Age and growth of Cape stumpnose Rhabdosargus holubi (Pisces: Sparidae) in the Eastern Cape,South Africa

Rhabdosargus holubi is a small (maximum weight=2.4?kg) yet important fishery species in the estuaries of the south-east coast of South Africa. Little is known of its biology and specifically its growth rate, which is essential for sustainable management of the fishery. We examined and counted the opaque zones in the sectioned otoliths of 134 R. holubi to determine its age and growth parameters. The otoliths from two recaptured fish marked with oxytetracycline confirmed that one opaque zone was deposited annually. The species reached a maximum age of 18 years and growth was adequately described by a von Bertalanffy growth function of the form: L_t = 358.1 (1 – e^{?0.24(t+0.77)}) mm fork length. There were no significant differences between any of the male and female growth parameters (likelihood ratio test: p = 0.3). The growth was slow (omega index: ω = 86.56); however, despite this, the unique life history of R. holubi may provide a degree of resilience to heavy fishing pressure in estuaries.     

Colonization,adaptations and temporal changes in diversity and biomass of a phytoplankton community in upwelled water off the Cape Peninsula,South Africa,in December 1979

The colonization, adaptations and temporal changes in the species diversity and biomass of phytoplankton in an upwelling plume off the Cape Peninsula are described. Certain community characteristics such as biomass, diversity and growth rates were investigated so that successional stages as a result of environmental changes could be characterized. A mixed phytoplankton bloom comprising 49 species developed with Chaetoceros compressus Laud, and Skeletonema costaium (Grev.) Cleve the dominant species. The factors responsible for these species successfully colonizing and dominating are examined with respect to their specific selective adaptations for growth. An attempt is made to determine the mechanism whereby these species dominated, by proposing several possible adaptations in terms of cell size, growth, nutrient absorption and buoyancy. Increase in cell size along the drogue trajectory is considered as a strategy in adapting to the changing environmental conditions.     

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.     

The influence of dissolved petroleum hydrocarbon residues on natural phytoplankton biomass

The effect of dissolved petroleum hydrocarbons in the environment on phytoplankton biomass measured as chlorophyll a was studied near the oil tanker route in the southern Bay of Bengal. In the transect from 5° N, 77° E to 5° N, 87° E the concentrations of dissolved petroleum hydrocarbons were negatively correlated with phytoplankton biomass, whereas in the 0° N, 87° E to 1° N, 79° E transect they were positively correlated with phytoplankton biomass. The mean petroleum hydrocarbon concentrations in the two transects were 12·12 ± 4·67 μg litre⁻¹ and 11·23 ± 4·5 μg litre⁻¹, respectively.It is surmised that the effect of dissolved petroleum hydrocarbons on phytoplankton biomass varies depending on the nature rather than the quantity of petroleum hydrocarbons present. Culture studies with unialgal Nitzschia sp. in seawater collected from selected stations in the study area as well as in artificial seawater spiked with the water-soluble petroleum hydrocarbon fraction of light Arabian Crude support this.     

Information on currents in the Cape Peninsula area,South Africa

Statistical information and some interpretative detail, based on currents measured at 28 mooring sites between Cape Point and Yzerfontein since 1981, are presented. The most striking feature is the continual presence of wavelike reversals of current close to the coast and in deeper water offshore. These currents have a periodicity of 3–4 days and exhibit a net poleward transport. The energy at these frequencies is greater than that at tidal frequencies and dominates the flow. The observation of equatorward flow over the 230-m contour confirms the presence of a jet there, identified previously by drogue tracks and current profiling studies.     

