On board the Walter Herwig III: In the eels’ spawning grounds

This regular research expedition focuses on the distribution and abundance of the willow leaf and leptocephalus larvae of the European eel in its spawning grounds – the Sargasso Sea in the western North Atlantic.

To date, the recruitment time series of arriving glass eels and young yellow eels on the coasts of Europe and North Africa following their transatlantic migration have been regarded as the key indicator of the species’ population trends. However, supplementary information on the numbers and spatial distribution of the youngest larvae directly in their area of origin can provide further insights. 

This enables us to better understand oceanic changes and identify potential impacts on recruitment trends at an early stage. The captured eel larvae are therefore being studied with regard to a wide range of biological and ecological parameters. The aim is to gain new insights into their ecology, diet and development.

Now, two weeks into the cruise, we have already sampled around two dozen stations for fish larvae and zooplankton. At almost every station, drifting algae of the genus Sargassum have been caught in our nets. Eel larvae and other organisms tend to get entangled in these plants and have to be carefully removed afterwards.

In addition to these “Sargassum by-catches” from the IKMT net, the crew of the Walther Herwig III has also been actively collecting algae with hand nets at the stations. All samples are carefully examined, labelled, photographed, and frozen by Dr. Florian Lüskow. Once back in Bremerhaven, they will be analysed with respect to the composition of their associated invertebrate communities. Comparable studies were already carried out in the southern Sargasso Sea in spring 2015 - making it particularly interesting to see if and how these communities have changed since then. Such changes could provide insights into the impacts of climate change in the Sargasso Sea.

Between stations, Florian can often be found on deck conducting systematic observations of the sea surface, following a protocol originally developed in 2015. A key focus is estimating the amount of Sargassum at the surface, as these drifting algae serve as habitat, nursery ground for many fish species, and as a food source. In practice, however, this is easier said than done: small-scale observations cannot easily be extrapolated to larger ocean areas. Conversely, satellite images - with resolutions of roughly 10 × 10 to 300 × 300 metres - often fail to reliably detect smaller patches of algae.

To address this, Lüskow follows a multi-approach strategy, combining satellite data with visual impressions and video observations. The goal is to better understand which methods tend to over- or underestimate the actual amount of algae - whether single fragments or larger floating mats.

In addition to leptocephalus larvae and a few other selected organisms that perform daily vertical migrations from the surface layers to deeper parts of the Sargasso Sea, our Czech colleagues from Charles University in Prague are focusing during this cruise mainly on mesopelagic fishes. These are specialised species that live in a depth range between the surface and the deep sea. Vít Kaufmann and Camilla Kidmose present their work here:

The mesopelagic “twilight zone” (200–1000 m depth) represents a very different visual environment compared to life on land. At these depths, only about 0.1 % of surface sunlight remains, and much of the available light originates from bioluminescent organisms, including both invertebrates and vertebrates. Because blue-green light (wavelengths around 480–570 nm) travels furthest through seawater, most animals in this zone are expected to have visual systems adapted to this part of the spectrum. Our work focuses on understanding how fish vision has evolved under these extreme light conditions.

In vertebrates, vision relies on photoreceptor cells - rods and cones - that produce light-sensitive proteins called opsins. Different opsins are tuned to different wavelengths and determine how animals detect light and potentially distinguish colours.

We investigate which retinal adaptations allow deep-sea fishes to remain functional under dim light conditions. The silver spinyfin (Diretmus argenteus) serves as a model species, as it possesses an unusually large diversity of rod opsins. With our work, we test two possible explanations: either this diversity enables some form of colour vision in very low light, or it primarily enhances light sensitivity, allowing even minimal amounts of light to be used. To investigate this, we analyse which opsins are active in individual photoreceptor cells and whether they are predominantly found in rods or cones.

A central task during our research expeditions to the Sargasso Sea is the collaborative sorting of the plankton samples we collect at every station using our pelagic larval net. Particular care is required to spot - and gently separate from the rest of the catch - the tiny leptocephalus larvae, some of which measure only a few millimeters in size.

Technician Tina Blancke, whose responsibilities on board include the morphological and molecular identification of the larvae, describes this process as follows: 

The search for leptocephalus larvae always begins with a ladleful of thin 'plankton soup', which sloshes rhythmically back and forth in the participants' glass sorting dishes with every swell. Under the light of the sorting lamp, these flat, semi-transparent larvae are visible even to the naked eye, allowing them to be carefully grasped with fine-tipped forceps and transferred to a separate collection dish. All told, since 2011, we have identified more than 60 different species of leptocephalus larvae here; consequently, at every new station, I eagerly set about the task of identification.

On this expedition - my sixth journey to the Sargasso Sea - I naturally know my usual larval 'suspects' very well. I take pleasure in every specimen I can quickly and confidently identify under the stereomicroscope, drawing upon my specialized expertise. Larvae that turn out to belong to the Anguillidae family are subsequently subjected to rapid genetic tests right here on board to definitively assign them to either the European or the American eel species.

Nevertheless, I find the larvae that remain elusive - whose specific identity remains hidden from me even after a first, and often a second, concentrated look - to be particularly fascinating. When encountering rare - or even previously unrecorded - specimens during our expeditions, the only recourse is to examine specific morphological markers, such as gut length, eye shape, pigmentation, and the count of muscle segments (myomeres).

To then attempt at least a rough taxonomic assignment - to a genus or, failing that, a family - I embark on a diligent search through the relevant literature, ever hopeful that on the very next page I might finally discover an illustration or description of the particular larva currently suspended in the drop of seawater within the Petri dish before me.

It is all the more thrilling when such a match turns up deep within the volume - specifically in the chapter dedicated to "hitherto unidentified larvae." And I grow all the more impatient at the thought that it will be several more weeks before I can return to the institute laboratory with my samples to conduct genetic sequence analyses - and perhaps, at long last, coax these larvae into revealing the closely guarded secret of their true species identity.

