Fish communities in temporarily open/closed estuaries from the warm- and cool-temperate regions of South Africa: A review

The majority of estuaries along the coastline of southern Africa are termed temporarily open/closed estuaries (TOCEs) and are closed off from the sea for varying periods by a sandbar which forms at the mouth. It is therefore important to understand the processes occurring within TOCEs and their importance to fishes in order to make sound management recommendations. Estuaries along the coast of South Africa and their associated fish assemblages are biogeographically distinct and occur in either a subtropical, warm-temperate or cool-temperate zone. There are 125 TOCEs found within the cool-temperate and warm-temperate zones. Most fish species found in TOCEs are the juveniles of marine taxa that breed at sea. Permanently open estuaries generally have a higher diversity of species than TOCEs, but TOCEs still provide important nursery areas for many marine species and numerically often have a higher proportion of estuarine resident species. Important taxa in terms of abundance and biomass in warm-temperate TOCEs include the sparids Rhabdosargus holubi and Lithognathus lithognathus, several mugilid species, estuarine residents (particularly Gilchristella aestuaria and Atherina breviceps) and the freshwater cichlid Oreochromis mossambicus. The diversity of fishes in cool-temperate TOCEs is low when compared with warm-temperate systems and Liza richardsonii tends to dominate catches by number and mass in most systems. Several species recorded in TOCEs show clear longitudinal distribution trends. For example Atherina breviceps is generally more abundant in the lower reaches of estuaries. Mouth state, particularly the frequency, timing and duration of mouth opening plays a key role in determining species richness, composition, diversity and abundance in TOCEs. Mouth state is directly linked to freshwater input. Reduced river inflow leads to prolonged mouth closure and shorter open phases, which inhibits immigration and emigration of marine fish species between estuaries and the sea. Understanding of the effects of various processes occurring within these systems, particularly variation in freshwater input, on the biota of these important systems facilitates the development of informed management recommendations.     

Life cycles and distribution of atherinids in the marine and estuarine waters of southern Australia

New and published data have been collated for the biology and distribution of atherinid species abundant in the coastal saline waters of Australia below 30°S. This information has been used to determine whether these species typically spawn at sea or pass through the whole of their life cycle in estuaries, and in one case, also lagoons and saline lakes. Length-frequency data, gonadosomatic indices and distribution records indicate that in south-eastern AustraliaCraterocephalus honoriae andAtherinosoma microstoma typically reach total lengths less than 90 mm, have a one-year life cycle and breed within estuaries. This parallels the situation recently described forAtherinosoma elongata, Atherinosoma wallacei andAllanetta mugiloides in south-western Australia (Princeet al., 1982a; Prince & Potter, 1983). The marine speciesAtherinosoma presbyteroides, which reaches a similar size and has a one year life cycle in both south-western and south-eastern mainland Australia, only enters estuaries in large numbers in the former region. WhileAtherinomorus ogilbyi is also found in estuaries and typically breeds at sea, it reaches total lengths as great as 189 mm and has a longer life thanA. presbyteroides. The limited data forAtherinason esox andAtherinason hepsetoides demonstrate that both these marine atherinids can attain total lengths of 139 and 108 mm respectively and live for longer than a year but do not enter estuaries in large numbers. The latter species is unique amongst southern Australian atherinids in having a distribution which extends into deeper water. It is suggested that landlocking may have played a role in the evolution and success of the estuarine mode of lifesensu stricto ofA. wallacei, A. elongata, A. microstoma, A. honoriae andA. mugiloides in southern Australian waters.     

