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 stageand
raised in large eel farms.
In other words, cultured eels are a natural resource, considering
Stocks of Japanese eel have been significantly decreased since the
1990s,and eels have now become an endangered species.
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
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℃.
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.