Contents of this page / Inhoudsopgawe van die bladsy:
Een van die grootste oseaniese “riviere”, die Agulhas-seestroom, vloei vanaf die ewenaar aan die ooskus tot by die suidpunt van Afrika. Dit vervoer ongeveer 75 miljoen kubieke meter water per sekonde suidwaarts verby Oos-Londen.
Aan die suidelikste punt van die vasteland verlaat die stroom die einde van die vastelandplat by die vlak Agulhas-bank en draai terug na die Indiese Oseaan. Hier waar die stroom omkeer, sluit die lus dikwels en vorm ‘n groot werwelstroom (die Agulhas-ring) ongeveer 300 km in deursneë, wat dan in die Suid-Atlantiese Oseaan inbeweeg.
Die diepsee-omgewing rondom Suid-Afrika word oorheers deur die Agulhas-seestroom wat ook die ooswaartsvloeiende Suid-Atlantiese en die Antarktiese Sirkumpolêre Seestroom ontmoet.
Die Agulhas-seestroom is warm, maar bevat min voedingsoute, terwyl die Antarktiese Sirkumpolêre seestroom koud is en baie voedingsoute bevat. Die Agulhas-seestroom bring warm water vanaf die subtropiese gebiede langs die ooskus wat aanleiding tot die verskeidenheid aan seelewe gee. In die omgewing van Oos-Londen word die vastelandplat wyer wat veroorsaak dat die stroom dieper die see in vloei en die kuswaters effens koeler word.
(BRON: Agulhas NP Argiewe, Blogspot)
Die Agulhas-bank strek 200 km verder suid van Kaap Agulhas langs die Kaapse suidkus; dit is ook die gebied waar ons vissers visvang.
Besoekers aan die Suidpunt word gereeld daarop attent gemaak dat die eintlike Suidpunt omtrent 250km see-in geleë is. Hierdie relatief vlak gebied (minder as 200m diep) is nie so produktief soos die weskus nie, maar die winde slaag daarin om die see genoegsaam te vermeng om sodoende aanvaarbare vlakke van voedingsoute toe te laat om die sonverligte dele binne te kom waar die primêre produksie plaasvind.
As gevolg van die invloed van die winde op sowel die Agulhas-bank as die water van die Kaapse Weskus, wissel die voedselvoorsiening en produktiwiteit van hierdie waters voortdurend van dag tot dag en ook van seisoen tot seisoen, net soos die winde.
Riviere is ‘n bron van organiese stof en bodemmodder op die Agulhas-bank. Die riviermodder dra by tot die habitat van diere soos die Agulhas-tongvis. Die Ansjovis skiet hoofsaaklik kuit op die Agulhas-bank tussen Oktober en Februarie. Dit is ‘n belangrike bron van voedsel vir stokvis bo die breë Agulhas-bank.
Die vermoede bestaan dat snoek op die Agulhas-bank kuit skiet. Vandag word die Agulhas-bank, soos alle oseane en seegebiede in die wêreld, deur plastiekbesoedeling bedreig. En helaas deesdae ook deur die gesoek na gas.
(BRON: Agulhas NP Argiewe)
Marine heritage along the Cape Agulhas coastline: coralline red algae – Prof Gavin W. Maneveldt, Department of Biodiversity and Conservation Biology, University of the Western Cape
New evidence points to the fact that within an already special place (western biogeographic transition zone), lies an extraordinary place. The meeting of the cold Benguela and warm Agulhas currents at the edge of the Agulhas Bank fuels the nutrient cycle for much marine life in the area, making this region one of the most productive areas in South Africa. Furthermore, the greatest calcite (the most stable form of calcium carbonate) gradient exists between our southwestern and southern shores (along the Agulhas Bank), and Antarctica.
This makes the region an area rich in calcareous derived and dependent organisms. One such group of organisms is the coralline red algae. These calcified seaweeds are the only algae in which most vegetative cell walls are impregnated with calcite, making them hard as rock. While coralline algae are typically ignored by most marine biologists and even specialist phycologists (people who study algae and seaweeds), they are important from an ecological, environmental and economic standpoint. Not only do these organisms help cement reefs together, but they are also important sources of primary production and food for very many marine organisms. Recent evidence has even suggested that coralline algae may be one of the largest stores of carbon in the biosphere, making them very important organisms considering the impacts of increased carbon emissions and their effects on global climate change.
