The European eel (Anguilla anguilla) is in such dire straits that it is included in CITES’ Appendix II, a list of species not necessarily threatened with extinction, but in which trade must be controlled in order to avoid utilization incompatible with their survival. Under EU legislation it is an Appendix B species, trade in which is permitted but under even stricter conditions than those in Appendix II. Following scientific advice from the International Council for the Exploration of the Sea (ICES) that warned that the stock of the European eel was outside safe biological limits and that urgently recommended the development of a recovery plan for the entire European eel stock, the EU enacted legislation in 2007 that required Member States to develop and implement eel management plans. Research into eel has a long history DTU Aqua, the National Institute of Aquatic Resources, is an institute at the Technical University of Denmark that educates and researches at university level and off ers advice on the sustainable exploitation of aquatic resources. A team comprising researchers from the institute in partnership with several representatives from the private sector is currently working on a project to breed the European eel in captivity. The project, ITS-EEL, builds on the work done in two previous projects, the first of which, PRO-EEL, started a decade ago. But the history of research into eel goes back at least to 2004 when, what was at the time the Danish Eel Producers’ Organisation approached the Danish Fisheries Research Institute, the precursor to DTU Aqua, to initiate research into a closed production cycle to breed eel. The work today is carried out in Hirtshals on the Danish west coast, where DTU Aqua manages a 650 sq. m facility. Denmark has a long history of eel farming and in those days production was significantly higher than it is currently. But already then eel farmers had foreseen they would face problems sourcing glass eels in the desired quantities and at affordable prices. They were not wrong—farmed production of eel in Denmark has been declining since 2009 when it was 1,700 tonnes to 450 tonnes in 2018, according to Statistics Denmark. The farmers were also highly interested in closing the breeding cycle as it removes dependence on wild catches of glass eels enabling a far more predictable production. The experience of farmers cultivating other species, whether trout, salmon, or seabass and seabream also suggested that independence from wild catches made eminent commercial sense. The challenge was reproducing the eel’s highly complex life cycle in captivity, when there were vast gaps in the knowledge about the species. Breeding eel was therefore unlike farming any of the other species widely cultivated in Europe. To address these issues an international consortium formulated the project PRO-EEL that aimed to breed eel in captivity with a view to contributing to the development of a self-sustaining farmed production. Among the challenges the project wanted to address was the improvement of methods to induce and finalise gamete development and to understand the nutritional requirements of the female broodstock necessary for the production of healthy eggs and larvae. Among the outcomes of the project was the stable production of viable larvae.
Commercially viable production of eel is the goal
PRO-EEL was followed by another project, EEL-HATCH, that also included partners from the private sector and that focused on developing the larvae to the stage where they begin to feed prior to their transformation into glass eels. The ambition was to develop larval feeding protocols and hatchery technology and to test them at a commercial scale. Dr Jonna Tomkiewicz, Senior Scientist at DTU Aqua, who coordinated both projects, points out that the industry does not benefi t from the production of glass eels that are too expensive to be commercially viable. It is critical therefore to carry out large-scale trials during the project itself rather than subsequently. EEL-HATCH concluded in December 2017 and the results are being followed up on in ITS-EEL, where the goal is to establish larval culture technology for a hatchery production of glass eels for closedcycle European eel aquaculture. The efforts that are going into studying the eel are intended both to throw light on a fish, parts of whose lifecycle remain an enigma, as well as to enable a captive breeding programme for aquaculture and possibly for restocking purposes. Restocking presently assists the wild catch sector whose capture of eel decreased significantly from 423 tonnes in 2010 to 182 tonnes in 2019, reports the Danish Fisheries Agency. For eel is a valuable species—medium-sized eel, smoked, wholesales for EUR47/ kg in France, according to FAO Globefish’s European Price Report. Denmark’s exports of eel (in different product forms) has also shrunk with the decline in farmed and capture production. Closing the cycle and breeding the fish in containment thus has potential economic, social, and environmental benefits. Th is however is more easily said than done. The European eel is catadromous (migrates from freshwater to the sea to spawn) and semelparous (spawns only once in its lifetime). It has a complex life history, parts of which are still shrouded in mystery, but it is fairly well established that eels spawn in the southwestern part of the Sargasso Sea. Th is is body of water within the Atlantic Ocean and the only sea in the world without a land boundary. Instead, its borders are defined by currents, the Gulf Stream to the west, the North Atlantic Current to the north, the Canary Current to the east, and the North Atlantic Equatorial Current to the south. When the fertilised eggs hatch, the larvae feed on the yolk sac initially and then graduate to feeding, though precisely what they feed on is yet to be established. Theories abound, says Dr Tomkiewicz, gelatinous plankton and marine snow (organic detritus falling from the upper layers of the ocean to the bottom) are possible candidates, but neither what they eat nor where in the water column they feed is known for certain. Th e lack of this kind of knowledge means that the researchers in the ITS-EEL project are often operating in the dark, all the information has to be created by the researchers themselves based on their observations and trials—of feeds, rearing conditions, and microbiology of the water.
