Factors affecting the distribution and abundance of Macrobrachium petersi (Hilgendorf) in the Keiskamma River and estuary,South Africa

A dry (1979–1980) and a wet (1980–1981) season had a marked effect on the freshwater inflow into the Keiskamma estuary. Under low inflow conditions, which results in elevated salinities in the upper reaches, an upstream migration of adult Macrobrachium petersi (Hilgendorf) to freshwater takes place. During periods of increased river inflow adult M. petersi move downstream to the more saline reaches of the estuary. These two migratory responses have been interpreted as (a) a breeding migration under high inflow conditions which ensures that larvae are in close proximity to salinities that favour growth and development, and (b) an adult upstream migration back to freshwater to escape elevated estuarine salinities as a result of the low freshwater inflow.     

The effect of a storm surge event on the macrophytes of a temporarily open/closed estuary,South Africa

Temporarily open/closed estuaries typically open to the sea due to freshwater inflow coupled with storm surge events. In September 2008, in the absence of freshwater inflow, the mouth of the East Kleinemonde Estuary breached in response to a storm surge. The mouth of the estuary closed the following day at a high level. Marine overwash events following the breach introduced large volumes of saline water into the estuary and raised the water level by 0.07–0.33 m. Salinity was significantly higher in the 15 month closed phase after the breach (31 ± 0.9) compared to 21.9 ± 0.9 in the closed brackish phase before the breach. The historical average salinity for the estuary during a closed period is 23–25. The increase in salinity has reduced submerged macrophytes Ruppia cirrhosa and Chara vulgaris cover by 38.1%. Macroalgal cover of species such as Dictyota dichotoma, Caulacanthus ustulatus, Codium tenue and Ulva spp. have increased by 7.9%. The saline high water levels have also significantly reduced supratidal salt marsh cover by 15.2%, and reed and sedge cover by 19.7%. Loss of these habitats may result in bank destabilisation and erosion. This is the first record of an extended saline period in the 15 years the estuary has been monitored. Sea level rise in association with climate change, together with localised freshwater inflow reduction is likely to result in an increase in marine overwash events. The frequency and duration of closed saline periods are likely to increase in this type of estuary. A loss of submerged macrophytes may have significant impacts on faunal composition and abundance and on the subsequent functioning of temporarily open/closed estuaries. This has serious ecological implications since these estuaries represent 70% of the different types of estuaries found in South Africa.     

A comparative study of microalgal production in a marine versus a river-dominated temporarily open/closed estuary, South Africa

Temporarily open/closed estuaries (TOCEs) account for almost 71% of the total number of South African estuarine systems. To date the dynamics of microalgal production and biomass in TOCEs is poorly understood. A survey was therefore conducted in two TOCEs along the KwaZulu-Natal coast: the river-dominated Mdloti and the marine-dominated Mpenjati. The aims of this investigation were: (1) to compare the variability of benthic and pelagic microalgal production and biomass in these two contrasting systems; (2) to determine the key environmental parameters influencing primary production in these TOCEs. Results show a similar pattern of primary production and biomass in both estuaries. High benthic microalgal biomass with low primary production and low phytoplankton biomass with high rates of pelagic primary production were observed in both estuaries. Possible explanations for this include: (a) optimum light conditions in the water column, compared to the sediment surface; (b) high impact of grazing by zooplankton in the water column; and (c) settling of phytoplankton. Significant correlations were observed between primary production and environmental parameters (K_d, PAR, temperature, DIN and DIP), during the open and closed phases of both estuaries. A BIOENV (PRIMER) analysis showed that patterns of primary production in both estuaries were influenced primarily by interactions of multiple independent parameters. Comparisons between measured hourly pelagic primary production rates at the Mpenjati and Mdloti and the composite parameter (P_c) of Cole and Cloern (1987) revealed highly significant relationships (r² = 0.8; p < 0.001) between these differently derived variables.     