Fishes and salinities in the St Lucia estuarine system—a review

The recorded salinity ranges of freshwater, estuarine and marine fish species in Lake St Lucia, a Ramsar and World Heritage Site, are documented. The freshwater group is most diverse and abundant under oligohaline conditions, although the Mozambique tilapia (Oreochromis mossambicus) was common under all salinity regimes. Estuary resident species also favoured oligohaline conditions but, in contrast to the freshwater taxa, were well represented in salinities up to 40 ‰. The marine group was most diverse and abundant within the salinity range 10–40 ‰, but a large number of species could also be found in salinities up to 70 ‰. Very few fish species were able to tolerate salinities between 70 ‰ and 110 ‰, with only O. mossambicus surviving for extended periods in salinities above 110 ‰. All the aquatic macrophytes and most of the zoobenthos within the lake appear to die out within the salinity range of 50–60 ‰, thus creating additional stress to those fish present under such conditions. The food resources least affected by extreme hypersalinity are the microphytobenthos and detritus food chains, with detritivorous fishes being dominant when the lake is in this state. Mass mortalities of fishes in Lake St Lucia have been recorded under both low (<5 ‰) and high salinity (>70 ‰) conditions. The fish kills are often triggered by exceptionally low or high water temperatures which affect the osmoregulatory abilities of these species. Hypersaline conditions and fish mortalities under the most recent closed estuary mouth conditions (2002–2005) are reviewed. If the surface area of St Lucia (35,000 ha) is compared to the total surface area of all South African estuaries (approximately 70,000 ha), then the possibility exists that the loss of the Lake St Lucia nursery area for estuary-associated marine fish species over the past few years may cause significant short-term declines in the future abundance of these taxa on both a local and regional scale.     

Molecular phylogeny,morphology and toxigenicity of Ostreopsis cf. siamensis (Dinophyceae) from temperate south‐east Australia

Ostreopsis is a genus of dinoflagellates that includes species producing palytoxin and structurally related compounds. The distribution of Ostreopsis species in Australia is largely unknown, but they have been reported from north Queensland (18^° S) to Tasmania (41–43^° S). Ostreopsis spp. have been recurrently reported from estuaries around New South Wales, with persistent occurrences in Merimbula Lake inlet throughout the year. We isolated and characterized a strain of Ostreopsis cf. siamensis using light and scanning electron microscopy as well as molecular sequences of small subunit (SSU), large subunit (LSU) and ITS regions of ribosomal DNA. The strain grew significantly faster in low nutrient concentrations. Palytoxin‐like compounds were produced by the strain, as determined by chemical analysis, and the LD ₅₀ of the cell extract by intraperitoneal injection in mice was 25.1 mg kg^?1. This is the first comprehensive molecular, morphological and toxicological study of an Ostreopsis species from Australian waters. Increasing reports of Ostreopsis from temperate waters suggest an empirical need to expand the knowledge of their diversity and distribution to aid aquaculture monitoring in Australian estuaries.     

Residency and movement patterns of Arctic charr Salvelinus alpinus relative to major estuaries

Estuarine residency and marine movements of 43 anadromous Arctic charr Salvelinus alpinus (mean ± s.d . fork length = 523 ± 97 mm) were examined using acoustic tracking in inner Frobisher Bay (IFB; 63° N; 68° W), Canada, from July to September 2008 and 2009. A mean ± s.d . migration duration of 63 ± 7 days occurred from late June to early September. Detected S. alpinus were either continuously (maximum 34 days) or intermittently present in estuarine zones, on average residing approximately one third of time tracked and returning once every 9 days. Significantly higher estuarine residency during the final 15 migration days suggested that a transition phase may occur prior to freshwater re‐entry. Low travel rates during flood tide suggested individuals staged before accessing intertidal and estuarine zones. Although the two main estuaries were c. 22 km apart, 19% of tagged individuals used both. Individuals remained relatively close to freshwater overwintering systems, although late‐migration inter‐estuarine movements may have indicated natal homing. Approximately half of the individuals exhibited extra‐estuarine travel, mostly during mid‐migration, but remained within 3 km of shore ranging < 30 km straight line distance (SLD) of either estuary. It was concluded that IFB S. alpinus (1) spent a significant portion of their migration within or adjacent to the estuaries and (2) had a restricted marine distribution within 30 km SLD of the river mouths.     

Why are there so few freshwater fish species in most estuaries?

