It's described as the "greatest shoal on earth" but KwaZulu-Natal's famous sardine run is occurring progressively later each year. In recent years, it has failed to happen at all. Climate change, according to a team of scientists at Wits University, is threatening this annual phenomenon.
And any delay in the sardine run will have significant implications for fisheries and tourism as the associated dolphin, whale and shark sightings are an important tourist attraction.
Their paper, Progressive Delays in the Timing of Sardine Migration, notes how the incidence of failed sardine runs has increased in frequency from 1946 until 2012.
"These changes in fish migration phenology coincide with a poleward shift in the position of the 21°C mean annual sea surface temperature (SST) isotherm (a line on a map linking places of equal temperature)," write the authors, from the school of geography, archaeology and environmental studies in the paper, which is published in the latest issue of the South African Journal of Science.
Phenology refers to the study of cyclic and seasonal natural phenomena, particularly in relation to climate and plant and animal life.
For the study, Dr Jennifer Fitchett, together with her colleagues, Prof Stefan Grab and Heinrich Portwig, explored changes in the timing of the sardine run between 1945 and 2012 and found that the delay - 1.3 days per decade - is related to regional sea surface warming, ENSO (the El Niño/Southern Oscillation) conditions and a reduced frequency of tropical cyclones, "all factors of a changing climate".
Their findings present one of the first phenological records of fish migration for the southern hemisphere, and one of the only phenological records for the southwest Indian Ocean.
"The sardine run is an annual winter migration of sardines, northeast of their summer spawning grounds on the Agulhas Bank off the coast of Durban ...The sardine run has received considerable media attention for more than 150 years in and near the city of Durban on the east coast of South Africa," they write.
"The sudden migration of sardines results in favourable fishing conditions in a region otherwise too warm for this species, and the heightened shark, dolphin and penguin activity in the region serves as a tourist attraction."
Newspapers contain a rich, temporally constrained record of the annual sardine migration, which had not yet been explored. "We investigated this record for the region, over the period 1947–2010. In so doing, we aimed to quantify the phenological shift of the sardine run over the given period, and interrogate a range of ocean–atmospheric factors that may explain any changes in the timing of this phenological event in the context of global climate change related phenological shifts."
Their analysis of the first arrival dates of sardines reveals a 1.3 day per decade delay over the period 1946–2012. "Although this phenological shift reveals a poor association with SST, it coincides with a poleward shift in the position of the 21 °C mean annual SST isotherm – the threshold temperature for sardine populations. The timing of sardine arrivals near Durban corresponds closely with the number of mid-latitude cyclones passing over the Durban coastline during the months of April and May.
"The strength of the run is strongly associated with ENSO conditions. The complex suite of factors associated with this phenological shift poses challenges in accurately modelling the future trajectory for this migratory event."
They note how a substantial body of literature has highlighted both advances and delays in phenological events as a direct response to global scale warming over recent decades, confirming most importantly "that such shifts are both location and species specific in nature.
"This, in turn, results in mismatches between flowers and pollinators predators and prey and food supply (or food peaks) and demand during breeding and feeding young offspring."
However, unlike plants which remain in a fixed location throughout their lifespan, and which can thus be observed daily on an individual basis, migratory events can often only be measured for the migrating population as a whole."
For the uncertain future of the sardine run, the authors cite three key factors of concern. "The first, in the short term, is the variability in timing of the sardine run, and the consequent lack of predictability, which can have a detrimental effect on the fishing and tourism sectors of the Durban region.
"While this study confirms and quantifies the rate of delay in arrival date, this shift is coupled with a significant increase in the variability in arrival dates and recent failure of arrivals. In the medium term, the increased incidence and last few years of failed runs presented in this study is of concern."
They note how it is unclear from the records available whether these incidents of failed runs represent a year in which the population did not migrate from the Agulhas Bank, a run in which the migration did not extend as far northeast as the Durban region, or a run which was thermally restricted to deeper waters, and thus not visible from the coastline.
"However, should this trend continue, it would heighten the probability that the run may have collapsed altogether, or shifted considerably further south. The delayed timing in the short term and possible collapse of the migration pattern in the long term present a third concern at an ecosystem scale, which relates to the predator–prey mismatches which may ensue."