Rhabdosargus Holubi Research Articles

Recruitment of ichthyoplankton and macrozooplankton during overtopping events into a temporarily open/closed southern African estuary

The composition and short-term temporal variation in the ichthyofauna and macrozooplankton entering the temporarily open/closed (TOC) West Kleinemond estuary (33°33′S, 27°02′E) during 7 overtopping events were investigated in June 2003. A total of 84 fish representing 7 taxa from 6 families and 456 macrozooplankton representing at least 16 species was collected from water overtopping the sandbar using a custom-built funnel trap (150 μm mesh). Larvae of estuarine dependant marine species, especially the sparid, Rhabdosargus holubi, which contributed 54% to the total fish catch, dominated the ichthyofauna. Also well represented among the ichthyofauna were Monodactylus falciformis and Mugilidae spp., which together accounted for a further 40% of the total fish catch. Among the macrozooplankton, larvae of Palaemon peringueyi and the mysid Mesopodopsis wooldridgei dominated numerically and by biomass. Numerical analyses using multidimensional scaling (MDS) revealed variability in the overtopping community on a diel scale. It is suggested that recruitment through overtopping is essentially a passive process governed by the physical environment with the composition of the recruiting community being a function of the patchy dynamics of surf zone plankton. Preliminary estimates indicate that an hour-long period of overtopping over spring high tide is capable of introducing between 8000 and 33 500 individuals of R. holubi into the TOC West Kleinemond estuary. The advantages and disadvantages of recruitment using overtopping events as well as impacts on the estuarine foodweb are discussed.

Habitat use by larval fishes in a temperate South African surf zone

Larval fishes were sampled in the Kwaaihoek surf zone on the south east coast of South Africa. On six occasions between February and May 2002, larval fishes were collected in two habitat types identified in the inner surf zone using a modified beach-seine net. The surf zone habitats were classified as either sheltered trough areas or adjacent exposed surf areas. Temperature, depth and current measurements were taken at all sites. Trough habitats consisted of a depression in surf topography characterised by reduced current velocities and greater average depth than adjacent surf areas. In total, 325 larval fishes were collected. Of these, 229 were collected in trough and 96 in surf habitats. At least 22 families and 37 species were represented in the catch. Dominant families were the Mugilidae, Sparidae, Atherinidae, and Engraulidae. Dominant species included Liza tricuspidens and Liza richardsonii (Mugilidae), Rhabdosargus holubi and Sarpa salpa (Sparidae), Atherina breviceps (Atherinidae) and Engraulis japonicus (Engraulide). Mean CPUE of postflexion larvae of estuary-dependent species was significantly greater in trough areas. The proportion of postflexion larval fishes in trough habitat was significantly greater than that of preflexion stages, a result that was not apparent in surf habitat sampled. CPUE of postflexion larvae of estuary-dependent fishes was negatively correlated with current magnitude and positively correlated with habitat depth. Mean body length of larval fishes was significantly greater in trough than in surf habitats. These results provide evidence that the CPUE of postflexion larvae of estuary-dependent fishes is higher in trough habitat in the surf zone and this may be indicative of active habitat selection for areas of reduced current velocity/wave action. The implications of this behaviour for estuarine recruitment processes are discussed.

Additional morphological information on two species of Caligus (Copepoda: Caligidae) parasitic on South African marine and estuarine fish

Two species of Caligus that are parasitic on fish were collected from Lake St Lucia, South Africa.Specimens of C. confusus Pillai, 1961, were removed from the gill chambers of both Caranx sexfasciatus Quoy & Gaimard, 1825, and Rhabdosargus holubi (Steindachner, 1881), and Caligus acanthopagri Lin, Ho & Chen, 1994, from the skin of both Acanthopagrus berda (Forsskål, 1775) and R. holubi. Except for the occurrence of Caligus confusus on Caranx sexfasciatus, all of the remaining host records are new in South Africa. Caligus acanthopagri is recorded for the first time from South African waters. The morphological features of these two species were studied with scanning electron microscopy. Patterns were observed on the dorsal side of the cephalothorax of the male C. confusus. Such patterns have not previously been observed. The fine structures on the exopod of leg 4 of both species were studied, as these features are important for distinguishing species.