As we come to a better understanding of the biodiversity of the coralline algae in South Africa, ongoing research has shown the Cape Agulhas region is particularly rich. Three species of encrusting coralline algae new to science have thus far been documented for the area. Even more remarkable is that two of these species are range-restricted endemics, occurring in an area no greater than 10 km of coastline at the southernmost tip of the African continent. This is unusual! The only other range-restricted endemic seaweed (a brown seaweed) recorded along the south coast with a limited distribution, occurs along about 60 km of coastline from De Hoop to just west of Cape Agulhas.
If research on such a neglected group of marine organisms such as the coralline algae, which are comparatively well researched in South Africa, is showing that the Cape Agulhas coastline is particularly rich in diversity, then the same is probably true of many other groups of marine organisms. Expanded research efforts into the Cape Agulhas region will no doubt provide the answers.
There are currently 284 known species of seaweeds in the coastal region that includes Cape Agulhas. This figure, however, excludes the encrusting coralline algae that amounts to a further 28 species from this region of the coast. (Information provided by Prof Robert Anderson of the Biological Sciences Department at UCT).
Bron: Two Oceans Aquarium
Die Galjoen (Eng: Blackfish, Black bream, Damba) is so ‘n gewilde strandhengelvis dat dit as Suid-Afrika se nasionale vis verklaar is. Die naam is waarskynlik afgelei van die Hollandse woord vir “gallei” of “galleon” wat verwys na die vis se legendariese terugveg wanneer dit gevang word. Die Galjoen is een van slegs twee spesies in die familie Dichistidae, wat albei endemies is aan suidelike Afrika. Die Gebande Galjoen, Dichistius multifasciatus, is kleiner en kom slegs voor in sub-tropiese water. Die Galjoen kom voor vanaf noordelike Namibië tot by die suidelike Kwa-Zulu Natal kuswaters.
Die riwwe aan die Indiese Oseaankus word met ‘n verskeidenheid kleiner seewiere of deur ‘n ryke verskeidenheid vasgehegde seediere bedek. Die Galjoen wat ‘n bankvis genoem word, kom algemeen op hierdie riwwe voor. Dit word ook as ‘n geharde spesie beskou wat onstuimige witwater verkies waar dit swartmossels, rooi-aas en ander bodemlewende organismes wat van die rotse losgespoel het, verslind.
Galjoene is gebiedsgebonde, maar beweeg soms weg. Gemerkte vis is al duisende kilometer weg van hul normale gebied gevang. Algemeen word aanvaar dat dit dalk te doen het met kosbeskikbaarheid en watertemperature.
Kuitskiet vind gewoonlik meer as een keer ‘n seisoen plaas, en strek vanaf Oktober tot Maart met die hoogtepunt in Desember. Wyfies groei vinniger as die mannetjies, en bereik ‘n maksimum lengte van 670 mm. Die mannetjies word 470 mm lank. Die vis word nie ouer as 13 jaar nie. Die voortplantingstadium word op vyf jaar bereik waar die wyfies ongeveer 34 cm lank is en die mannetjies 31 cm.
Oor die laaste drie dekades het die Galjoen se getalle weens oorbenutting deur kommersiële en sportvissers dramaties verminder. Treknette is in 1974 verbied. Vandag word die Galjoen beskerm deur ‘n minimum-grootteperk, sakbeperking en ‘n geslote seisoen waar dit nie verkoop mag word nie. Die vangseisoen vir Galjoen is tussen 1 Maart en 15 Oktober jaarliks. Hengelaars met permitte mag net ‘n maksimum van vier visse per dag vang en die minimum grootte wat gevang mag word, is 30 cm.
‘n Nasionale opname wat in 2000 van die visstapel van verskillende lynvis gedoen is, het gevind dat galjoen se getalle ineengestort het en dat dringende heropbou nodig is. (Kuitvis biomassa per aanwasindeks was minder as 25%).
