RiverWise 

Issue 15 ~ June 2019

The monthly newsletter of the Lower Breede River Conservancy Trust

Shark Awareness Day

14 July was shark awareness day for 2019. Did you know that there are over 500 species of shark found in our planet’s waters?


And yet, you’re unlikely to ever cross their paths, unless you’re directly seeking them out. Sharks generally tend to keep to themselves, with many species being solitary and most of them being active at night, rather than during the day. Some of them are predatory – the great white shark, for example, will typically feed on species such as seals, but others will spend their days feeding on the ocean floor. Whether feasting on larger animals such as seals or filter-feeding on tiny plankton and krill, sharks keep the ocean in balance – something which we cannot afford to go without, especially given the fact that they have been an integral part of our oceans for some 450 million years. To see species of shark disappearing is to witness monumental, devastating change to our planet – and we are already seeing the beginnings of this detriment, even with regards to man-made industries such as tourism and the restaurant trade. A North Carolinian study showed that as sharks began to disappear, ray populations began to grow, with the rays eating up all of the scallops and forcing local fisheries to close down. Restaurants have had to remove favourite dishes, such as chowder, from their menus since clam populations have become more sparse. The economy itself is taking a gradual hit but, bad as this may seem, it is just a fraction of the long-term damage that could be yet to come, particularly with regards to the environment.


Since many shark species are regarded as ‘apex predators’, their position atop the food chain proves vital when keeping their prey and the overall ecosystem in check. An Australian study showed that as shark numbers began to decline around coral reefs, mid-level predators (such as snappers) would increase, and herbivorous fish populations would decrease. With fewer fish around, algae began to overwhelm the reef system, making it much more difficult for it to recover from disturbances such as coral bleaching.

Elsewhere, sharks have been proven to encourage an even distribution of seagrass, their intimidation tactics of dugongs and green sea turtles resulting in keeping the grasses at consistent levels: in areas where tiger shark abundance is high, for example, species destined to be prey will change where they feed, moving to lower levels of seagrass or even to different plots altogether. Particularly in the case of green turtles, the grazing of seagrass blades leads to the stimulation of rapid growth and an increased rate of nutrient recycling. Something which is helpful in terms of habitat and food distribution, but without the sharks to ‘move’ these turtles from plot to plot, fewer habitats and food sources would exist…meaning fewer turtles, dolphins and so forth to frequent the area. And speaking of dolphins…well, sharks are largely responsible for removing the old or ill from their populations, ultimately preventing the spread of infectious disease. Without sharks, fisheries, habitats and general fish populations would suffer a huge detriment.


The above are but a few examples to demonstrate the necessity of sharks in our waters, but they certainly point the way towards understanding why we must continue to protect these animals if we wish for our economies, ecosystems, and overall planet to survive.

The Spotted Ragged-Tooth Shark

The grey nurse shark, the name used in Australia and the United Kingdom, is the second-most used name for the shark, and in India it is known as blue-nurse sand tiger. The most unambiguous and descriptive English name is probably the South African one, spotted ragged-tooth shark, or scientifically as Carcharias taurus. Also colloquially known as Raggies.


They are often associated with being vicious or deadly, due to its relatively large size and sharp, protruding teeth that point outward from its jaws. These sharks are however quite docile, and are not a threat to humans. Their mouths are not large enough to cause a human fatality. Raggies roam the surf, sometimes in close proximity to humans, and there have been only a few instances of unprovoked shark attacks on humans, usually associated with spear fishing, line fishing, or shark feeding.


Physical characteristics and appearance

This species has a sharp, pointy head, and a bulky body. The can grow to a length can reach 3.2 m. They are grey with reddish-brown spots on their backs.

Their diet consists of bony fish, crustaceans, squid, skates and other sharks. Unlike other sharks, they can gulp air from the surface, allowing it to be suspended in the water column with minimal effort. During pregnancy, the most developed embryo will feed on its siblings, a reproductive strategy known as intrauterine cannibalism i.e. "embryophagy" or, more colorfully, adelphophagy—literally "eating one's brother".

Behaviour

Raggies in South Africa undertake an annual migration that may cover more than 1,000 km. Raggies are nocturnal feeders. During the day, they take shelter near rocks, overhangs, caves and reefs often at relatively shallow depths. This is the typical environment where divers encounter them, hovering just above the bottom in large sandy gutters and caves. However, at night they leave the shelter and hunt over the ocean bottom, often ranging far from their shelter. They hunt by stealth and are the only shark known to gulp air and store it in the stomach, allowing the shark to maintain near-neutral buoyancy which helps it to hunt motionlessly and quietly. Aquarium observations indicate that when it comes close enough to a prey item, it grabs with a quick sideways snap of the prey.