Occurrence of Larval and Early Juvenile Fishes in the Surf Zone Adjacent to two Intermittently Open Estuaries, South Africa

I monitored larval and early juvenile fishes in the surf zone adjacent to two warm temperate, intermittently open estuaries in South Africa. Mugilidae (59–67%) dominated fish assemblages in the surf zones adjacent to the Van Stadens and Kabeljous estuaries. Other dominant families included the Atherinidae, Sparidae and Soleidae. The most abundant species caught during the study were the catadromous mugilid Myxus capensis, in a postflexion or early juvenile stage of development, and the sparid Rhabdosargus holubi, in a postflexion stage of development. Species displaying a degree of dependence on estuaries during the early phase of their life cycle dominated (96–98%) surf zone catches. Most fish encountered were at the postflexion stage of larval development. Larval and early juvenile fishes were more abundant during the summer and autumn months with density peaking several-fold during natural estuary opening events, particularly in the surf zone adjacent to the Kabeljous Estuary. Decreases in salinity showed a significant relationship with increases in fish density during a March opening event in the surf adjacent to the Kabeljous Estuary where catches were concentrated along a current-driven estuarine plume. Catches adjacent to the Van Stadens Estuary also increased considerably during two smaller opening events in March and November although salinity played no statistically significant role in these increased catches. Results from the surf adjacent to the Van Stadens Estuary suggested that either very small amounts of estuarine water entering the surf zone can elicit an accumulation response by estuary-dependent larval and early juvenile fishes or that an additional characteristic of estuary/river water, other than salinity per se, is aiding the cueing process.

Biomass and production estimates of a fish community in a small South African estuary

The fish community of the small (17·5 ha) intermittently open East Kleinemonde estuary was sampled between 1994 and 1997 to estimate population size, standing stock, growth and productivity. The estuarine‐spawning species were numerically more abundant (n c. 750 000) but due to their small size contributed only 11·7% to the total biomass. The total annual productivity of all fishes in the estuary (n c. 890 000), with a standing stock of 28·44 g m −2, was calculated at 55·89 g m−2 year−1. The small sparid Rhabdosargus holubi with a production estimate of 41·35 g m−2 year−1 accounted for <74% of the total fish production in this estuary.

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 d...

The Surf Zone Ichthyoplankton Adjacent to an Intermittently Open Estuary, with Evidence of Recruitment during Marine Overwash Events

The composition, structure and seasonality of ichthyoplankton in the surf zone adjacent to the mouth of the intermittently open East Kleinemonde Estuary (33°32′S, 27°03′E) were investigated over a period of 2 years. Altogether 451 fishes, representing at least 21 taxa from 14 families, were collected. The assemblage was dominated by postflexion larvae of euryhaline marine species that are dependent on estuaries as nursery areas. The sparid Rhabdosargus holubi was the most abundant taxon and constituted more than 77% of the total catch. A distinct modal size class was identified forR. holubi , while the mean individual size of this and other abundant taxa was comparable to the observed recruitment size range reported from a wide variety of estuarine nursery habitats in southern Africa. Periodic regression analyses revealed significant peaks in abundance of larval R. holubi during late winter (August), at down and dusk, at new and full moon (spring tides), and on the flood stage of the tidal cycle. Evidence for estuarine immigration during marine overwash events (surging rough seas that enter the estuary) is provided by (1) the stranding of postflexion larvae in the region of the closed estuary mouth following these events, and (2) back extrapolation from length modes within the estuary to coincide with such an event. The advantages and disadvantages of such a recruitment strategy are discussed.

Stable isotope (∂13C) composition of the food web of the temperate Kariega estuary (Eastern Cape)

