Alan K. Whitfield

Comparisons between the roles played by estuaries in the life cycles of fishes in temperate Western Australia and Southern Africa

SynopsisThe rivers of south-western Australia and south-eastern Africa lie at similar latitudes, open into the Indian Ocean and frequently have estuaries that are periodically closed off from the sea by sand bars at their mouths. The present study has compared the species, genera and families represented in the fish assemblages of estuaries in south-western Australia and temperate southern Africa, i.e. below 31°S, and the way in which these estuaries are used by fish. The Clupeidae, Mugilidae, Atherinidae and Gobiidae were important families in both regions. However, the Terapontidae and Tetraodontidae, and the tropical families Apogonidae and Gerreidae, were represented by large numbers of individuals only in the estuaries of south-western Australia. Although 45 out of a total of 112 families and 32 of 233 genera occurred in both south-western Australia and temperate southern Africa, only 15 of the 326 species were found in both regions. The contributions made by the number of marine species which regularly enter estuaries in large numbers (marine estuarine-opportunists) to the total number of species recorded in the estuaries of south-western Australia and temperate southern Africa were similar (13.4 and 12.2% respectively) and the same was also true of species capable of completing their life cycles in estuaries (8.8 and 8.2%). The number of fresh water and diadromous species recorded in both regions was small. By contrast, the species of marine stragglers contributed approximately 70% to the total number of species in both regions. The adaptations of marine estuarine-opportunists and estuarine spawners to life in estuaries, and particularly to the effects of the closure of estuary mouths, is discussed. Although only one marine species was restricted to estuaries at any particular interval of its life cycle in south-western Australia, the juveniles of a number of marine species were confined to estuaries in temperate southern Africa. It is suggested that this difference can be attributed to the presence of a greater area and quality of alternative nursery habitats in the inshore marine environments in south-western Australia than in southern Africa.

A CHARACTERIZATION OF SOUTHERN AFRICAN ESTUARINE SYSTEMS

Summary Modifications to the structure and functioning of southern African estuarine systems can be placed into two major categories; those changes that have been driven by global forces such as climatic and sea-level fluctuations, and those that have resulted from direct human interference and are of a local or regional nature. Both the above categories will determine the rate and magnitude of physiographic and hydrodynamic changes within southern African estuaries, and therefore have a major impact on the characterization of individual systems in the future. Although the fluctuating nature of southern African estuaries (on both a spatial and a temporal scale) makes it difficult to place individual systems in distinctive categories, a classification system is possible if the dominant conditions in a particular estuary are assessed. Using a combination of physiographic, hydrographic and salinity features, five types of systems are recognised in this review, viz. permanently open estuaries, temporarily ope...

A global review of the cosmopolitan flathead mullet Mugil cephalus Linnaeus 1758 (Teleostei: Mugilidae), with emphasis on the biology, genetics, ecology and fisheries aspects of this apparent species complex

This study reviews published information on Mugil cephalus from around the world, with recent genetic studies indicating that the flathead mullet may indeed be a species complex. Disciplines that are covered range from the taxonomy, genetics and systematics, through a variety of biological and ecological attributes, to biomarker and fisheries studies. The eurytopic nature of M. cephalus is emphasized, with the migratory life history covering a succession of very different aquatic environments (e.g. rivers, estuaries, coastal lakes/lagoons, marine littoral, open ocean), each of which is occupied for varying lengths of time, depending on the population characteristics within a region and the life-history stage of the species. Interpretation of these movements over time has been greatly enhanced by the use of otolith micro-chemistry which has enabled scientists to map out the different habitats occupied by individual fish at the different life stages. The range of physico-chemical attributes within these environments necessitates a wide tolerance to differing conditions, especially with regard to salinity, turbidity, dissolved oxygen and temperature, all of which are discussed in this review. The importance of M. cephalus to the ecological functioning of coastal systems is emphasized, as well as the pivotal role that this species fulfills in fisheries in some parts of the world. The parasites range from internal trematode and cestode infestations, to external branchyuran and copepod parasites, which use M. cephalus as either an intermediate or final host. The value of the flathead mullet as a biomarker for the monitoring of the health of coastal habitats is discussed, as well as its potential as an indicator or sentinel species for certain ecosystems.