Reproduction

Raggie mating occurs around August–October in the southern hemisphere. The courtship and mating has been best documented from observations in large aquaria. In Oceanworld, Sydney, the females tended to hover just above the sandy bottom ("shielding") when they were receptive. This prevented males from approaching from underneath towards their cloaca. Often there is more than one male close by with the dominant one remaining close to the female, intimidating others with an aggressive display in which the dominant shark closely follows the tail of the subordinate, forcing the subordinate to accelerate and swim away. The dominant male snaps at smaller fish of other species. The male approaches the female and the two sharks protect the sandy bottom over which they interact. Strong interest of the male is indicated by superficial bites in the anal and pectoral fin areas of the female. The female responds with superficial biting of the male. This behaviour continues for several days during which the male patrols the area around the female. The male regularly approaches the female in "nosing" behaviour to "smell" the cloaca of the female. If she is ready, she swims off with the male, while both partners contort their bodies so that the right clasper of the male enters the cloaca of the female. The male bites the base of her right pectoral fin, leaving scars that are easily visible afterwards. After one or two minutes, mating is complete and the two separate. Females often mate with more than one male. Females mate only every second or third year. After mating, the females remain behind, while the males move off to seek other areas to feed, resulting in many observations of Raggie populations comprising almost exclusively females.


Protection status

They are categorized as vulnerable on the International Union for Conservation of Nature Red List. It is the most widely kept large shark in public aquariums owing to its tolerance for captivity. They are listed in the South African National Environmental Management: Biodiversity Act (10/2004) in terms of Section 56(1) and may not be targeted, caught or killed.

The LBRCT and The Anchorage Beach Restaurant and Bar organised a Beach Clean-up at Witsand Main beach on Friday 28 June 2019. We invited anyone who would be interested in participating to get our beaches clean and do our bit to help the environment!

We had a great turnout and collected many bags of rubbish with everything from microplastics, plastic pipes through to fishing line recorded. Thanks to everyone who showed up yesterday morning and hopefully there will be many more successful beach clean-ups in the area!

A special thanks to The Anchorage Beach Restaurant & Bar!

June water quality

The salinity values for June show an increase in freshwater in the system due to the rain that fell in the area. The salinity by Mudlark River Lodge was 30.75 PSU (Practical Salinity Units) which is lower than that of seawater.

Some mixing of freshwater taking place in this area even at high tide. The two largest differences in salinity between sites occurred between the Breede River Lodge and Goudmyn, and White House and Bobbejaanskraans.

The salinity from Kemp to the pont at Malgas was very low and ranged from 0.92 PSU to 0.26 PSU with freshwater conditions occurring from Ronnies House.

June bird count

We were lucky enough to find 33 species between the mouth and the Malgas pont. Overall 459 birds were recorded with the usual suspects being the most abundant: Egyptian Geese (109 birds), Yellow-billed Ducks (81 birds), Sacred Ibis (38 birds) and Kelp Gulls (30 birds). Other interesting birds that were recorded were African Fish Eagle, Black-Crowned Night Heron, Black-winged Stilts, Cape Shoveler, Little Grebe and juvenile Common Greenshanks.


Yet again some special birds were found on the trip. A single Terek Sandpiper was found between Powerlines and White House. Some of these summer migrants overwinter in southern Africa and hopefully these birds hang around the Breede mudflats throughout the year. A single Crowned Cormorant juvenile was seen at Kraaltjie. These endemic cormorants are uncommon to rare over their distribution range (Namibia to Tsitsikamma) with about 1900 breeding pairs in South Africa. They are listed as near-threatened on the IUCN red-data list.

June marine debris

Marine debris surveys for June were the first surveys for a more in-depth data collection. The GPS points of litter were recorded in more extensive categories which will allow us to determine if there are litter hotspots or if the type of litter changes between months.


The clean-up on Main beach took the form of a public beach clean-up on 28 June 2019 which was organised between the LBRCT and Anchorage Restaurant and Bar. We had a good turn out and 6 bags worth of litter. The usual litter was recorded with small plastic pieces smaller than 2.5 cm dominating the litter found.


Blokke to the Tidal Pool was dominated by plastic-related litter with was over half of all litter picked up microplastics (plastic smaller than 2.5 cm) accounted for half of the plastic litter followed by large plastic pieces and plastic beverage bottle caps. Fishing-related litter was the next most abundant litter category with 11.4% of all litter picked up, followed by glass-related items (6.4%), particularly small glass pieces (5.67%).


The area from Oysterbeds to Groen Punt was dominated by glass-related items (47%), with glass pieces smaller than 2.5 cm dominating the litter found (30.3%). Plastic-related items (19%), particularly plastic bags (4.6%) and Beverage Bottles (4%) were the most numerous plastic items on this stretch of beach. Food wrappers e.g. chip packets, chocolate wrappers etc. accounted for 6% of the litter that was found.


Gov. slip to Skuitbaai showed similar trend to the Oysterbeds clean-up with glass-items (64.5%) and plastic-related litter (16.5%) being the most numerous. This was dominated by small glass pieces (62.5%) and microplastics (9.9%). Smoking-related litter was the third most abundant litter group with all of the litter picked up being cigarette butts (4.6%).

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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