Saldanha Bay,South Africa I: the use of ocean colour remote sensing to assess phytoplankton biomass

The efficacy of ocean colour remote sensing in assessing the variability of phytoplankton biomass within Saldanha Bay is examined. Satellite estimates of chlorophyll a (Chl a) were obtained using the maximum peak-height (MPH) algorithm on full-resolution (300?m) data from the Medium Resolution Imaging Spectrometer (MERIS). Subsurface Chl a maxima often occur within Saldanha Bay below the mean detection depth of the satellite (1.5?m) during periods of thermal stratification. Consequently, the MPH product was poorly correlated to in situ data from 4?m depth (r² and average relative percentage difference [RPD] of 0.094 and 53% respectively); however, the coefficient of determination was much improved if limited to in situ data collected under conditions of mixing (r² and RPD of 0.869 and 89%, respectively). Composites of monthly MPH Chl a data reveal mean concentrations consistent with in situ seasonal trends of phytoplankton biomass, confirming the capability of the MPH algorithm to qualitatively resolve surface Chl a distribution within the bay.     

Dynamics of recruitment of larval and juvenile Cape stumpnose Rhabdosargus holubi (Teleostei: Sparidae) into the Swartkops and Sundays estuaries,South Africa

Recruitment of early life stages into estuaries is an integral part of the life cycle of many marine fish species. Although estuaries are naturally environmentally dynamic, they also are subject to anthropogenic disturbances, including land use and climate change, which may affect recruitment. Rhabdosargus holubi is an endemic marine-spawning species predominantly associated with freshwater-rich estuaries which serve as nursery areas for postflexion larvae and juveniles. This study assessed the effect of environmental variables on the dynamics of recruitment of R. holubi larvae and juveniles into the Swartkops and Sundays estuaries, South Africa. Over a period of two years, fyke nets were set at each estuary mouth to monitor movement into the estuaries at each tidal phase over a 24 h cycle during two sampling sessions per season. Rhabdosargus holubi larvae recruited into estuaries primarily in summer and autumn and during the ebb tide at night, while juvenile movements showed no pattern. Salinity, turbidity and temperature were significantly important factors affecting R. holubi recruitment, with pH having no significant effect. Turbidity and salinity are affected by rainfall and freshwater abstraction which may thus influence recruitment. Rhabdosargus holubi is dependent on estuaries, so a combination of future changes in turbidity, salinity and temperature due to predicted climate change may place populations at risk.     

Bycatch in the gillnet and beach-seine fisheries in the Western Cape,South Africa,with implications for management

Interview questionnaires and access point surveys were conducted in order to describe and quantify the catch composition of the inshore net-fisheries in the Western Cape, South Africa. A total of 138 562 fish, representing 29 species from 20 families, was recorded in 141 monitored commercial gillnet fishing operations between February 1998 and October 1999. Numerically, the legal target species, harders Liza richardsonii, dominated the catches, contributing 94.87% of the total gillnet catch. Elf, Pomatomus saltatrix, horse mackerel Trachurustrachurus capensis, gurnard Chelidonichthys capensis and barbel Galeichthys feliceps were the most common bycatch species, and contributed 4.2% to the total catch numerically and occurred in 12–47% of the marine 44–64-mm gillnet catches that were monitored. Five species most frequently targeted by shore-anglers on the West Coast: galjoen Dichistius capensis, white stumpnose Rhabdosargus globiceps, hottentot Pachymetopon blochii, silver kob Argyrosomus inodorus and white steenbras Lithognathus lithognathus, also occurred in gillnet catches. Most of the bycatch consisting of immature, undersized fish that were often injured during entanglement and were not released alive. L. richardsonii also numerically dominated the beach-seine hauls that were monitored (>99%) with only four bycatch species being recorded in low numbers. Beach-seine questionnaire respondents, however, reported sporadic catches of at least 17 bycatch species, including occasional appreciable catches of the important linefish species L. lithognathus and A. inodorus.     

Occurrence and feeding of three shark species,Carcharhinus brachyurus,C. obscurus and Sphyrna zygaena,on the Eastern Cape coast of South Africa

The shark species Carcharhinus brachyurus, C. obscurus and Sphyrna zygaena, which attain more than 2 m when adult, occur in coastal waters of the Southern Cape as juveniles. They use these coastal waters as nursery areas and their relative abundance there varies through the year, possibly because of seasonally varying oceanographic conditions. Adult C. brachyurus occur in the same geographic area but usually deeper, although there is some overlap in juvenile and adult depth ranges. Adult C. obscurus prefer the warmer subtropical and tropical waters north of the study area. Adult S. zygaena are found over deep reefs at the edge of the continental shelf. There was considerable dietary overlap between the three species, the prey of all three being dominated by small schooling fish, such as pilchard, and squid. For the three species, animals of total length >2 m took larger prey, such as elasmobranchs, with greater frequency, although they continued to prey on squid and schooling fish. It is postulated that the nursery areas are in the study area because of the abundance of food there, and that avoidance of cannibalism would be enhanced by the use of such nurseries.     

