A review and tests of hypotheses about causes of the KwaZulu-Natal sardine run

  • P Fréon
  • JC Coetzee
  • CD van der Lingen
  • AD Connell
  • SH O’Donoghue
  • MJ Roberts
  • H Demarcq
  • CG Attwood
  • SJ Lamberth
  • L Hutchings

Abstract

The term ‘sardine run’ is part of the cultural heritage of the South African nation and refers to a natural phenomenon that is well known to the general public but still poorly understood from an ecological perspective. This lack of understanding has stimulated numerous hypotheses, often contradictory, that try to explain why (ultimate factors) and how (proximate factors) the run occurs. Here, we provide a new definition of the term sardine run, review the various hypotheses about the run, and propose ways to test those hypotheses. Where possible, the results of tests that have been conducted thus far are presented and discussed. Our interpretation of the causes is that the sardine run most likely corresponds to a seasonal (early austral winter) reproductive migration of a genetically distinct subpopulation of sardine that moves along the coast from the eastern Agulhas Bank to the coast of KwaZulu-Natal (KZN) as far as Durban and sometimes beyond, in most years if not in every year. This eastward migration is constrained close to the coast by the thermal preference of sardine and the strong and warm offshore Agulhas Current. The run is facilitated by  the presence of a band of cooler coastal water and by the occurrence  of Natal Pulses and break-away eddies that enable sardine shoals to overcome their habitat restrictions. These enabling mechanisms are most important in the area where the shelf is at its narrowest and feature most prominently off Waterfall Bluff, which has led to the coining of the ‘Waterfall Bluff gateway hypothesis’. Based on the collection of eggs off the KZN coast, sardine remain there for several months and their westward, return migration during late winter to spring is nearly always unnoticeable because it likely occurs at depth as the fish avoid warmer surface waters. Years in which the sardine run is not detected by coastal observers could reflect either its real absence due to high water temperatures and/or other hydrographic barriers, or an eastward migration that is farther offshore and possibly deeper and is enabled by hydrographical anomalies.

Keywords: migration, proximate factors, Sardinops sagax, South Africa, ultimate factors

African Journal of Marine Science 2010, 32(2): 449–479

Author Biographies

P Fréon
IRD, UMR 212 EME, CRHMT, avenue Jean Monnet, 34209, Sète, France
JC Coetzee
Branch Fisheries, Department of Agriculture, Forestry and Fisheries, Private Bag X2, Rogge Bay 8012, South Africa
CD van der Lingen
Branch Fisheries, Department of Agriculture, Forestry and Fisheries, Private Bag X2, Rogge Bay 8012, South Africa; Marine Research Institute, University of Cape Town, Private Bag X3, Rondebosch 7701, South Africa
AD Connell
South African Institute for Aquatic Biodiversity, Private Bag 1015, Grahamstown 6140, South Africa
SH O’Donoghue
South African Institute for Aquatic Biodiversity, Private Bag 1015, Grahamstown 6140, South Africa
MJ Roberts
Oceans and Coasts, Department of Environmental Affairs, Private Bag X2, Rogge Bay 8012, South Africa
H Demarcq
IRD, UMR 212 EME, CRHMT, avenue Jean Monnet, 34209, Sète, France
CG Attwood
Marine Research Institute, University of Cape Town, Private Bag X3, Rondebosch 7701, South Africa
SJ Lamberth
South African Institute for Aquatic Biodiversity, Private Bag 1015, Grahamstown 6140, South Africa
L Hutchings
Marine Research Institute, University of Cape Town, Private Bag X3, Rondebosch 7701, South Africa; Oceans and Coasts, Department of Environmental Affairs, Private Bag X2, Rogge Bay 8012, South Africa
Section
Articles

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eISSN: 1814-2338
print ISSN: 1814-232X