Systematics of the grey mullets (Teleostei: Mugiliformes: Mugilidae): Molecular phylogenetic evidence challenges two centuries of morphology-based taxonomy

The family Mugilidae comprises mainly coastal marine species that are widely distributed in all tropical, subtropical and temperate seas. Mugilid species are generally considered to be ecologically important and they are a major food resource for human populations in certain parts of the world. The taxonomy and systematics of the Mugilidae are still much debated and based primarily on morphological characters. In this study, we provide the first comprehensive molecular systematic account of the Mugilidae using phylogenetic analyses of nucleotide sequence variation at three mitochondrial loci (16S rRNA, cytochrome oxidase I, and cytochrome b) for 257 individuals from 55 currently recognized species. The study covers all 20 mugilid genera currently recognized as being valid. The family comprises seven major lineages that radiated early on from the ancestor to all current forms. All genera that were represented by two species or more, except Cestraeus, turned out to be paraphyletic or polyphyletic. Thus, the present phylogenetic results generally disagree with the current taxonomy at the genus level and imply that the anatomical characters used for the systematics of the Mugilidae may be poorly informative phylogenetically. The present results should provide a sound basis for a taxonomic revision of the mugilid genera. A proportion of the species with large distribution ranges (including Moolgarda seheli, Mugil cephalus and M. curema) appear to consist of cryptic species, thus warranting further taxonomic and genetic work at the infra-generic level.

FEEDING ECOLOGY OF PISCIVOROUS BIRDS AT LAKE ST LUCIA, PART 3: SWIMMING BIRDS

Summary Whitfield, A. K. & Blaber, S. J. M. 1978. Feeding ecology of piscivorous birds at Lake St Lucia, Part 3: Swimming birds. Ostrich 50:10-20. The diets, foraging periodicities and feeding behaviour of the Reed Cormorant Phalacrocorax africanus, Whitebreasted Cormorant Phalacrocorax carbo and White Pelican Pelecanus onocrotalus were studied at Lake St Lucia, Natal, South Africa, during 1975 and 1976. The Reed Cormorant fishes in shallow water within 100 m of the shore and mainly caught Sarotherodon mossambicus and Solea bleekeri, while the Whitebreasted Cormorant caught Mugilidae, Rhabdosargus sarba and Thryssa vitrirostris in deeper water. The diet of the White Pelican followed three distinct phases: a pre-incubation phase when the birds followed and preyed heavily on migrating Mugil cephalus shoals; an incubation and post-incubation phase when the adults flew a round trip of 200 km to the north to obtain freshwater fish, mainly cichlids, from the Pongolo pans where fishes were concentrated and densi...

Salinity ranges of some southern African fish species occurring in estuaries

The recorded salinity ranges of 96 fish species occurring in southern African estuaries are documented. Factors influencing the tolerance of fishes to low and high salinity regimes are discussed, with most species tolerant of low rather than high salinity conditions. This is important since most systems are subject to periodic freshwater flooding, especially during summer. The penetration of freshwater teleosts and elasmobranchs into estuaries is examined and the occurrence of marine fishes in rivers documented. Mortalities arising from salinity extremes in southern African estuaries are discussed, with temperature a key factor initiating such fish kills. Only eight fish species have been recorded spawning in estuaries although the fry of many species are attracted to estuarine waters. It is concluded that a small percentage of southern African fishes have succeeded in utilizing estuaries and this may be attributed to the paucity of strong osmoregulators.

