L. Hutchings

Comparative trophodynamics of anchovy Engraulis encrasicolus and sardine Sardinops sagax in the southern Benguela: are species alternations between small pelagic fish trophodynamically mediated?

The results of detailed morphological, experimental, field and modelling studies on various aspects of the trophic ecology of sardine Sardinops sagax and anchovy Engraulis encrasicolus in the Benguela ecosystem are synthesised, and differences in the trophodynamics of these two species are highlighted. Anchovy possess a relatively coarse branchial apparatus; feed predominantly by particulate-feeding and maximise their net energetic gain by using this feeding mode; show higher weight-standardised clearance rates than do sardine for prey >580μm; are most efficient at assimilating nitrogen from zooplankton and excrete <50% of ingested nitrogen; feed inefficiently on phytoplankton and derive the bulk of their dietary input from larger zooplankton; and maximise their scope for growth on mesozooplankton. In contrast, sardine possess a relatively fine branchial apparatus; feed predominantly by filter-feeding and maximise their net energetic gain by using this feeding mode; show higher weightstandardised clearance rates than do anchovy for prey <580μm; are most efficient at assimilating nitrogen from zooplankton but excrete >50% of ingested nitrogen; are able to feed on phytoplankton but derive the bulk of their dietary input from smaller zooplankton; and maximise their scope for growth on microzooplankton. These differences provide compelling evidence that anchovy and sardine are trophically distinct, and indicate that the two species show resource partitioning based on zooplankton size. The implications of these trophic differences for ecosystem functioning are discussed, and support the hypothesis that species alternations between anchovy and sardine, both in the southern Benguela and in other upwelling ecosystems, are likely to be trophodynamically mediated.

Mesozooplankton dynamics in the Benguela ecosystem, with emphasis on the herbivorous copepods

Recent research developments on the ecology, dynamics and trophic position of copepods in the Benguela ecosystem are synthesized. Attention is focused on herbivorous species of the southern Benguela and how they cope with the physical and biological variability characteristic of this upwelling region. Copepods constitute on average approximately half of the total zooplankton carbon and. are most abundant during the upwelling season. They are able to maintain large population densities within local coastal upwelling areas by combining ontogenetically based vertical migration behaviour with features of the current system. Some species have developed finely tuned strategies to overcome periods of starvation between upwelling bouts by storing lipid reserves or by entering temporary developmental arrest. In situ measurements of production rates of local species are among the highest recorded for copepods. Despite an apparent excess of food, copepods exert only limited impact on the phytoplankton, removing on a...

Multiple factors affecting South African anchovy recruitment in the spawning, transport and nursery areas

Despite high primary productivity, the yield of pelagic fish in the southern Benguela is relatively low compared to that in the Humboldt system. Part of the constraint may be the ability of pelagic fish to reproduce successfully in a strongly pulsed upwelling environment, where enrichment, retention and concentration mechanisms are less compatible than in Peru-Chile. Anchovy Engraulis capensis spawn upstream of the main upwelling centres on the food-poor, thermally stratified western Agulhas Bank, over a protracted summer season (October–February) when high wind speeds of 7–8 m˙s−1 are prevalent. Eggs spawned farther east, on the central or eastern Agulhas Bank, may be subject to increased cannibalism and advective losses, whereas those spawned farther west could be susceptible to heavy advective losses offshore during periods of strong southerly winds. Copepod concentrations are negatively correlated with spawner biomass on the western Bank and are inversely linked to high rates of gonad atresia in ancho...

St Helena Bay (southern Benguela) then and now: muted climate signals, large human impact

The development of suitable reference states for ecosystem-based management requires documentation of changes in structure and functioning of marine ecosystems, including assessment of the relative importance of bottom-up and top-down processes as drivers of change. We used monitoring data available from St Helena Bay, the most productive bay and an important nursery area situated on the west coast of South Africa, during 1950–2010 to reveal changes in the abiotic and biotic components. St Helena Bay in the 1950s showed similarities to 2000–2010 in terms of wind patterns, hydrology and phytoplankton. Upwelling, oxygen and nutrient concentrations in subthermocline water displayed pronounced decadal-scale variability. Primary production in St Helena Bay is variable, but consistently higher than that on the adjacent Namaqua shelf. Zooplankton size composition and biomass in August have changed markedly since the 1950s. During 2001–2010, mesozooplankton biomass in autumn was considerably lower than in summer, probably due to predation by small pelagic fish. Pelagic fish catch patterns and distributions have altered dramatically. Conservation measures, implemented to reverse past negative human impact, have benefitted marine mammals, the abundance of which has increased in the area, but additional conservation measures are necessary to reverse the decline in African penguins Spheniscus demersus. St Helena Bay shows a muted response to long-term change in the southern Benguela, with marked decadal variability but no clear long-term trend in oceanography and biogeochemistry. Changes in ecosystem boundary conditions and fishing pressure cannot be ignored as important drivers of change in the southern Benguela since the 1950s.