Foraging ecology of an endemic shorebird, the African Black Oystercatcher (Haematopus moquini) on the south–east coast of South Africa

We investigated small–medium (1–300 km) scale variation in the foraging ecology of the African Black Oystercatcher during its breeding season, using traditional diet analysis coupled with carbon and nitrogen stable isotope analysis. Fieldwork was conducted between January and March 2006 and 2007, on rocky shores on the south–east coast of South Africa at East London, Kenton and Port Elizabeth. Middens of shelled prey left by adults feeding their chicks were collected from five territories and the abundances of the collected prey on the foraging areas were estimated using quadrats. Blood samples from 45 birds (16 females, 10 males and 19 chicks) and tissues from the predominant prey species on the territory of each breeding pair were collected for isotope analysis. The Manly–Chesson selectivity index revealed that adults feed their chicks preferentially with the limpet Scutellastra cochlear and the Mediterranean mussel Mytilus galloprovincialis, if available. A slight enrichment in the ¹⁵N stable-carbon isotope signature was observed towards the west in both prey and oystercatchers. Differences in isotope signatures between males and females from the same breeding pair indicate sex-related differences in the diet. Both had signatures indicating a mixed diet, but with males exhibiting a signature closer to that of limpets and females closer to that of mussels. In the single case where mussels were rare on the feeding territory, the two members of a pair showed carbon signatures which were identical and very similar to that of limpets. These results indicate dietary partitioning between genders in breeding pairs.     

Estimation of the non-seasonal production of Spartina maritima (Curtis) Fernald in a South African Estuary

Net aerial production of Spartina maritima (Curtis) Fernald, in a warm temperature estuary in Algoa Bay, South Africa, occurring possibly as an exotic, was estimated as 523–680 g dry weight m^?2 year^?1. The method of production measurement designed here used community structure data and accounted for the species' extremely slow shoot production (3.1–6.7 dry g m^?2 year^?1) and continuous leaf turnover (516–676 dry g m^?2 year^?1). Standard methods of production measurement for other Spartina species failed by not accounting for the non-seasonal growth of S. maritima. N, C, organic and energy content of live and dead shoots remained constant through the seasons. The P : B ratio is 1.1, which is lower than for other Spartina species, but agrees with the low vigour shown by negligible regrowth a year after clipping. Decomposition rates were 90 mg g^?1 month^?1 in winter and 305 mg g^?1 month^?1 in summer. Monthly decomposition values showed significant correlation with air temperature (r = 0.86; P < 0.01), and water temperature (r = 0.80; P < 0.01), suggesting that monthly litterbag losses are a valid means of determining decomposition rate.     

Stomach content analysis of juvenile Cape stumpnose Rhabdosargus holubi in the Swartkops Estuary,South Africa

Stomach content analyses were performed on the Cape stumpnose Rhabdosargus holubi. The contents were identified to species level and the dietary importance of each component assessed by employing unrelated quantifying methods, which were subsequently combined to determine an index of relative importance (IRI). The index was determined on a seasonal basis as well as for individual sampling stations. The aquatic macrophyte Zostera capensis was the single most abundant item and, together with filamentous algae, constituted the major proportion (IRI = 7 314) of the diet. Crustacea were the next most abundant (IRI = 6 534), followed by Mollusca (IRI = 160). Stomach contents from the sampling stations in the lower reaches were dominated by aquatic macrophytes and algae, whereas those from the middle and upper reaches had a higher incidence of Crustacea. Mollusca in the diet were uniformly distributed throughout the estuary. There was considerable flexibility in the feeding pattern of R. holubi.     

Estuarine resources use by juvenile Flagfin mojarra (Eucinostomus melanopterus) in an inverse tropical estuary (Sine Saloum,Senegal)

The Flagfin mojarra, Eucinostomus melanopterus, is a marine spawner whose young individuals are common in the Sine Saloum inverse estuary (Senegal). The species offers the opportunity to study both the use of the estuarine nursery resources and the impact of the particular environment of the inverse estuary on these resources. This will lead to a better understanding of the functioning of the nursery. We investigated the resources used by juvenile Flagfin mojarra by coupling stomach contents and stable isotopes methods.     

