Trial and error

The releases were easier said than done. In the early days, the survival rate of freed sturgeons was stubbornly low. Immature breeding techniques meant researchers had to release newly spawned fish as quickly as possible, even before they were old enough to fend for and feed themselves in natural waters.

Jiang said in 1985 his predecessors started researching the sturgeon's breeding techniques with almost no literature available.

Over the ensuing decade, they figured out the crucial factors affecting the well-being of the young fish being raised in captivity, such as food and water temperature and quality. They also worked on experiments to prevent the spread of deadly fish diseases. "In 1995, the body length of the released fish had reached more than 30 centimeters and their survival rate in the wild rose," Jiang said.

The progress in breeding knowledge also meant that the young sturgeons became sexually mature in captivity and provided sperm and eggs for human-controlled reproduction. That eased the strain on the wild population of adult sturgeons, which were caught to harvest sperm cells and eggs.

"In 2009, the institute managed to incubate young sturgeons using sperm and eggs from domestically bred parents. That was a milestone because it meant we had managed to breed sturgeons with no reliance on the wild species," Jiang said.

In recent years, Jiang and his colleagues have used gene-sequencing technologies to prevent breeding among close relatives, which can harm genetic diversity and increase vulnerability to disease.

"With the genetic data, we are now able to match sperm and eggs in the best possible ways to ensure genetic diversity," he said.

He added that the decadelong fishing ban imposed on the Yangtze has proved a boon for sturgeon releases and other efforts to replenish the fish population.