Large-scale Mortalities Research Articles

Comparative analysis of oyster (Crassostrea gigas) immune responses under challenge by different Vibrio strains and conditions

Screening a virulent Vibrio strain to study the interactions between bacterial infection and host immune defence is important to resolve large-scale summer oyster mortalities. Using adductor muscle injection we tested the oyster responses to phosphate-buffered saline (PBS), lipopolysaccharide (LPS), or Vibrio strains. Mortalities and stress gene expressions were used to characterize oyster immune responses. At 5 days post injection, the cumulative mortality rates in PBS, LPS, Vibrio tubiashii, Vibrio anguillarum, Vibrio alginolyticus and Vibrio aestuarianus groups were 8%, 16%, 36%, 40%, 40% and 76%, respectively, indicating that V. aestuarianus induced the highest death rate. Two-factor analyses of variance revealed that expression of SOD, CAT, GPX and HSP70 was influenced by bacterial injections in a time-dependent manner. The expression of all genes increased and reached their peak 3 or 12 h after bacterial injection and then decreased. These genes could be applied as immune responsive biomarkers to monitor early changes in oysters in response to bacterial infection. The greatest changes were observed in the V. aestuarianus-injected group, which may indicate that V. aestuarianus could be used as one of the more virulent strains for experimental infections.

Emergence of Epizootic Ulcerative Syndrome: Large-Scale Mortalities of Cultured and Wild Fish Species in Uttar Pradesh, India

Epizootic ulcerative syndrome (EUS), a disease listed by World Organisation for Animal Health (OIE) has been reported in 26 countries across 4 continents. Till date, 94 fish species have been found to be naturally infected with EUS and its host range is gradually expanding. In the year 2010-11, outbreaks resulting in heavy fish mortality were recorded in wetland districts of Uttar Pradesh, India, and EUS was confirmed as the cause of mortality on the basis of histopathology, isolation of Aphanomyces invadans , bioassay and PCR. A prevalence of ∼69% (371/540) was recorded and 13 fish species were found to be infected. Interestingly, EUS was observed in seven new species ( Aristichthys nobilis, Channa punctatus, Wallago attu, Mastacembelus armatus, Mystus cavasius, Anabas testudineus and Puntius conchonius ) for the first time in natural outbreaks. Furthermore, the disease was observed even in the month of May when the mean water temperature was 31.6±0.65°C. This suggests that the disease can result in severe losses even after two decades of its emergence.

Occurrence, seasonality and infectivity of Vibrio strains in natural populations of mussels Mytilus galloprovincialis

Widespread and large-scale mortalities of bivalve molluscs significantly affect their production. A number of pathogens have been identified as the primary causes of death in oysters or clams, especially bacteria of the genus Vibrio. We evaluated the occurrence, seasonality and infectivity of Vibrio strains associated with natural mussel (Mytilus galloprovincialis) populations. In particular, different isolates of V. splendidus and V. aestuarianus were analysed because they were associated with major oyster mortalities in areas where mussels are cultured without presenting mortalities. The presence of both Vibrio spp. was analysed bimonthly in mussels, water, sediment, plankton and other associated fauna from 2 sites in Galicia (NW Spain), the region with the highest mussel production in Europe. Environmental factors were also considered. The pathogenicity of different Vibrio isolates was analysed by performing experimental infections in mussels with strains isolated from the field. Results showed that Vibrio populations were mainly influenced by changes in water temperature and salinity. V. splendidus was dominant during the warm months and V. aestuarianus was predominant throughout the cold season. The sediment was the most important natural reservoir for bacteria. Experimental infections showed the extreme resistance of mussels to bacterial pathogens. Isolates of V. splendidus and V. aestuarianus were only moderately pathogenic for mussels in intramuscular infections and bath infections, and mortalities only occurred when animals were infected with a high bacterial concentration in adverse environmental conditions (hypoxia and 25°C). Although the pathogenicity of the Vibrio strains isolated from the wild was low for mussels, their potential risk for other bivalves cannot be ignored.

