RiverWise 

Issue 32 ~ May 2021

The monthly newsletter of the Lower Breede River Conservancy Trust

In this issue

There were heavy rains early in May which brought an immense amount of water downriver, but luckily the system avoided flooding. This restricted our movement on the river as broken jetties and dead trees floated down, and the turbidity was extremely high due to all the sediments. Apart from our monthly water quality and bird count monitoring projects, the LBRCT removed two snakes from houses in Witsand, performed estuary clean-ups, and removed hazardous dead trees and floating logs out of the river. In this issue we have included two articles; one on the most common reeds and sedges which occur here, and another on how sharks use their multiple senses to survive, hunt, and reproduce.

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Reeds and Sedges

Reeds and sedges are represented by a few species: Common Reed (Phragmites australis), Bulrush (Typha capensis), Papgras (Schoenoplectus scirpiodes) and Umbrella Sedge (Cyperus textilis).


Common Reeds (Phragmites australis) are a widespread and robust reed. Reaching a height of 4 meters this reed is found in freshwater, brackish water, estuaries as well as areas with freshwater seepage. This species can tolerate water with a salinity of up 40 PSU (seawater is 35 PSU) if their roots are in freshwater seepage areas. However, Common reeds typically occur in estuaries where the salinity is less than 15 PSU. In the Breede River estuary these reeds first occur by Powerlines (12km from mouth). Dense stands are found in the upper reaches of the estuary. The flowering season is between December and June. Within systems, this species is ecologically important as it protects the system from flooding, filters water and can control soil erosion. It also offers shelter to a variety of birds and other animals.


Bulrushes are found in most freshwater bodies with slow-flowing or stagnant waters. This includes marshes, stream banks, dams and lakes. The muddy substrate to which this species is adapted helps the plant anchor properly to withstand strong winds and currents. These plants colonize areas very quickly. They are perennial, leafy aquatic plants with distinctive velvety-brown flower spikes. The leaves are long and blueish-grey to green that vary in length, and can be taller than 1.5 meters. This species has a flowering season in December and January. The flowers are initially yellow and then turn brown. The male flowers are the upper portion while the female flowers are the lower portion of the flower spike. The seeds are dispersed by wind. This species is important for a variety of bird species such as the Common Moorhen and Southern Red Bishop which use the dense stands for shelter and nesting. The submerged stems also provide nurseries for a variety of species such as frogs, toads and terrapins. Bulrushes are found in the upper freshwater reaches of the Breede.


Papgras is a sedge found from eastern and southern Africa. This vegetation is an important stabilizer of sandbanks, islands and coastal lagoons. It is also used to make building materials, mats, baskets and ornaments for the tourist trade. This species is adapted to being inundated with water and thrives under waterlogging (Naidoo and Naidoo 2014).


The Umbrella Sedge/ matjiesgoed or kooigoed has stems that are 1-3 meters tall. It grows in riverbanks, streams, dams, marshes, coastal wetlands and brackish estuaries. In the Breede River Estuary these plants are found in the upper freshwater reaches. The seeds are probably distributed in mud that is carried by waterfowl. Several bird species such as Weavers build their nests in umbrella sedge clumps.


In the Breede River Estuary these species are found in the freshwater and brackish areas, often associated with freshwater seepage areas. There has been a decrease in the total area covered by reeds and sedges in the Breede between 2009 (229.3 ha) and 2020 (132.9ha). Reeds are extremely important in stabilising the banks of systems and increasing the habitat availability for fish by acting as a nursery area. The reeds and sedges in the Breede are an important source of organic matter and nutrients in the system which is important for higher trophic levels (position of an organism in the food chain) to be supported in this estuary.


Localised loss of reeds and sedges particularly in the upper reaches is due to direct removal and could have a negative impact by affecting the estuarine biodiversity and productivity. Bank destabilisation would also increase. It is recommended that the removal of reeds should not take place.


References


Adams JB, Bate GC. 1999. Growth and photosynthetic performance of Phragmites australis in estuarine waters: a field and experimental evaluation. Aquatic Botany 64: 359-367.


