Life-history traits displayed by neritic fish in the Benguela Current ecosystem

Three fish species, anchovy, hake and lanternfish, occupy different habitats in the neritic zone of the Benguela Current ecosystem. Compatibility between the predicted environmental variability in each habitat and the collection of life history traits found in each species is examined with simulation models. Variability in early-stage survival is introduced into the models by means of both random and non-random ("red") spectra. Traits exhibited by lanternfish are shown to be inefficient in filtering out random variability and the species therefore cannot prevent collapse in a non-randomly variable environment. By comparison, the traits found in anchovy and hake filter out part of both random and non-random variability in survival and allow a fairly stable population size to be maintained. It is argued that, as life-history traits are adapted to the spectrum of variability associated with the habitat, only anomalies with respect to this spectrum are likely to have a large influence on stock size.     

Biophysical models of larval dispersal in the Benguela Current ecosystem

We synthesise and update results from the suite of biophysical, larval-dispersal models developed in the Benguela Current ecosystem. Biophysical models of larval dispersal use outputs of physical hydrodynamic models as inputs to individual-based models in which biological processes acting during the larval life are included. In the Benguela, such models were first applied to simulate the dispersal of anchovy Engraulis encrasicolus and sardine Sardinops sagax ichthyoplankton, and more recently of the early life stages of chokka-squid Loligo reynaudii and Cape hakes Merluccius spp. We identify how the models have helped advance understanding of key processes for these species. We then discuss which aspects of the early life of marine species in the Benguela Current ecosystem are still not well understood and could benefit from new modelling studies.     

The ecology of sandy beaches of the Benguela ecosystem

Sandy beaches constitute nearly 46 per cent of the coastline between the Cape of Good Hope and the Orange River along the west coast of South Africa. In addition, shores of mixed sand and rock make up a further 24 per cent although these are not considered here. Sandy beaches are therefore the dominant shore type along the coastline, and most are subject to high wave energy. There are two main ecological beach types along the study coastline: those that receive a high input of organic matter in the form of stranded kelp and those that do not. Neither type appears to support large stocks of surf-zone phytoplankton, but despite this, even beaches receiving no stranded kelp bear high standing stocks of infauna. This fact may be related to the location of the beaches alongside a highly productive upwelling region. Existing ecological information on sandy beaches along the Benguela coastline is reviewed and integrated to form a composite picture of present understanding of these beaches. The definition of a sandy beach includes not only the sandy intertidal zone but also the surf zone and sand dunes associated with it. Sandy beaches are characterized by the absence of attached primary producers, although in some parts of the world primary production by surf-zone phytoplankton has been found to be important. Secondary production by the infauna usually depends on matter imported into the system, except on beaches supporting important stocks of surf-zone phytoplankton. Imported organic matter is retained by beach sediments which act as a physical sieve, filtering large quantities of water with each wave and tide.     

Environmental influence on phytoplankton communities in the northern Benguela ecosystem

An investigation of surface phytoplankton communities was undertaken on the shelf of the northern Benguela upwelling ecosystem during austral autumn (May) and spring (September), along latitudinal transects at 20° S and 23° S, from 2 to 70 nautical miles offshore, as well as on a zigzag grid located between these transects. Microscopic identification of the phytoplankton and CHEMTAX analysis of pigment biomarkers were used to characterise the community composition. During May 2014, warmer, more-saline water with a shallower upper mixed layer corresponding to periods of less-intense offshore Ekman transport was encountered on the shelf. Satellite imagery indicated high phytoplankton biomass extending for a considerable distance from the coast, and CHEMTAX indicated diatoms as dominant at most of the stations (52–92%), although dinoflagellates were dominant at some inshore localities (57–74%). Species of Chaetoceros, Bacteriastrum and Cylindrotheca were the most abundant, with abundance of the Pseudo-nitzschia ‘seriata-group’ being particularly high at a number of stations. In September 2014, more-intense wind-forcing resulted in a deeper upper mixed layer and stronger upwelling of colder, less-saline water. Elevated phytoplankton biomass was confined close to the coast, where diatoms accounted for most of the population (54–87%), whereas small flagellates, such as prasinophytes, haptophytes and cryptophytes, as well as the cyanobacterium Synechococcus, dominated the communities (58–90%) farther from the coast. It is hypothesised that stronger upwelling and deeper vertical mixing in September of that year were not conducive for widespread diatom growth, and that small flagellates populated the water column by being entrained from offshore onto the shelf in the upwelled water that moved in towards the coast.     