The freshwater fish assemblage in most estuaries is not as species rich as the marine assemblage in the same systems. Coupled with this differential richness is an apparent inability by most freshwater fish species to penetrate estuarine zones that are mesohaline (salinity: 5·0–17·9), polyhaline (salinity: 18·0–29·9) or euhaline (salinity: 30·0–39·9). The reason why mesohaline waters are avoided by most freshwater fishes is difficult to explain from a physiological perspective as many of these species would be isosmotic within this salinity range. Perhaps, a key to the poor penetration of estuarine waters by freshwater taxa is an inability to develop chloride cells in gill filament epithelia, as well as a lack of other osmoregulatory adaptations present in euryhaline fishes. Only a few freshwater fish species, especially some of those belonging to the family Cichlidae, have become fully euryhaline and have successfully occupied a wide range of estuaries, sometimes even dominating in hyperhaline systems (salinity 40+). Indeed, this review found that there are few fish species that can be termed holohaline (i.e. capable of occupying waters with a salinity range of 0–100+) and, of these taxa, there is a disproportionally high number of freshwater species (e.g. Cyprinodon variegatus, Oreochromis mossambicus and Sarotherodon melanotheron). Factors such as increased competition for food and higher predation rates by piscivorous fishes and birds may also play an important role in the low species richness and abundance of freshwater taxa in estuaries. Added to this is the relatively low species richness of freshwater fishes in river catchments when compared with the normally higher diversity of marine fish species for potential estuarine colonization from the adjacent coastal waters. The almost complete absence of freshwater fish larvae from the estuarine ichthyoplankton further reinforces the poor representation of this guild within these systems. An explanation as to why more freshwater fish species have not become euryhaline and occupied a wide range of estuaries similar to their marine counterparts is probably due to a combination of the above described factors, with physiological restrictions pertaining to limited salinity tolerances probably playing the most important role.     

Influence of Tribulus terrestris extract on the survival and histopathology of Oreochromis mossambicus (Peters, 1852) fry before and after Streptococcus iniae infection

The aim of this study was to determine the effects of Tribulus terrestris extract (TT) on growth performance, disease resistance and histopathological changes in intestine and liver tissues of Oreochromis mossambicus (Peters, 1852) first‐feeding fry before and after exposure to Streptococcus iniae. Five isonitrogenous and isocaloric diets were formulated to contain 0 (control), 200, 400, 600, and 800 mg kg^?1 TT. After feeding for 45 days, fish were infected with S. iniae and mortalities recorded. Final weight, weight gain and SGR of tilapia fry fed the 400 mg kg^?1 TT diet were significantly greater than that of control diet. In the challenge experiment, the best survival rate was obtained with 400 mg kg^?1 TT supplementation. Infection by S. iniae appeared to have a negative effect on histopathological findings and outcome than did TT‐800 used alone. However, administration of TT (200 or 400 mg extract kg^?1) resulted in overall improvement in the intestine and liver histopathology, emphasizing the protective potential of TT. The present study suggests the protective potential of TT in alleviating intestinal and hepatic damage that can occur after a S. iniae infection. It was concluded that 400 mg kg^?1 TT can enhance growth and disease resistance during first–feeding of O. mossambicus fry. This suggests that TT may be an alternative to antibiotics in controlling streptococcal disease in tilapia culture.     

Age,growth and reproduction of Marcusenius pongolensis,Oreochromis mossambicus and Schilbe intermedius in an oligotrophic impoundment in Swaziland

The age, growth and reproductive biology of Marcusenius pongolensis, Oreochromis mossambicus and Schilbe intermedius were investigated in the Mnjoli Dam, Swaziland. Otolith annulus formation occurred in winter for M. pongolensis, and in spring/summer for O. mossambicus and S. intermedius. Maximum ages of 8, 6 and 8 years were recorded for M. pongolensis, O. mossambicus and S. intermedius, respectively. Growth was described by the von Bertalanffy growth model as L_t = 238.73(1 − exp^{−0.27(t+2.27}) mm fork length (FL) for M. pongolensis, L_t = 226.83(1 − exp^{−0.45(t+2.02)}) mm total length (TL) for O. mossambicus, and L_t = 214.59(1 − exp^{−0.60(t+1.20)}) mm FL for S. intermedius. Sexual maturity was estimated for male and female M. pongolensis at 134 mm FL and 119 mm FL, respectively. Marcusenius pongolensis matured within their first year. Female O. mossambicus and S. intermedius matured at 239 mm TL and 205 mm FL, corresponding to 2 and 4 years of age, respectively. Extended spawning periods, with two spawning peaks was observed for M. pongolensis, one in spring (September) and the second in autumn (March) and one peak over late‐summer for S. intermedius.     

