The time it takes for seaweed to grow from deployment to harvest varies significantly between tropical and temperate species. Seaweeds are low trophic species and generally do not need to be fed during the grow out process. Yet once the lines are in the water, some species require more attention than others. Risks during the grow out phase includes epiphytes, diseases, pests, aquatic animal grazing and even strong weather events. Technology usage for production planning, communication or monitoring indicates how widespread digital solutions are.
Explore Grow Out by Species
The grow out period of Pyropia starts around October when the seawater temperature drops below 23°C and multiple harvests can be taken from a single seeding throughout the winter months.
In the fixed net or fixed pole method and the semi-floating method, the tide fluctuation naturally exposes the cultivation nets to the air for a period of time every day. This improves growth and quality of the crop and reduces contamination from other algae.
Cultivation with fully floating nets keeps the Pyropia submerged in the water during both high and low tides and the farmer has to flip the nets over for a few hours every 4-5 days to ensure air exposure. This is very labour intensive work. In South Korea, a floating net method was developed that flips the nets over automatically using the tidal cycle (floating turnover net method or flip-over floating net method). This method was also employed in China, but the flip-over is not fully automated yet.
Nutrient depletion, especially lack of nitrogen in seawater, causes a chlorosis-like phenomenon that is called “yellow bleaching”. The farmers can anticipate this phenomenon by seeing clear seawater instead of the usual turbidity.
In South Korea and Japan, using an organic citric acid-based fertiliser treatment between harvesting operations is common practice. Harvesting and treating usually takes place at the same time or at least during the same process.
During grow out, the young Pyropia plants are especially susceptible to diseases and changing water conditions. Farmers can usually tell from their experience and intuition favourable from unfavourable conditions.
In South Korea, environmental water quality controls are carried out by the government every 2 months, which measure among other things the iodine, nitrogen and phosphorus content.
In Japan, farmers reported that they use portable sensors for temperature, pH and salinity every now and then.
In China, it is not typical for farmers to monitor and measure seawater conditions regularly.