Impact of predation by Cape fur seals Arctocephalus pusillus pusillus on Cape gannets Morus capensis at Malgas Island,Western Cape,South Africa

Cape fur seals Arctocephalus pusillus pusillus were estimated to kill some 6 000 Cape gannet Morus capensis fledglings around Malgas Island in the 2000/01 breeding season, 11 000 in 2003/04 and 10 000 in 2005/06. This amounted to about 29%, 83% and 57% of the overall production of fledglings at the island in these breeding seasons respectively. Preliminary modelling suggests this predation is not sustainable. There was a 25% reduction in the size of the colony, the second largest of only six extant Cape gannet colonies, between 2001/02 and 2005/06. There has been a large increase in predation by Cape fur seals on seabirds around southern African islands since the mid-1980s, coincidental with both an increase in the seal population, altered management of the islands and an altered distribution of prey for gannets and seals. At Malgas Island, most gannet fledglings were killed between 10:00 and 18:00, the period when most are in the water around the island, from mid-January to mid-March, the main fledging period. The Cape gannet is classified as Vulnerable.     

Biochemical adaptation of phytoplankton to salinity and nutrient gradients in a coastal solar saltern,Tunisia

The distribution of protein and carbohydrate concentrations of the particulate matter (size fraction: 0.45–160 μm) was studied, from 22 January 2003 to 02 December 2003, in three ponds of increasing salinity in the Sfax solar saltern (Tunisia). The coupling of N/P: DIN (DIN = NO₂⁻ + NO₃⁻ + NH₄⁺) to DIP (DIP = PO₄³⁻) with P/C: protein/carbohydrates ratios along salinity gradient allowed the discrimination of three types of ecosystems. Pond A1 (mean salinity: 45.0 ± 5.4) having marine characteristics showed enhanced P/C ratios during a diatom bloom. N/P and P/C ratios were closely coupled throughout the sampling period, suggesting that the nutritional status is important in determining the seasonal change in the phytoplankton community in pond A1. In pond A16 (mean salinity: 78.7 ± 8.8), despite the high nitrate load, P/C ratios were overall lower than in pond A1. This may be explained by the fact that dinoflagellates, which were the most abundant phytoplankton in pond A16 might be strict heterotrophs and/or mixotrophs, and so they may have not contributed strongly to anabolic processes. Also, N/P and P/C ratios were uncoupled, suggesting that cells in pond A16 were stressed due to the increased salinity caused by water evaporation, and so cells synthesized reserve products such as carbohydrates. In pond M2 (mean salinity: 189.0 ± 13.8), P/C levels were higher than those recorded in either pond A1 or A16. N/P and P/C were more coupled than in pond A16. Species in the hypersaline pond seemed paradoxally less stressed than in pond A16, suggesting that salt-tolerant extremophile species overcome hypersaline constraints and react metabolically by synthesizing carbohydrates and proteins.     

Assessment of the recreational linefishery in selected Eastern Cape estuaries: trends in catches and effort