Carter RA. 1983. Estuaries of the Cape. Part II: Synopses of available information on induvial systems. Report No. 21: Breё (CSW22). Eds: Heydorn AEF, Grindley JR. CSIR Researcjh Report 420. Stellenbosch.


Naidoo G, Naidoo Y. 2014. Waterlogging responses of Schoenoplectus scirpoides (Shrad) Browning (Cyperaceae). African Journal of Ecology 53: 36-43.


Riddin T, Adams J. 2020. Updated Vegetation assessment of the Breede River Estuary. Nelson Mandela University.


SANBI. http://pza.sanbi.org/cyperus-textilis

SANBI. http://pza.sanbi.org/typha-capensis

SANBI. http://pza.sanbi.org/phragmites-australis

Shark senses

Sharks have a brain of two cerebral hemispheres with well-developed senses of smell, vision, taste, touch, hearing, electroreception and vibration reception.

Smell

Sharks are able to smell a drop of blood in an Olympic sized swimming pool. Sharks have nostril like holes under their snout called nares. Each nare is split by a nasal cap into two openings. The olfactory sac contains a series of folded olfactory lamellae to increase the surface area, allowing the shark more opportunity to register the odour. Odours that pass over the olfactory lamellae stimulate the endings of neuro-sensory cells. Dissolved water molecules flow into the incurrent opening, pass over the olfactory lamellae, bind to the receptor cells which then send a signal to the brain, and then exit the olfactory sac through the excurrent opening. Nares are not used for respiration as they are separate from the mouth and throat. A shark will swim moving its head from side to side once it has picked up a scent and on the hunt, this allows the snout to move backwards and forwards through the scent helping the shark determine the direction of the source. If the shark loses the scent it will swim in a wide zig-zag pattern until the scent is picked up again. Some shark species are able to pump water over the snares while resting on the ocean floor. Some species, like the angel shark, rely less on smell and more on vision in order to locate prey and will have less sensitive olfactory sacs. Sharks don’t only use their sense of smell for hunting – they also release pheromones which signal their reproductive state and potential mates will use their sense of smell to locate that shark.

Touch

Most sharks nose an object before biting it to determine whether it is edible or not. Biting is also considering an extension of touch as well as taste. Sharks are also able to pick up distant touch through a series of interconnected canals that start at the back of their head and end on the upper lobe of the tail; this is called their lateral line. On the sharks head is the infraorbital canal (extends ventrally from behind the eye and along the snout), the supraorbital canal (passes above the eye towards the snout) that connects to the infraorbital canal and the supratemporal canal (leads back over the top of the head). Tiny pores that allow water to penetrate open the canals to the outside of the skin. As other animals move in the sharks surroundings, small waves are sent over the hairs along the lateral line canals which then send a signal to the brain. The sharks own movement will create waves that hit obstructions and return, this creates a vibration echo map for the shark and helps the shark determine whether there are any sick or injured animals around.

Taste

Sharks have small pits that contain rod-shaped gustatory sensory cells lining the mouth and throat. A signal of whether to reject or consume reaches the brain after the dissolved chemicals from the bitten object attach to the gustatory cells. Other shark senses play a more vital role in locating prey and are therefore more adapted than the taste organs. A very small amount of sharks rely heavily on taste, an example would be the Nurse shark which use their whisker-like barbells on the upper lip to rake through the sand to taste for a possible meal. Great whites require a high quantity of fat in their diet and that is why three out of four shark attacks on humans aren’t fatal, because the shark will take a bite then realize that there won’t be enough fat in the meal and swim away.