Stable isotope (∂13C) composition of the food web in the temperate Kariega estuary in summer and winter was analysed to determine the trophic relationships between 19 taxa. There was no seasonality in the stable isotope ratios of the dominant producers and consumers in the estuary. Two distinct zooplankton assemblages were identified. The first assemblage, comprising the different developmental stages of the copepods Acartia longipatella, A. natalensis and Pseudodiaptomus hessei and the mysid Mesopodopsis wooldridgei, had intermediate ∂13C values ranging between −16 and −18‰. These values indicate a carbon source relatively depleted in carbon, most likely particulate organic matter derived mainly from the salt marsh and riparian vegetation. The second zooplankton assemblage, consisting of the caridean shrimp Palaemon peringueyi, the cumacean Iphinoe truncata and the amphipod Grandidierella lignorum, had ∂13C values (range −9 to −12‰) enriched in carbon. The most likely sources of carbon for these zooplankters were the eelgrass Zostera capensis, its associated epiphytes and/or Spartina maritima. The isotope compositions of the seven most abundant fish species were also divided into two groups. The first group, comprising Atherina breviceps and Gilchristella aestuaria, had isotope values ranging between −14 and −15‰, suggesting that they consume mainly zooplankton. The ∂13C values of the second group, comprising Rhabdosargus holubi, Liza richardsonii, L. dumerilii, Caffrogobius natalensis and C. gilchristi ranged between −11 and −12‰. These values suggest that these species assimilate a carbon source enriched in carbon derived either from eelgrass, Zostera capensis, its associated epiphytes or S. maritima. Results of the study suggest that there are two main carbon pathways for plankton and nekton in the Kariega estuary, carbon derived from the eelgrass and its associated epiphytes and carbon which has its origins in the salt marsh riparian vegetation and zooplankton.

Fish population size estimates from a small intermittently open estuary in South Africa, based on mark-recapture techniques

Population sizes of the migratory marine-spawning fishes in the small intermittently open East Kleinemonde Estuary (33º32′S, 27º03′E) were estimated during two independent mark–recapture studies between 1994 and 1996. Seines were used to sample fishes during the closed-mouth phase of the estuary. All individuals above a stipulated minimum size were marked by fin-clipping. Population estimates were obtained by use of two mark–recapture models (Schnabel and a maximum-likelihood estimator) and a derived method. The total population size was estimated at ~18 000 and ~133 000 individuals during the two study periods respectively. The large temporal (interannual) variability was ascribed to both abiotic (estuary mouth conditions) and biotic-conditions. The dominant species during both periods was Rhabdosargus holubi, which accounted for ~75% and ~80%of the total population of marine fishes in the estuary. The success of this species is attributed to an extended breeding season and the ability to recruit during mouth overwash events.

Influence of prey availability on the distribution of dusky kob Argyrosomus japonicus (Sciaenidae) in the Great Fish River estuary, with notes on the diet of early juveniles from three other estuarine systems

The stomach contents of 391 juvenile dusky kob Argyrosomus japonicus (23–805 mm total length, TL) from the Great Fish River estuary were analysed. Early juveniles ( 100 mm TL, but the dominant species varied with predator size: early juveniles of 100–149 mm TL preyed mostly on the late larvae and early juveniles of other fish species, particularly the Cape stumpnose Rhabdosargus holubi; juveniles of 150–400 mm TL fed largely on early juvenile mugilids and on the small, pelagic estuarine round herring Gilchristella aestuaria; juvenile dusky kob of >400 mm TL fed on several teleost species, including early juvenile conspecifics, but their most important prey item was G. aestuaria. Based on the distribution patterns of A. japonicus and their predat...

INFLUENCE OF FRESHWATER FLOW REGIME ON FISH ASSEMBLAGES IN THE GREAT FISH RIVER AND ESTUARY

Summary Two and a half years of data were collected from the lower Great Fish River, head region and estuary to determine the fish species composition within these areas. Gilchristella aestuaria, Liza dumerilii, Rhabdosargus holubi and Pomadasys commersonnii were the four most abundant species captured, with riverine flow rate having an important effect on both species composition and numbers of fishes in the different regions. Most marine species displayed a strong inverse relationship between catch per unit effort and elevated freshwater inputs. Euryhaline marine species dominated the catches at all sampling sites during low flows but were less common during high flow periods when catadromous species were most abundant. Based on the available evidence it is suggested that for most marine species in the river this decline in abundance is related to low conductivity levels following floods rather than avoidance of elevated flows. The impact of elevated suspensoid concentrations and lowered dissolved oxyge...

