Sturgeon Breakthrough Could Revolutionize Caviar Industry

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Male chromosomes, female eggs. Researchers at Kindai University have achieved a breakthrough in sturgeon aquaculture. They have artificially fertilized and hatched eggs from genetically male sturgeon.

Sturgeon, which produce eggs for luxury caviar, are mostly farm-raised. However, determining their sex before they reach egg-bearing size is difficult. This means farmers must raise males alongside females, driving up costs while they don't know which fish is which. However, if eggs can be harvested from males, it could completely transform caviar production.

'Ancient Fish' with Pricey Eggs

Sturgeon are living fossils. Their form has remained largely unchanged across 300 million years of evolution. Despite their ancient roots, however, they bear no relation to sharks. Meanwhile, their salted roe — caviar — ranks alongside foie gras and truffles among the world's most prestigious delicacies.

Many species exist. Beluga sturgeon, often called the "king of caviar," produces the largest eggs at 3.5-4.0mm in diameter. Russian sturgeon eggs (3.0-3.5mm) are prized for their refined flavor profile. Siberian sturgeon, favored in Japanese aquaculture, produce slightly smaller eggs at 2.5-3.0mm.

*Siberian sturgeon raised at Kindai University. (Courtesy of Kindai University)*

While males do enter the market as fillets, caviar dominates the sturgeon industry in terms of value. Females command substantially higher prices than males. In nature, sturgeon reproduce equal numbers of males and females. However, the industry needs a method to obtain females efficiently or render males economically viable.

Sorting the Sexes for Valuable Eggs

The core difficulty lies in determining the fish's sex at an early stage. Sturgeon sex is determined by chromosomes — ZZ for males, ZW for females — producing a natural 1:1 ratio. But young sturgeon appear identical regardless of genetic sex, with no external characteristics to distinguish them. Fish farmers must raise entire cohorts until they can assess the sex at maturity — typically around three years.

The identification process itself is labor-intensive. Workers remove each fish individually and perform a surgical incision to visually inspect the gonads. After determining sex, they suture the opening and return the fish to rearing ponds. The procedure is time-consuming and places significant stress on the fish.

Meanwhile, costs for feed, space, and labor accumulate. Beluga sturgeon require over 20 years to mature in natural environments or a minimum of seven years under optimal farm conditions. Russian sturgeon need approximately five years. Even Siberian sturgeon, considered relatively fast-maturing for aquaculture, require roughly four years before egg production begins. Each year of maintenance adds substantially to production costs.

Turning Unpopular Males into Valued Resources

Against this backdrop, the need to secure eggs efficiently without wasting males has grown urgent.

The research team first tackled giving all individuals female gonads. In 2017, they artificially hatched imported fertilized eggs. They fed the 130-day-old fry with female hormone-supplemented feed for six months. In 2019, they observed the gonads of 45 fry. Not only genetically female fish, but even ZZ chromosome individuals — genetically male — showed egg cells. All gonads were female. The male bodies had "feminized."

The team then raised some feminized males on regular feed, waiting for maturation. After seven years and two months, in 2025, one fish carried mature eggs. They successfully harvested approximately 1,050 grams (about 68,000 eggs).

*Eggs harvested from a feminized male sturgeon (Courtesy of Kindai University)*

These eggs were then fertilized with sperm from another male, and they hatched in about 5 days. By late August, approximately 5,000 fry had grown to about 20cm. The team called this Japan's first case of harvesting and hatching eggs from feminized Siberian sturgeon.

Random genetic analysis of the fry revealed that all were ZZ males. Theory had long predicted this outcome — offspring from two males would all be genetically male — but this was the first experimental proof anywhere in the world.

*Hatched Siberian sturgeon fry (Courtesy of Kindai University)*

Previous sturgeon farming technology focused on efficiently securing females and obtaining massive egg quantities. This latest breakthrough demonstrated that normal, viable eggs can be produced solely from males. It is a revolutionary discovery for the industry.

Japan Leading the Way

The researchers' achievement creates a concrete path from "males to caviar." It is currently pursuing two approaches.

First, it is pursuing feminization using food-derived components like soy isoflavones and enzyme-treated soy residue. This would create a system for harvesting eggs from male populations.

Second, "all-female" fry would become the farming foundation, eliminating sex determination and surgical confirmation burdens. This approach would use not only feminized males but also "super-females" — long theorized as possible.

Super-females have a WW genotype that produces only female offspring. They had long been considered hypothetical—almost mythical. In 2022, Ryuhei Kinami, Assistant Professor at Kindai University, became the first in the world to create one using UV-treated sperm. His work confirmed their reality.

*Farmed caviar from Kindai University (Courtesy of Kindai University)*

Looking ahead, the team plans to verify the safety of the feminization method and labeling standards for commercial products. They also aim to assess effectiveness across species, shorten rearing times, develop automated early sex identification, and stabilize fry supply. Public-private collaboration is already underway. Joint research with Miyazaki Prefectural Fisheries Research Institute and private companies focuses on super-female selection, evaluation, and testing protocols.

Toshinao Ineno, an associate professor at Kindai University who led the research, noted: "No published research has previously demonstrated egg harvesting and hatching from artificially feminized sturgeon. Our Japanese team considers itself at the forefront of this field globally. I expect caviar production from males will become possible, establishing a sturgeon farming industry where males are no longer discarded."

(Read the report in Japanese.)

Author: Juichiro Ito, The Sankei Shimbun

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