Biology, Host Range, Pathogenesis and Diagnosis of White spot syndrome virus

White spot syndrome virus (WSSV) is the most serious viral pathogen of cultured shrimp. It is a highly virulent virus that can spread quickly and can cause up to 100% mortality in 3-10days. WSSV is a large enveloped double stranded DNA virus belonging to genus Whispovirus of the virus family Nimaviridae. It has a wide host range among crustaceans and mainly affects commercially cultivated marine shrimp species. The virus infects all age groups causing large scale mortalities and the foci of infection are tissues of ectodermal and mesodermal origin, such as gills, lymphoid organ and cuticular epithelium. The whole genome sequencing of WSSV from China, Thailand and Taiwan have revealed minor genetic differences among different strains. There are varying reports regarding the factors responsible for WSSV virulence which include the differences in variable number of tandem repeats, the genome size and presence or absence of different proteins. Aim of this review is to give current information on the status, host range, pathogenesis and diagnosis of WSSV infection.

PATHOGENECITY OF GROUPER SLEEPY DISEASE IRIDOVIRUS (GSDIV: Megalocytivirus, FAMILY Iridoviridae) TO CORAL TROUT GROUPER Plectrophomus leopardus

Grouper sleepy disease iridovirus (GSDIV), a member of the genus Megalocytivirus in the family Iridoviridae, has been known to cause large scale mortalities resulting in severe economic losses in grouper industries in south-east Asia including Indonesia. In this study, experimental infection of coral trout grouper Plectrophomus indicus with GSDIV was performed to evaluate the viral pathogenecity to this fish species. After virus exposure, the mortalities of coral trout grouper injected with primary and 10-1 dilution of spleen homogenates derived from tiger grouper Epinephelus fuscoguttatus were 100% and 90%, respectively. Histopathology revealed that moribund fish receiving GSDIV inoculum displayed massive formation of enlarged cells in the spleen and hematopoitic tissues. Under electron microscopy, the enlarged cells were observed as inclusion body bearing cells (IBCs) and necrotic cells allowing virus propagation within an intracytoplasmic virus assembly site (VAS). GSDIV virions were 167-200 nm in size. These findings confirmed that GSDIV has severe pathogenicity to coral trout grouper and IBCs as well as necrotic cells were determined to be the pathognomonic sign of megalocytivirus-infected coral trout grouper.

ASSOCIATION OF AN UNUSUAL MARINE MAMMAL MORTALITY EVENT WITH PSEUDO-NITZSCHIA SPP. BLOOMS ALONG THE SOUTHERN CALIFORNIA COASTLINE

During 2002, 2,239 marine mammals stranded in southern California. This unusual marine mammal stranding event was clustered from April to June and consisted primarily of California sea lions (Zalophus californianus) and long-beaked common dolphins (Delphinus capensis) with severe neurologic signs. Intoxication with domoic acid (DA), a marine neurotoxin produced during seasonal blooms of Pseudo-nitzschia spp., was suspected. Definitively linking harmful algal blooms to large-scale marine mammal mortalities presents a substantial challenge, as does determining the geographic extent, species composition, and potential population impacts of marine mammal die-offs. For this reason, time series cross-correlation analysis was performed to test the temporal correlations of Pseudo-nitzschia blooms with strandings occurring along the southern California coastline. Temporal correlations were identified between strandings and blooms for California sea lions, long-beaked common dolphins, and short-beaked common dolphins (Delphinus delphis). Similar correlations were identified for bottlenose dolphins (Tursiops truncatus) and gray whales (Eschrichtius robustus), but small sample sizes for these species made associations more speculative. The timing of the blooms and strandings of marine mammals suggested that both inshore and offshore foraging species were affected and that marine biotoxin programs should include offshore monitoring sites. In addition, California sea lion-strandings appear to be a very sensitive indicator of DA in the marine environment, and their monitoring should be included in public health surveillance plans.

Domoic acid exposure in pygmy and dwarf sperm whales ( Kogia spp.) from southeastern and mid-Atlantic U.S. waters

The neurotoxin domoic acid (DA) was detected in urine and fecal samples recovered from pygmy sperm whales ( Kogia breviceps) and dwarf sperm whales ( Kogia sima) stranding along the U.S. Atlantic coast from 1997 to 2008. Of the 41 animals analyzed from Virginia, North Carolina, South Carolina and Florida, 24 (59%) tested positive for DA at concentrations of 0.4–1.8 ng/mL in urine and 12–13,566 ng/g in feces as determined by liquid chromatography–tandem mass spectrometry (LC–MS/MS). Feces appeared to be the best indicator of DA exposure in Kogia spp., with 87% of all fecal samples analyzed testing positive for this toxin. Additional stranded animals ( n = 40) representing 11 other cetacean species were recovered from the same region between 2006 and 2008 and analyzed by LC–MS/MS, however DA was not detected in any of these individuals. DA is produced naturally by diatoms in the genus Pseudo-nitzschia. Although blooms of DA-producing Pseudo-nitzschia have been associated with repeated large-scale marine mammal mortalities on the west coast of the U.S., there is no documented history of similar blooms on the southeast U.S. coast, and there were no observed Pseudo-nitzschia blooms in the region associated with any of these strandings. The feeding habits of Kogia spp. are poorly documented; thus, the vector(s) for DA exposure to these deep-diving species remains to be identified. Toxin accumulation in these pelagic whale species may be an indication of cryptic harmful algal bloom activity in offshore areas not currently being monitored. This study highlights the need for a better understanding of the role of toxigenic algae in marine mammal morbidity and mortality globally.