The ichthyofauna in the Port of Ngqura,South Africa

This study provides one of the first quantitative assessments of a fish community within a pre-operational deep-water port system. Between September 2006 and September 2007 a total of 4 559 fish, representing 47 species and 27 different families, were caught within the Port of Ngqura, Africa's deepest port, in 1 713 h of shore-angling (2.3 fish angler^?1 h^?1) and 239 h of boat-based angling (2.8 fish angler^?1 h^?1). Catches included species representative of both estuarine and shore fisheries. The majority (83.0%) of species caught were marine and most were less than size-at-50% maturity (71.4%). The most abundantly caught species were dusky kob Argyrosomus japonicus (25.5%), elf Pomatomus saltatrix (24.9%), garrick Lichia amia (17.7%) and dusky sharks Carcharhinus obscurus (10.7%). Fish assemblages differed significantly (p < 0.05) between three habitats within the port: Dolosse, Quay Wall and Sandy Shore. The Dolosse habitat supported the greatest abundance (3.1 fish angler^?1 h^?1) and diversity (43) of fish species. The Port of Ngqura is thus functioning as an important habitat for both juvenile and adult fish. The diversity and abundance of fish suggests the potential use of ports and other artificial structures for enhancing local fisheries in South Africa.     

Temporal variation in the vertical distribution of microphytobenthos in intertidal flats of the Nakdong River estuary,Korea

The vertical distribution of microphytobenthos (MPB) was investigated in intertidal flats of the Nakdong River estuary over different temporal periods. The MPB biomass were measured as chlorophyll a concentration in the sediment, and monitored at two different depth scales. From August 2006 to August 2007, monthly sampling was carried out at four sites to a depth of 10 cm. The vertical distribution exhibited an exponential decline with depth and the slope was closely related to sediment composition and MPB biomass in the surface sediment. The two slightly muddy sand (5–25% mud, <63 μm) sites presented similar trends of seasonal variation to a sediment depth of 10 cm, and no statistical difference was observed in their initial values and the slope of the exponential curves applied to each data set. The site with higher mud content also had higher surface biomass, and the fitted exponential curve had a larger slope than sites with more sand content. The seasonal variations of phaeo-pigments compared to chlorophyll a revealed higher grazing stress on the MPB in summer at the three slightly muddy sand sites, and low grazing stress all year around at the sand (<5% mud) site. In January, April and October 2007, and January 2008, diurnal sampling was conducted at hourly intervals at one slightly muddy sand site. The investigation in January 2007 which was conducted at 1 cm intervals to a depth of 10 cm sediment showed the migration mainly existed in the upper 1 cm. The other diurnal sampling was undertaken to a 1 cm depth at 0.5 mm intervals. An evident migratory rhythm was present, with migration up to the surface during daytime emersion and down into deeper sediment during night and high tide submersion. Migrating cells could reach sediments as deep as 6 or even 10 mm. When the visible thick biofilms formed, the migratory rhythm changed by keeping higher biomass at the surface even at night and during high tide submersion. Generally, the MPB surface biomass was an important factor in structuring the MPB vertical distribution. The vertical distribution to depths of 10 cm was closely related to the sediment composition over the long term, while the MPB migration influenced the vertical distribution over the short term in the top 1 cm of the sediment.     

Distribution and nature of organic matter in recent sediments of Lake Nokoué, Benin (West Africa)

The distribution and nature of organic matter clearly identify the principle sectors of the lagoon which vary according to hydrological and sedimentological conditions. The central area frequently includes coarse ancient fluviatile sediments. The rare organic matter there offers aliphatic and nitrogenous characters which may be derived from both the origin of organic matter (algae and meiofauna) and the level of oxygenation prevailing during deposition. In the southern sector, highly oxygenated conditions resulting from sea currents generated by the Cotonou outlet could cause the elimination of labile compounds, and hence be responsible for the high degradation state of organic matter. In the western sector, still water conditions allow fine sediments to be deposited. This frequently leads to the accumulation of organic matter showing such reducing conditions that they allow the integration of sulphur in humic compounds and a decrease of their solubility. In the northern area, the contribution of terrestrial material supplied by the Ouémé and Sô rivers is clearly indicated by the low average percentages of hydrolyzable fraction, C/N ratios ranging between 9 and 15, while H/C and N/C ratios are low in humic acids.     

