MUSSELS ARE SAVING THE BAY!

by Thabo Sefike, thabos@elsenburg.com

SSaldanha Bay has been earmarked as a hub for the West Coast industrial development zone to contribute to economic growth and the creation of jobs. Enormous quantities of human and industrial effluent have been discharged into the bay as a result of the increased human activity. This situation poses a significant risk to the nearshore ecosystem. Coastal and estuarine ecosystems are essential habitats for many species and contribute to the economic health of many coastal communities.

Nutrients play a significant biogeochemical role in nearshore and estuarine systems. They control densities and toxicities of micro- and macroalgae. In combination with temperature, nutrients will also drive non-toxic and toxic algal blooms. Human activity from the industrial and agricultural revolutions has increased the incidence of natural blooms. At optimal levels nitrogen is a vital nutrient to the marine environment. However, an accelerated rate of nitrogen enrichment from increased human activity subsequently causes eutrophication of coastal waters. Eutrophication (an enrichment of water bodies by nutrients and minerals, which induce excessive growth of algae) leads to increased anaerobic dead zones, resulting in various communities of nearshore marine ecosystems to die out in great quantities.

Shellfish, especially mussels, have the potential to remove significant amounts of nitrogen from the water column. Firstly, they bury nitrogen in sediment and enhance denitrification processes through increased microbial activity. Secondly, the bivalves consume phytoplankton and detritus, incorporating the nitrogen into their shells and tissues. Harvesting of bivalves for human consumption removes the incorporated nitrogen and other nutrients from the marine ecosystem. Mussels are a healthy source of energy, protein, sodium, selenium, vitamin B12, zinc, and foliate, and are low in fat and calories. Consumers appreciate their nutritional benefits and their mediumtextured meat with a strong earthy flavour.

The bulk of South African mussel production consists of the Mediterranean (Mytilus galloprovincialis) and the indigenous black mussel (Choromytilus meridionalus). Culture begins by collecting seed stock from natural beds or ropes placed in areas with an abundance of microorganisms. Seed stock is then grown out on hanging ropes for at least seven months.

The mussels are farmed on rafts and longline systems, but until recently, rafts were preferred because of the high-water energy of the bay in the winter season.

Recently, longlines have been prominent because of the advancement in technology and design.

At favourable sites like Saldanha Bay, the highly productive southern Benguela upwelling system brings cold nutrient-rich water to the surface. These conditions provide a highly productive environment for phytoplankton growth to sustain the biomass and rapid growth of bivalves throughout the year. In combination with ideal temperatures, 1,7 harvests per annum are achieved. Several previously disadvantaged mussel farmers and former fishermen have been supported by the Western Cape Department of Agriculture with production input to boost joint production for the bay to more than 12 000 tonnes per annum.

Feed is a major constraint to aquaculture development. Only a third of global food fish production is achieved without the use of artificial feed. On the other hand, mussels feed on naturally occurring organisms in the bay such as kelp and phytoplankton. Two-thirds of the food particles consumed by mussels consist of kelp-derived particles. Phytoplankton constitutes the bulk of the remaining third in their diet. Mussels filter the food by pumping water through an enlarged sieve-like gill. The food particles accumulate on the gill lamellae, are sorted, and transported towards the mouth by cilia.

The increased mussel production in the bay has multiple roles in the region. Ecologically, the production of mussels assists with the removal of excess nutrients in the bay. Mussel production is labourintensive with every ten tonnes of live mussel weight produced creating at least one permanent job on the farm and four permanent jobs in the processing factory.

Finally, mussel production contributes to the food security of the country. Feel free to indulge in mussels that are sustainably farmed locally.

Enquiries: thabos@elsenburg.com