Spatial characterisation of the Benguela ecosystem for ecosystem-based management

The three countries of the Benguela Current Large Marine Ecosystem (BCLME), namely Angola, Namibia and South Africa, have committed to implementing ecosystem-based management (EBM) including an ecosystem approach to fisheries (EAF) in the region, to put in practice the principles of sustainable development in ocean-related matters. There is also recognition of the need for marine spatial planning (MSP) as a process for informing EBM with regard to the allocation and siting of ocean uses so that ecosystem health is ensured and trade-offs between ecosystem services are appropriately dealt with. Marine spatial planning is both an integrated and an area-based process, and this paper produces a spatial characterisation of the BCLME for achieving a common basis for MSP in the region, focusing on the oceanography, biology and fisheries. Recognising spatial variation in physical driving forces, primary and secondary production, trophic structures and species richness, four different subsystems are characterised: (1) north of the Angola–Benguela Front, (2) from the Angola–Benguela Front to Lüderitz, (3) from Lüderitz to Cape Agulhas, and (4) from Cape Agulhas to Port Alfred on the south-east coast of South Africa. Research and monitoring requirements of relevance for MSP and EBM in the region are identified, focusing on understanding variability and change, including with regard to the boundary areas identified for the system. To this end, 14 cross-shelf monitoring transects are proposed (including seven that are already being monitored) to estimate fluxes of biota, energy and materials within and between the subsystems. The usefulness of models for understanding ecosystem variability and changes is recognised and the need for fine-scale resolution of both sampling and modelling for adequate MSP as input to EBM for the often-conflicting interests of conserving biodiversity, and managing fisheries, recreation, offshore oil and gas exploration and exploitation, offshore mining and shipping routes, is emphasised.     

Mesopelagic fish derivatives in the southern Benguela upwelling region

Discussion of mesopelagic fish within the southern Benguela system necessitates recognition of both oceanic and neritic realms, demarcated by the continental shelf-break. In the eastern South Atlantic (FAO Area 47), the stock size of two oceanic, mesopelagic families (Myctophidae, Sternoptychidae) have been estimated at 8–12 million and 750 000 metric tons respectively. Each possesses pseudoceanic species, populations of which constitute a possible alternative fishery resource to conventional pelagic stocks. Of these upper-slope/shelf derivatives, Lampanytodes hectoris (Myctophidae) and Maurolicus muelleri (Sternoptychidae) are most abundant. In the southern Benguela system (Orange River mouth to Cape Point), both species appear to be restricted mainly to the region east of the offshore divergence, corresponding roughly to the continental shelf-break. Maurolicus muelleri has not been taken by the fishery, but purse-seine landings of Lampanyctodes hectoris have accounted for up to some 10 per cent of the total South African pelagic catch. Anomalies in the commercial catch (sexual disparity, sexual immaturity) have been linked to expatriatism. This phenomenon is re-examined in the light of more recent data from research sampling in the southern Benguela region, and alternative biological hypotheses are presented to explain the anomalies. It is suggested that the energy "bonus" derived from vertical migration in summer is enhanced both by a physical component, termed "cycle riding", and by a biochemical component, linked to long-chain wax esters. Seasonal variation in percentage lipid concentration also appears to be associated with reproductive activity in the late winter-spring spawning period. Vertical migration may be reduced at this time, in tandem with the cessation of upwelling within the system and paucity of food in the upper water column. Comparative data for Maurolicus muelleri are presented.     