BRONNE:
1. COAST CARE file.
2. OSEANE VAN OORVLOED. (Dept. Omgewingsake en Toerisme, Hoofdirektoraat Seevissery, g.d.)
3. Two Oceans Aquarium website
Bron: Wikipedia
Die Geelstert is ‘n baie gesogte lynvisspesie aan die Agulhaskus, soveel so dat ‘n fees elke jaar in Struisbaai ter ere van die vis gehou word. Dit is gesog onder hengelaars omdat dit heerlik smaak en ‘n sterk vegter is.
Die Geelstert is ‘n aflandige migrant wie se bewegings deur watertemperatuur en prooiverspreiding bepaal word. Visspesies soos die Geelstert trek na gebiede waar voedsel volopper is of na omgewings wat gunstiger is vir die kleintjies om voort te plant. Hulle spandeer baie van hul tyd aflandig, maar beweeg tog nader aan die kus gedurende die somermaande wanneer die kuswaters warmer word. Kuitskiet vind gewoonlik in die somer plaas, ongeveer 30-40 km van die kus af. Die volwasse vis (kleiner as 65 cm) beweeg voortdurend aanlandig/aflandig gedurende hierdie periode.
Geelstert is op ‘n stadium met beursnette gevang, maar as gevolg van ‘n groot afname in getalle, is dit gestaak. Gelukkig, volgens die Seekosverbruikersgids, val Geelstert nog onder hul groenkleurkode. Dit beteken dat die Geelstert nog in die mees gesonde en besbestuurde visgroep val en die huidige visvangdruk kan hanteer. Volgens ‘n 2000 nasionale opname van die Geelstert se getalle was dit nog aanvaarbaar (Kuitvis biomassa per aanwasindeks naby aan 40%), maar getalle is besig om kommerwekkend laag te daal.
BRONNE:
1. COAST CARE file.
2. OSEANE VAN OORVLOED. (Dept. Omgewingsake en Toerisme, Hoofdirektoraat Seevissery, g.d.)
Haliotis midae
Abalone is a species of large sea snail endemic to South African waters.
Identification: It has a large heavy shell with strong, irregular corrugations running more or less parallel to the growing edge. The margins of the foot have a dense fur of fleshy projections. It grows up to a size of 190mm
Reproduction: Abalone is a slow-growing animal and reaches sexual maturity only after eight to 10 years. They shed their eggs and sperm into the water, where the larvae undergo a brief planktonic development.
Diet: It is a marine herbivore that feeds on seaweeds (mostly kelp and red seaweeds). Abalone was part of man’s diet for ages.
Conservation status
Abalone is seriously under threat due to poaching (illegal, unregulated and unreported fishing). The recreational abalone season has been closed until further notice. (SOURCE: BRANCH, GRIFFITHS, BRANCH, BECKLY. 2005. Two Oceans: a guide to the marine life of Southern Africa. Cape Town: David Philip; Coast Care fact sheet. n.d.);
Abalone are important in marine ecosystems. As herbivores they consume macro-algae (seaweeds) and so are important in that they have the ability to prevent the monopolisation of space by a few fleshy seaweeds that have the potential to competitively exclude other seaweeds. Their feeding behaviour often allows a greater diversity (bearing in mind that diversity is not simply a function of the number of species, but also their relative abundance) of seaweed species to co-exist. Areas with a high density of abalone can actually have a higher diversity of seaweeds, particularly those that grow epizoically (on the shells of the abalone). In the absence of abalone, seaweed diversity can be comparatively low.
This has a number of implications because it also affects the diversity of other animals that potentially are dependent on a select number of seaweeds. In short, a healthy ecosystem is dependent on a number of interacting trophic (primary producers [seaweeds], primary consumers [herbivores], suspension feeders, secondary consumers [omnivores, predators, etc.], and decomposers) levels, and abalone form but one link in that trophic chain for it to function most effectively.
Source: Abalone important in marine ecosystems - Prof. Gavin W. Maneveldt, Department of Biodiversity & Conservation Biology, University of the Western Cape
Bronze Whaler Shark & Ocean Sunfish