Muddy Shelves


Muddy Shelves, Ishelufa zodaka, Imigede yodaka, Modderbanke

Muddy seabed relying on fresh water and sediments transported by rivers to support fisheries for sole, prawn and line fish

What are Muddy Shelves?

Muddy shelves are coastal ecosystems characterised by muddy sediments (natural material broken down by weathering and erosion), mostly originating from land. Mud has some of the properties of sand but is finer (smaller grain size) and this makes it stickier than sand. Accumulations of mud, also known as a mud bank, have travelled from coastal catchments through our rivers and estuaries over millions of years to reach the sea. Even though mud may occur far offshore, it depends on the connection between land and the ocean through our freshwater ecosystems with rivers being the transport vehicle for mud and sand to the coast. Where settled on the seabed, muds are enriched by nutrients and mineral particles that have been collected from the land, rivers and estuaries (where rivers enter the sea) along the way. Muddy shelves are rare across the South African shelf and slope.

Unlike sandy sediments found on sandy shelves, muds are made up of fine silt and clay with high mineral content, making them rich and fertile. Also in contrast to sandy shelves, muddy shelves are more stable, often having weaker currents and lower wave energy which allows the fine sediment to settle on the seabed. Deeper muddy ecosystems receive little to no light and rely on energy flowing into the system from bottom currents, falling from the surface layer or from freshwater input.

The rich, muddy ecosystems offer refuge opportunities for burrowing animals, who hide from predators like fish, sharks and rays. Burrowed mud shows the wealth of animals and activities on the shelf where the surface of the seabed is full of tracks and burrows with associated holes and mounds. The process of creating these features on the seabed is called bioturbation where burrowing animals (bioengineers) change and turn over the sediments. Bioturbation contributes to nutrient cycling and helps to exchange minerals and oxygen between the seabed and the water column.

Who lives on Muddy Shelves?

Species found within the muddy shelf include those that live on the seabed (epifauna) and those animals that live between the tiny grains of mud or in holes, burrows and mud tubes beneath the surface (infauna). Animals who live within the rich muddy sediment play an important role in the ecosystem, not only as prey items. Crabs, burrowing worms, mussels, snails and sea cucumbers live in the mud, consuming bacteria in mud and organic matter and aid in cycling nutrients. Other animals on muddy shelves include filter feeders who grab food particles from the water column like sea pens (soft corals adapted to anchoring in soft sediment), tube-dwelling anemones (anemones with a double ring of tentacles) and other invertebrates able to live on the soft muddy seabed. Many animals that live in mud cannot survive in sandy or rocky areas. Muddy ecosystems must be maintained if we are to have these animals in the future.

The animals in muddy shelves rely on energy from rivers, plankton or bacteria. Seaweeds and grazers are usually absent because the water column appears murky (turbid) and the low light prevents photosynthesis (making food from sunlight). Deposit feeders, predators and scavengers are more common in these ecosystems. Starfish, brittle stars, prawns, langoustines, crabs and urchins feed on organic matter or other animals that live on or in the muddy sediments.

Muddy ecosystems are important habitats for some fish species especially commercially important fish like sole, hake and kob or kabeljou. Feeding on the surprisingly high diversity on muddy shelves are various species of skates and rays that create giant holes or feeding pits on the seafloor when they forage for buried animals, turning over sediments creating new habitats and tiny seafloor features for creatures to colonise.

Why are Muddy Shelves important?

Muddy shelves play a key role in food provision, nutrient cycling, carbon storage and hence mitigation of climate change.

A range of fisheries including those for sole, prawns, langoustines and some linefish are supported by freshwater and sediment from estuaries and rivers that supply muddy shelves. Additionally, muddy shelves play important roles in the completion of the life cycles of many seafood species and many fish are dependent on muddy habitats for food (feeding on burrowing animals) or to hide in the turbid water. Where muddy habitats are used by young fish and sharks to hide and feed these areas are known as nursery areas. Examples include muddy shelves off the uThukela River, the Orange River, the Breede River and even off Port St Johns. Reduced freshwater entering the sea affects fisheries with lower catches of line fish and prawns in times of reduced river input.

Muddy shelves are rich in nutrients and support many commercially important animals but also play a role in the carbon cycle helping to maintain a healthy climate. Muddy shelves play a key role in nutrient cycling. Without muddy shelf ecosystems, there would be a reduction in the amount of nutrients and oxygen cycled between coastal ecosystems affecting food webs and the survival of many animals. Without muds, climate change could be worse.

How do we care for Muddy Shelves?

In South Africa, most muddy ecosystem types are considered threatened because of a combination of their small area and the multiple pressures they experience. Reduced freshwater flow, pollution and bottom trawling are three of the main pressures on muddy shelves. To look after these ecosystems and the benefits they deliver to people we need to;

Ensure sufficient quantity and quality of freshwater flows to marine ecosystems– Unpolluted, freshwater flowing from rivers through estuaries into the sea is not wasted and is essential for coastal and marine food production, livelihoods, tourism and climate resilience. Through appropriate management, South Africa can maintain the vital freshwater flows that reach the coast. Water volume and quality and sediment inputs from rivers, estuaries and groundwater must be well managed to sustain muddy ecosystems. Decreased sediment supply onto the shelf may cause loss of muddy habitat and can alter food webs and impact commercial and small-scale fisheries. Reductions in freshwater input and quality can reduce catches of commercial and small-scale fishers -affecting businesses, livelihoods and food supply and quality. The marine environment needs to be recognised as a receiving environment in the National Water Act to support flow allocation for freshwater flows. This is essential not only for water but also for sediment and nutrient delivery and to address pollution issues in freshwater flows and marine outfalls. Coastal sediment requirements are critical for healthy functional beaches and dunes and offshore muddy habitats that support key fisheries.