Boat-based creel surveys were used to collect catch and effort data on the shore- and boat-based linefisheries of the Kromme, Gamtoos, Swartkops, Sundays, Bushmans, Kariega, Kowie and Great Fish estuaries in the Eastern Cape, South Africa, from January 1996 to April 1997. The surveys, which were performed primarily during routine fisheries law enforcement patrols by a regional conservation agency, recorded the catch and effort of 2 468 individual angler outings (12 840 angling hours) during 337 angler-count patrols. Although 26 species were recorded, a large proportion of the catch (85%) comprised only four species. Spotted grunter Pomadasys commersonnii (43%) was most commonly caught, followed by Cape stumpnose Rhabdosargus holubi (16%), dusky kob Argyrosomus japonicus (14%) and white seacatfish Galeichthys feliceps (12%). In terms of mass, the overall catch was dominated by A. japonicus (50%) and P. commersonnii (33%). Overall catch per unit effort (cpue) by number was highest during summer and lowest during winter, and vice versa for cpue according to mass. Angler counts showed the Sundays Estuary had the highest mean angling effort for both weekdays and weekends (10.1 and 22.9 anglers count^?1 respectively). The Gamtoos Estuary had the lowest count for weekdays (5.5 anglers count^?1) and the Kariega Estuary the lowest for weekends (9.8 anglers count^?1). Shore-based was more popular than boat-based angling (59% compared to 41%), and all estuaries had substantially higher angling effort on weekends than on weekdays. Angling with bait (93.3%) was more popular than lure- or fly-angling (6.7% combined). Considerations for management of A. japonicus in the Eastern Cape estuarine linefishery are presented.     

Spatial and temporal variability in South San Francisco Bay (USA). I. Horizontal distributions of salinity, suspended sediments, and phytoplankton biomass and productivity

The horizontal pattern of mesoscale (1–4 km) variability in salinity was a poor predictor of mesoscale patterns in chlorophyll a, suspended particulate matter, and daily primary productivity in the South San Francisco Bay estuary during spring 1987. The tidally-averaged salinity distribution varied over weekly time scales, reflecting inputs of freshwater as well as transport processes. Spatial distributions of the other quantities also varied weekly, but not in concert with the salt field. Spatial patterns of phytoplankton biomass (chlorophyll a) deviated from the salinity patterns, largely reflecting in situ production of phytoplankton biomass during the spring bloom. The tidally-averaged distribution of suspended particulate matter (SPM) was highly dynamic and responded to (1) the riverine input of suspended sediment during a freshet, (2) neap-spring variations in tidally-driven resuspension, and (3) resuspension in shallows following a period of wind mixing. Two-dimensional distributions of primary productivity P′, derived from maps of biomass and turbidity (SPM), also varied weekly, but the spatial variability of P′ was only about half that of SPM and chlorophyll. Since the magnitude and patterns of spatial variability differ among nonconservative quantities, at least in part because of local sources and sinks, we conclude that the spatial distributions of nonconservative quantities cannot be predicted from distributions of conservative tracers, such as salinity.     

Socio-economic characteristics of gillnet and beach-seine fishers in the Western Cape,South Africa

Data collected by questionnaire and telephone surveys conducted during 1998 and 1999 are used to describe the socio-economic characteristics of inshore netfishers in the Western Cape. Approximately two-thirds of netfishers work or have worked in other fishing sectors and a further 6–50%, depending on the area surveyed, are retired. Very few (0–11%) permit-holders in most areas classified their occupations as netfishers and the majority claimed to make <5% of their income from netfishing. Estimated costs and returns to net permit-holders suggest that, in most areas, commercial netfishing at current levels of catch and effort is not economically sustainable in the long-term. Only Saldanha-Langebaan gillnetters and beach-seine permit-holders, on average, manage to cover their opportunity costs and make an economic profit. The lack of profits in other areas is compelling evidence that the net fisheries are at or beyond the open access equilibrium point, suggesting that effort reduction in the order of 60% is necessary if maximum economic yield is to be obtained from the fishery. The netfisheries provide part-time employment for approximately 2 000 crew in the Western Cape. Additional economic benefits and employment directly related to the fishery in the form of equipment and fuel purchases made by fishers, maintenance of fishing gear and the sale of fish are estimated to contribute at least R15 million to the regional economy annually. Between 42 and 76% of respondents felt that their catches had declined since they had started netfishing and most felt that no new permits should be issued. Knowledge of catch restrictions among respondents was low (53–73%), indicative of a lack of communication between management and fishers, poorly defined permit conditions and a lack of enforcement. Many fishers interviewed feel it is unfair that they are restricted to catching only low-value target species and do not adhere to the catch restrictions, even if they do know them. The importance of the netfishery for participants varies greatly between and within areas. In order to reduce effort equitably, current and potential new permit-holders should be assessed on an individual merit basis.     