Diet of sardine Sardinops sagax in the southern Benguela upwelling ecosystem

The diet of sardine Sardinops sagax in the southern Benguela was investigated by microscopic examination of stomach contents. The relative dietary importance of prey size and prey type was assessed by calculating the carbon content of prey items. Sardine is an omnivorous clupeoid, ingesting both phytoplankton and zooplankton, with the relative importance of these two food types varying both spatially and temporally. Stomach contents were numerically dominated by small prey items, principally dinoflagellates, followed by crustacean eggs, cyclopoid copepods, calanoid copepods and diatoms. Virtually all prey items ingested by sardine were <1.2 mm maximum dimension, the particle size below which sardine only filter-feed. Despite the numerical dominance by phytoplankton, zooplankton contributed the major portion to sardine dietary carbon, small calanoid and cyclopoid copepods, anchovy eggs and crustacean eggs being the primary prey types. These results indicate that, like anchovy Engraulis capensis, sardine in the southern Benguela are primarily zoophagous, and contrast with earlier dietary studies on sardine in the region. However, the two species appear to partition their prey on the basis of size; sardine consume small zooplankton, whereas anchovy consume large zooplankton. This difference has been observed in other upwelling ecosystems where the two genera co-exist and is likely to contribute to the regime shifts observed between sardine and anchovy.     

Pigment signatures of phytoplankton composition in the northern Benguela ecosystem during spring

Pigment indices were used to investigate the distribution and composition of phytoplankton in the northern Benguela during the austral spring of 2000, with sampling being conducted on five transect lines between 19°S and 25°S and at other inshore stations on the shelf. Total chlorophyll a concentrations (TChla) at the surface ranged from 18.4mg m^?3 at shallow inshore stations to 0.58mg ^m?3 at offshore localities in deeper water. Phytoplankton communities in the inshore environment were characterised by elevated biomass levels and the dominance of diatoms (high Fuc_DP indices). These diatom populations were associated with cool, higher density, nutrient-rich waters near the coast. Phytoplankton biomass declined with increasing distance offshore and the communities changed to a mixed population where small flagellates were generally dominant (high Flag_DP indices). These flagellate communities were associated with warmer, lower density waters. Whereas dinoflagellates (Per_DP Index) and prokaryotes (Zea_DP Index) were observed throughout the region, their contribution to the communities was of secondary importance.     

Application of the sequential t-test algorithm for analysing regime shifts to the southern Benguela ecosystem

Long-term ecosystem changes, such as regime shifts, have occurred in several marine ecosystems world-wide. Multivariate statistical methods have been used to detect such changes. A new method known as the sequential t-test algorithm for analysing regime shifts (STARS) is applied to a set of biological state variables as well as environmental and anthropogenic forcing variables in the southern Benguela. The method is able to correct for auto-correlation within time-series by a process known as prewhitening. All variables were tested with and without prewhitening. Shifts that were detected with both methods were termed robust. The STARS method detected shifts in relatively short time-series and identified when these shifts occurred without a priori hypotheses. Shifts were generally well detected at the end of time-series, but further development of the method is needed to enhance its performance for auto-correlated time-series. Since 1950, two major long-term ecosystem changes were identified for the southern Benguela. The first change occurred during the 1960s, caused predominantly by heavy fishing pressure but with some environmental forcing. The second change occurred in the early 2000s, caused mainly by environmental forcing. To strengthen these findings, further analyses should be carried out using different methods.     

Zooplankton dynamics in the northern Benguela ecosystem,with special reference to the copepod Calanoides carinatus

Main features of the zooplankton distribution and the ecological characteristics of the dominant species in the northern Benguela during different phases of upwelling are discussed. The composition of the zooplankton between 17 and 27°S was similar each year. Among the 20°30 most abundant species, 3°4 copepods dominated, influencing the distribution of total zooplankton biomass. During quiescent upwelling, zooplankton abundance was low and there were no significant differences in the inshore-offshore distribution of zooplankton biomass, the maximum occurring over the slope. During active upwelling, zooplankton biomass increased significantly, the maximum over the shelf being constituted almost entirely of developmental stages of herbivorous copepods. Over the inner shelf, all stages of the copepod Calanoides carinatus were feeding actively, removing up to 5 per cent per day of the standing stock of phytoplankton. Comparison of daily ration, respiration rate and biochemical composition of C. carinatus revealed active storage of energy inshore. Offshore populations of C. carinatus, found deeper than 200 m, comprised mainly copepodite stage V, which were not feeding and were characterized by decreased mobility and respiration and a high lipid content. It is estimated that the energy stored during active upwelling enables copepods to survive up to six months without any additional source of energy.     

