The KwaZulu-Natal 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 Mozambique. Fisherman are sometimes observed singing songs while hauling in the fishing nets in typical South African style. It is estimated that the sardine run is the biggest Biomass migration in terms of numbers.
In terms of biomass, researchers estimate the sardine run could rival East Africa's great wildebeest migration.Marine Scientists Scratch Heads Over Sardines. Reuters. August 30, 2004 However, little is known about the phenomenon. It is believed that the water temperature has to drop below 21 °C in order for the migration to take place. In 2003, the sardines failed to 'run' for the third time in 23 years. While 2005 saw a good run, 2006 marked another non-run.Sardine Run
The shoals are often more than 7 km long, 1.5 km wide and 30 metres deep and are clearly visible from spotter planes or from the surface.
Sardines group together when they are threatened. This instinctual behaviour is a defence mechanism, as lone individuals are more likely to be eaten than when in large groups.
Causes
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.
Genomic and transcriptomic data indicate that the sardines participating in the run originate from South Africa's cool-temperate Atlantic coast. These are attracted to temporary cold-water upwelling off the south-east coast, and eventually find themselves trapped in subtropical habitat that is too warm for them.
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.
Oceanographic regions of the KwaZulu-Natal coast The KwaZulu-Natal coast includes varied oceanographic regions, each influenced by distinct environmental forces. The continental shelf waters of the KwaZulu-Natal Mid to Lower South coasts are dominated by the warm Agulhas Current which flows toward the south west. This water has a mean winter temperature of 23 °C and the current speed is often more than 1 m/s within 5 km of the coast. The Agulhas Current follows a very constant path. The main stream is just offshore of the continental shelf break most of the time, which suggests that conditions are normally unsuitable for sardines along that part of the coast. Local winds do not appear to have much effect on the currents. Sardine move closer to shore as they travel northwards along the coast, but it is not known whether this is due to environmental conditions or biological conditions. There is a persistent cyclonic gyre known as the Durban Eddy, where warm Agulhas Current water flows onto the shelf and the resulting inshore current direction is from south to north. This section of coast may be considered a transition from the wind-dominated section of the continental shelf to the north, to the Agulhas Current dominated section of shelf to the south. The North Coast section of continental shelf is considerably wider (>40 km) than that of the south coast (roughly 15 km). This causes the Agulhas Current to flow farther offshore, and current conditions over the shelf are more variable. Wind appears to be a dominant influence in the region. Longshore north-easterly or south-westerly winds precede currents of similar direction by roughly 18 hours. Sea temperature is often lower and nutrients higher than along the South Coast. The North Coast would seem to be more suitable habitat for sardine, but it is not known to what extent they use it.
These distinct regions may affect sardine distribution and movement.
Oceanographic variables and sardine presence Some oceanographic variables have been found useful for describing conditions influencing sardine presence.
Water temperature has an inverse and highly significant influence. This is consistent with the preferred temperature range of sardine. Sea currents have a significant effect, with calm current conditions most favourable for sardine presence and moderate current speeds from north to south most detrimental. As sardine movement during the run is northwards, this counter-current effect is expected. Other conditions associated with sardine presence are: Increasing atmospheric pressure: sardine presence appears to be higher during periods between the cold fronts along the KwaZulu-Natal coast. These periods have calm atmospheric conditions and slow nearshore currents. Large swells and low water clarity associated with cold fronts have a negative effect on sardine presence. Wind direction, wind speed, current direction, air temperature and rainfall all significantly affect sea surface temperature and consequently sardine presence. *Current and wind direction effects dominated, with north-easterly wind and currents from north to south resulting in cooler sea surface temperatures. North-easterly winds cause the surface water layer to move away from shore (Ekman veering), allowing the cool water to reach the surface, and south-westerly winds push warm Agulhas Current surface water towards the shore causing inshore temperatures to increase, which would negatively impact upon sardine presence. Increasing maximum air temperature, south-easterly (onshore) winds, wind speeds in excess of 6 m/s, and rainfall, all result in warmer sea surface temperatures. Strong south-easterly winds and rainfall are associated with the passage of frontal systems, which would push warm surface waters shoreward resulting in warmer sea surface temperatures. Frequent light north-westerly land breezes: When north-westerly land breezes are the strongest winds of the day they have a cooling effect on sea surface temperature. This cooling should be greatest in the vicinity of the surf zone where mixing is most effective. Sardine are often sighted close inshore during early mornings, suggesting that they could be attracted by cooler conditions found there.
Summary: Oceanographic predictors of sardine presence Favourable: Decreasing sea surface temperature Calm current conditions Light north-westerly land breezes Stable atmospheric conditions. Unfavourable: Increasing sea surface temperature Moderate north to south currents Large swells Turbid water North-easterly and north-westerly winds and north to south currents have a cooling effect upon nearshore sea surface temperatures, but south-easterly winds and increasing air temperatures cause nearshore sea surface temperature warming.
Predators
Dolphins (estimated as being up to 18,000 in number, mostly the common dolphin (Delphinus capensis)) 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), and birds (like the Cape gannet), and Bryde's whales take advantage of the opportunity. Other whale species, regardless of whether they do or not join the run, may appear in the vicinity such as humpback, southern right, and minke whales.Smith D.. 2017. Southern Right whales sardine run 2017 . Offshore Africa Port St. Johns. Retrieved on October 01, 2017世界の海からこんにちは～ダイバーの為の現地情報～. 2017. クチコミとダイビング自慢 - 2017/07/11 行ってきました 南アフリカ・サーディンラン No.2. Retrieved on October 01, 2017
Predators as predictors of sardine presence The Cape gannet is the predator species most closely associated with sardine presence along the Eastern Cape and KwaZulu-Natal coastline and is the most useful indicator of sardine run activity.
Sharks and large gamefish presence is also strongly associated with sardine presence during the run, but as they are not as easily observed from the surface they are not as useful a predictor of sardine presence.
The presence of common dolphins inshore along the east coast during winter is significantly associated with sardine presence, and the common dolphin can be considered the third most useful species for predicting sardine presence.
The resident population of bottlenose dolphin does not appear to associate with the sardine run, whereas the migrant stock does. This may explain why the bottlenose dolphin is less likely to predict sardine presence.
Record of predators 2005 records: In June and July 2005 the avian and mammal predators included Bryde's whale (Balaenoptera edeni), African penguin (Spheniscus demersus), Cape cormorant (Phalacrocorax capensis), which were predominantly found in the cooler southern part of the region. Peak sardine run activity occurred within 4 km of shore at the northward limit of a strip of cool water (<21 °C) stretching along the East Coast. The principal predators at this stage were common dolphins (Delphinus capensis) and Cape gannets (Morus capensis).
Economic importance
Tourism The recent interest in the sardine run has had significant impact on the local economy. International and domestic divers join local tour operators on sardine run diving expeditions. Such expeditions run from Eastern Cape towns, including East London, Port Saint Johns, and Port Elizabeth. The run has become important to tourism and is considered to be one of the main attractions in KwaZulu-Natal during the winter holiday period. Both local and international tourists are attracted to the spectacle and are provided with opportunities to participate in activities such as dive charters and boat based predator viewing tours. The KwaZulu-Natal Sharks Board and East Coast Radio, facilitate a 'Sardine Run Hotline', which provides information on the position and movement of sardine shoals. Information is also provided on the internet.
The Sardine Run Association (www.thesardinerunassociation.org) has been formed to provide a link between tour operators, tourists, non-governmental organisations,