Gracilaria in Brackish-Water Ponds: The Agar Seaweed of South Sulawesi

Not all Indonesian seaweed is grown in the open sea. Gracilaria, the main raw material for agar, is largely farmed in brackish-water ponds known as tambak, the same coastal ponds used for milkfish and shrimp. For a buyer this matters because the growing system shapes the quality risks: pond seaweed tends to carry more mud and silt than line-grown cottonii, and its agar properties depend on how the pond is managed.

Why Gracilaria Suits Ponds

Gracilaria tolerates a wider range of salinity than the carrageenan seaweeds and grows well in sheltered, shallow water. That makes it a natural fit for tambak, which receive seawater through tidal channels and freshwater from rivers or rain, and whose salinity therefore varies through the year.

Farming is simple in principle. Pieces of healthy seaweed are spread across the pond bottom or, in some systems, tied to lines. Farmers manage water depth and exchange through sluice gates, weed out unwanted algae and harvest by hand, usually taking most of the growth and leaving or replanting part of it as seed for the next cycle. Like other seaweeds, it needs no feed or fertiliser of its own.

Polyculture With Milkfish and Shrimp

Many pond farmers in South Sulawesi and elsewhere grow Gracilaria alongside milkfish (bandeng) or shrimp rather than on its own. The arrangement has practical logic:

• Nutrient uptake: the seaweed takes up dissolved nutrients from fish or shrimp culture, which can help keep pond water in better condition.
• Shade and shelter: seaweed mats give cover to fish and shrimp and can limit the growth of unwanted algae.
• Diversified income: farmers have more than one crop, which spreads the risk of a poor season or a disease outbreak in one of them.
• Trade-off: the pond must be managed for both crops, and practices that suit shrimp, such as chemical treatments, may not suit seaweed quality. Buyers should ask what else is grown and how the pond is treated.

How far the benefits hold depends on stocking density, water exchange and local conditions. They are well described in aquaculture practice, but the results on any given farm vary.

What Determines Agar Quality

Agar (E406) is extracted by boiling the seaweed, usually after an alkali treatment that improves gel strength, then filtering, gelling and drying the extract. Several raw-material factors affect how much agar a processor recovers and how strong its gel is:

1. Species and strain. Different Gracilaria species and local strains give agars with different gel properties, so buyers usually want consistency of source.
2. Growing conditions. Salinity, light and nutrient levels during the cycle influence agar content and quality.
3. Harvest timing. Seaweed harvested too young or left too long may perform less well in the factory.
4. Cleaning. Mud, shells, sand and other algae are the most frequent defects in pond seaweed and add weight without adding agar.
5. Drying and storage. As with any dried seaweed, uneven drying and damp storage lead to mould, discolouration and loss of quality.

For a side-by-side view of the two main seaweed gums, see carrageenan versus agar. For the contrast with open-sea methods used for carrageenan seaweeds, see long-line versus off-bottom farming.

What Buyers Typically Check

When evaluating dried Gracilaria, processors commonly look at species identity, moisture content, the level of foreign matter and other algae, colour and smell, and an agar yield and gel strength test on a sample. Because pond seaweed can pick up whatever is in the pond water, some buyers also ask about the water source and any chemicals used in the pond. Clear, consistent moisture testing matters here just as it does for cottonii.

Frequently asked questions

Working with Kospermindo

Kospermindo has supported seaweed farmer groups in South and Central Sulawesi since 2002, offering mentoring and technical assistance through Farmer Group Centres. Read our history or contact us.