Successful production of Japanese eel juveniles through complete aquaculture using feed without egg yolk

Kindai University Aquaculture Research Institute and San-Ei Gen F.F.I. Co., Ltd. (Toyonaka City, Osaka Prefecture) announced on July 10 that they have successfully produced over 100 glass eels (juveniles) through joint development of feed for Japanese eel larvae, using an original feed without egg yolk (patent pending). By using thickening agents, which are food additives, viscosity can be easily adjusted according to growth stages, and this provides a more stable supply than chicken eggs.

Feed used for rearing eel larvae must contain certain amounts of protein and lipids, and these components must maintain a slurry-like state without separating even in water. To achieve this, the developed feed removed egg yolk from conventional feed and utilized thickening agents.

Chicken eggs not only compete with food resources, but in recent years their supply has become unstable, and prices have continued to rise due to avian influenza and other factors. Additionally, conventional feeds adjusted viscosity by adding water, limiting the selection of nutrients and causing difficulties in new feed development.

Eel larvae were reared with long-term use of the newly developed feed, and glass eels were successfully obtained in May last year. Furthermore, rearing with improved feed meant that larvae that began metamorphosis to glass eels from 140 days of age appeared, and at 282 days of age, more glass eels were obtained compared with larvae given conventional feed.

In complete eel aquaculture, rearing during the larval period, from obtaining fertilized eggs to glass eels, is considered the most difficult. Larval period rearing greatly affects survival rate, growth speed, and the number of days required for metamorphosis to glass eels. The long period and enormous costs required to obtain glass eels suitable for general eel aquaculture make it difficult to achieve complete aquaculture.

Japan depends on aquaculture for over 99% of domestic eel consumption. All aquaculture seedlings use wild glass eels, but resources are declining and securing seedlings necessary for eel aquaculture has become an issue.

Research aimed at complete eel aquaculture has been conducted for a long time, and Hokkaido University succeeded in artificial hatching in 1973. However, development of suitable feed for larval rearing was difficult, and larvae could not be grown for over 20 years thereafter.

In 2002, the Fisheries Research and Education Agency (FRA) succeeded in rearing larvae to glass eels for the first time by developing slurry-type larval feed with shark eggs and krill as the main ingredients. Thinking of the stable quality and sustainable supply of raw materials, the FRA developed slurry feed in 2017 with egg yolk, milk protein, and enzyme-treated fish meal as the main ingredients, and that feed has been widely used to this day. However, larvae survival rates to the glass eel stage remain low even now, with growth tending to be slower than in the wild, so development of better feed was desired.

The researchers will continue to aim for the development of feed that realizes metamorphosis to glass eels in a shorter period of time.

Kindai University's Professor Hideki Tanaka commented as follows: "Since eel larvae take more than half a year from hatching to become juveniles, it takes a long time to see results, and feed improvement also requires years. Developing feed that enables metamorphosis to juveniles as quickly as possible is considered the most important future challenge for reducing juvenile production costs."