WWW.WAS.ORG • WORLD AQUACULTURE • MARCH 2024 69 4) and, hopefully, in the near future, the glass eel stage. These parameters need to be improved simultaneously, to approach the metamorphosis to glass eel in the least amount of time possible and with the highest number of larvae. One year ago, the team was struggling with high mortality rates at the 28 days post-hatching bottleneck: the moment when the yolk sac is completely exhausted and larvae have to rely only on exogenous feeding. Nevertheless, after optimising the composition of the diet and adjusting the feeding method, the researchers have managed to significantly increase survival and growth rates. Average survival rates increased from 16 percent to 46 percent, whereas growth rates increased from 0.12 mm/day to 0.18 mm/day, which allowed many leptocephalus larvae to reach 200 days of survival. In order to achieve commercial production of glass eels in the future, higher growth and survival rates are required to shorten the larval stage and lower production costs. Future Prospects for Reaching the Glass Eel Stage The next steps for Glasaal Volendam include the standardisation of protocols for optimizing leptocephalus production. The research will then focus on the induction of metamorphosis into the glass-eel stage. At the same time, the team is already working on rearing methods and systems for the future up-scaling of production. This is a very important issue, since researchers don’t want their advances in larval rearing to be confined by protocols which only work in the highly controlled conditions of the laboratory. In conclusion, the results that have been obtained at Glasaal Volendam over the past year show promise for closing the life cycle of the European eel in captivity. The team managed to develop hatchery production protocols that will allow larvae to grow into glass eels in the very near future. This step will be essential for a sustainable European eel aquaculture industry and provide farmers with high quality, year-round hatchery-produced glass eels. This will reduce the pressure on wild caught eels, promoting the recovery of natural stocks. Notes Camillo Rosso, Nijverheidsstraat 5u | 1135 GE Edam Netherlands. camillo@volendamglasaal.com Hatchery Technology Development At Glasaal Volendam a group of researchers is working hard to develop technologies and protocols to make the production of glass eels in aquaculture possible, to reduce pressure on wild eel populations. Full-grown eels are matured in the research facility through artificial reproduction protocols to stimulate maturation of the gonads. In the past years a lot of attention has been given to broodstock management, to continuously produce high quality eel eggs. Fertilized eggs are incubated for 48 hours (Fig. 3) and, after hatching, the larvae are transferred to larger rearing tanks. Newly hatched eel larvae don’t have mouths, eyes, nor digestive systems and during the first two weeks they survive by consuming their yolk sac while their eyes, mouth and anus are developing. When these organs are fully formed, exogenous feeding starts. These first two weeks are a crucial period in larval life: larvae are so fragile that mortality can reach almost 100 percent when the optimal rearing conditions are not met precisely. In the last two years, the research team managed to increase the survival rate of the larvae in this period from an average of 3.8 percent to 21.7 percent, with some batches reaching over 50 percent. This huge improvement has allowed more larvae to enter the exogenous feeding stage and allowed the researchers to focus on the development of the most suitable diet, feeding methods and protocols to reach the leptocephalus and glass eel stages. Improving Survival and Growth Rates Because eel larvae are characterised by a peculiar and unique mouth morphology, feed ingestion has only been observed through the use of a slurry-type diet, inspired by protocols developed to rear Japanese eel larvae. Nevertheless, there are many other parameters affecting larval feeding behaviour, growth and survival. These include diet composition and size, feeding regime, amount of food and moment of the onset of feeding. Furthermore, environmental parameters such as water quality, temperature, salinity and light intensity vastly influence ingestion and survival. With every batch of larvae, the team runs different feeding experiments on one or more parameters, in order to improve survival and growth rates to achieve the leptocephalus stage (Fig. FIGURE 3. Development of eel eggs from fertilization to hatching, in hours post fertilization (hpf). a) fertilized eggs at 3hpf, b) early embryo at 30 hpf, c) embryo with spinal cord appearing at 36 hpf, d) embryo at 43 hpf and e) hatching larva at 48 hpf. FIGURE 4. Growing larvae during one of the feeding experiments. a) larvae at the beginning of the feeding trial (7.5 mm). b) larva at day 42 post hatch (11.2 mm), c) detail of the mouth of leptocephalus larva at 85 days post hatch, d) leptocephalus larva at day 105 post hatch (16.7 mm)
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