Unique seasonal forage bases within a local population of bottlenose dolphin (Tursiops truncatus)

Using photo‐identification data, bottlenose dolphin (Tursiops truncatus) populations can be differentiated based on their use of particular estuaries or coastal habitats. Questions remain, however, about the validity of such fine‐scale population partitioning and whether the resulting assemblages utilize unique forage bases. To address the issue of forage base use, stable isotopes of carbon (δ¹³C), nitrogen (δ¹⁵N) and sulfur (δ³⁴S) were analyzed from skin tissues (n= 74) of bottlenose dolphins sampled seasonally along the coast and in three estuaries near Charleston, South Carolina. Autumn values of δ³⁴S, δ¹⁵N, and δ¹³C and summer values of δ³⁴S indicated that dolphins sampled from these four assemblages utilized unique forage bases, despite limited sample sizes. Likewise, autumn and spring differences in δ¹⁵N and δ¹³C values were evident in the North Edisto River, and in δ³⁴S from dolphins sampled from all three estuarine assemblages; no seasonal differences were identified in the coastal assemblage. Results demonstrate the importance of considering spatial and temporal variation in forage base when developing local management plans for bottlenose dolphin and highlight the discriminatory power of δ³⁴S for estuarine and coastal marine mammals. These results also suggest that stable isotopes could be developed as a complementary tool for photo‐identification based partitioning of bottlenose dolphin populations.     

Biomass and productivity of fishes in estuaries: a South African case study

Estuaries are well known for their role as nutrient and detrital sinks that stimulate high levels of both primary and secondary production which, in turn, support a large biomass of fishes per unit area. This study reviews available information on coastal fish biomasses (g m^?2 wet mass) and productivity (g m^?2 wet mass year^?1) in order to place South African data on these topics into a global perspective. Using biogeographic fish productivity estimates, together with estuarine water area, the approximate annual teleost production in South African estuaries was calculated at 585, 1706 and 13 904 t in the cool temperate, warm temperate and subtropical regions, respectively. Total annual fish production in estuaries on the subcontinent is conservatively estimated at 16 195 t, but this figure is likely to fluctuate widely, depending on recruitment success and annual environmental conditions pertaining to these systems. Approximately 2000 t of fish are estimated to be harvested by fishing activities in South African estuaries each year, which represents c. 12% of annual fish production. Although this figure may appear sustainable, the reality is that there are a few heavily targeted estuary‐associated marine species at the top of the food chain that are being overexploited by both anglers and subsistence fishermen. Natural mortalities due to piscivorous fish and bird predation has been estimated at c. 3% of total fish biomass per month in the East Kleinemonde Estuary, but this figure will vary considerably depending on bird abundance and foraging patterns along the coast. In contrast to catches made by the fishermen, piscivorous fishes and birds are targeting mainly juvenile marine fish and small estuarine resident species that are very abundant and generally low down in the food web.     

Are nest sites a limiting resource for speleophilic nesting fishes in sand dominated temporarily open/closed estuaries?

Southern African temporarily open/closed estuaries are often characterized by sand dominated mouth regions, a feature in contrast to many permanently open estuaries of the region which regularly have substantial rocky areas. Here we explore how habitat characteristics may reduce nesting opportunities and potentially explain population trends for a common estuarine fish species, the speckled sand goby Psammogobius knysnaensis, in many temporarily open/closed estuaries in South Africa. The present study is the first to document that P. knysnaensis is a speleophilic nester, utilizing the underside of rocks and other hard substrata for egg attachment. Furthermore, our results suggest that ideal nesting habitat for the speckled sand goby is potentially a limiting resource in sand-dominated temporarily open/closed estuaries. We postulate that this is a factor contributing to the relatively low numbers of adult P. knysnaensis found in sandy temporarily open/closed estuaries of the region, although the role of marine connectivity, food availability, predation pressure and other environmental factors in this regard still remain largely unassessed.     