Vision

Requiem sharks have no eyelids over their eyes but rather a protective membrane covering called the nictitating membrane. The nictitating membrane is a toughened layer that rises from below the eye, covering it during feeding. Other sharks, like the Great White shark don’t have nictitating membrane but can roll their eyes back into their sockets, exposing a hardened pad, to protect their pupils. Their retina contains only rods (in most sharks), making sharks adapted to dim light. Some sharks do rely on vision for hunting and most species the eyes are well developed, large complex structures. Many shark species have a thin area of skin on top of the head leading directly to the pineal gland in the brain; this allows them to sense light and dark. They don’t possess lenses to focus the light so there is no shape perception, but the shark will use the differing light levels to plan its movement throughout the day. Many shallow water sharks are able to dilate their pupils to control the amount of light entering their eyes. Sharks are able to focus when the rectus muscles pull the lens closer or further away from the retina. A shark’s focus is different to higher vertebrates which have distorted lenses making them able to focus light from different distances. Sharks that live in the deeper, darker waters may lack the ability to stop light entering their eyes; for example sixgill sharks. About twenty shark species have rods and cones in their eyes, but whether they are able to see colour is questionable. The tapetum lucidum structure in their eyes is a layer of mirrored crystals behind the retina that can be adjusted to reflect light back into the retina, amplifying the strength of the image; this gives sharks an excellent vision in low light conditions. An interesting fact about Great White sharks is that they have been observed sticking their heads out the water and look at their surroundings, but it is not proven that they are adapted to site above water.

Electroreception

Ampullae of Lorenzini are sensory organs around the nose that pick up fluctuations in electric fields. They are large pores filled with a gelatinous substance; cylindrical canals which store the gelatinous secretions are connected to the pores. Sensory nerves that transports the electrical senses to the brain are found at the base of each pore. Sharks that actively hunt may have up to 1500 ampullae around their snout while the rest may have just a few hundred. Sharks like hammerheads rely on these organs to pick up electrical signals coming from prey hiding in the sand. The electrical sense of sharks takes over from sight or smell once they are close to their prey. Wounded animals give off erratic electrical signals causing sharks to react aggressively – this is the reason that sharks will continually bite and attack victims while remaining disinterested in the rescuers swimming next to the victim. It has amazed scientists how sharks can migrate extreme distances and the conclusion is that many senses are in use during navigation. One theory is that the combination of the saline water and the magnetic fields around the earth creates an electrical gradient that helps lead the shark in the correct direction.

Hearing

It is unclear whether sharks have the same hearing as humans. They have internal ears that are embedded within the chondrocranium (frontal skull). The spiracles which are used in respiration are often confused for ears as they are holes behind the eyes. Sharks use their ears to remain in balance and equilibrium. A sharks swaying is picked up on its sensory hair cells which will send signals to the brain, the shark will then correct the imbalance.

In the data

Monthly routine monitoring

May bird counts

Our monthly bird count was conducted on 24 May. We are proud to have added the first sightings of 5 species to the Overberg bird list this month. These species include the Grey plover, Common greenshank, Marsh sandpiper, Caspian tern, and the South African shelduck. Fewer birds were observed in May (480) compared to April (1092), with the most common birds being the Yellow-billed duck, Sacred ibis, and the Kelp gull. Ten Pied kingfishers were also recorded! The photograph in this sections header is of an immature Fish eagle.

May water quality

May’s water quality run was conducted shortly after the heavy rains that threatened to flood the river. For this reason the river was extremely fresh. Despite this, the deep hole in front Moddergat slipway still revealed higher salinity readings than anywhere else. The visibility (turbidity) data this month was negligible as the secchi disk could not be seen more than 0.1 m under the water across all sites.

Estuary

clean-ups

The LBRCT removed floating and submerged dead trees out of the river. After the heavy rains there were many ‘hazards’ in the river. However, two spring tides since then has pushed most of these logs above the high water mark or out to sea. Dead trees that remain stuck in the mud are indicative of shallow waters. Please be safe and vigilant on the water!

Snake removals

Brown house snake (Boaedon capensis)

Puff adder (Bitis arietans)

Photos of the month

The photo-of-the-month competition is an initiative which the LBRCT is promoting whereby members of the public can send us their photographs of anything related to the natural beauty of the Breede River! Please send your photos to conservation@breede-river.org.

Southern double-collared sunbird

(Cinnyris chalybeus)

African hoopoe

(Upupa africana)

Thank you for reading

We hope you enjoyed this months’ issue. Should you have any feedback, questions, or matters you would like us to cover in a future issue, please do not hesitate to write to us at news@breede-river.org. 

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