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Since 1977, the estuaries of the Minjiang River have seen the practice of seaweed and algae aquaculture using floating frog-like systems. By 1998, farmers in Guanling Village had refined this method into a more advanced model, which could be applied to shallow seas up to 10 meters deep. This technique not only makes full use of the nutrient-rich waters from the two rivers but also helps purify and improve water quality. It ensures safe production, delivers high-quality yields, is easy to manage, and has strong resistance to diseases. The resulting "beautiful, clean, and green" products are popular both domestically and internationally. Below is an introduction to the techniques and experiences of this seaweed cultivation system.
The selection of aquaculture areas is crucial. Ideal locations include estuarine waters with natural conditions such as large waves, unobstructed currents, and fresh water inflow. These features help exchange water bodies, regulate temperature and salinity, renew water quality, and make it easier to remove algae and debris, reducing the risk of disease. The farming area is located near the mouths of the two rivers, taking advantage of unique geographical resources. However, fixed-column cranes should be placed in shallow seas with about 10 meters of depth, on sandy or muddy seabeds. They should avoid busy shipping lanes, docks, and shelters to prevent damage to the curtains. Also, they should not be set up directly in straight river channels but should use buffer zones.
Seaweed larvae grow best in water temperatures around 25°C. Temperatures above 28°C for extended periods can slow their growth, while below 20°C, growth becomes significantly slower. Once the fronds develop, they can tolerate a wider range, from 20°C to 150°C. In this culture zone, from September to October, the water temperature ranges between 28.5°C and 24.5°C. There are few or short periods above 28°C, and from November to December, the temperature drops to 23–15°C, then to 10–15°C from January to March. Therefore, managing and adjusting water temperature during these periods is essential to promote growth and increase yield.
The nutrient levels in the culture area are ideal: nitrates range from 50 to 180 mg/m³, nitrite nitrogen from 4 to 12 mg/m³, ammonium nitrogen from 10 to 32 mg/m³, and phosphate from 6 to 39 mg/m³. These nutrients support robust algal growth. However, areas far from the river mouths may have less than 20 mg/m³ of nitrogen, which can hinder growth and even cause lesions. The seawater pH in the culture area is between 7.56 and 8.75, with salinity ranging from 32 to 16 ‰. Freshwater from the Minjiang River is regularly injected, which benefits the growth of the algae and improves product quality. However, vigilance is needed to prevent flood impacts that can lead to disease outbreaks.
For seedling curtain setup, materials like vinyl nylon and polyethylene blended ropes are used. These are durable, have moderate water absorption, and are suitable for seed attachment and growth. Their lightweight nature also facilitates management. Curtain heads and tails are made with similar materials. Fixed bamboo columns are selected based on water depth, typically planted about 1.5 meters into the seabed. When tides are high, at least 1 meter of the column should remain above water, so most bamboo poles used are 8–10m or 11–14m long. Pontoon bamboo is φ8cm in diameter and 3.5–4m in length.
Floating rafts, foot piles, and ropes are made from durable polyethylene. Foot piles are made from pine wood, about 15cm in diameter and 1.8m in length. A 5-horsepower engine is installed on the boat for transportation, management, and harvesting. Water pumps are also used to spray water, remove silt, and reinforce posts.
Each curtain is about 3.5m long and 1.0–1.3m wide. Seedling ropes are 5–10cm in length, with three frames per curtain, attached to the pontoon rope. Each frame has nine sections, with two sets per mu (approx. 0.067 hectares). Two to three boxes are set per mu, each containing a bamboo pole. The poles are close to the main rope and tied with 1–3 adjustment ropes, each about 7–8m long. Between the poles, there's a 5m gap for ships to pass through. The bottom is sprayed vertically with a water gun, and the bamboo is inserted at an angle, about 1.5m deep. It is then straightened and filled with sediment naturally. Each foot pile has four roots, and the upper end is tied with a cross-stitching rope. The tidal range is about 6m, so the foot rope can be adjusted accordingly.
Daily management involves proper layout of the curtains to avoid overcrowding. They should be spread out with sufficient distance to prevent disease and promote faster growth, as well as ease of operation by boats. During the frond growing phase, four key practices are emphasized: checking the firmness and tidiness of the curtains and platforms, especially during severe weather; washing the seedlings with a water jet to keep them clean; inspecting the bamboo poles and foot piles; and monitoring for signs of disease.
In the early stage, algae require more light and grow better in surface layers. After December, light requirements decrease, and the curtain can be lowered. If high temperature or low nitrogen levels occur, the water layer should be reduced or the curtain temporarily submerged to reduce photosynthesis and maintain normal growth. Proper drying time is also important. Seedlings are sensitive to drying, so they should not be exposed for too long, especially when temperatures are high. As the algae grow, their tolerance to drying increases, allowing longer exposure times. Before and after the winter solstice, many curtains are dried, which helps eliminate moss and diatoms, extending the growth period and improving yield and quality.
Diseases and pests are common challenges. Barnacle larvae attach to the lower parts of the curtains, causing damage. Early removal is critical. Wood and bamboo can be damaged internally by organisms, so regular inspections and repairs are necessary. Omnivorous fish like black sea bream and yellow croaker can cause serious damage, so triple nets are used to catch them and reduce losses. Red bubble disease, caused by bacteria, appears as red spots that expand into vesicles, leading to rot. Prevention includes drying the curtains for 1–2 days if detected. Green disease causes softening and discoloration, often due to low nitrogen levels. Treatment involves lifting the curtains, adding ammonium sulfate, and adjusting water conditions. Bad water events, such as red tides, require immediate curtain lifting to protect the crop.
Seedlings are kept under net curtains for about 30 days before being cut to 20 cm. Harvesting occurs every 15–20 days. If density is too high, some seedlings can be removed or cut horizontally to allow drifting culture, which helps prevent disease and increase yield. Cutting lengths vary depending on the season: 10 cm in early and mid-term (October to February) for faster growth, and 5 cm in late seasons (March to April) to optimize harvest timing.