Showing papers by "Alan K. Whitfield published in 1989"

Ichthyoplankton in a southern african surf zone: Nursery area for the postlarvae of estuarine associated fish species?

Abstract: The surf zone ichthyoplankton of Swartvlei Bay was studied between February 1986 and June 1987, with particular emphasis on its potential role as a nursery area for estuarine associated marine fish species. Larvae and/or postlarvae of 16 families were identified from the surf zone, with the Gobiidae, Soleidae, Sparidae and Mugilidae comprising 85·7% of all teleosts sampled. The postlarvae of several taxa (including the six most common species), which utilize the Swartvlei estuary as a juvenile nursery area, were abundant in the surf zone. Conversely, species which are common in nearshore marine waters as juveniles and adults, but seldom enter estuaries, totalled less than 8% of the surf zone ichthyoplankton assemblage. Larval and postlarval densities peaked during summer when water temperatures exceeded 19°C and the estuary mouth was open. Concentrations of ichthyoplankton were highest at those sampling stations closest to the estuary mouth during the summer period. Diel changes in total catches revealed no significant difference between day and night densities; but of the four major taxa, the Mugilidae and Sparidae tended to be more abundant during the day, the Gobiidae at night and the Soleidae showed no distinct pattern. Results from a 24 h sampling session indicated that tidal phase may also be important in governing ichthyoplankton abundance in the surf zone.

Ichthyoplankton interchange in the mouth region of a southern African estuary

Abstract: Migration patterns of ichthyoplankton in the seasonally open Swartvlei estuary were monitored during 1986/87 with particular emphasis on exchange between the estuary and sea. Four groups of fishes are recognised in the area accorhng to breeding and recruitment strategies. Group 1 consists of species (e.g. Rhabdosargus holubj and Monodactylus falciformis) which spawn at sea, enter the estuary mainly as postlarvae, and congregate along the margins or on the bottom where water current speeds are reduced. Group 2 species (e.g. Spondyljosorna ernarginatum and Etrumeus whitehead]] also spawn at sea, their larvae are swept into the estuary on the flood tide, but are returned to the marine environment on the ebb hde. Breeding and larval development of Group 3 species (e.g. Hippocampus capensis and Syngnathus acus) occur within the estuary, although some eggs and larvae are lost to the sea following opening of the mouth. Group 4 larvae (e.g. Psarnrnogobius knysnaensjs and Caffrogobius spp.), which hatch from demersal eggs in the estuary, leave the system on the ebb tide before returning several weeks later as postlarvae. Die1 ichthyoplankton density changes in the lower reaches of the Swartvlei estualy revealed that movements of larvae and postlarvae between the estuary and marine environment occurred malnly during twilighWnocturna1 hours. The above data, together with a hydrodynamic model of the estuary, has enabled the quantification of ichthyoplankton exchange over specific 24 h periods.

Composition, species richness and similarity of ichthyofaunas in eelgrass Zostera capensis beds of southern Africa

Abstract: 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...

The benthic invertebrate community of a southern cape estuary: structure and possible food sources

Abstract: SUMMARY Cluster analysis of invertebrate communities in the Swartvlei estuary revealed that vegetated (Zostera capensis) sites were distinct from unvegetated ones. ‘Indicator species’ for the eelgrass group included Melita zeylinaca, Loripes clausus, Natica tecta and Palaemon pacificus, whereas those for the bare sand community were Urothoe pulchella, Callianassa kraussi, Iphinoe truncata and Pontogeloides latipes. Infaunal bivalves comprised >60 % of the invertebrate biomass at Zostera sites but 80 %) and was a minor component (<5 %) at eelgrass sites. The supratidal invertebrate community was dominated by Orchestia spp. which live and feed on wrack detritus. Litterbag experiments revealed that degradation of Zostera leaf wrack was rapid during the first thirty days after deposition but slow between 40 and 140 days. Laboratory experiments indicated that Orchestia consumption of wrack material could not accoun...

Fish larval composition, abundance and seasonality in a southern African estuarine lake

Abstract: The ichthyoplankton community of Swartvlei, an estuarine lake on the southern Cape coast, has a low species diversity (< 10 species) owing to the tenuous link with the marine environment and paucity of fishes breeding in South African estuaries. The larval assemblage was dominated by the clupeid Gilchristella aestuaria, which was distributed across the entire lake and comprised 78% of the total catch. Mean annual ichthyoplankton abundance was relatively low (38 larvae 100 m−3), possibly as a result of the extremely low phytoplankton productivity and poor zooplankton biomass. Fish larvae and zooplankton peak in abundance between October and March, with very low densities recorded for the remainder of the year. During a 24 h sampling session In February 1988, ichthyoplankton abundance increased significantly (p < 0,001) in surface waters after sunset and declined following sunrise.

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

Abstract: 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 areas. The species richness (R) of fish within eelgrass habitats along the southern African coast tended to decrease from north-east to south-west, a trend attributable to an attenuation in the number of tropical/subtropical Indo-Pacific species. The numerically dominant fish families in eelgrass beds in South Africa showed a greater degree of overlap with those of south-eastern Australia than with those of either southern Japan or eastern North America.

The response of Grandidierella lignorum (Barnard) (Crustacea: Amphipoda) to episodic flooding in three eastern Cape estuaries

Abstract: The response of the euryhaline amphipod Grandidierella lignorum to changing freshwater inflow was investigated in three estuaries with differing river discharges. In the Kariega and Keiskamma estuaries an increase in the density of G. lignorum was correlated with an increase in river inflow. The sharp reduction in salinity during flooding is suggested as a possible trigger which stimulates G. lignorum to rise into the water column. The position of stations along the Kariega and Keiskamma estuaries, together with seasonal effects, also influenced G. lignorum abundance. In the Great Fish estuary, which has a continuous large freshwater input, changes in the abundance of G. lignorum were not correlated with inflow. Possible reasons for this anomalous situation, as well as the biological implications of becoming part of the zooplankton during flooding, are discussed.

The preliminary design of radial inflow turbines

Abstract: A procedure is described which develops the non-dimensional design of a radial inflow turbine rotor. The design is developed, for any specified non-dimensional power ratio, with the objective of minimising the inlet and discharge Mach numbers so that the passage losses are minimised. Initially state of the art efficiencies are assumed but are later modified through the specification of empirical losses. The resultant non-dimensional design can be transformed to absolute dimensions through the specification of the inlet stagnation conditions and the mass flow rate of the working fluid.Copyright © 1989 by ASME

Distribution and diversity of mkuze swamp fishes during a summer flood

Abstract: SUMMARY Very little information is available on the fishes of the Mkuze swamps and this paper presents the results of an ichthyofaunal survey conducted in March 1988. The study, which coincided with a major river flood, determined that the fish fauna was considerably more diverse than expected and 25 species were collected from 31 localities. Several distinct communities are recognized including a river channel/floodplain community, lagoonal community, feeder stream community and lower swamp community. The subtraction of tropical freshwater fish species in the region is documented and a complete checklist for the Mkuze River system, including several new records from this survey, is presented.

The Preliminary Design of Radial Inflow Turbines

Abstract: A procedure is described which develops the non-dimensional design of a radial inflow turbine rotor. The design is developed, for any specified non-dimensional power ratio, with the objective of minimising the inlet and discharge Mach numbers so that the passage losses are minimised. Initially state of the art efficiencies are assumed but are later modified through the specification of empirical losses. The resultant non-dimensional design can be transformed to absolute dimensions through the specification of the inlet stagnation conditions and the mass flow rate of the working fluid.Copyright © 1989 by ASME