Artificial Seedling Production of Japanese Eel × SDGs
The Japanese eel has been assessed as an endangered species by IUCN. The reason can be found from deterioration of the river environment, and also the overfishing of juveniles. It is expected that this crisis can be improved by the success of artificial seedling production. We would like to contribute to the SDGs by maintaining the biodiversity of the ocean and rivers.
The United Nations has designated the decade starting in 2021 as the "Decade of Ocean Science," with the aim of contributing to SDGs. Ocean science, as defined by the UN, includes the field of fisheries.
Why artificial seedling is important for Japanese eel?
For Japanese eel, the “cultured eels” on the market were captured during the glass eel stage and raised in large eel farms.
In other words, cultured eels are a natural resource, considering their origin.
For the conservation of eels, aquaculture that does not depend on natural resources is required.
In this system, the eels that were born at an eel farm grow and mature enough to spawn eggs, and then those that hatch from the eggs grow and spawn.
This system is called complete aquaculture, and the fish born and reared at the aquaculture site are called artificial seedlings.
Feminization of eels
Unlike humans, the sex of eels is not determined when they are born.
The sex of eels is determined only after they have grown to the yellow eel stage (approximately 25–35 cm in length).
Strangely, most of the eels captured during the glass eel stage and reared in an eel farm become males.
However, female is necessary for the seedling production.
Estradiol-17β is added to the feed for glass eels, promoting the eels to become females.
(Estradiol is a type of estrogen, which is a female hormone)
Such artificial development of female eels is called “feminization.”
Sexual maturation of eels
For breeding, sexual maturation is induced in the eels that have reached a certain size (weight and length).
The induction of sexual maturation by hormone administration is called “artificially induced maturation.”
Weekly injections of salmon pituitary extract (SPE) and human chorionic gonadotropin (hCG) to females and males, respectively, promote their sexual maturation.
Eels matured by artificially induced maturation require the administration of a maturation-inducing hormone (17,20β-Dihydroxy-4-pregen-3-one : DHP) for spawning.
However, DHP is not administered to all individuals.
The condition of the eggs should be checked before the injection of DHP.
Oocytes (follicles) are sampled from the cloaca of eels for size check, and DHP is only injected into the abdomen of individuals with an oocyte diameter of 750 μm or more.
The amount of DHP injected into each individual at this time is 2 μg/g (body weight), so the body weight needs to be measured before the injection.
Injection of DHP into the abdomen of an eel
Eels complete egg maturation and ovulation approximately 18 hours after the administration of DHP and become ready for spawning.
For fertilization, sperms taken from male were added to the eggs taken from female, and then placed in seawater for fertilization.
Collection of unfertilized eggs from eels
Fertilized eggs in good condition are empirically known to float.
Sunken eggs are not used for the subsequent observations.
The floating rate can be calculated by counting the number of floating eggs out of all the eggs.
Floating rate (%) = number of floating eggs / number of artificially inseminated eggs x 100
Development and hatching of fertilized eggs
Approximately one and a half hours after fertilization, eggs enter the 2-cell stage and blastomeres can be observed under a microscope.
As cleavage progresses, eggs form a mulberry-like mass, which is called the morula stage.
The embryo is formed to hold the egg yolk and oil droplet approximately one day after fertilization, followed by the hatching of the larvae from the egg in 15–20 hours.
The fertilization rate is calculated by observing the artificially inseminated eggs under a microscope and excluding those that do not show the formation of an egg membrane or cleavage over time.
Fertilization rate (%) = number of eggs fertilized normally / number of eggs artificially inseminated x 100
The development rate of eggs and their hatching time vary depending on the water temperature, which are faster at higher water temperatures.
The time described above is based on the water temperature of 22℃.The optimum water temperature for hatching eel eggs is 25℃, which is the water temperature of the spawning area. However, eggs are kept and hatched at 22℃ in some aquaculture facilities to reduce bacterial activity.
Not all fertilized eggs hatch out.
The hatching rate is calculated by counting the number of hatched larvae out of the fertilized eggs incubated under the same conditions.
Hatching rate (%) = number of larvae hatched / number of fertilized eggs x 100
Eggs are very fragile. They can be easily damaged or even die from the shock that occurs when transferred to a housing container.
Therefore, only the number of live fertilized eggs is used as the denominator for the calculation of the hatching rate.