The biology, specificity and habitat of the species of Eubothrium (Cestoda: Pseudophyllidea), with reference to their use as biological tags: a review

The literature pertaining to the distribution, habitat, specificity, life cycles and population biology of all the species of Eubothrium is reviewed. Records in which scolex morphology is used to identify species cannot be used as this character is too variable to be of value in specific recognition. Study of the valid records suggests that E. crassum comprises three races. There is a freshwater race, whose preferred host is non-migratory Salmo trutta fario, but which also infects parr, smolts and landlocked adult S. salar and parr of migratory S. trutta trutta. Its distribution is confined to Europe and Eurasia and is co-extensive with that of S. trutta: it does not occur in N. America or E. Asia. It may be carried to sea in migratory salmonids, where it dies out in S. salar but may survive in S. trutta. Its life cycle only takes place in fresh water, and requires a copepod as the only intermediate host. Other fish species may serve as paratenic or accidental hosts. The preferred host of the marine Atlantic race of E. crassum is S. salar, but S. trutta is also infected. Its distribution is co-extensive with that of S. salar on both sides of the Atlantic Ocean. It may be carried into fresh water by both hosts, when its numbers decline in S. salar but not in S. trutta. The life cycle is unknown, but infection probably takes place in marine coastal waters. The preferred hosts of the marine Pacific race are all species of Oncorhynchus, with which its distribttion is co-extensive on both sides of the Pacific Ocean. It may also be carried into fresh water, and its life cycle is also unknown. E. salvelini comprises two races. The European freshwater race is specific to Salvelinus alpinus, and does not infect Salmo spp. Its life cycle takes place in fresh water and involves only one intermediate host, a copepod. It may be carried to sea in migratory fish, and survive to return to fresh water. The American race, found throughout North America and East Asia, infects species of Salmo, Salvelinus, Oncorhynchus, Cristivomer and other genera. Its biology is otherwise similar to that of the European race, although its return from the sea has not yet been confirmed in America. The other species of Eubothrium are more local in their distribution, some being endemic; do not comprise different races, and infect marine and anadromous fish. Nothing is known of their biology or life cycle. Eubothrium is recognised as being a marine genus that has invaded fresh water with anadromous fish. The inability to distinguish the races of E. crassum and E. salvelini on morphological characters, their low rate of survival in the other medium and their wide distribution renders them unsuitable as biological tags.     

The nematode fauna of the Murray River estuary,South Australia; the effects of the barrages across its mouth

The littoral nematode fauna of the Murray River, South Australia, above and below the barrages that control the outflow of the river into the Southern Ocean has been investigated by taking cores of the sediment at monthly intervals for one year. Samples of river water were taken at the same time for chemical analysis and sediment particle size has also been investigated.At two sites below the barrages the salinity varied depending on the volume of water discharged through the barrages. It rose almost to that of sea water while they remained closed, but fell to that of the river water when they were all open and discharging large volumes of river water. The nematode population fluctuated from about 12 200 to 180 000 m^-2 for nine months, but the numbers of live nematodes fell sharply in July through to September, accompanied by heavy nematode mortality, reaching 100% at one site in August. We attribute the mortality to a prolonged discharge of river water. More than 40 genera have been identified, most with cosmopolitan distributions.Above the barrages, at Mundoo and on Lake Alexandrina the water is fresh river water. The fauna was less dense than that below the barrages; with a maximum of 10000 m^-2. It comprised genera typical of freshwaters overseas (no comparable studies of Australian freshwater nematodes have been reported), such as Eutobrilus, Tripyla and Ironus, together with two species from typical marine genera, i.e. Mesacanthion and Enoploides. The latter species is very similar to E. fuviatilis from the Volga river.It is suggested that low population density below the barrages, when compared with other estuaries, reflects periodic mortality in estuarine nematodes when salinity falls drastically, whilst above the barrages there is a mixture of tolerant estuarine and typical freshwater nematodes.     

