Population Biology of Nile tilapia (Oreochromis niloticus) in Lake Naivasha,Kenya

The Nile tilapia (Oreochromis niloticus) was introduced into Lake Naivasha to provide exploitable fisheries, being one of the commercially important fish providing livelihoods to communities around the lake. Its stocks in the lake have been declining, however, because of intense fishing, ecological changes and effects of other exotic species. The present study was undertaken to investigate the population and biological parameters of Oreochromis niloticus. Fish samples (1,021) were caught monthly from January to December 2017, collecting a total of 1,021 O. niloticus, with an overall sex ratio of 2.21:1.0 (male: female). The length of the fish ranged from 9.0 to 40.0 cmTL and weighed between 20 and 1,220 g. The mean length and weight for all fish was 22.6 cm and 272.6 g. The slope b of length–weight relationship was 2.86 for all fish, indicating negative allometry. The mean condition factors were 0.99, 1.04 and 1.01 for male, female and all fish, respectively. The length at first maturity (Lm₅₀) was estimated to be 28.0 cmTL for all sexes, indicating the fish matures at smaller sizes. The length at first capture (Lc₅₀) was calculated to be 19.38 cm, a value much lower than its Lm₅₀, reflecting the capture of juvenile fish. The asymptotic length (L∞) was 42 cm, the growth coefficient (K) was 0.21/year, and the growth performance index (Ф) was 2.57. The natural (M), fishing (F) and total mortality (Z) coefficients were estimated to be 0.55, 0.26 and 0.80/year, respectively. The optimum sustainable yield (E_0.5), maximum sustainable yield (E_max) and maximum economic yield (E_0.1) indices of 0.23, 0.35 and 0.26, respectively, imply an optimal exploitation of O. niloticus. The results of the present study provided some important insights into the biology and management needs of the O. niloticus fishery in Lake Naivasha, as well as revealing a gap for further research on its reproductive biology.     

A comparative study of Labeo victorianus (Bouelenger, 1901) and Oreochromis niloticus (Linnaeus, 1758) grown in polyculture systems

A polyculture experiment with African carp (Labeo victorianus) and Nile tilapia (Oreochromis niloticus) was conducted for 24 weeks in 12 earthen ponds measuring 150 m² to assess their compatibility. Monoculture treatments T1 and T4 for L. victorianus and O. niloticus, respectively, were the controls for the two species, while treatments T2 and T3 were a polyculture of L. victorianus and O. niloticus at a ratio of 1:2 and 2:1, respectively. All treatments were carried out in triplicate, with feedings performed twice daily at 09:00 hr and 16:00 hr at 10%, 5% and 3% body weight distributed evenly over the 24‐week growth period. Fish sampling was conducted monthly, while water quality was measured biweekly. Treatment T3 (1L: 2T) had a significantly (p < .05) higher survival (L = 49% and T = 87%), compared to T2 (L = 31% and T = 69%, 2L: 1T). O. niloticus monoculture treatment (T4) exhibited a significantly higher survival (p < .05) and higher growth (80%; 79 ± 20.04 g) than L. victorianus monoculture (T1) (50%; 13.12 ± 4.49 g). Dawn and afternoon temperatures ranged between 20.16–24.49°C and 23.76–31.07°C, respectively. The lowest temperatures were recorded in the months of June, July and August. The NH₃ concentration was zero at 14:00 hr for all treatments throughout the growth period, but significantly higher (p < .05) in T1 at 06:00 hr. The pH and DO levels at 06:00 hr and 14:00 hr were significantly different in all the treatments (p < .05). The slow growth of L. victorianus can be associated with the low temperatures and DO levels. However, there is need to identify other factors that may have contributed to the poor growth of L. victorianus. Accordingly, this study recommends L. victorianus growth trials in other culture systems, coupled with different feed formulations, to identify optimal culture conditions.     

A review on the status of some major fish species in Lake Victoria and possible conservation strategies

Lake Victoria, the second largest freshwater lake in the world, supports an enormous flora and fauna biomass, with a large human population around the Lake. The lake is a source of food (fish), water for domestic use and crop and livestock production, transportation and tourism, among other uses. As a result of these varied uses, human intervention within the lake and its catchment has resulted in several ecological changes in the lake in recent years, with profound effects on its fish resources. One of the most notable changes is the reduction and risk of extinction of some fish species, including the endemic tilapiine species (Oreochromis esculentus and Oreochromis variabilis), catfishes (Xenoclarias eupogon), haplochromines and cyprinids (Labeo victorianus and Barbus altinialis) in the lake. The reduced biodiversity, as well as extinctions within the lake, has been attributed to the introduction of alien fish species (e.g. Nile perch (Lates niloticus) and Nile tilapia (Oreochromis niloticus) into the lake, habitat loss and cultural eutrophication. If left unchecked, these changes will have devastating effects on the lake's resources, as well as to people living around the lake and beyond who depend on the lake fisheries for their livelihood. Thus, there is an urgent need for management measures based on sound scientific research to be implemented in order to curb the loss of ichthyodiversity within the lake, as well as to save the livelihoods of those stakeholders who are either directly or indirectly dependent on the lake.     