Tectonic evolution of the Cape and Karoo basins of South Africa

The Cape and Karoo basins formed within the continental interior of Gondwana. Subsidence resulted from the vertical motion of rigid basement blocks and intervening crustal faults. Each basin episode records a three-stage evolution consisting of crustal uplift, fault-controlled subsidence, and long periods of regional subsidence largely unaccompanied by faulting or erosional truncation. The large-scale episodes of subsidence were probably the result of lithospheric deflection due to subduction-driven mantle flow. The early Paleozoic Cape basin records the combined effects of a north-dipping intra-crustal décollement (a late Neoproterozoic suture) and a right-stepping offset between thick Rio de la Plata craton and Namaqua basement. Following the Saldanian orogeny, a suite of small rift basins and their post-rift drape formed at this releasing stepover. Great thicknesses of quartz sandstone (Ordovician–Silurian) and mudstone (Devonian) accumulation are attributed to subsidence by rheological weakening and mantle flow. In contrast, the Karoo basin is a cratonic cover that mimics the underlying basement blocks. The Permian Ecca and lower Beaufort groups were deposited in a southward-deepening ramp syncline by extensional decoupling on the intra-crustal décollement. Reflection seismic and deep-burial diagenetic studies indicate that the Cape orogeny started in the Early Triassic. Deformation was partitioned into basement-involved strike-slip faults and thin-skinned thrusting. Uplift of the Namaqua basement resulted in erosion of the Beaufort cover. East of the Cape fold belt, contemporaneous subsidence and tilting of the Natal basement created a late Karoo transtensional foreland basin, the Stormberg depocentre. Early Jurassic tectonic resetting and continental flood basalts terminated the Karoo basin.     

Comparison of the abundance of pilchard in Cape gannet diet and commercial catches off the Western Cape,South Africa

A hypothesis is presented that, in the absence of quota restrictions, commercial catches of pilchard Sardinops ocellatus in the western Cape and the abundance of pilchard in the diet of Cape gannet Morus capensis obtained at three South African colonies are directly related to pilchard biomass when the latter is low. There were significant correlations (p < 0,05) between the abundance of pilchard in gannet diet at Lambert's Bay and Malgas Island combined, and the total catch and CPUE at annual, though not monthly, time-scales. Trends in levels of correlation are discussed in relation to short-term variability, sampling intensity, statistical chance and the distribution of pilchard. Gannet diet and commercial catches reflect regional, but not local, changes in biomass. The 1983 pilchard year-class was relatively strong. Gannet diet is suggested to be a reliable monitor of the trend in pilchard biomass at low pilchard biomasses, a time at which direct surveys are least reliable.     

Growth, Production, and a Partial Energy Budget for the Amphipod, Talorchestia capensis (Crustacea; Talitridae) in the Eastern Cape, South Africa

Abstract. .A population of Talorchestia capensis was sampled in dune slacks over the period of one year. Two hatching periods, one in March and one in September, were observed. Growth was rapid, the amphipods reaching a size of 8.4 mm standard length in 18 months. Maximum growth occurred in spring and minimum growth in winter. Annual mortality rates for summer- and winter-hatched individuals were low, 0.22-y“¹ and 0.16-y”¹. Average production for the summer classes was 7.2g-y?‘ (dry mass) and for the winter classes 2.9-y“¹, with a P/B ratio of 2.25-y”’. Production values peaked during autumn and summer. Abundance and biomass estimates showed maxima during summer and minima in early spring. Reproductive output was low, 0.1 kJ ? m^-2 y“¹, indicating that 7 % of the total production is used for reproduction and 93 % for somatic production. The assimilated energy flow through the population was estimated to be 16.3 kJ m?² y?‘ and consumption about 81.6 kJ m^-2 y^_’.     