Laval and Juvenile Fishes in the Surf Zone Adjacent to the St Lucia Estuary Mouth, KwaZulu-Natal, South Africa

Composition, seasonality and developmental stages of larval and juvenile fish in the surf zone adjacent to the St Lucia Estuary mouth, KwaZulu-Natal, were investigated. Samples were collected monthly, from February 1992 to January 1993, at six stations along the beach north of the estuary mouth. A 24-h study was also undertaken in the surf zone when the estuary mouth was closed. In the 12-month study a total of 2931 larvae, representing 88 taxa and 47 families, was collected. The most abundant families were the Sparidae, Haemulidae, Ambassidae, Tripterygiidae and Chanidae, together comprising 64.2% of the total catch. The most abundant species were Pomadasys olivaceum, Ambassis sp. and Rhabdosargus holubi. In the 24-h study a total of 13731 larvae, representing 43 taxa and 22 families, was collected and was dominated by Ambassis sp., Croilia mossambica and Chanos chanos. Temperature and turbidity were the most important environmental variables related to fish densities of estuary-associated species. Temporal changes in fish densities were most significant for daylnight catches, with densities being significantly higher at night than during the day. Fish species associated with estuaries peaked in abundance in mid autumn and late winter. Spatial trends in fish densities indicated significantly higher densities of estuary-associated species at Stations 2 and 3, with densities decreasing thereafter away from the estuary mouth. Larval stages were predominantly postflexion for all months sampled. The total catch was dominated by marine spawner species, with 36% of all taxa having some degree of estuarine association. Species with some degree of estuarine association dominated the catch in terms of density during both the 12-month study (60%) and the 24-h study (83%). Results from this study indicate that the surf zone adjacent to the St Lucia Estuary functions as a transit route and a nursery habitat for certain estuary- and non-estuary-associated fish species.

Fish abundance in the Wilderness and Swartvlei lake systems: changes relative to environmental factors

Fish communities throughout the Wilderness and Swartvlei lake systems were sampled in the winters of 1991 and 1993 using gill and seine nets. Gill net catches indicated substantial increases in the abundance of large juveniles and adults of the majority of marine fish species in both systems. Seine net catches indicated increases in the abundance of juvenile Liza richardsonii, but a decline in the abundance of other fish species. The recruitment of most fish is unlikely to have been negatively affected by the recorded fluctuations in salinity or turbidity. The senescence of macrophytes, however, may have increased exposure of recruiting Monodactylus falciformis and Rhabdosargus holubi to piscivorous birds and fish, The artificial breaching of estuaries during optimum fish recruitment periods in 1991 and 1992 may have contributed to the recorded increases in the abundance offish in 1993. A longer duration tidal phase in the Swartvlei system during 1992 and 1993, compared to the Wilderness lake system, did not result in greater abundance of fish sampled. There appears to be no justification for the artificial maintenance of permanently tidal conditions in the Swartvlei and Touw River estuaries on the grounds of benefit for the maintenance of viable fish communities.

New morphological information on the parasitic copepod <i>Lernanthropus sarbae</i> Kensley & Grindley, 1973 (Copepoda: Lernanthropidae) from Lake St. Lucia, South Africa, using scanning electron microscopy

New and additional morphological information on both sexes of Lernanthropus sarbae Kensley & Grindley is given. The morphology of a postantennal process, a new discovery from the family Lernanthropidae, is given. Material examined was sampled from the gills of three host species, Acanthopagrus berda Forsskal, Rhabdosargus holubi Steindachner and Rhabdosargus sarba Forsskal, collected from Lake St. Lucia, South Africa. Observations were made with the aid of scanning electron microscopy (SEM).

The distribution of littoral fish associated with eelgrassZostera capensisbeds in the Kariega Estuary, a southern African system with a reversed salinity gradient

Eelgrass Zostera capensis has expanded its distribution in the Kariega Estuary mainly in response to the reduced salinity gradient in that system. Seine-net sampling showed that Atherina breviceps, Rhabdosargus holubi, Glossogobius callidus and Mugilidae were the most common fish taxa in the Zostera beds. Fish species richness was maximum in the lower reaches, but highest densities (17,3 individuals·m−2) occurred in the upper reaches. There was increased domination by marine opportunist species in the lower reaches, but estuarine taxa predominated the middle and upper reaches. There was a significant similarity between the species in the middle and upper reaches of the estuary, but not between the lower and middle, or lower and upper reaches. No significant correlation was evident between fish density and salinity, temperature or turbidity. The distributional patterns of most fish associated with Zostera beds in the Kariega Estuary were similar to other estuaries in the Eastern Cape with normal axial salinity gradients, indicating that factors other than salinity influence fish distribution in estuaries.