Manage activities that can impact the muddy seabed– Trawl nets, mining, petroleum activities and pollution can damage the seabed. Some of these activities cause physical damage to animals on the seafloor reducing the complexity of the seafloor, making it more uniform, thereby, reducing the diversity of animals able to live on or in the substrate. As trawling is not selective in the animals it catches, there is a high number of animals accidentally caught in nets. Many threatened animals like sharks and rays are caught as bycatch in crustacean trawl fisheries. Polluted waters can carry heavy metals, pesticides and herbicides. The chemistry of these pollutants is such that they concentrate into muddy sediments and are easily accumulated or concentrated up food chains to top predators. We can’t see, taste or smell these pollutants which are dangerous to us, so unmanaged land-based activities can cause us to be unhealthy if we consume sea products from polluted areas. Our choices concerning food make a difference in the demand for certain species (See SASSI). Mining operations should avoid sensitive areas and threatened ecosystem types. Pollution needs to be monitored and well managed with due consideration of types of pollutants, location of outfalls and pipelines and polluting activities and mitigation measures. Help raising awareness about the impacts of pollution and avoid consuming seafood from polluted areas.

Protect portions of muddy ecosystemsSetting aside portions of muddy ecosystems in protected areas or seabed management areas is important to understand the natural state and functioning of these ecosystem types and protect vulnerable species. A small proportion of South Africa’s muddy shelves are protected within the Benguela Muds, Agulhas Muds and uThukela Banks Marine Protected Areas. To improve the protection of muddy ecosystems and management of the connected coastal ecosystems on the west coast, the protection of mud habitats linked to the Orange Cone off the Orange River is a priority. This area has threatened mud ecosystem types and is recognised as an Ecologically and Biologically Significant Area. Protected Area management plans need to account for the freshwater flow requirements of Muddy Shelves and ensure the critical linkages are maintained in managing such MPAs.

How do we learn more about Muddy Shelves?

Shakira Rylands

MSc Student Marine benthic ecology, Department of Biological Sciences, University of Cape Town

Studied: Bachelor of Science (University of Cape Town), Bachelor of Science, Honours (University of Cape Town

What is interesting about the ecosystem you are working in?

The ecosystems I work in support well-known species of fish such as soles, kabeljou and the Cape hake (hake and chips!). These fish species are important to our commercial fisheries and sustain the livelihoods of many people involved like fishermen and fish factory workers but we still know so little about the ecosystems that support them. It is interesting to explore these ecosystems and to start to understand the many connections between the many components. Rivers on land deliver rich nutrients to the muddy shelf, which are transported through currents. Nutrients and food from the water column allow many marine animal communities to survive in this ecosystem. The muds afford shelter to young fish and eggs and many animals that burrow into the seafloor. Understanding the links and energy flow between water, sediment, invertebrates, fish and people is fascinating to me.

What are the challenges with working in your ecosystem?

Muddy shelf ecosystems are found at depths that are naturally inaccessible to humans. We often work many kilometres from the shore off a boat using camera equipment that must be sent remotely to the seafloor. Boats and equipment such as remotely operated vehicles and sediment grabs are expensive and require experts to operate. Working offshore to access these ecosystems is also highly weather-dependent. Apart from being dangerous to work in weather that causes large wave swells, there is also a possibility of having poor visibility and it may be difficult to see and identify animals in photographs and video.

What are you working on and why is it important?

My research is focused on understanding the role of Table Mountain National Park MPA in prIn my current research, I am investigating the impacts of inshore trawl fishing on the structure and complexity of seafloor habitats in sandy and muddy ecosystems. Trawl fishing involves dragging a large net on the seabed by a trawling vessel to capture fish such as sole and hake. However, at the same time, the trawl nets and trawl doors can disturb or damage communities of attached and slow-moving animals living on the seabed or in the surface layer of sediment. Trawling may remove or injure animals that are essential to the functioning of these ecosystems and can even change the sediment characteristics. Trawling-induced changes should be measured so that we can have a better understanding of its effects on biodiversity and information to better manage the fishery. Muddy shelf ecosystems are important in cycling nutrients and oxygen from the water column and they store carbon from the atmosphere which helps reduce climate change so it is important to understand how to maintain these ecosystem services.


The ACEP Deep Forests (NRF Grant 110765), ACEP Agulhas Bank Connections (NRF Grant 119213), ACEP Deep Connections (NRF Grant 129216), ACEP KZN Bight (NRF Grant 67373) and the ACEP Spatial Solutions (NRF Grant 97968se projects are acknowledged for helping build the knowledge base for better management of muddy shelves. The CoastWise project of the MeerWissen initiative, funded by the German Federal Ministry for Economic Cooperation and Development (BMZ) and implemented by the Deutsche Gesellschaft für Internationale Zusammenarbeit (GIZ) GmbH, is acknowledged for supporting improved communication about coastal ecosystems. We thank Fiona Mackay, Natasha Karenyi and Lara Atkinson for inputs and review of the information in this fact sheet.

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