Temperature and salinity as primary determinants influencing the biogeography of fishes in South African estuaries

Relationships between key fish taxa and various abiotic factors in South African estuaries have been examined. Of the physico-chemical parameters recorded, temperature and salinity appeared to be the main factors affecting the distribution and occurrence of estuarine fishes. Tropical estuarine-associated species appear to prefer warm, turbid, brackish water conditions with many species being largely restricted to subtropical estuaries. The decline in sea temperatures as well as upwelling events limits the dispersal of tropical species into warm-temperate estuaries. Some eurythermal tropical species, however, do extend into warm-temperate estuaries. Warm-temperate estuaries are characterised by cool, more saline waters with low turbidity and are dominated by endemic taxa. Some warm-water endemic species are common in both warm-temperate and subtropical estuaries but are generally not a major component of the fish community of cool-temperate estuaries. Other endemic species appear to prefer cooler waters and occur in both warm- and cool-temperate estuaries but are uncommon in subtropical systems. Temperate species also occur in warm-temperate systems but are generally absent from subtropical estuaries. Widespread or cosmopolitan species occur in all biogeographic regions.     

Riverine influence determines nearshore heterogeneity of nutrient (C,N, P) content and stoichiometry in the KwaZulu-Natal Bight,South Africa

Riverine influences on nearshore oceanic habitats often have detrimental consequences leading to algal blooms and hypoxia. In oligo- to mesotrophic systems, however, nutrient delivery via rivers may stimulate production and even be a vital source of nutrients, as may nutrient supplements from upwelling. We investigated the nutrient content (C, N, P) and stoichiometry of sediment, and several pelagic, benthopelagic and benthic species in the KwaZulu-Natal (KZN) Bight, a narrow shelf area on the south-east coast of South Africa, bordering the Agulhas Current. Three suggested nutrient sources to the bight are the Thukela River in the central region of the bight, upwelling in the northern part and a semi-permanent eddy (Durban Eddy) in the southern part. Elemental content of the various groups studied showed significantly higher values for most groups at the site near the Thukela River. C:P and N:P were highest in the southern part of the bight, and lowest near the Thukela Mouth or at Richards Bay in the north, indicating the latter were the P-richer sites. Sediment organic matter showed lowest elemental content, as expected, and zooplankton stoichiometry was highest compared to all other biotic groups. Environmental heterogeneity played a greater role in organismal C, N and P content and stoichiometry compared to phylogeny, with the exception of the differences in C:P and N:P of zooplankton. From this bight-wide study, the higher elemental content and lower ratios at the Thukela Mouth site supported previous findings of the importance of coastal nutrient sources to the bight ecosystem. Reductions in river flow for water use in the catchment areas may therefore have negative consequences for the productivity of the entire ecosystem.     

The feeding ecology of skates (Batoidea: Rajidae) off the Cape south coast,South Africa

Six species of skates were trawled commonly on the continental shelf off the Southern Cape during routine surveys of demersal fish between 1986 and 1990. Raja miraletus is a small skate found principally in shallow (<50 m) water that feeds mainly on small crustaceans, such as mysids and macrurans. The large Raja alba occurs over most of the continental shelf, is a piscivore throughout its life and preys heavily on flatfish, especially Cynoglossus zanzibarensis. R. cf. clavata is ubiquitous on the shelf but most abundant between 50 and 150 m deep. Feeding predominantly on crustaceans, this skate switches from carids, penaeids and mysids when small to brachyurans and small benthic fish when larger. R. pullopunctata is found over a similar depth range but attains a much larger size. It eats small mysids and macrurans when small, but brachyurans and fish become more important in the diet with increasing size of the predator. R. wallacei is most commonly taken at depths of 80–>200 m, feeds initially on small crustaceans, but switches to fish, including eels, as it grows. The small Cruriraja parcomaculata is found at the edge of the continental shelf and over the upper slope; it preys on small crustaceans, including mysids, isopods and macrurans. The distribution pattern of the skates and their prey suggest overlap in prey taken, the overlap being greatest in those species which feed on crustaceans. The large biomass of skates in the area suggests that they are an important component of the demersal faunal assemblage of the continental shelf there, and also that they may have a significant impact on the benthic fauna.