Aspects of the upwelling regime between Cape Point and Cape Agulhas,South Africa

A progressive increase in mean ovary mass (standardized by the cube offish length) over time was observed in specified length classes of pilchard Sardinops ocellatus sampled from purse-seine catches off the west coast of South Africa. This, in earlier investigations, was considered to have been a density-dependent response following the collapse of the stock in the mid 1960s, and it appeared to result mainly from the increasing frequency of occurrence of pilchard entering the batch-spawning cycle, particularly among fish of 16–20 cm L_c . The possibility that the observed trend was an artifact of changing distribution of fishing or shifting seasonality of spawning relative to the three-month period of sample collection was rejected following analysis of variance on spatially disaggregated data and inspection of seasonal patterns in ovary mass. Other causes of the increase in mean ovary mass could have been density-dependence, declining age structure, changes in the environment and selection for early maturity under prolonged high rates of mortality. The data are critically examined to evaluate the likelihood of each of these explanations.     

Studies on the population ecology of Upogebia deltaura (Leach) (Crustacea, Thalassinidea)

Upogebia deltaura was collected quantitatively and qualitatively between June 1980 and August 1982 from a level, sandy bottom at a 12 m depth in the archipelago of Lysekil (on Gullmarsfjorden) about 70 km north of Göteborg, in western Sweden. A total of 347 individuals were obtained. The deepest burrows reached a depth of more than 65 cm into the substrate. During the cold season, U. deltaura was situated much deeper in the substrate than it was during the summer, probably as a result of ‘hibernation’. The average density was 5·0 individuals 0·1 m⁻², and the maximum of 10 specimens was found in one 0·1 m² sample. Out of the 341 individuals identifiable as to sex, 186 (54·6%) were females and 155 (45·4%) were males. Berried females were found between May and August, and the average number of eggs carried was 4757. Ecdysis took place between May and August; most females moulted in the middle of June and most males about one month later, in mid July. The chelipeds of large males were proportionally bigger than were those of large females. Maximum total body length of the females was 65·7 mm, and that of the males was 65·3 mm.     

Abundance and growth of larval and early juvenile cod (Gadus morhua) in relation to variable environmental conditions west of Iceland

Around Iceland, the west- and north-flowing coastal current, induced by freshwater runoff, provides a transport mechanism for pelagic eggs and larvae derived from the main spawning grounds off the southwest coast to the main nursery grounds off the north coast. In the present study, abundance and growth of larval and juvenile cod were recorded during a series of cruises conducted in June/July of 1998–2001 along the drift route southwest and west of Iceland. The cruises provided information on approximately 2–8-week-old individuals. Hatch dates and abundance varied greatly between years. Hatch dates ranged from Julian Day 92 to 167. Growth rate differed also between the years studied. Relative abundance was generally greatest in temperatures above 7.5 °C and in low-salinity waters, characteristic for the coastal current. The study demonstrates the link between the coastal current and larval/juvenile distribution, thus providing evidence for its importance in promoting successful recruitment of the Icelandic cod stock.     

Benchmarking the first decade of post-apartheid fisheries reform in South Africa

The paper uses the three legislative criteria of resource sustainability, economic stability and social equity to measure the past decade of fisheries reform in South Africa. All three components of the reform process have been successful, although this judgement is subject to a number of important qualifications. First, the data available to measure success are limited. Second, both sustainability and stability are vulnerable to volatile factors external to the reform process (particularly variability in recruitment, climate change and exchange rate fluctuations) capable of derailing the process. Substantial alteration of the fishing industry by internal transformation of existing companies and the reallocation of access rights to new black entrants has led to a composition that closely reflects that of society as a whole and therefore meets the goal of equity. In the process, however, bona fide traditional fishers with a historical dependence on fishing have largely been denied legitimate access to the fisheries, and meeting their aspirations remains a challenge.     

The recovery of microalgal production and biomass in a South African temporarily open/closed estuary, following mouth breaching