Does vertical migratory behaviour retain fish larvae onshore in upwelling ecosystems? A modelling study of anchovy in the southern Benguela

A spatially explicit individual-based model (IBM) forced by 3D temperature and current fields simulated by a hydrodynamic model of the southern Benguela upwelling region was used to test two hypotheses concerning the role of diel vertical migration (DVM) by Cape anchovy Engraulis encrasicolus larvae and pre-recruits. These hypotheses were that: (1) DVM enhances alongshore transport of anchovy eggs and larvae from the spawning grounds to the nursery area while avoiding the lethal effect of low water temperatures in the upwelling system, and/or (2) DVM enhances the transport of larvae and pre-recruits from the offshore to the onshore domain of the nursery area, and then counteracts offshore advection by favouring retention. We tracked the trajectories of virtual particles in the model and calculated a pre-recruitment index as a proxy for transport success to the nursery area (onshore and offshore) and found that the index increased from 10% to 20% after the incorporation of larval vertical migration into the IBM, with virtual individuals held at depths of around 60 m showing maximal pre-recruitment index values. Hence, DVM does appear to enhance transport to the nursery area (offshore) for early and late larvae. Model outputs showed coarse-scale horizontal distribution patterns of larvae by age/size class that are similar to field observations for early, small larvae but not for large larvae and pre-recruits. Observations show that early/small larvae are located offshore whereas older/larger larvae and pre-recruits are found closer to the continental shelf and the inner nursery grounds. This disparity between model results and field observations does not support the hypothesis that DVM is one of the mechanisms involved in the onshore movement of early life-history stages, especially for large larvae.

Overview of the KwaZulu-Natal sardine run

This paper provides an introduction to, and overview of, the natural phenomenon known as the KwaZulu-Natal sardine run. Previous literature on this topic and hypotheses about the reasons why, and the mechanisms how, the run occurs are briefly synthesised and described. Papers contributing to this suite that detail more recent work on a variety of aspects of the sardine run, ranging from physical oceanography through sardine biology and ecology to socio-economic and ecological consequences, are outlined. Such studies will lead to improved understanding of the factors that regulate the timing and intensity of the run, which may permit predictions of whether it will occur, when fish will arrive on the KZN beaches, and how long it will persist. Such predictions would have substantial benefits for this ecologically and economically important event.

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

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.

Application of a chlorophyll index derived from satellite data to investigate the variability of phytoplankton in the Benguela ecosystem

The spatial and temporal variability in phytoplankton biomass in the Benguela ecosystem was investigated over an 8-year period using a chlorophyll index (Chl Index) derived from low resolution SeaWiFS ocean colour data. Monthly composite images revealed that the surface chlorophyll a concentration was high near the coast and generally decreased with distance offshore, allowing observation of the spatial extent of phytoplankton distribution in relation to coastal upwelling. The Chl Index was estimated as the integration of the chlorophyll a concentration from the coast to an offshore minimum level of 1mg m−3. The index captured both the cross-shelf structure and the major part of the chlorophyll-rich areas and was used to examine the average seasonality and annual variability in phytoplankton biomass. The data indicated that the Benguela ecosystem may be divided into two major sectors, north and south, with the divide occurring at the Lüderitz upwelling cell where the Chl Index, the chlorophyll a concentration, and the offshore extent of phytoplankton were at a minimum. The index revealed a clearer and more contrasting seasonal pattern than simple coastal averaged values of chlorophyll. The latitudinal gradient and seasonal change in the index and wind forcing showed that the northern and southern sectors were out of phase and that the Benguela ecosystem appears to have a dipole structure. The interannual variation in biomass was highlighted and showed a high variability in the dipole, including shorter periods of synchronicity during 2001 and 2002. The simple Chl Index derived for the Benguela appears to be robust and generic enough to be applied to other upwelling systems as a reasonably good index of variability in phytoplankton biomass for comparative studies.

The effect of sea temperature and food availability on the spawning success of Cape Anchovy engraulis capensis in the Southern Benguela

Data on the thermal structure, copepod biomass and production, and total number of eggs of the Cape anchovy Engraulis capensis were obtained from monthly surveys during the periods August 1993 – March 1994 and September 1994 – March 1995 on the western Agulhas Bank and off the South-Western Cape, South Africa. Previous work suggested that anchovy spawn on the western Agulhas Bank in temperatures between 16 and 19°C, where they feed predominantly on copepods. This study shows that the western Bank is a more suitable spawning area for anchovy, having greater thermal stability, a larger area of 16–19°C water and a more consistent food environment than off the South-Western Cape. Also, copepod production on the western Bank was highest in 16–19°C water. To identify factors controlling the area of this water mass, a cluster analysis was used on a suite of hydrographic variables. Three periods were identified: winter (August-September), spring (October-December) and summer (January-March), reflecting changes in...

Estimating the lipid content of pelagic fish in the southern Benguela by visual assessment of their mesenteric fat

A technique to estimate the lipid content of anchovy Engraulis encrasicolus and sardine Sardinops sagax using visual assessments of mesenteric fat is described. Anchovy are allocated to one of five and sardine to one of seven fat stages, depending upon the amount of fat associated with their intestine and stomach respectively. The accuracy of this technique was evaluated by relating body (less gonad) lipid content to fat stage and other morphometric parameters (fish length and mass) through general linear modelling; significant relationships for both species were derived. Fat stage by itself is a good predictor of anchovy body lipid content and accounts for 75% of the observed variability in body lipid content, whereas fat stage and fish mass together account for most of the variability (89%) in body lipid content of sardine. Application of the fat staging technique has revealed both spatial and temporal variability in pelagic fish condition. Spatially, the condition of anchovy recruits is higher off the South African west coast than the south coast, whereas the condition of anchovy adults increases in an eastward direction over the Agulhas Bank. Temporally, anchovy recruits exhibit marked interannual variability in their fat stage distributions, and sardine spawners show clear seasonal changes in their mean fat stage that are inversely related to their mean gonadosomatic index. This technique has the advantages of being quick and easy to apply, requires no specialised equipment and is cheap, and is therefore well-suited for use at sea. These factors, together with good reproducibility and accuracy, demonstrate its efficacy as a method for assessing the condition of pelagic fish.