The composition,nutritional status and digestibility of the diets of Sarotherodon mossambicus from nine man-made lakes in Sri Lanka

Synopsis The dietary composition and the nutritional status and the digestibility of the diets of Sarotherodon mossambicus from nine reservoirs in Sri Lanka were evaluated. The feeding habits of S. mossambicus were variable from reservoir to reservoir; they ranged from herbivory to total carnivory. The protein, total lipid, carbohydrate and total organic matter content of the ingested material were related to the dietary composition and ranged from 18.53% to 35.15% (x−24.18%), 5.94% to 9.84% (x−7.91%), 11.6% to 34.7% (x−22.34%) and 34.4% to 64.4% (x−45.71%), respectively. Irrespective of the feeding habits, the diet contained a significant proportion of organic material which cannot be accounted for by protein, total lipid and carbohydrate. As much as the ingested material was related to the feeding habit, the digestibility of the nutrient components was related to the food material devoured. For example, the mean digestibility of the total organic matter in S. mossambicus feeding on detritus, plants and animal were 36.85, 33.5 and 29.5 respectively, and compared well with observations from elsewhere. It is hypothesised that the favourable nutrient quality of the available dietary material in the reservoirs of Sri Lanka, which could be and is effectively utilized by S. mossambicus, may have been, at least partially, responsible for its almost unprecedented success in Sri Lanka.     

Microfossils in surface sediments of brackish waters on the west coast of South Africa and their palaeoecological implications

A faunistic survey covering 25 sites in estuaries, coastal lakes and ponds along the west coast of South Africa between the Cape of Good Hope in the south and the Olifants River in the north was carried out in May 2014. The study aimed to establish a dataset with ecological and distribution data of ostracods and foraminifers for later palaeoenvironmental reconstructions. Canonical correspondence analysis showed that the distribution of the 19 foraminifer and 32 ostracod taxa was controlled mainly by habitat structure, but that specific conductivity (salinity) was best and solely described by the second axis, highlighting the usefulness of the two microfossil groups for salinity reconstructions. Habitat structure was demonstrated by the foraminifer species Trochamminita irregularis’ preference for stillwater areas of lakes. Sarscypridopsis aculeata and Sarscypridopsis glabrata were the dominant ostracod species in coastal ponds and lakes. However, their living distributions excluded each other, with S. aculeata preferring areas of lower salinity and S. glabrata dominating areas of higher salinity up to hyperhaline conditions in small, closed water bodies.     

Riverine and near coastal migration performance of hatchery brown trout Salmo trutta

To study migration performance and return rates of hatchery brown trout Salmo trutta smolts the first 5 months after release, 50 fish in each year (fork length, L_F, 158–288 mm) were in two subsequent years tagged with acoustic transmitters and recorded by automatic listening stations in the River Nidelva (central Norway), its estuary and in the marine environment. More than half of the smolts became anadromous migrants (52% in 2011 and 70% in 2012). The fish spent longer time in the estuary than in the marine environment and the results suggest that migratory behaviour of S. trutta smolts is not only restricted to be resident or anadrome–lacustrine, but that there is also an intermediary strategy of estuarine feeding. There were no differences in L_F or mass between groups of smolts with different migration patterns. Return rates from the sea within the first 5 months after release were in both years 16%. Median progression rate in the river was 0·090 L_F s^?1 but decreased significantly as the smolts entered the estuary (0·015 L_F s^?1). The long residential time in the estuary may increase the risk of negative effects of anthropogenic activities in estuaries, such as harbours and industrial development, and special attention should be given to evaluate effects of such activities.     

