Starting a field test of Ulva prolifera, which grows even at high water temperatures, in Yoshino River

The flood discharge (design flood) of the Yoshino River in Tokushima Prefecture is 24,000 cubic meters per second, the largest in the country.
Once it floods, the Yoshino River goes wild. From a long time ago, the river has often caused great damage to the nearby areas, but at the same time it has carried fertile soil and brought a lot of benefits. One of the benefits is Ulva prolifera farmed in the Yoshino River basin. A rise in the seawater temperature is affecting the farming of Ulva prolifera. We interviewed about challenges to maintain the regional specialty.

---

A rise in seawater temperature is seriously affecting Ulva prolifera famed in the Yoshino River basin. Ulva prolifera is a familiar green seaweed sprinkled on okonomiyaki, a Japanese savory pancake. It is a streaky algae species inhabiting brackish waters combining seawater and freshwater. Tokushima Prefecture is the nation’s largest producer having a share of 70 to 80% in the domestic market. In recent years, the water temperature does not fall very much in winter and the yield has been unstable. In 2015, particularly, the water temperature after mid-October was higher than usual by 2°C and, besides that, the shipment volume dropped to 84.5% year on year due to torrential rain in December.
To solve the problem, the Fisheries Research Division of Tokushima Prefecture launched the development of a species resistant to high water temperature in 2015 and discovered a strain that grows with high water temperatures in 2016.

*
The temperature suitable for the growth of Ulva prolifera ranges between 15°C and 17°C. In that environment, some algae grow as long as over one meter. If the temperature rises to 20°C or more, poor growth results. With a temperature of below 10°C, the growth is slowed. Naturally, the harvest season is limited to a short period. After seedlings are planted in October, they are usually cultivated for two months in November and December. These days, according to the division, as the water temperature does not fall very much, the start of farming is delayed.

“If the harvest season is shortened, the yield will be reduced and the farmers’ incomes will decline. Ulva prolifera farmers have asked us before whether it would be possible to produce Ulva prolifera growing at high water temperatures. Over the past several years, demand for a new species has been increasing.”
*

The Fisheries Research Division took seeds called “zoospores” from natural strains picked from the Yoshino River and strains provided by fishermen. The Division cultured the spores in a laboratory and studied the growth rate at different water temperatures. They found a strain that grew at 35°C.

“We gathered as many strains as possible and conducted a laboratory test again and again. It was simply lucky to discover a strain resistant to high water temperatures.”

*

Ulva prolifera farming greatly depends on salt concentration and changes in the farm environment. It is known empirically that the salt concentration of the Yoshino River falls and the yield decreases if it rains a lot. However, if the river swells and floods so badly as to flow away the whole farms as in 2015, the damage will be devastating. Increases in torrential rains and in the size of typhoons, likely to occur in the future, will also have serious effects.

“One of the reasons that Tokushima’s Ulva prolifera is the best in Japan is the good environment of the Yoshino River. We will continue our research to help achieve harmonious coexistence between the environment and humans.”

Studies on the high-temperature-resistant strain discovered by the division are moving onto the phase of trial farming in the Yoshino River. If it succeeds, a new species of Ulva prolifera will be developed for the first time.