Background
Glasaal Volendam is a private research company founded in 2012 in Volendam, a former eel-fishing city in the north of the Netherlands. The company aims to close the eel life cycle in captivity and set up a large-scale production facility to farm glass eels (juvenile European eel). Glass eels can be sold to eel grow-out companies, which are now solely dependent on wild caught glass eels. Replacing these wild glass eels with farmed ones could relieve the legal and illegal fishing pressure on this already endangered species.
The life cycle of European eel A. anguilla begins with hatching in the Sargasso Sea. From here larvae drift as leptocephali back to the coasts of Europe and then transform into glass-eels as they enter freshwater rivers in Europe. They migrate upstream, maturing into elvers and developing pigmentation in freshwater. Over several years, they become sexually mature adult eels and undergo a final transformation into silver eels before migrating back to the Sargasso Sea to spawn, completing the cycle.
The life cycle of European eel has not yet been closed in captivity, unlike the one of Japanese eel A. japonica, which was completed in 2003 (Tanaka et al. 2003). However, considerable progress has been made in recent years towards closing the life cycle of European eel in captivity. This translates in the production of high-quality gametes, embryos, yolk-sac larvae and pre-leptocephali feeding larvae.
Materials and methods
To conduct our research, we obtain the broodstock from the wild through local fishermen. Eels are housed in 500l tanks with a closed re-circulation system. During the maturation period, they are kept at low density (20 kg/m3), with 36‰ salinity and 18 °C of temperature. Eels are maintained under dimmed light conditions to mimic daily photoperiod. No feed is provided during maturation. To induce vitellogenesis, females receive weekly injections of Carp Pituitary Extract (CPE) for 15-20 weeks (Kagawa et al., 2005) and weekly biopsies are taken to evaluate oocyte development (Paalstra et al., 2005). To induce ovulation, females are injected 12 hours before spawning with DHP (7α,20ß-dihydroxy4-pregnen-3-one; 2 mg per/kg body weight) according to Ohta et al. (1996). Male broodstock is obtained from commercial eel farms and receives weekly injections of recombinant human chorionic gonadotropin (Perez et al. 2005). Milt is collected after 10 weeks of hormonal treatment, evaluated for quality under the microscope and mixed with eggs at the time of spawning. Fertilized eggs are incubated for 48 hours in 200 l tanks and then hatching larvae are moved to 500 l tanks. On day 14 after hatching, when mouth, primitive digestive system and eyes are formed (Sørensen et al. 2016), larvae are moved to feeding systems where experimental diets of different compositions are being tested. Larvae are fed five times a day a slurry-like diet (Tanaka et al. 2003). Different diet compositions are tested in order to improve survival and growth rates, to reach leptocephalus stage first and glass eel stage ultimately. Through pictures larval growth is measured and developmental changes in larval morphology are monitored. Ingestion is evaluated by direct observation after each feeding session.
Results and future steps
At Glasaal Voendam we made significant progress towards closing the life cycle of the European eel. At first, the research was primarily focused on the artificial maturation of the broodstock. Currently we can produce vast numbers of fertilized eggs weekly. From 2018 onwards, the focus shifted to larval feeding. We improved larval survival rates at first feeding (14 days post-hatching) from an average of 0.3% in 2018 to 3.8% in 2021 and 21.7% in 2022. This year, we have observed trials with over 50% of the larval batch surviving until first feeding. These improved survival rates allowed us to focus on the larval feeding. After obtaining the first signs of food ingestion by larvae in 2018, we gradually improved feeding technique and larval diets, to enhance survival and growth rates. Being the average length at first feeding 0.7 cm and the maximal survival 30 days, in 2022 we successfully grew our first batch of larvae up to 1.4 cm. The oldest larvae of the batch survived for 94 days. We then focused on nutritional values of the larval diets, to enhance growth rates and reach the leptocephalus and the glass eel stage.
Alongside, to obtain growing larvae, we conducted experiments on parameters such as temperature, light, salinity, water quality, type of flow and tank design, to identify optimal rearing conditions. We also developed a method to analyze larvae pictures and we weekly process measurement data to extrapolate growth curves, to monitor larval growth rate and to compare it with the ones from past experiments.
In the first quarter of 2023 the larval growth rate was improved by 60%. The primary and most compelling goal for the next years is to reach the glass eel stage. The second phase of our company will then involve upscaling to commercial size.
References
Kagawa, H., Tanaka, H., Ohta, H., Unuma, T., Nomura, K., 2005. The first success of glass eel production in the world: basic biology on fish reproduction advances new applied technology in aquaculture. Fish Physiol. Biochem. 31, 193.
Ohta, H., Kagawa, H., Tanaka, H., Okuzawa, K., & Hirose, K. (1996). Changes in fertilization and hatching rates with time after ovulation induced by 17, 20P101 dihydroxy-4-pregnen-3-one in the Japanese eel, Anguilla japonica. Aquaculture, 291–301.
Palstra AP, Cohen E.G.H., Niemantsverdriet P.R.W., Van Ginneken V.J.T., Van den Thillart G.E.E.J.M. (2005) Artifcial maturation and reproduction of European silver eel: development of oocytes during fnal maturation. Aquaculture 249(1–4):533–547.
Pérez L, Aturiano J.F., Tomás A., Zegrari S., Barrera R., Espinós F.J., Navarro J.C., Jover (2000) Induction of maturation and spermiation in the male European eel: assessment of sperm quality throughout treatment. J. Fish. Biol. 57(6):1488–1504.
Sørensen S.R., Tomkiewicz J., Munk P., Butts I.A., Nielsen A., Lauesen P., Graver C. (2016b) Ontogeny and growth of early life stages of captive-bred European eel. Aquaculture 456:50–61.
Tanaka, H., Kagawa, H., Ohta, H., Unuma, T., Nomura, K., 2003. The first production of glass eel in captivity: fish reproductive physiology facilitates great progress in aquaculture. Fish Physiol. Biochem. 28, 493–497.