Pan‐steatitis in farmed northern bluefin tuna, Thunnus thynnus (L.), in the eastern Adriatic

Clinical, gross and histopathological investigations were carried out into large-scale mortalities on eastern Mediterranean bluefin tuna, Thunnus thynnus (L.), farms. Fish showed only nervous signs and darkened colour. At post-mortem the liver was bronze coloured and the pyloric area waxy in consistency. There was no evidence of any other gross pathology. Histopathology showed severe hepatic necrosis and lipidosis. Peri-pancreatic lipoid tissue was heavily infiltrated with an inflammatory round cell infiltrate. Fish on all three farms had been fed on a North African pilchard diet rather than traditional local or Baltic species. Once the diet was modified, losses ceased. A diagnosis of pan-steatitis as seen in other farmed fish species, as well as in terrestrial animals, on particular fish-based diets was made, although the actual factor within the diet which induced the inflammatory effect is not known.

Chemotaxis of the amphibian pathogen Batrachochytrium dendrobatidis and its response to a variety of attractants

Batrachochytrium dendrobatidis is a fungal pathogen of amphibians that is increasingly implicated as a major cause of large-scale mortalities of amphibian species worldwide. Previous studies indicate that motile zoospores of B. dendrobatidis colonize the keratinized tissues of susceptible amphibians. Infections spread to adults and cause destruction of epidermal tissue. In an effort to understand how the chytrid cues into its host we developed an assay to study chemotaxis in the fungus. Here we show that zoospores exhibit positive movement toward a variety of attractants including sugars, proteins and amino acids. These observations suggest that the chytrid can respond to nutritional cues, including those of host origin. Implications of these observations to amphibian susceptibility to infection and chytrid virulence are discussed.

Chemotaxis of the amphibian pathogen Batrachochytrium dendrobatidis and its response to a variety of attractants

Batrachochytrium dendrobatidis is a fungal pathogen of amphibians that is increasingly implicated as a major cause of large-scale mortalities of amphibian species worldwide. Previous studies indicate that motile zoospores of B. dendrobatidis colonize the keratinized tissues of susceptible amphibians. Infections spread to adults and cause destruction of epidermal tissue. In an effort to understand how the chytrid cues into its host we developed an assay to study chemotaxis in the fungus. Here we show that zoospores exhibit positive movement toward a variety of attractants including sugars, proteins and amino acids. These observations suggest that the chytrid can respond to nutritional cues, including those of host origin. Implications of these observations to amphibian susceptibility to infection and chytrid virulence are discussed.

Status of the recreational fisheries in two Australian coastal estuaries following large fish-kill events

