January 2009 Shark Bytes
Posted on Saturday, 31 January 2009
We have not had a lot of sharky encounters this past month mostly due to windy conditions but also because we decided to go on an impromptu trip to The Kalahari, a protected nature reserve in the Upper Northern regions of South Africa.
We have been managing at least 1 pelagic shark trip each week and as in December we have been seeing large numbers of Blue sharks and also a couple of mako sharks.
The water visibility has been very varied and often we have had to go close to 30 miles off Cape Point in order to find clean water to dive in.
Due to the large numbers of blue sharks in the area, on the flat days we have had regular sightings of blue sharks cruising on the surface even before getting to our destination. So, it has been fun trying to spot them on our outward bound journey.
As a general rule we find that a current line can be a rich source of marine life and gamefish as well as sharks can be found in this area.
These current lines to the trained eye can be fairly easy to see and the surface of the water can look a little disturbed. As I said, to the trained eye, so as most would not know what to look for it can be difficult to see.
On one particularly flat morning we had a very smooth ride up to Cape Point and beyond. At about 15 miles from the point we all picked up on white caps in the distance and immediately thought it was wind further out. Once in this area it became apparent that it was not wind but in fact an extremely powerful current line. The water was so disturbed that Chris had to slow the boat right down as we pounded from one cresting wave to another. Mixing current lines do not normally cover a large distance but this time it was a good 6 miles before we came out of it.
Unfortunately a couple of guests were sea sick and I myself was sporting a green tinge to the cheeks...not pleasant!
The Agulhas Current
Coming across this incredible current line has inspired me somewhat into writing about how special this is off the coast of Cape Town.
The Agulhas current is a large body of water that moves in a general north east to south west direction along the east coast of South Africa from the Mozambique Channel to Cape Town. When this water comes close to the Agulhas plateau near Struisbaai the water is either forced over the plateau following the contour of the coast or is pushed around the 200m contour line off the continental shelf, either way usually finding its way off the West of Cape Town. This is normally warm temperate water and during the summer months between 19-23c.
Off Cape Town, especially during our summer months, we have strong offshore winds that blow. This wind pushes the warm surface water offshore and creates an upwelling of the colder water from deeper below. This up welled water is normally dirty as it contains disturbed sediment from below, comprising of either living or non living organisms. It is also normally full of nutrients. This is because this water is highly oxygenated from all the movement and contains good amounts of chlorophyll. The chlorophyll in turn supports a variety of marine life from plant matter to small fish larvae.
Generally cold water does not support a great diversity of species but the bio mass is large comprising of bait fish species such as sardines, and anchovies. Of course the bait fish attract the gamefish as well as sharks and from above the pelagic birds are also able to survive. It is amazing to me that just the offshore winds are such an important catalyst to our oceans ability to produce food.
Now if I can finally get to the point, the warm water in the Agulhas Current will in various spots off Cape Town meet with the colder water. These two bodies of water are moving in different directions and this is what creates such troubled water. There is never only one meeting spot as is so often thought … but there will be mixing in many different areas creating little hot spots of activity.
In order to understand why a current line is particularly a good spot for sharks and gamefish it is because these predators are attracted to the current line by the bait fish and other food. The pushing ends of a current line will generally contain all the nutrients (plankton, fish larvae etc) which have attracted the bait fish. The mixing of the currents causes the disturbed area and allows all these things to stay “trapped” in the same area until the different waters have sorted themselves out.
This situation is pretty unique and there are only a few other areas where something like this happens, California being one of them.
We have traveled to many ocean destinations around the world but we have yet to come across an area such as the waters off Cape Point that provides so many different possibilities to view a wide variety of marine life at different times of the year. This is what makes our pelagic shark trip so special. It can only be described as a modern day ocean adventure and although we are pretty certain we will find sharks out there we can happen along almost anything!