Eel breeding: from biological mystery towards controlled eel reproduction

This news item is an English summary of a Dutch article written by Arjan Palstra, Leon Heinsbroek, Pauline Jéhannet, Laura Gentile, Alexandra Dobričić and William Swinkels that was recently published in VISionair magazine.

The lead picture shows eel larvae that are feeding on the new diet. The picture was made by Sander Boer.

History of artificial eel reproduction

Modern history of artificial eel reproduction started with the French researcher Maurice Fontaine. In 1936 he published a study in which he managed to induce maturation in male eels by injecting urine of pregnant women. Later, in the 1960's, he also succeeded in inducing sexual maturation in female eels by injecting pituitary extract of spawning carps.

In the 1970's, Japanese researchers managed to produce larvae of the Japanese eel, and in 1996 they discovered the steroid hormone that could induce ovulation in females. In 2003 they published the composition of the shark egg diet that was successfully used to feed captive-bred eel larvae to grow into glass eels. The cycle was closed in 2010. Now, several generations later, the main goal is to reduce the costs for glass eel production.

In the meantime, Danish and Dutch researchers have successfully produced larvae of the European eel. Thanks to the Japanese knowledge and new insights into our own species, the glass eel stage is drawing closer and closer at the Technical University of Denmark and 'Glasaal Volendam' in the Netherlands.

EELRIC and EEL SUPPORT

To speed up the process and close the cycle as soon as possible, the Eel Reproduction Innovation Centre (EELRIC) was set up in 2016. The centre is a collaboration between the Dutch 'Stichting Duurzame Palingsector Nederland' (DUPAN) and Wageningen Livestock Research (WLR). The initial aim of EELRIC was to solve the existing bottlenecks and to start a global network to share knowledge. In 2024, the collaborating parties were granted with the European COST Action EEL SUPPORT, a network which now consists of 73 members from 20 European countries, Japan, New Zealand and the United States.

Innovations and triumphs

So what were the bottlenecks that needed to be solved? "First of all, we needed to develop a method to make good broodstock eels out of juvenile glass eels," says Arjan Palstra, senior researcher fish physiology at ABG and chair of EEL SUPPORT. "This is to shorten the generation time drastically and make sure that in the future we are not dependent on wild eels to provide offspring. To make the glass eels more susceptible to sexual maturation later in life, they are fed pellets that were coated with the hormone estradiol. Consequently, they receive a diet aimed to optimise the production of high-quality eggs. Finally, they are subjected to a simulated migration during which they swim 3,000 kilometres at alternating temperatures, as they would experience during migration in the ocean. The result of these combined treatments is that it is now possible to produce broodstock animals within a time span of just one and a half years."

A second issue that needed to be addressed was the way in which sexual maturation was induced in the female animals. "The 'Fontaine' method, where females receive weekly injections with pituitary extract from carp or salmon, is suboptimal because the composition and quality of the extract cannot be controlled," says Palstra. "To solve this issue, a collaboration was set up with the Spanish company RARA Avis that can produce eel-specific FSH and LH hormones. An additional advantage is that these eel-specific hormones remain active in the body for a longer period of time, which means that the animals do not need to be injected on a weekly basis."

This directly solves a different issue, namely that the female animals experience stress from the weekly injections, which has a negative effect on reproduction. Instead of injecting weekly, it was found that male eels can be matured with just a single gonadotropin injection.

Another problem the researchers faced was determining when the female eels would ovulate. "Determining the ideal moment to administer the hormone injection that induce ovulation can be tricky," says Palstra. "The steroid hormone that is usually used also has negative impact on the fertility of the female if it does not ovulate rapidly. We found that we can induce eels to make their own steroid hormone by stimulating upstream in the reproductive axis on brain level."

The lack of a natural reference of the process also posed a problem for the researchers. Everything that is known about the European eel's reproductive cycle comes from artificial eel reproduction. But there have been some promising developments that may help solve this issue. In 2019 a 43-year-old female eel spontaneously matured in an aquarium for fresh water fish in Finland. This was big news, as it was the first time a female eel that naturally matured in a place that was not the Sargasso Sea. The eel – and the group of eels she was a part of – was examined extensively, and ever since this discovery was made, the EELRIC researchers travel to Finland every year to measure the eels, take blood samples for hormone measurements, and take ultrasound images in order to keep track of the developments of the gonads.

Current focus is on the rearing and first feeding of the larvae. In the past, the mortality rates among the larvae in the first week was quite high. In addition, many of the larvae showed abnormalities. While some abnormalities are a direct result of the egg quality, others can be attributed to the breeding conditions (e.g. light, water temperature and salinity, pH value, oxygen levels, etc.). "Over the past years, the breeding conditions have improved significantly, which means that a larger percentage of the larvae survive," says Palstra. "As a result, we can now do large scale feeding trials to test new diets on which larvae grow fast without drastically lowering water quality. A first experimental diet that we tested showed significant improvement over the regularly used Japanese shark egg diet."

Challenges

While there's been a lot of progress already, there are still many challenges up ahead. "To reproduce the produced glass eels will be easier as they are already used to an aquaculture environment," says Palstra. "Broodstock selection then becomes very important. An industrial protocol should allow up-scaling and increase practicability in order to produce glass eel that will outcompete wild glass eel. Eel aquaculture in Europe is highly restricted because of quota for catching wild glass eels. If the industry has its own glass eel production, aquaculture is not limited anymore. The industry could grow and help with recovery of the natural population. If we can, and actually want to, use the produced glass eels for restocking the natural population is still an important consideration."