Severe flooding occurred during early 2001 over a widespread region of northern New South Wales (NSW), Australia. This flooding was followed by large scale mortalities of invertebrates and finfish in the Richmond and Macleay estuaries. These estuaries were then closed to all forms of recreational and commercial fishing to enhance the recolonisation and recovery of their fish populations. The lower reaches of these estuaries were re-opened to limited fishing after closure periods of 4.5 months (Richmond) and 3.5 months (Macleay). Complemented angler surveys, based on stratified random sampling procedures, were used to assess the subsequent recovery of these important estuarine recreational fisheries. Historical information from the Richmond and Clarence estuarine fisheries and data collected during recent estuarine recreational fishing surveys in other areas of NSW were used as relative benchmarks to compare the post-fish-kill survey data. These comparisons indicated that at the time of the post-fish-kill surveys these fisheries were characterised by normal temporal trends in fishing effort and typical seasonal harvests. Similarly, three indicators of recreational fishing quality were shown to be of similar magnitude to comparable measures from other estuarine fisheries in NSW. All comparisons indicated that the recreational fisheries in the lower Richmond and Macleay estuaries had recovered and were providing good quality recreational fishing opportunities. Luderick ( Girella tricuspidata), yellowfin bream ( Acanthopagrus australis) and dusky flathead ( Platycephalus fuscus) together accounted for 93.9% (Richmond) and 91.7% (Macleay), by weight, of the recreational harvest. Length frequency distribution data for these species taken by boat-based fishers were compared between estuaries. No significant differences between estuaries were detected for luderick or yellowfin bream indicating that legal sized recruits recolonising these estuaries after the fish-kill events were derived from larger meta-populations which spanned a spatial scale of at least 200 km (the distance between the estuaries). These results suggest that for luderick and yellowfin bream the deleterious impacts of localised catastrophes, even on the spatial scale of a large estuary, can be mitigated rapidly by immigration from other areas. In contrast, the length frequency distributions of dusky flathead that were present in the Richmond and Macleay estuaries during the survey period were highly significantly different indicating that they had originated from different source populations. The more localised scale of recolonisation evident for dusky flathead indicates that this species may be less resilient to localised catastrophic events than luderick or yellowfin bream. This study highlights the value of recreational fishing surveys for providing insights about the factors that may influence the recolonisation and recovery of estuarine finfish populations following large fish-kill events.

Apoptosis in Crassostrea virginica and its modulation by Perkinsus marinus

Apoptosis has been implicated as a defense mechanism against intracellular parasites whereas the parasite-induced manipulation of host apoptosis is a means to extend survival and proliferation. Perkinsus marinus is an intracellular protozoan parasite which causes large scale mortalities in the eastern oyster, Crassostrea virginica. We studied whether apoptosis of oyster immune cells (hemocytes) is increased to counteract P.marinus infection and whether P.marinus can inhibit this increase. Our data show that following in vitro exposure of hemocytes, hemocyte apoptosis significantly increased by 45 minutes, remained elevated up till 3 hours and returned to the control levels by 4 hours. In vivo, infections of oysters with P.marinus for one day resulted in elevated apoptotic levels of hemocytes accompanied by a significant decrease in circulating hemocyte concentration. By day two, apoptosis levels returned to control, and infections in oyster tissues were clearly established. Together, these results suggest that an increase in apoptosis may be an early defense mechanism employed by oysters to control infection and P.marinus successfully establishes infection by suppressing this response. Supported by NOAA/North Carolina Sea Grant and UNC Charlotte Faculty Research Grant.

TOXICITY OF CLOSTRIDIUM BOTULINUM TYPE E NEUROTOXIN TO GREAT LAKES FISH: IMPLICATIONS FOR AVIAN BOTULISM

Since 1999, large-scale mortalities of fish-eating birds have been observed on the Great Lakes, and more specifically on Lake Erie. Type E botulism has been established as the primary cause of death. The mechanism of type E botulism exposure in fish-eating birds is unclear. Given that these birds are thought to eat live fish exclusively, it seems likely that their prey play a key role in the process, but the role of fish as potential transport vectors of botulinum neurotoxin type E (BoNT/E) to birds has not been adequately investigated. Between June 2003 and April 2004 a methodological model for exposing fish to Clostridium botulinum was developed and used to compare the sensitivity of rainbow trout (Oncorhynchus mykiss), round goby (Neogobius melanostomas), walleye (Stizostedion vitreum), and yellow perch (Perca flavescens) to four doses (0, 800, 1,500, and 4,000 Mouse Lethal Doses) of Clostridium botulinum type E neurotoxin. Each fish species expressed unique changes in both behavior and skin pigmentation prior to death. Yellow perch survived significantly longer (P < 0.05) than the three other species at all toxin treatments. Results of this study suggest that live fish can represent a significant vector for transfer of BoNT/E to birds.

Vibrio harveyi and other bacterial pathogens in cultured summer flounder, Paralichthys dentatus