MASS MORTALITY OF MARINE AND ESTUARINE FISH IN THE SWARTVLEI AND WILDERNESS LAKE SYSTEMS, SOUTHERN CAPE.

Summary Two incidences of mass mortality of fish were recorded in two saline waterbodies (Swartvlei estuary and Rondevlei) in the Wilderness National Park. Species affected were the Knysna seahorse (Hippocampus capensis) and longsnout pipefish (Syngnathus acus) in the Swartvlei estuary, and Cape stumpnose (Rhabdosargus holubi) and white steenbras (Lithognathus lithognathus) in Rondevlei. Comparison of water quality parameters during fish mortalities to past ranges and reported species tolerances indicated that the probable causes for the mortalities were high water temperature in the Swartvlei estuary, and low dissolved oxygen concentration in Rondevlei.

Shore-angling catches in the De Hoop Nature Reserve, South Africa, and further evidence for the protective value of marine reserves

Data on the species composition, size, seasonality and capture rates of surf-zone fish sampled by shore-angling at two sites in the De Hoop Marine Reserve are presented. In all, 21 969 fish of 33 species were caught during 11 344 angler-hours of effort. The overall catch rates were 163 and 233 fish·100h−1 at Koppie Alleen and Lekkerwater respectively. Coracinus capensis (57%N) and Diplodus sargus (33%N) were the most frequently captured species and four others (Lithognathus lithognathus, D. cervinus, Umbrina canariensis and Rhabdosargus holubi) each contributed >1% of the combined catch. Catch rates of most species varied seasonally, the overall catch rate being high (175–215 fish·100h−1) between March and November and lower (135-150 fish-100h−1) during summer. Differences in the species composition of catches, catch rates and seasonality at the two sampling sites are attributed to differences in the seasonal distribution of sampling effort and exploitation histories. Despite the fact that comparisons with other angling catch data from around southern Africa are complicated by a number of factors, a clear relationship between catch per unit effort and degree of exploitation is evident, suggesting that abundance of angling fish is particularly sensitive to exploitation and that marine reserves are a viable management option.

Stomach content analysis of juvenile Cape stumpnoseRhabdosargus holubiin 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.

Composition, distribution and abundance of ichthyoplankton in the Sundays River Estuary

The species composition, distribution and seasonal abundance of ichthyoplankton in the Sundays River estuary was investigated at five stations from March 1986 to March 1987. Two estuarine taxa, the clupeid Gilchristella aestuaria and gobies Caffrogobius spp., accounted for 86% of the ichthyoplankton community, while the marine Mugilidae and sparid Rhabdosargus holubi together comprised 12% of the total catch. Ichthyoplankton densities were highest in the middle and upper reaches of the estuary where zooplankton stocks were greatest. Juvenile G. aestuaria predominated in the bottom waters throughout the system but the larvae were more evenly distributed (vertically) within the water column. In contrast, the mugilids were concentrated in surface waters throughout the estuary. Larvae were most abundant during spring and summer months, coinciding with the documented spawning period of most species and the peak in zooplankton biomass.

Composition, species richness and similarity of ichthyofaunas in eelgrassZostera capensisbeds of southern Africa

This study collates published and unpublished data on the ichthyofaunas associated with beds of eelgrass Zostera capensis in eight estuaries and one lagoon in southern Africa. These macrophyte beds are utilized predominantly by small teleosts, both species which breed in the estuary and juvenile marine fish. By far the most abundant of the 97 species of fish recorded in these beds were Atherina breviceps, Gilchristella aestuaria, Liza dumerilii, Liza richardsonii and Rhabdosargus holubi. Classification grouped the ichthyofaunas of the six warm-temperate estuaries in the South-Eastern and Southern Cape at a similarity level >50 per cent. Ordination produced a similar clustering and also demonstrated that the subtropical Richards Bay and warm-temperate Mngazana estuary, together with the cold-temperate Langebaan Lagoon, were outliers. Classification and ordination showed that, within an estuary, the ichthyofaunas of dense and sparse Zostera areas were more similar than either was with those of non-vegetated...