Responses of the PAS-positive pars intermedia cells in the cichlid fish Sarotherodon mossambicus to ambient calcium and background adaptation

Summary The function of the PAS-positive pars intermedia cells in fish has been connected with control of background adaptation and of plasma calcium levels. Since background reflectivity and illumination influence calcium metabolism in S. mossambicus, we have tested the hypothesis that the effects of background reflectivity on the PAS-positive cells are mediated by changes in plasma calcium. However, total plasma calcium does not correlate with the activity of the PAS-positive cells as estimated by morphometrical criteria. Transfer of S. mossambicus to a white background leads to a drop in plasma calcium, and a marked reduction of the activity of the PAS-positive cells. Adaptation to low-calcium freshwater, on a neutral background, has the same effect on plasma calcium but has no effect on the PAS-positive cells. The characteristic structural features of highly active PAS-positive cells in fish from a black background are not due to the slight hypocalcemia that occurs in these fish, since addition of calcium to the water in concentrations that fully prevent the drop in plasma calcium does not suppress the PAS-positive cells. These findings make it very unlikely that these cells produce a hypercalcemie factor in S. mossambicus.     

Estuarine Typology and the Structuring of Fish Communities in South Africa

Synopsis Some 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.     

Genomic organization and evolution of the 5S ribosomal DNA in Tilapiini fishes

Summary 5S rDNA sequences present an intense dynamism and have proved to be valuable as genetic markers to distinguish closed related species and also in the understanding of the evolutionary dynamic of repetitive sequences in the genomes. In order to identify patterns of 5S rDNA organization and their evolution in the genome of fish species, such genomic segment was investigated in the tilapias Oreochromis niloticus and Tilapia rendalli, and in the hybrid O. urolepis hornorum × O. mossambicus. A dual 5S rDNA system was identified in the three analyzed tilapia samples. Although each 5S rDNA class was conserved among the three samples, a distinct 5S rDNA genome organization pattern could be evidenced for each sample. The presence of a dual 5S rDNA system seems to be a general trait among non-related teleost fish orders, suggesting that evolutionary events of duplication have occurred before the divergence of the main groups of teleost fishes.     

Ammonia in estuaries and effects on fish

This review aims to explore the biological responses of fish in estuaries to increased levels of environmental ammonia. Results from laboratory and field studies on responses of fish to varying salinity and their responses increased ammonia will be evaluated, although studies which examine responses to ammonia, in relation to varying salinity, pH and temperature together are rare. In a survey of British estuaries the continuous measurement of total ammonia showed values that ranged from background levels increasing up to c. 10 mg N l^?1 although higher values have been noted sporadically. In outer estuaries pH values tended to stabilize towards sea water values (e.g. c. pH 8). Upper reaches of estuaries are influenced by the quality of their fresh waters sources which can show a wide range of pH and water quality values depending on geological, climatic and pollution conditions. In general the ammonia toxicity (96 h LC₅₀) to marine species (e.g. 0·09–3·35 mg l^?1 NH₃) appears to be roughly similar to freshwater species (e.g. 0·068–2·0 mg l^?1 NH₃). Ammonia toxicity is related to differences between species and pH rather than to the comparatively minor influences of salinity and temperature. In the marine environment the toxicity of ionized ammonia should be considered. The water quality standard for freshwater salmonids of 21 μg l^?1 NH₃–N was considered to be protective for most marine fish and estuarine fish although the influence of cyclical changes in pH, salinity and temperature were not considered. During ammonia exposures, whether chronic or episodic, estuarine fish may be most at risk as larvae or juveniles, at elevated temperatures, if salinity is near the seawater value and if the pH value of the water is decreased. They are also likely to be at risk from ammonia intoxication in waters of low salinity, high pH and high ammonia levels. These conditions are likely to promote ammonia transfer from the environment into the fish, both as ionized and unionized ammonia, as well as promoting ammonia retention by the fish. Fish are more likely to be prone to ammonia toxicity if they are not feeding, are stressed and if they are active and swimming. Episodic or cycling exposures should also be considered in relation to the rate at which the animal is able to accumulate and excrete ammonia and the physiological processes involved in the transfer of ammonia. In the complex environment of an estuary, evaluation of ammonia as a pollutant will involve field and laboratory experiments to determine the responses of fish to ammonia as salinity and temperature vary over a period of time. It will also be necessary to evaluate the responses of a variety of species including estuarine residents and migrants.