What Is Sardine Run?
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| Sardine Run |
The sardine
run of southern Africa occurs from May through July when billions of sardines
– or more specifically the Southern African pilchard
Sardinops
sagax – spawn in the cool waters of the Agulhas Bank
and move northward along the east coast of South Africa.
Their sheer numbers create a feeding
frenzy along the coastline. The run, containing millions of individual
sardines, occurs when a current of cold water heads north from the Agulhas Bank
up to Mozam
Causes of the sardine run
Netting
a shoal of sardines (dark patch in the water)
The
sardine run is still poorly understood from an ecological point of view. There
have been various hypotheses, sometimes contradictory, that try to explain why
and how the run occurs.
A recent
interpretation of the causes
is that the sardine run is most likely a seasonal 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
in most years if not in every year.
The
migration is restricted to the inshore waters by the preference of sardine for
cooler water and the strong and warm offshore Agulhas Current, which flows in
the opposite direction to the migration, and is strongest just off the
continental shelf. A band of cooler coastal water and the occurrence of Natal
Pulses and break-away eddies make it possible for sardine shoals to overcome
their habitat constraints. The importance of these enabling factors is greatest
where the continental shelf is narrowest.
The
presence of eggs off the KwaZulu-Natal coast suggests that sardine stay there
for several months and their return migration during late winter to spring is
nearly always unnoticeable because it probably occurs at depths where the water
is cooler than at the surface.
In some
years there does not appear to be a sardine run. This may be because it is not
detected by coastal observers either because it actually does not occur due to
high water temperatures and/or other hydrographic barriers, or the migration
may occur farther offshore and possibly deeper due to unusual conditions.
Oceanographic influences
Sardine
prefer water temperatures between 14 and 20 °C.
Each southern winter the nearshore sea temperature along the South African
south east coast drops to within this range. Along the KwaZulu-Natal coast,
sardine may be found in water warmer than 20 °C. It was hypothesized that
factors beside temperature may influence the movement of sardine along the
KwaZulu-Natal coastline, One of these factors may be predation pressure.
Predators
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| Sharks take advantage of The Sardine Run |
Dolphins
(estimated as being up to 18,000 in number, mostly the Common
Dolphin Delphinus capensis, but also the Bottlenose Dolphin Tursiops aduncus) are
largely responsible for rounding up the sardines into bait balls. These bait balls
can be 10–20 metres in diameter and extend to a depth of 10 metres. The bait
balls are short lived and seldom last longer than 10 minutes. Once the sardines
are rounded up, sharks (primarily the Bronze Whaler,
but also Dusky Shark,
Grey Nurse
Shark, Blacktip Shark, Spinner Shark
and Zambezi Shark),
game fish (including Shad/Elf a.k.a. Bluefish,
King Mackerel,
Garrick,
Geelbek and Eastern
Little Tuna) and birds (like the Cape Gannet,
cormorants,
terns and gulls) take advantage of
the opportunity.
Source :
Wikipedia
