Mariculture

We are stopped just outside the gates of the Alexcor offices in Kleinzee. Several "No entry" signs assure that the message is clear, you cannot proceed by accident or use the excuse that you didn't know...  They do not beat around the bush: you are not allowed in here! And if you are one of the really fortunate people to get permission to enter, you will not by any means be allowed to take pictures...

And no, these are not the gates to a restricted diamond field, a nuclear plant or a secret arms testing site... 

Just behind these high walls oysters are being cultured... and looking at the fences and signs, this is a highly secret venture!

Oyster-, mussel- and abalone farming are collectively named mariculture.

Mariculture generally is an aquaculture in marine environments – the raising of marine plants and animals in the ocean itself.  It includes a wide range of species and culture methods and is growing fast on a global scale.  This is due to the fact that many fish stocks are over-fished and catches are declining.  At the same time the world population is rising and with it the need for dietary protein.  The expansion of mariculture can reduce pressure on wild fish, shrimps and mollusks, because they reduce their market price and by this the investments in fishing fleets, or they can increase the pressure due to the use of fishmeal in feed for some mariculture-species.  Some forms of mariculture provide good quality food and the production is more efficient than that of terrestrial animals (roughly half the level of feed input per unit output is necessary.)  Products obtained from mariculture are not only used directly as food items, but also as inputs for e.g. cosmetics, nutraceuticals, medicines, food additives and many more.

Mariculture can play an important role, especially in rural areas, for food security, economic and social welfare.  Its practices are many and varied and mariculturists may use experimental laboratories, shore-based systems or farms in sheltered bays and estuaries to produce seafood products such as oysters, mussels, abalone, fish, prawns and seaweed.  

In heavily populated coastal areas mariculture is in competition with other human activities for space and other resources.  These activities can for example be fisheries, tourism, harbour operations, nature conservation and industry.

Mariculture accounts for less than 1% of South Africa`s seafood production, but a recent surge in the cultivation of abalone has shown that the industry has a high growth potential.  South Africa does not have a coastline suited for mariculture, and the few enterprises that have flourished are located in physically protected bays and estuaries.  Land-based impoundments with a pumped seawater supply are used to grow abalone.

Saldanha is the mariculture headquarters of the West Coast, with mussels, seaweed and oysters being farmed there.   It has the potential to grow even bigger (10 to 28 times its current size) and could provide direct employment for 940- 2500 people in the area, provided certain regulatory and market requirements are met.  Based on research it is estimated that in the 345 hectares now allocated for bivalve culture in the Bay, Saldanha is capable of producing 12 470 to 333 253 tonnes of live bivalves per year, much more than the 1176 tonnes it is now producing per year.  South African oyster production has the advantage that when northern countries are experiencing summer mortality, South African oysters are in peak condition in the middle of the southern hemisphere winter.  Culture of bivalve mollusks like oysters, mussels and clams can produce a high-value product without some of the drawbacks of farming other species.  Bivalves do not require feeding as they remove phytoplankton from the water and convert it to edible protein.  In 2010, annual production of oysters and mussels was R14,4 and R9.1 million respectively, making these the second and third most valuable mariculture products in South Africa, with the R355 million abalone sector dominating. 

But mariculture can also have some disadvantages: there are several environmental problems that are associated with it.  The extent of the problem depends on species, culture method, stocking density, feed type, husbandry practice, hydrodynamic site conditions and the sensitivity of the receiving ecosystem.

Eutrophication defined as nutrient enrichment is considered by some the most important pollution threat to marine waters.  This problem is often mentioned in the context of intensive culture of fish and shrimp, where a lot of artificial feed is used.  Here waste consisting of uneaten feed and faeces move down into the benthos.  This can likely cause the development of (non-) toxic algal blooms, like red tide. 

Water-based mariculture practices, such as oyster cultivation on racks, have also caused habitat transformation by obstructing the natural flow of water in estuarine environments, increasing sediment accumulation and smothering the benthos underneath the racks.  

Toxic chemicals, such as water treatment compounds, disinfectants, antibiotics and vitamins, are used by fish farms and may have an impact on the environment if they are released in the natural systems.  

The introduction of exotic species for mariculture purposes is common practice worldwide.  The most important ecological concern is that some of these species might escape into the natural environment where they might out-compete natural species and introduce foreign parasites.  In a previous post I have already mentioned how the Mediterranean mussel is rapidly displacing our indigenous black mussels along our western and southern Cape coasts.