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What is the history of the South African shelf?

This case study has focused on the Benguela Current with the aim of establishing whether physical forcing caused the dramatic changes in productivity over the last century. Having assembled an array of relevant historical data from government-generated sources (the ‘Blue Books'), the research team have compiled a volume of analytical papers (edited by Charles Griffiths) will shortly be submitted for publication. The project will be completed once the data has been modeled.

Here are answers to two major questions by the HMAP team leader Dr Charles L Griffiths:


Right whales
Right whales


How have Benguela marine mammal and seabird populations fluctuated with time?
Before colonization by Europeans, the coastal waters of Southern Africa supported dense populations of southern right whales and the few offshore islands teamed with breeding colonies of Cape fur seals, African Penguins and other sea birds. These large and readily accessible animals were obvious targets for early settlers, who exploited them for food, oil and other products, such as baleen and eggs. As a result these populations have undergone dramatic fluctuations over the past 300 years.

Graf 1

Whale and seal populations were driven to low levels by the end of the 19th century, but have subsequently shown remarkable recoveries. At its lowest point, in the 1930s, the South African southern right whale population is thought to have contained a few as 35 adult females, but since becoming fully protected (effectively from about 1971) the population has been growing at a rate of about 7% per annum and now numbers over 3000 individuals. Since the females calve in inshore bays, where they are readily observed from the shore they now form the basis of a flourishing whale watching industry.  Common though this species now is, it is remarkable to realise that pristine populations were probably 10 times this level, at which time every bay must have teamed with whales!

graf2

Seals continued to be exploited well into the 20th century, with over 3 million individuals harvested between 1900 and 2001, but despite this the population has grown from only about 100,000 in 1900 to between 1.5 and 3 million today. One of the main reasons for this is that seal colonies were previously restricted to the few islands in the region, with mainland predators preventing the establishment of colonies there. With the elimination of most predators, the largest colonies are now found on the mainland, where breeding space is essentially unlimited. Seals also scavenge from commercial fishing vessels.

Penguins
Penguins

African penguin have been protected since the 1960s, but have fared less well. This is partly because the guano caps that formed their preferred nesting habitats have been permanently removed from the islands, but is also partially due to their vulnerability to oil pollution and because their preferred food, pilchards and anchovies, have been depleted by the commercial fishing industry.
by Charlie Griffiths

A midden

A midden of abalone shells

Who catches abalone?
South African abalone stocks have been exploited by subsistence fishers for at least 125,000 years, but the commercial fishery only became established in the 1950s.  Although this fishery expanded rapidly in the 1960s and, in the absence of any limiting quota, reached unsustainable levels, the introduction of catch limits from about 1970 resulted in a stable and sustainable take.


Graf3

From about 1994, however, the stock has faced two problems – uncontrolled poaching and ecological change – and is now in severe crisis.  Abalone poaching is carried out by organized and well armed gangs and the illegal catch is thought to be at least as large as that taken by the entire legal fishery! If this was not bad enough, the most productive regions within the fishery have recently been invaded by dense populations of rock-lobsters. These have consumed most of the smaller invertebrates in the region, notably the sea urchins, under the spiny canopy of which juvenile abalone used to shelter from their predators. As a result abalone recruitment in this area is now less than 10% of normal.

Graf4


Under the joint onslaught of reduced recruitment and illegal exploitation the future of the abalone fishery looks bleak. Indeed, the recreational fishery, which was once equivalent to more than 80% of the commercial catch, has now been completely closed and unless poaching can rapidly be brought under control the commercial fishery appears doomed.
by Charlie Griffts
Reference (graphs):
Grifftihs, Charlie et al. IMPACTS OF HUMAN ACTIVITIES ON MARINE ANIMAL LIFE IN THE BENGUELA: A HISTORICAL OVERVIEW in Oceanography and Marine Biology: An Annual Review 2004, 42, 303–392.

Teamleader:
Associate Professor Charlie L. Griffiths, Zoology Department, University of Cape Town, Rondebosch 7700, South Africa
Charlie Griffiths has a broad knowledge of and interest in South African coastal ecology, but his research has focused on crustacean systematics, predator-prey interactions, the ecophysiology of bivalve molluscs and coastal fisheries management.
Read more:
Grifftihs, Charlie et al. IMPACTS OF HUMAN ACTIVITIES ON MARINE ANIMAL LIFE IN THE BENGUELA: A HISTORICAL OVERVIEW in Oceanography and Marine Biology: An Annual Review 2004, 42, 303–392.

Read more:

Grifftihs, Charlie et al. IMPACTS OF HUMAN ACTIVITIES ON MARINE ANIMAL LIFE IN THE BENGUELA: A HISTORICAL OVERVIEW in Oceanography and Marine Biology: An Annual Review 2004, 42, 303–392.