New vertical farming technique doubles seaweed output without expanding ocean space

High-density longline suspended-rope aquaculture technology. [Korea Institute of Ocean Science and Technology (KIOST)]

South Korean researchers have developed a technique that more than doubles seaweed output without expanding existing farm areas, by using vertical space beneath the ocean surface. Field trials produced about 5.6 tons of seaweed per 200 meters of longline.

The Korea Institute of Ocean Science and Technology (KIOST) announced Tuesday that it had developed a high-density "longline suspended-rope" aquaculture technique capable of more than doubling farmed seaweed output compared with conventional methods.

Conventional seaweed farming relies mainly on the longline method, in which kelp and sea mustard are grown on horizontally strung ropes. The approach is relatively resistant to tidal flat sediment and pollution, but large-scale installations can obstruct shipping lanes, limiting how much farming space can be added.

KIOST developed the new technique together with Kim Nam-gil, an emeritus professor at Gyeongsang National University, by adapting the suspended-rope method used in oyster and sea squirt farming for use with seaweed. The system hangs 3-meter vertical ropes — called suspended lines — at 1-meter intervals from a 200-meter horizontal longline, with seed strings carrying juvenile seaweed wound around each vertical rope.

The key innovation is using vertical underwater space rather than claiming additional horizontal area, raising cultivation density per unit area. Farmers can increase output by working within existing farm boundaries, even in waters where aquaculture sites are already saturated.

The ropes are made from polypropylene with a diameter of at least 20 millimeters. The material resists corrosion and decay, is lightweight and has a low raw-material cost, improving durability in marine environments while reducing maintenance expenses.

To prevent the suspended lines from tangling or twisting underwater, the system also attaches a lower longline and anchor weights at the bottom. Metal supports maintaining the gap between the upper and lower longlines are placed every 50 meters, keeping the facility intact against strong currents and high waves.

Workers harvesting seaweed. [Korea Institute of Ocean Science and Technology]

Field trials were conducted in waters off Tongyeong, South Gyeongsang Province, home to KIOST's Tongyeong Maritime Demonstration Base, using sea mustard and kelp. Measuring output across the upper, middle and lower thirds of the 3-meter suspended lines, researchers recorded average yields per meter of 12.11 kilograms at the top, 8.29 kilograms in the middle and 7.78 kilograms at the bottom.

Total output per suspended line came to 28.18 kilograms. Scaled to a 200-meter farming facility, that translates to about 5.6 tons.

A survey of local fishers found that conventional longline farming yields about 3 tons per 200 meters in Wando, South Jeolla Province, and about 2 tons in Gijang, Busan. The new technique's output is 1.9 times higher than Wando's and 2.8 times higher than Gijang's.

KIOST said the productivity gains could benefit not only fishers' incomes but also carbon neutrality efforts. Seaweed absorbs carbon dioxide from seawater through its entire body and has a short growth cycle, allowing annual harvesting and regrowth. Expanding farmed seaweed output and coverage would also broaden the carbon-absorption base that seaweed provides, the institute said.

Interest in seaweed as a new blue carbon resource is growing internationally. KIOST said the technology's value would increase as the carbon absorption capacity of seaweed, tidal flats and subtidal sediments may eventually be reflected in national greenhouse gas inventories and nationally determined contributions (NDCs).

"Seaweed is not only a next-generation blue carbon resource but also a strategic asset that can simultaneously drive marine ecosystem recovery, raise fishers' incomes and create high-value industries," KIOST Director Lee Hee-seung said. "We will work to enhance the productivity and utility of seaweed and develop sustainable ways to use marine and fishery resources in the era of carbon neutrality."

adastra@heraldcorp.com