Parts of the eel’s life cycle is still a mystery
In the wild, the European eel larvae are carried by currents from the Sargasso Sea to the shores of western Europe. It is a long and hazardous journey. As larvae they are feed for predators, such as cod, herring and mackerel, and when they develop into glass eels, the next stage of their lifecycle, they are hunted by fishers in particular from Portugal, Spain, the UK, and France as the fish leave the sea and head upstream. They now enter the growth phase of their lives, when they are called yellow eels, which can last from two to 25 years and culminates when they transform into silver eels. At this stage they begin their long migration back to the Sargasso Sea, whence they came as larvae, to spawn and then die, an act that has so far not been witnessed. Researchers have established that the development of the ovaries and the testes is controlled by a complex hormonal mechanism. At the start of the migration to the Sargasso Sea, an inhibitory mechanism kicks in to prevent the development of these organs. As the fish approach their destination this mechanism is deactivated allowing the development of these organs to continue and the gametes (eggs and sperm) to form. In captivity (or in European waters), eels do not breed naturally because of this inhibition of the development of their reproductive organs.
Assisted Reproductive Technologies used to breed eels
Dr Tomkiewicz explains how she and her colleagues overcome this hurdle. For about two years, until they reach an appropriate size for broodstock, female fi sh are fed a specially formulated diet which combines essential fats, proteins, and vitamins in the proportions necessary for the eggs to have the correct ratio of these nutrients. The fish are then transferred to a system where they are subjected to assisted reproduction procedures. This involves treating them with hormones to induce them to produce gametes and then further with a steroid to provoke maturation and ovulation of the eggs. The eggs are then stripped and mixed with sperm removed from male fish. These male fish are usually from a local farmer. In a paper (*) in which she and her colleagues explore four issues: broodstock establishment and dietary requirements; assisted reproduction procedures; fertilisation techniques and incubation technology; larvae culture techniques and dietary requirements, Dr Tomkiewicz notes that comparisons between wild and farmed broodstock reproductive success, such as fertilisation and hatching success and larval deformities, show that while the provenance of females matters, the male fish’s origin does not have a significant influence on these factors. Therefore, studies focus on female diets and assisted reproduction protocols in order to enhance egg quality and the production of healthy off spring, thereby reducing proportions of off spring with deformities— an issue to be considered when breeding eels. The fertilised eggs are incubated and hatch after two days. Th e larvae feed on the yolk sac for the first 10 to 14 days (within the limits, the lower the water temperature the longer the yolk sac lasts). During this phase, the researchers studied the influence of temperature, light, salinity, and microbial activity with a view to improving rearing conditions and thereby survival rates. They established that temperature influenced several traits including time to hatch, hatching success, and incidence of deformities, and that 18-20 degrees C was the optimal level for higher growth, fewer deformities, and lower stress.
Mapping the factors that affect successful embryo/larva production
Light affected both embryos and yolksac larvae, and after studying the reactions the researchers concluded that both did best under a 12 hour:12 hour light/dark photoperiod and low-intensity light regime, and, in addition, larval survival was better under red light than under green or white light. Although eels can tolerate a wide range of salinities (euryhaline) and, being catadromous, their eggs and larvae develop in salt water, the researchers found that gradually reducing salinity improved growth and led to a four-fold increase in survival. This they attributed to the creation of an energy surplus due to the reduction in metabolic demands from osmoregulation. This excess energy may be used for somatic development resulting in improved survival and growth efficiency. Lowering salinity also reduced deformities such as spinal curvature and emaciation. These results enabled the researchers to improve incubation and larval culture technologies and produce large numbers of healthy larvae. In spring 2019 over a period of five months they produced 5.5m larvae, while this year they have more than 200,000 going into feeding experiments.
Identifying the first feeding diet starts with studying mouth parts
Scientists have analysed the stomach contents of eel larvae living in their natural environment, but so far have been unable to fully identify what they feed on. In ITS-EEL, efforts are aimed at improving larval culture technologies with a focus on rearing conditions and on feeds suitable for on-growing into the leptocephalus larval stage—the migratory stage that transforms into glass eels on arrival at continental waters. Dr Tomkiewicz and her colleagues are therefore studying the morphology of the larval feeding apparatus for clues as to what European eel larvae might eat. By estimating the biting force and the size of the particles they can ingest, they concluded that larvae have a preference for very soft and/or small food organisms and/or particles. They also tested potential diets, studied swimming behaviour, looked at the effect of light on feeding, and investigated physiological mechanisms at the molecular level. For instance, a diet of enriched rotifers, concentrated and ground into a paste was consumed by up to 50% of the larvae in the experiment, throwing light on larval feeding biology. In addition, the researchers noted that larvae were able to execute complex swimming behaviours to capture their food. At higher light intensities ingestion improved, suggesting that larvae use light to detect their food. Other tests showed that they also used other stimuli (taste and smell) to capture prey. At the molecular level, larvae on the enriched rotifer diet, showed higher levels of protein digesting enzymes compared with enzymes to digest carbohydrates or fats indicating a nutritional predisposition for proteins. Th e results explain some of the physiological changes the larvae undergo as they develop from newly hatched animals feeding on the yolk sac to creatures that must actively catch their own food. Research into eel has made significant progress over the last decade or so. But more needs to be done before the goal of a completely closed culture of eel at a commercial scale is achieved. Dr Tomkiewicz is confident though, that within 10 years eel farmers will be able to base their production on glass eels produced from broodstock in a hatchery.
(*) Tomkiewicz, J., Politis, S. N., Sørensen, S. R., Butts, I. A. E., & Kottmann, J. S. (2019). European eel - an integrated approach to establish eel hatchery technology in Denmark. In A. Don, & P. Coulson (Eds.), Eels - Biology, Monitoring, Management, Culture and Exploitation: Proceedings of the First International Eel Science Symposium (pp. 340-374). 5M Publishing.