Mouth breaching is a recurrent event in temporarily open/closed estuaries (TOCEs). Such disturbances result in flushing and sediment scouring, reducing the microalgal biomass stock. The depletion of these microalgae may have negative repercussions in the form of depleted stocks of commercial fish, game fish, crustaceans and mollusks. The aim of this investigation was therefore: (1) to monitor the recovery of microalgal biomass and production following a breaching event; and (2) to determine the key environmental parameters influencing primary production during the open and recovery phases. Phytoplankton and benthic microalgal production was measured (¹⁴C-uptake method) successively during the closed, open and recovery phases of the Mdloti TOCE (South Africa). Upon breaching, 94–99% of microalgal biomass was washed out to sea through flushing and sediment scouring. A temporary recovery of phytoplankton and benthic microalgal biomass was observed during the open phase, but this was not sustained because of continual flushing and scouring of the sediment. During the re-closure (recovery phase), microalgal biomass immediately increased, reaching pre-breaching levels 35–40 days following the breaching event. In contrast to biomass, autochthonous pelagic primary production reached a maximum level (341 mg C m⁻² h⁻¹) during the open phase. Pelagic primary production normalized to biomass (P^B) significantly increased during the open phase. This is attributed to a favorable combination of optimum light conditions, high influx of macronutrients and high water temperatures (33 °C). Similarly, benthic primary production normalized to biomass (P^B) peaked during the open phase (35 mg C mg chl-a⁻¹ h⁻¹). Multivariate analysis showed that major variations in primary production were mainly controlled by temperature, dissolved inorganic nitrogen (DIN) to phosphorus (DIP) molar ratios (water-column and pore-water) and light extinction (K_d), all of which were regulated by the state of the mouth.     

A prominent colour front in False Bay, South Africa: Cross-frontal structure, composition and origin

Colour fronts are a frequent occurrence in False Bay, South Africa, and their occurrence has been the subject of previous study and anecdotal conjecture. The opportunity arose to make a cross-frontal study of this feature in November 2005. Photographs were taken and, subsequently, satellite imagery was obtained. Measurements were made of temperature, salinity, turbidity, dissolved oxygen, suspended solids, plant nutrients and chlorophyll a. Cross-frontal comparisons were also made on particulate material using a scanning electron microscope (SEM) and elemental dispersive x-ray (EDX) analysis. Frontal waters were milky white-green in colour, in stark contrast to the adjacent clearer, green-blue waters. The milky white-green water (MW-GW) was found to be warmer, (apparently) less saline, more turbid, richer in nitrate and silicate and had a higher chlorophyll a concentration. The dissolved oxygen signal was less pronounced, both water types being supersaturated. Paradoxically, in spite of the higher turbidity in the MW-GW, both water types had similar weights of suspended solids, although the MW-GW material was found to be more abundant and fragmentary when compared with its green-blue water (G-BW) counterpart. The MW-GW was rich in calcium whereas the G-BW was silicon enriched. The central findings of this study are that the strong southerly, onshore wind conditions prior to MW-GW formation introduced calcium-rich, fine particulates into the waters of the surf-zone. The sources of these particulates are thought to be the sea bed sediments and the sea/land interface. The particulates are close to neutral buoyancy enabling the MW-GW to persist over the time-scale of days. A mechanism reinforced by the warming of this water in the nearshore zone. The water was then advected by wind-forcing and subsequently, its own inertia around the north-west corner of False Bay, at which stage it was easily observed and sampled. It is suggested that the eventual collapse of the front was due to the slowing down of inertial movement in combination with the passive sinking of fine particulates and evaporative cooling resulting in downward convection.     

Philopatry and dispersal of juvenile leervis Lichia amia (Teleostei: Carangidae) tagged in a warm-temperate South African estuary

Understanding and characterising movement and area-use patterns of fishes within estuaries, as well as understanding the degree of connectivity between estuaries and the marine environment, can provide important insights into a species’ ecology, which is fundamental for effective management and conservation. Mark-and-recapture data obtained from the 082 TAG FISH Project, a dart-tagging programme, were used to describe movement patterns of juvenile leervis Lichia amia in the permanently open Swartkops Estuary, South Africa, as well as the degree of connectivity with the adjacent marine environment. A total of 628 juvenile leervis were tagged from 2008 to 2015, of which 77 fish (12.3%) were recaptured, with juveniles comprising 85.7% of all measured recaptures. The majority of recaptures (75.3%) were made within the Swartkops Estuary, indicating estuarine philopatry, with 23.4% having moved less than 1 km, revealing site fidelity, but most (51.9%) displaying estuarine roaming. A smaller portion of recaptures (27.7%) had dispersed to other habitats; 16 (20.8%) displayed multiple-habitat connectivity, while only 3 (3.9%) had undertaken long-distance movements (>100 km) and were recaptured up to 825 km northeast of the Swartkops Estuary. Lichia amia that had moved greater distances were characterised by a greater mean size and age at the time of recapture. This study provides valuable information on the philopatry and dispersal of juvenile L. amia.