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 average <25 per cent of that available daily. Indirect estimates of carbon flux indicate that 11–25 per cent of copepod daily ration is used for egestion of faecal pellets. Copepods are the preferred prey of a wide variety of invertebrate and vertebrate predators. Large copepods in particular are important in the diet of commercially exploited pelagic fish. Localized areas of low abundance of copepods have been found in association with high densities of anchovy during peak spawning and recruitment periods. Copepods may therefore constitute a central limiting factor for pelagic fish production in the southern Benguela.     

Birthdate distribution of juvenile anchovy Engraulis capensis caught in the southern Benguela ecosystem

Birthdate distributions of anchovy recruits caught during research surveys in June 1985 and June 1989 were compiled from weighted length frequency distributions, aged samples and length frequencies from commercial catches. Birthdates ranged from July to March with a peak in October for both year-classes. However, birthdate distribution for the 1989 year-class showed comparatively few fish spawning after October 1988. Also, the growth rate of recruits was slower in 1989 than in 1985. It is proposed that the very weak 1989 year-class of anchovy and the low spawning biomass subsequently observed were a result of poorer-than-average egg production and survival of prerecruits spawned after October 1988, and the slow rate of growth of juvenile anchovy recruited till June 1989.     

Phytoplankton absorption and pigment adaptation of a red tide in the Benguela ecosystem

Phytoplankton absorption and pigment characteristics of a red tide were investigated in coastal waters of the southern Benguela. Diagnostic indices indicated that dinoflagellates were the dominant phytoplankton group, with diatoms and small flagellates being of secondary importance. Very high biomass was observed close to the coast where chlorophyll a concentrations of up to 117 mg m^–3 were measured. Both measured (a _ph) and reconstructed pigment absorption (a _pig) displayed an increasing trend with chlorophyll a, while the package effect index (Q^* _a) decreased, indicating increased packaging with an increase in biomass. Proportioning of the total pigment absorption between 400 and 700 nm revealed that chlorophyll a accounted for 39–65% of the absorption, while photosynthetic carotenoids (15–30%) and chlorophyll cs (15–30%) were also prominent in absorbing light for photosynthesis.     

Interaction between Cape hake spawning and the circulation in the northern Benguela upwelling ecosystem

Cape hake in Namibian waters are demersal and mesopelagic spawners, spawning peaking offshore between 100 and 400 m deep, depending on local environmental conditions. The cross-shelf circulation, low-oxygen layers and mesoscale gyres are three important environmental factors influencing hake spawning behaviour and subsequent transport of the spawning products. Normally, hake spawn offshore near the bottom at depths of 150–400 m. However, during one cruise, spawning was concentrated below several subsurface mesoscale gyres, resulting in reduced dispersion of the eggs and larvae. When the low-oxygen layer above the bottom is pronounced, hake spawning has been observed close to the top of the layer at oxygen concentrations as low as 0.2–0.3 m? ?^?1. The relatively small size of the eggs and their high specific gravity make them ascend quite slowly from the spawning depths, 10–40 m per day. Consequently, hake eggs spawned deeper than 200 m hatch before they reach the upper mixed layer. The newly hatched larvae are relatively undeveloped, without functional eyes or mouth, and display little swimming activity during their first hours, but laboratory observations have revealed subsequent periods of downward swimming activity. Based on current field observations, on buoyancy measurements of eggs and larvae and on observed larval behaviour, it is concluded that hake eggs and larvae are transported onshore by features of the upwelling subsurface circulation that compensate for offshore movement of surface water. This may be the basic mechanism concentrating early juvenile hake nearshore. Spawning activity near the low-oxygen layer might be a behavioural adaptation to minimize egg predation, because few other species are expected to survive such low concentrations of oxygen.     

