Kelp

Botanical Information

Botanical Classification

Laminaria, a plant of the Laminariaceae family.

Morphological Characteristics

The thallus is olive-brown, turning dark brown when dried. When mature, it is leathery and ribbon-like, typically 2–6 m long and 20–50 cm wide. Two parallel, shallow longitudinal grooves run down the center of the thallus; the thicker section between the grooves is the “midrib,” approximately 2–5 mm thick, while the edges on either side gradually thin and feature wavy folds.The base of the thallus is wedge-shaped; in the thickened stage, it becomes flattened and circular. Below this is a short, cylindrical or flattened-circular stalk, 5–15 cm long. Both the stalk and the thallus consist of a medulla, cortex, and epidermis.Inside the outer epidermis lies a mucilage cavity containing secretory cells that secrete mucilage onto the leaf surface, forming a gelatinous layer that makes the algal body slippery and serves a protective function. The pith consists of numerous filaments, with the cells at one end of each filament enlarged into a trumpet-shaped tube. In the early stages of development, the leaf surface is smooth; during the “young kelp” stage, the leaves develop a bumpy texture.On the lower part of the fronds of one-year-old plants, sporangia clusters are usually visible, appearing as nearly circular patches; on two-year-old plants, sporangia clusters grow on almost the entire surface of the fronds. The holdfast consists of fork-branched pseudorhizae. Spores mature in the fall.

Distribution Area

In the wild, its distribution in China is limited to the fertile coastal waters of the Liaodong and Shandong peninsulas. Cultivation has been expanded to the coastal areas of Zhejiang, Fujian, Guangdong, and other regions. However, it is a cold-temperate species.

Growth Habits

Kelp is inherently a cold-water seaweed. Its growth temperature range is 0–13°C, with 2–7°C being the optimal range.Kelp requires sufficient light energy for photosynthesis and absorbs nutrients from seawater. Depending on seawater turbidity or transparency, the suitable growth depth varies: deeper growth occurs 2–3 m below the tidal line, while shallower growth occurs 1 m below the water surface (applying nitrogen and phosphorus fertilizers to the seawater can increase kelp yield).It grows well in areas with strong currents; conversely, growth is slow and it is prone to disease in areas with weak currents. Generally, a current speed of 50–80 cm/s is most suitable.

Propagation Methods

Asexual reproduction.

Cultivation Techniques

Although there are many types of kelp seedlings, summer seedlings cultivated in June and July serve as the primary source for kelp aquaculture in China. In kelp farming areas, nursery rooms should be constructed on rocky reefs in unpolluted waters, equipped with a refrigeration system, a water supply system that draws and purifies water from the open sea, and a glasshouse structure capable of regulating light exposure.Due to significant temperature differences between northern and southern China, spores should be collected between June and August, before the water temperature rises to 23°C. Select individual kelp from the sea that have straight, broad, thick, and robust fronds; a dark brown color; numerous spore sacs of moderate maturity; and no signs of rot to serve as breeding stock, then transfer them to indoor water for continued cultivation.Maintain the water temperature between 13–18°C under natural light, and apply supplemental fertilizers at 400 mg/m³ of nitrogen and 520 mg/m³ of phosphorus. After 14 days, clusters of spore sacs will form on the leaf surfaces.To induce massive release of zoospores, hang the seed kelp in a shaded area to air-dry for 2–4 hours, then place it in a tank with a water temperature of 20–22°C to stimulate zoospore release. The zoospores will attach to a growth substrate made of bamboo or palm rope, after which cultivation can proceed.The cultivation water temperature should be kept below 10°C, with at least 10 hours of light exposure per day. Light intensity should be adjusted using shading equipment according to different growth stages, gradually increasing from 500 lx to 3,000 lx.Fertilizer application rates during the seedling stage are as follows: in the early stage, 3 × 10⁻⁶ nitrogen and 0.15 × 10⁻⁶ phosphorus; in the middle stage, 4 × 10⁻⁶ nitrogen and 0.2 × 10⁻⁶ phosphorus; and in the late stage, 5 × 10⁻⁶ nitrogen and 0.15 × 10⁻⁶ phosphorus.Partially replace the seawater regularly and maintain a certain flow rate to ensure that 1/6 to 1/3 of the water volume is replaced per hour, thereby maintaining clean water quality.After approximately 100 days, when the seedlings reach 1–2 cm in length, transfer them to offshore holding pens. Promptly install shade curtains, apply fertilizer, and adjust the water layer so that the seedlings can be harvested once they reach 10–15 cm in length. After sorting the seedlings using the “large-handful shaking and pulling” method, attach them to harvesting ropes 1.5–2 m in length.For individual seedlings, space them 2.5–3 cm apart; for clustered seedlings, group 2–3 seedlings per cluster with 5–7 cm spacing between clusters. Then, hang the seedling ropes on rafts, using one of three cultivation methods: vertical, horizontal, or sloped.

Pest and Disease Control

Major diseases during the seedling stage include green rot and white rot. These can be effectively controlled by maintaining an appropriate spore attachment density (which should not be too high), increasing light exposure, intensifying rinsing, and soaking the seedlings in a 0.05% or 0.01% ammonium sulfate solution.During the rearing period, focus on high-density cultivation in the early stages; thin out the stock when it reaches about 1 m in length. Depending on the different growth stages of the kelp, implement measures such as adjusting light intensity, flushing away suspended sediment, applying chemical fertilizers like ammonium nitrate and ammonium sulfate, and adjusting water layers to prevent and control green rot, white rot, and spotted white rot.