Estuarine Typology and the Structuring of Fish Communities in South Africa

SynopsisSome 190 South African estuaries, covering all biogeographic provinces within the region, were classified into three types based on a combination of mouth condition and estuary size (surface area). The fish communities of the estuary types within each zoogeographic region were described and compared. Multivariate analyses revealed that each estuary type contained somewhat distinct fish communities. In addition, the study identified common patterns in species richness and ichthyofaunal composition. Open estuaries have relatively high species richness; this is a reflection of a permanent or near-permanent connection with the sea which allows access into these estuaries by all marine migrant species within the region. Intermittent connection with the sea limits the recruitment and utilisation of closed estuaries by marine migrant species; this results in reduced species richness in moderate to large closed estuaries. Small closed estuaries exhibit the lowest species richness and this is probably a result of their limited habitat and increased isolation from the sea. The key fishes that utilise estuaries could also be categorised into a number of groups based on their relative importance within each estuary type. Some species are largely restricted to predominantly open systems. Other taxa, while important in predominantly open estuaries, also occur in moderate to large closed systems. Some estuarine-associated species are well represented in all estuary types but exhibit a greater importance in closed estuaries. This study has shown that South African fish communities not only reflect estuarine typology but also respond to these differences in a consistent manner that spans all zoogeographic regions. The prevalence of similar patterns in other parts of the world suggests that estuarine typology is a major driver in the structuring of global estuarine fish communities.

The Influence of Selected Environmental Variables on Fish Assemblage Structure in a Range of Southeast African Estuaries

This study compared the ichthyofaunal assemblages in 10 Eastern Cape Province, South Africa, estuaries to identify whether there were any structural variations between the assemblages and to determine the environmental and physical factors linked to these variations. Species belonging to both the marine migrant and estuarine resident groups, by consistent representation within each estuary grouping, were responsible for the separation of fish assemblage structures between the different types of systems. The data further revealed that fish assemblages in the three main types of estuaries investigated were distinct, linked primarily to estuary mouth status and estuary size.

A quantitative study of the trophic relationships within the fish community of the Mhlanga estuary, South Africa

The importance of benthic floc, zoobenthos, zooplankton and epiphytic flora and fauna as energy sources for the fishes of the Mhlanga estuary was studied between January and December 1978. Benthic floc (detritus and associated microorganisms) was the most important food resource with an annual standing crop of 161 kJ m −2 (9·46 g m −2 ). More than 90% of the fish biomass utilized this energy source compared to 1·8% feeding on zoobenthos (26·4 kJ m −2 , 1·5 g m −2 ), 1·7% on zooplankton (1·7 kJ m −3 , 0·08 g m −3 ) and 3·1% on epiphytic flora and fauna (4·0 kJ m −2 , 0·27 g m −2 ). Since much of the detritus entered the Mhlanga estuary from the fringing Phragmites swamp, the biological productivity of the estuary was closely linked to that of the reed beds. During the closed phase of the estuary the standing crops of the fish food resources increased due to the relative stability of the physical environment. The highest biomasses of zooplankton, zoobenthos, epiflora and epifauna were recorded after extended periods of closure and decreased significantly when the estuary opened. This was due to two main factors: firstly prolonged exposure of vast areas of the lagoon and reed swamp, and secondly large scale substrate movement caused by floodwaters leaving the estuary.

A comparison of the ichthyofaunas in two permanently open eastern Cape estuaries

The Kowie and Great Fish estuaries are situated less than 30 km apart, yet they differ considerably in terms of riverine inflow, turbidity, food resources and habitat availability. The ichthyofauna of the two estuaries were sampled using plankton, seine and gill nets. A greater ichthyofaunal richness (R) was recorded in the Kowie estuary and this is attributed to the wider range of habitats and greater degree of marine influence in this system. In contrast, all three sampling gears revealed an approximate 3:1 ratio between fish abundance in the Great Fish and Kowie estuaries. The higher abundance of fishes in the Great Fish estuary is partly attributed to the large organic and nutrient inputs into this system when compared with the Kowie system, and the influence of these inputs on estuarine primary and secondary production. Individual fish species are affected differently by turbid water conditions. Indications from this study were that piscivorous fishes (e.g. Lichia amia) which rely mainly on visual fo...