Estimates of standing stock,production and consumption of meso- and macrozooplankton in the Benguela ecosystem

Attempts are made to consolidate current information on estimates of standing stock, production and consumption of meso- and macrozooplankton from the shelf region of the west and south coasts of southern Africa for inclusion in a network analysis of carbon flow in the Benguela system. The meso- and macrozooplankton communities consist chiefly of copepods and euphausiids respectively. Although geographic and seasonal differences in standing stock are reasonably well described for the inner shelf (<200 m), knowledge of production and rate processes affecting standing stock is limited, and extrapolation to the whole shelf area (<500 m) provides only a crude appraisal of the real situation. Despite the uncertainties, direct measurements have improved on estimates previously inferred. It seems likely that grazing by meso- and macrozooplankton plays a minor role in phytoplankton losses in the Benguela system. However, it would appear that predation by macrozooplankton, particularly under swarming conditions, has an appreciable impact on mesozooplankton biomass.     

Assessment of the likely sensitivity to climate change for the key marine species in the southern Benguela system

Climate change is altering many environmental parameters of coastal waters and open oceans, leading to substantial present-day and projected changes in the distribution, abundance and phenology of marine species. Attempts to assess how each species might respond to climate change can be data-, resource- and time-intensive. Moreover, in many regions of the world, including South Africa, species may be of vital socioeconomic or ecological importance though critical gaps may exist in our basic biological or ecological knowledge of the species. Here, we adapt and apply a trait-based sensitivity assessment for the key marine species in the southern Benguela system to estimate their potential relative sensitivity to the impacts of climate change. For our analysis, 40 priority species were selected based on their socioeconomic, ecological and/or recreational importance in the system. An extensive literature review and consultation with experts was undertaken concerning each species to gather information on their life history, habitat use and potential stressors. Fourteen attributes were used to estimate the selected species’ sensitivity and capacity to respond to climate change. A score ranging from low to high sensitivity was given for each attribute, based on the available information. Similarly, a score was assigned to the type and quality of information used to score each particular attribute, allowing an assessment of data-quality inputs for each species. The analysis identified the white steenbras Lithognathus lithognathus, soupfin shark Galeorhinus galeus, St Joseph Callorhinchus capensis and abalone Haliotis midae as potentially the most sensitive species to climate-change impacts in the southern Benguela system. There were data gaps for larval dispersal and settlement and metamorphosis cues for most of the evaluated species. Our results can be used by resource managers to determine the type of monitoring, intervention and planning that may be required to best respond to climate change, given the limited resources and significant knowledge gaps in many cases.     

Macroparasites of angelfish Brama brama (Bonnaterre, 1788) in the southern Benguela Current ecosystem

The angelfish Brama brama is a mesopelagic species distributed circumglobally in temperate to warm-temperate waters, including continental-shelf-edge and upper-slope waters of the Benguela Current ecosystem. Little is known about the parasite assemblage of Benguela B. brama, with only three parasite taxa having previously been documented from this species in the southern Benguela. This study describes the macroparasites recorded from 35 B. brama collected during research surveys off the west coast of South Africa in 2015 and 2016. A total of six macroparasite taxa were documented, including the nematode Anisakis pegreffii, the copepod Hatschekia conifera, the cestode Hepatoxylon trichiuri, an acanthocephalan from the genus Rhadinorhynchus, a monogenean from the family Diclidophoridae, and an unidentified species. Three of these (He. trichiuri, Rhadinorhynchus sp. and the unidentified species) had not previously been found to infect B. brama. The most prevalent macroparasite taxa were A. pegreffii (94%), the unidentified species (71%) and Ha. conifera (60%). Two of the parasites, Ha. conifera and He. trichiuri, showed seasonal variation in infection, and infection with the latter was positively correlated with host length. These findings increase our knowledge of B. brama biology and contribute to our understanding of the biodiversity of the southern Benguela ecosystem.