The Sargasso Sea is probably the only ocean region without fixed geographical boundaries. Unlike many other parts of the ocean, it is not defined by landmasses but by large-scale currents: the Gulf Stream to the west, the North Atlantic Current to the north, the Canary Current to the east, and the North Equatorial Current to the south. Together, these currents form a vast circulation system that effectively encloses the Sargasso Sea. At the same time, this also illustrates why research in this region is particularly challenging. Ocean currents are highly dynamic systems that constantly change, causing the boundaries of the Sargasso Sea to shift. In addition, smaller-scale structures such as eddies and fronts can form between the main currents due to wind, weather, and other physical processes, further influencing the movement and mixing of water masses.

With these water masses, plankton drifts – an immense diversity of organisms ranging from microscopic algae to crustaceans, fish larvae, cephalopods, and large gelatinous animals. Most of these organisms are not able to actively swim against the prevailing currents and instead follow the large-scale movement of the ocean. However, many of them exhibit another important movement pattern: a daily vertical migration through the water column. During the day, when sunlight penetrates deep into the clear waters of the Sargasso Sea, they remain at greater depths. At night, they migrate upwards into the upper layers. This behaviour, known as diel vertical migration, is one of the most widespread phenomena in the ocean.

The reasons for this migration are manifold. Descending during daylight likely reduces the risk of predation by visually hunting predators. At night, many organisms move into layers where food is available. The foundation of this is primary production – the growth of algae – which forms the base of marine food webs. Smaller organisms concentrate in these layers, followed by progressively larger components of the food web. Our catches clearly reflect this pattern. Night-time hauls in the upper 300 metres typically yield higher abundances and a greater diversity of organisms than those taken during the day. This behaviour is also shown by the leptocephalus larvae of eel species, which are the main focus of our research. What becomes visible in our samples is a highly dynamic, layered ecosystem drifting through the ocean. The sheer abundance and diversity of forms we encounter in each haul remain fascinating every single time.

After a short delay due to necessary maintenance work on the vessel, we were finally able to set sail and head towards our survey area. About 17 hours later, we reached the first station – marking the start of the actual work on board. Following a brief introduction, the Thünen team and our international guests were divided into two shifts, allowing operations to continue around the clock. This well-established system has developed and proven itself over several years. Since 2011, this is already the sixth expedition to the Sargasso Sea for the Thünen Institute (and the fifth aboard the Walther Herwig III). Accordingly, workflows are now highly routine, and the core team is well practiced in their assigned tasks. From the very beginning of this cruise, everything has been running smoothly.

The central focus of this year’s expedition is once again the core distribution of the larval stages (Leptocephali) of the European eel. Based on findings from previous surveys, we are focusing on an area between 30°N and 24°45′N and from 67°W to 52°W. This region covers approximately 860,000 km² – about two and a half times the size of Germany – and will be sampled at a total of 48 evenly distributed stations.

As this is part of a recurring survey with a time-series character, procedures are kept as consistent as possible. At each station, basic hydrographic parameters such as temperature, salinity, oxygen, and chlorophyll are first measured using a CTD probe. Chlorophyll serves as an indicator of primary production in the ocean, i.e. algal growth, which forms the foundation of marine food webs. This is followed by standardized plankton sampling using the IKMT net (Isaacs-Kidd Midwater Trawl) to collect eel larvae and other organisms of the pelagic community. Once the net is back on deck, the detailed work begins: samples are carefully sorted, and Leptocephali and selected fish larvae are separated from crustaceans, jellyfish, salps, and other organisms. Even within the first hours, it becomes clear how diverse life is in what appears to be an otherwise empty ocean.

In addition to the standard programme, new approaches are also being tested this year. These include the use of a larger-meshed net to target small mesopelagic fishes and larvae, as well as a newly developed multi-closing net system for the IKMT. The latter will allow us to investigate more precisely at which depths eel larvae occur during different times of day and night – a key question for understanding their early life in the open ocean. All preparations are in place – the coming weeks will show which new insights we can gain into the life of eels in their still largely inaccessible spawning area.

As part of this research expedition, we welcome several international guest scientists on board the Walther Herwig III. They contribute their own research approaches and questions, which closely complement the main objective of the cruise – investigating the distribution and abundance of European eel larvae in the Sargasso Sea. These additional perspectives create a collaborative research environment, providing further insights into ecological processes and enhancing the overall scientific value of the expedition.

On 17 March, the Walther Herwig III leaves the port of St. George’s with a total of 36 people on board – 24 seafarers and 12 scientists – and sets course for the Atlantic. Following the mandatory safety briefings, the first work in the planned station operations is scheduled to begin after approximately 16 hours’ sailing in the western Sargasso Sea.

A few words from the expedition leader, Prof. Dr Reinhold Hanel:

“This is my seventh voyage to this sea area, and yet my expectations are almost the same as they were in 2007, when I began my research into the early life stages of Atlantic eels as a member of the Danish Galathea expedition. Now, my crew and I are drawing on the experience gained from previous voyages and hope this will lead to further important insights into the distribution and frequency of occurrence of European and American eel larvae.

As on previous voyages, we have supplemented our core team of staff from the Thünen Institute of Fisheries Ecology with international guests who will assist us with sampling and investigate additional aspects of the samples taken. PhD students and postdocs from Sweden, the Netherlands, France and the Czech Republic will carry out genetic and genomic analyses on eel larvae and will also examine by-catches of deep-sea fish and drifting brown algae.

“After a slightly delayed departure from Bermuda, we are now all delighted to have reached our first sampling station at 67°W, 30°N and to be analysing our first catches. Let’s see what lies ahead...”