A monitoring program for Vibrio harveyi and other potential bacterial pathogens of summer flounder was conducted at two facilities in the Northeast United States. Bacterial samples were collected from larval and juvenile fish and live-feed, and identified using API 20E biochemical profiles and 16S rDNA sequencing. Histopathological examinations were conducted in order to relate histological changes with the presence of potential bacterial pathogens. V. harveyi, Vibrio ichthyoenteri, and Photobacterium damselae subsp. damselae, three known pathogens of flatfish, were isolated from diseased summer flounder. Although a high proportion of juvenile summer flounder showed microscopic signs of disease, the presence of these potential bacterial pathogens in fish was not associated with large-scale mortalities. An outbreak of flounder infectious necrotizing enteritis with 30% cumulative mortality occurred when juveniles were transported to a new facility. Isolates of V. harveyi from the disease outbreak were genetically different from the isolates from the commercial hatchery. V. harveyi isolates from both facilities were pathogenic to summer flounder by intraperitoneal injection. P. damselae subsp. damselae, V. ichthyoenteri, and Vibrio scophthalmi, also found in fish with gross lesions, were not pathogenic to juvenile summer flounder by intraperitoneal injection. Our research shows that several potential bacterial pathogens are associated with morbidity and mortality in summer flounder larvae and juveniles, especially in situations of stress. Increased knowledge about the environmental conditions that lead to disease, as well as the interactions of V. harveyi with other microbial species, could lead to the development of management strategies in summer flounder farms.

Sea temperature variability and Paracentrotus lividus (Echinoidea) population fluctuations

Observations and censi of the echinoid Paracentrotus lividus over 40 y at Lough Hyne marine nature reserve have revealed population fluctuations of &gt;4 orders of magnitude. The P. lividus population has been anomalously low since the mid 1980s. The population age–size structure has also changed considerably and is now dominated by older individuals. Sea temperature is a known and important determinant of spawning (and recruitment) in this species. In certain years sea surface temperatures (SST) may not reach critical values for P. lividus mass-spawning to be triggered. In addition, toxic dinoflagellate blooms may have been responsible for large-scale mortalities within short periods. Years of anomalously low SSTs match the timing of critical decreases in the Lough Hyne P. lividus population. Years with low maximum SSTs coincide with the timing of major El Niňo Southern Oscillation (ENSO) events. We suggest a SST and possibly ENSO link to unfished (Lough Hyne) P. lividus population changes and propose such links as potential aggravating factors in the decline of the fisheries.

The nutrient enrichment of Lake Victoria (Kenyan waters)

Lake Victoria is an international freshwater resource of great ecological and economic importance. Large changes that have occurred recently in the environment and the ecology of the lake are associated with natural processes as well as anthropogenic activities in the catchment's area and the Lake itself. Data collected in the Kenyan waters of the Lake in June 1998 indicate that nutrient concentrations and algal biomass are higher than previously reported and that Secchi transparency has decreased. These are evidence of increasing nutrient enrichment and general decline in water quality. Increased algal blooms, depletions of dissolved oxygen, frequent large-scale mortalities of fish, decline of endemic fish species, predominance in the fishery by introduced species, changes in other biotic species composition, proliferation of the water hyacinth and increasing human health risks also manifest the ecological degradation of the Lake. Long-term conservation efforts for rehabilitation and sustainable management of the Lake are suggested.

The effects of pollution on fish health

Potentially harmful substances-e.g. pesticides, heavy metals and hydrocarbons-are often released into the aquatic environment. When large quantities of pollutants are released there may be an immediate impact as measured by large-scale sudden mortalities of aquatic organisms, e.g. fish kills resulting from contamination of waterways with agricultural pesticides. Lower levels of discharge may result in an accumulation of the pollutants in aquatic organisms. The end results, which may occur long after the pollutants have passed through the environment, include immunosuppression, reduced metabolism, and damage to gills and epithelia. However, the link between adverse water quality and fish diseases is not proven. Alleged pollution-related diseases include epidermal papilloma, fin/tail rot, gill disease, hyperplasia, liver damage, neoplasia and ulceration. Many surveys have indicated a greater proportion of diseased fish in polluted compared to non-polluted marine sites. Yet, the value of such surveys may be questioned. Specific examples of fish diseases thought to reflect the effects of pollution include surface lesions attributed to Serratia plymuthica, fin and tail rot caused by Aeromonas hydrophila and Pseudomonas fluorescens, gill disease resulting from the actvity of Flavobacterium spp., vibriosis as caused by Vibrio anguillarum, and enteric redmouth (causal agent, Yersinia ruckeri). Research indicated that some of the diseases caused by Aeromonas, Flavobacterium and Pseudomonas resulted from generally adverse water quality, i.e. higher than usual quantities of organic material, oxygen depletion, changes in pH values and enhanced microbial populations. Some infections with Serratia and Yersina may well have reflected contamination of waterways with domestic sewage, e.g. leaking septic tanks. At least one outbreak of vibriosis was linked to high concentrations of copper, which may have debilitated the fish making them more susceptible to disease.

