton outlines. In any case, the larvae don’t stay in the Sargasso for long, they leave soon after they have hatched, a process in which ocean currents play an important role. “The eel larvae can’t swim to Europe on their own- they need to take advantage of ocean currents,” continues Righton.

“The most important current is the North Atlantic drift, to the north of the Sargasso Sea, which gets them travelling east and north towards Europe at the beginning of their journey. Amazingly, we still don’t know exactly how long their journey to Europe takes to complete; there is a lot of debate about this. Some scientists believe it might only take six months, others think that it might be as long as three years. What all are agreed on, however, is that only a tiny fraction of the larval eels that leave the Sargasso Sea will make a successful migration to Europe; many larvae will lose their way, eventually succumbing to either predation or starvation. Even the larvae on the right route are likely to meet the same fate.”

Those eels which do survive the journey to Europe metamorphose on arrival into glass eels: tiny, see-through creatures which are caught in their billions. Glass eels have been the basis of traditional fisheries for generations and formed an important part of many regional economies. The decline of eel stocks is especially evident in the numbers of glass eels arriving from the Sargasso Sea, which has dropped alarmingly in recent years. This is worrying for all eel fishermen and scientists alike, not just those that fish glass eels, because fewer glass eels means that fewer will grow into the yellow or silver eels that are the basis of other traditional and important fisheries in rivers and estuaries across Europe.

To preserve eel populations and eel fisheries, scientists have recommended that more eels need to be allowed to survive and migrate back to the Sargasso Sea. Defining how many eels actually need to migrate is very difficult because populations are very complex, and vary regionally. “Each year, the cohort that undertakes the migration to the Sargasso is of mixed ages and sizes, but we don’t really understand what determines the chances of an eel getting to the Sargasso Sea successfully. Many factors can prevent eels from achieving a successful migration, even if they escape from the coast successfully.

For example we don’t know the rates of predation on eels once they leave their rivers, or the long-term impact of persistent organic pollutants like DDT or PCB’s. Conversely, we also know very little about how many eggs each successful migrant can contribute to the next generation, and how many of the resulting larvae might survive the journey home to Europe.” Consequently, scientists have recommended significant precautionary reductions in eel fisheries and fishery management.

Migrating population

One particular challenge that the scientists in the eeliad project are hoping to overcome is to map the spawning migration of the eel. “No-one has ever caught a spawning eel in the Sargasso Sea, and no eel egg has ever been discovered, which is an enduring mystery in marine science. Where are the eels? What do they do? Why can’t we find them? This is not a new problem; the origins of eels have been pondered without solution for more than two millennia, starting with Aristotle in around 350 BC. We can make predictions about what we expect eels to do, based on their physiology and on swimming trials conducted in laboratories, but direct evidence of the oceanic migration is something that marine biologists have been trying to collect since Schmidt described the spawning area in 1912. The the basic problem is that tracking marine animals, especially in the deep ocean waters that eels migrate through, is technically challenging and can be prohibitively expensive. However, in 2006, an American company called Microwave Telemetry produced a prototype of a fish tracking device, now marketed as the X-tag, which is small enough to deploy on European eels. These tags can be fitted to eels and, after a user-set interval, the tags are released and float to the ocean surface, where they beam data on the eel’s behaviour and position to us through the ARGOS satellite system.”

The prototype tags were used on 20 eels as part of the Danish Galathea 3 expedition and proved more successful than expected. As a result, Righton and colleagues were able to describe in detail the routes that eels take to the Sargasso, and the depths and temperatures that eels experience on a day by day basis. 80 more tagged eels were released in 2008, and more releases are planned for 2010, which will result in a comprehensive database of eel behaviour and advance the field significantly.

Righton explains, “While the data from each individual eel tells its own story, the data from many individual eels can be brought together to gain a deeper understanding of the challenges that migrating eels face, such as predation and natural mortality. If the eels can overcome those hurdles, the data can then help us to assess whether eels use deep-ocean currents to help them migrate, and estimate how much energy eels actually use up during their migration. In turn, this will help us determine how much energy eels have left when they arrive at the Sargasso Sea, and how much they can invest in reproduction.”

To date, the furthest that the project team has tracked an eel is about halfway to the Sargasso Sea, and Righton is optimistic that the project will be able to track them further. “We’re the first group to ever track European eels across the ocean as they migrate to the Sargasso Sea, which is a result that we’ve published in Science magazine. In doing that, we’ve already advanced the field and raised expectations. Our next steps are to track eels even further and to identify what are the characteristics an eel needs to make it a successful migrator, and what features might contribute to failed migrations.”

One thing is clear- managing and protecting the European eel population requires a European solution. “For decades, there has been a debate about whether all eels in Europe originate from a single spawning population (panmixia), or if there are several spawning populations that supply glass eels to different parts of Europe”, says Righton.

“So far, the majority of the evidence, including ground-breaking genetic studies on larval eels that have been undertaken in the eeliad project, suggests that eels are indeed panmictic and that management of eel populations needs to be coordinated at the EU level. Accordingly, the biology of eels isn’t just ‘blue sky’ science, or even ‘blue ocean’ science,” he jokes. “Finding out more about the biology of eels will benefit the way that we manage the European eel population, and could make the difference between sustaining a commercial eel stock, or watching the stock decline further and our historic fisheries with it.”

For more information, please visit the eeliad wesite.

Published: Wednesday, 3rd November 2010