Broad-scale distribution patterns of sardine and their predators in relation to remotely sensed environmental conditions during the KwaZulu-Natal sardine run

The existence and strength of the annual KwaZulu-Natal (KZN) sardine run has long been a conundrum to fishers and scientists alike particularly that the sardine Sardinops sagax migrate along the narrow Transkei shelf against the powerful, warm Agulhas Current. However, examination of ship-borne acoustic Doppler current profiler (S-ADCP) data collected during two research surveys in 2005 indicated that northward-flowing coastal countercurrents exist at times between the Agulhas Bank and the KZN Bight, near Port Alfred, East London, Port St Johns and Durban. The countercurrent near Port Alfred extended as far east as the Keiskamma River, within an upwelling zone known to exist there. An ADCP mooring at a depth of 32 m off Port Alfred indicated that the countercurrent typically lasted a few days, but at times remained in the same direction for as long as 10 days. Velocities ranged between 20 and 60 cm s -1 with maximum values of ~80 cm s -1 . The S-ADCP data also highlighted the existence of cyclonic flow in the Port St Johns-Waterfall Bluff coastal inset, with a northward coastal current similarly ranging in velocity between 20 and 60 cm s -1 . CTD data indicated that this was associated with shelf-edge upwelling, with surface temperatures 2-4°C cooler than the adjacent core temperature (24-26°C) of the Agulhas Current. Vertical profiles of the S-ADCP data showed that the countercurrent, about 7 km wide, extends down the slope to at least 600 m, where it appeared to link with the deep Agulhas Undercurrent at 800 m. S-ADCP and sea surface temperature (SST) satellite data confirmed the existence of the semi-permanent, lee-trapped, cyclonic eddy off Durban, associated with a well-defined northward coastal current between Park Rynie and Balito Bay. Analysis of three months (May-July 2005) of satellite SST and ocean colour data showed the shoreward core-boundary of the Agulhas Current (24°C isotherm) to commonly be close to the coast along the KZN south coast, as well as between the Kei and Mbhashe rivers on the Transkei shelf. The Port St Johns-Waterfall Bluff cyclonic eddy was also frequently visible in these satellite data. Transient cyclonic eddies, which spanned 150-200 km of shelf, appeared to move downstream in the shoreward boundary of the Agulhas Current at a frequency of about once a month. These seemed to be break-away Durban eddies. Data collected by ADCP moorings deployed off Port Edward in 2005 showed that these break-away eddies and the well-known Natal Pulse are associated with temporary northward countercurrents on the shelf, which can last up to six days. It is proposed that these countercurrents off Port Alfred, East London and Port St Johns assist sardine to swim northwards along the Transkei shelf against the Agulhas Current, but that their progress north of Waterfall Bluff is dependent on the arrival of a transient, southward-moving, break-away Durban cyclonic eddy, which apparently sheds every 4-6 weeks, or on the generation of a Natal Pulse. This passage control mechanism has been coined the 'Wat...

Section: Sardine Migration and The Waterfall Bluff Gateway Hypothesissupporting

confidence: 74%

Section: Was the Gate Open Or Closed During The June-july 2005 Sardinmentioning

confidence: 99%

Shelf currents, lee-trapped and transient eddies on the inshore boundary of the Agulhas Current, South Africa: their relevance to the KwaZulu-Natal sardine run

Roberts¹,

Lingen

Whittle

et al. 2010

“…The average occurrence of eggs in samples does not show a strong bimodal pattern. Nonetheless, there is no evidence of adult sardine in the KZN area and farther north during late summer (March-April), and sardine are very seldom observed off KZN from January to May (Connell 2010, O'Donoghue et al 2010a, van der Lingen et al 2010a, which can be explained in part by the unsuitable range of water temperatures off the coast of KZN and Mozambique (Figure 7) at this time of year. Therefore, it is very likely that sardine participating in the sardine run mix with other sardine after the run, which means that the answer to T9 is negative.…”

Section: T9: the Kzn Fdaa Should Not MIX With Other Fdaasmentioning

confidence: 97%

“…The sardine run is usually associated with foraging top predators including seabirds, mammals (O'Donoghue et al 2010a(O'Donoghue et al , 2010b, and sharks and gamefish Cliff 2010, Fennessy et al 2010) that facilitate its visual detection. The north-east displacement is usually initiated from late May to early June and is mostly visually detected and commercially exploited (via beach-seine netting and tourism operations) when it occurs within a few hundred metres of the coast.…”

mentioning

confidence: 99%

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

Fréon

Coetzee²,

Lingen

et al. 2010

Self Cite

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.

“…Shore based tracking of bottlenose dolphins which were recorded as part of a humpback whale study at Cape Vidal in the early 1990s has recently been published (Photopoulos et al, this volume). Some recent data on the distribution of bottlenose dolphins along the east coast of South Africa are available as part of several broad ecological studies investigating the 'sardine run' and its associated predators (O'Donoghue et al, 2010a;O'Donoghue et al, 2010b). …”

Section: Southwest Indian Oceanmentioning

confidence: 99%

Cetacean research in the southern African subregion: a review of previous studies and current knowledge

Elwen

Findlay

Kiszka

et al. 2011

Cetacean research, in terms of the number of papers, research groups and areas for which data are available has expanded considerably in the Southern African Subregion (SAS) in the last decade, especially in the South West Indian Ocean (SWIO). Here we review cetacean research in the SAS from the 1800s to the present to provide an overview of findings, investigate trends and identify knowledge gaps. Data are presented separately for large whales (those subject to commercial whaling) and smaller cetaceans and are separated by era and ocean basin. Over 550 peer-reviewed papers and books were identified relating to research on cetaceans within the subregion. More than half (284) have been produced since 1990 and 193 relate specifically to South African waters. The most studied species are those that are most accessible due to their coastal distributions