Decreased lymphocyte responses in free-ranging bottlenose dolphins (Tursiops truncatus) are associated with increased concentrations of PCBs and DDT in peripheral blood.

Since 1987, large-scale mortalities of dolphins have been reported along the Atlantic coast of North America, in the Gulf of Mexico, and in the Mediterranean Sea. Autopsied bottlenose dolphins, Tursiops truncatus, which were collected from the large-scale mortality along the Atlantic coast in 1987 to 1988, exhibited opportunistic infections indicative of immune dysfunction. Further, these animals had high levels of chlorinated hydrocarbons, such as PCBs and DDT, that can suppress immune functions. The purpose of this study was to determine whether there is a relationship between chemical contaminant exposure and immune response in free-ranging dolphins. In June of 1991, peripheral blood was obtained from members of a bottlenose dolphin population that resides along the west coast of Florida. Peripheral blood lymphocyte responses to Concanavalin A (Con A) and phytohemagglutinin (PHA) were determined in vitro and compared by regression analysis with contaminant concentrations in whole blood from a small subset of these animals (n = 5). These data indicate that a reduced immune response in these bottlenose dolphins was correlated with increasing whole blood concentrations of several contaminants. Specifically, inverse correlations were found between Con A-induced lymphocyte proliferation and tetrachlorinated to octachlorinated biphenyls (r2 values ranged from 0.70 to 0.87). Con A-induced lymphocyte responses also correlated inversely with p,p'DDT (r2 values of 0.73 and 0.79); o.p'-DDE (r2 values of 0.93 and 0.96); and p,p'-DDE (r2 values of 0.73 and 0.81).

Impact of a major cyclone on a southeast African estuarine lake system

The St Lucia lake and estuarine system in northern Natal, South Africa was struck by Cyclone Domoina on 31 January 1984. This is the only cyclone known to have traversed part of South Africa. The ensuing flood levels in the St Lucia system (which have been exceeded perhaps only one other time in recorded history) provided an opportunity to assess the effect of a major disturbance on a subtropical estuarine system. Major geomorphological changes occurred in the mouth area where all man-made structures were obliterated. The two river channels were scoured from 2–3 m to 10–14 m in depth and widened by up to 300 m, while the shoreline between the two channels retreated in places up to 100 m. An estimated 16∗10 6 m 3 of sediment were removed from the lower reaches of the system including areas of mangrove and Phragmites australis swamp. One-metre tall mangrove saplings died following inundation lasting days to weeks. Effects on the fauna included redistribution of the tanaid Apseudes digitalis, the mysid Mesopodopsis africana, the copepod Pseudodiaptomus stuhlmanni, the polychaete Scololepis squamata and the bivalve Solen cylindraceus, all typical lake species which appeared in the tidal channel linking the lake with the sea. Responses of other species such as the penaeid prawns, the crab Scylla serrata and the sole Solea bleekeri suggested that physiological tolerance, extended larval or juvenile recruitment periods and prey switching minimized long-term flood effects. No large-scale mortalities were noted, and the absence of a temperature shock is considered to be a significant feature.

The demography and management of large cyprinids in a reservoir on the Orange River, South Africa

The population dynamics of smallmouth yellowfish, Barbus holubi, and Orange River labeo, Labeo capensis, was studied in a large reservoir from 1978 to 1983. These two species made up over 90% of the catches in a fleet of experimental gillnets, but there were considerable fluctuations in catch and in the size and species composition of the catch. These variations largely resulted from changes in environmental conditions caused by hydrological management. Yellowfish breed in the inflowing Orange River, which is regulated through a large reservoir upstream. Spawning success is variable but generally good, and depends on time of spawning which is influenced by pattern of water release. The juveniles disperse well throughout the 80-km-long lake and sub-adults are generally most abundant near the dam wall where the water is clearest. Sharp decreases in water transparency caused by the short retention time of flood waters in the two impoundments greatly reduced the carrying capacity of the lake for yellowfish, and resulted in declining body condition and large-scale mortalities. This species could probably withstand intensive exploitation in the lake. Orange River labeo breed throughout the lake, but conditions for spawning and early survival are variable, leading to variable reproductive success. This species is relatively stationary, but dispersal in the lake is enhanced when decreased water transparency reduces food production. Although growth is very poor at times of high turbidity, mortality remains low and the ultimate size reached by individual fish varies little. Therefore, this species cannot sustain an intensive fishery, but lends itself well to exploitation on an accumulated stock basis. A fishery for labeo should complement a commercial fishery for yellowfish.