Trends in interannual yield variation of reservoir fisheries in Bangladesh, with special reference to Indian major carps

Interannual variation in yield per species from a freshwater reservoir in Bangladesh was analysed by using long‐term landing data. Each species was categorized according to characteristics in interannual yield variation. The characteristics were defined with respect to size of the interannual variation in terms of absolute and relative variation around the mean yield and long‐term trends in catch levels. Annual variation was directly (t₁₈ = 7.84; P < 0.01) related to the yield and can serve as a good indication of the status of the fishery when catches are low. The target fish, Indian major carps (IMC) Labeo rohita, Labeo calbasu, Catla catla and Cirrhinus mrigala, showed large variation in annual yield. This might have occurred because of variation in annual recruitment. Illegal use of gears is mostly related to variation in year class strength, and is assumed to be the most important source of change of stock variation. An increase in catch rate was recorded for Corica soborna. Such a stable combination may only be possible for a certain period without a long‐term trend. Trends in the yield rate of the major fish category for three decades reveal a sharp decline in the catch of IMC. In contrast, small clupeid‐like C. soborna have the highest yield rate. Fishing gears can play a crucial role in this fishery. Fisheries managers must always be alert when issuing fishing licenses and on the control on gears. The conclusion is that the IMC fishery of Kaptai Reservoir is an overexploited fish stock. In this fishery, the same gears are usually employed indiscriminately for different species and size classes. To stabilize the fishery, the existing policy needs to be strengthened and more effectively implemented. Also, some fishing gears and traps need to be restricted. An effective stock protection policy is suggested. Further study on catch size of individual species is essential prior to the development and implementation of any management scheme.     

Biological aspects of Schilbe intermedius (Ruppell, 1832) in the Nyanza Gulf of Lake Victoria,Kenya

Silver butter catfish (Schilbe intermedius) is an indigenous fish species in Lake Victoria. It has a good economic value because of the quality and freshness of its flesh, increasing its demand for consumption. It is potamodromous, with its stock in the lake declining because of overexploitation by the use of illegal fishing gears at the river mouths on their way to spawning areas. Fish samples (321) were obtained from commercial gill net (1.5–3 in.) fishers within Nyanza gulf. The fish samples were weighed (g) and measured (cm), dissected and sex determined by visual inspection of the gonads. Ovaries for mature female fish specimens were preserved for analysis of fecundity. Fecundity was determined by the gravimetric technique. The mean (±SE) length and weight for all fish were 18.2 ± 0.2 cm TL and 51.9 ± 1.9 g, respectively. The female fish (20.0 ± 0.2 cm) were significantly (F = 199.80, p < .05) larger than the males (15.9 ± 0.1 cm). Similarly, the females (68.8 ± 2.6 g) were significantly (F = 152.61, p < .05) heavier than the males (30.93 ± 1.02 g). The overall sex ratio was 1.23:1.0 (female:male). The length frequency exhibited a unimodal distribution, with the modal class between 16 and 18 cm for either sex. The regression slope (b) of the length–weight relationship was 3.2, 3.1 and 3.0 for all fish, females and males. The absolute fecundity ranged from 6,510 to 59,818 eggs, with a mean of 34,473 eggs. The length at 50% maturity was estimated to be 16.0 cm (female) and 18.0 cm (male) fish. The mean length and size at maturity of S. intermedius have declined, likely attributable to fishing effects, changes in food resources, competition and predation. Thus, the present study provided relevant biological data of S. intermedius, which is currently limited for this region.     

Implications of increasing pollution levels on commercially important fishes in Lake Victoria

Lake Victoria receives huge quantities of effluent from domestic, agricultural and industrial sources. We used fish condition factor (K), vitellogenin (VTG) production and liver lesions as biomarkers to assess pollution levels in the lake. We tested the hypothesis that pollution levels do not affect the selected biomarkers. Beach seine and cast nets were used to collect Oreochromis niloticus (n = 230), Lates niloticus (n = 99) and Protopterus aethiopicus (n = 37) in areas presumed to be less or more polluted, both inshore and offshore. K was lower in more polluted compared to less polluted areas of the lake. VTG production was high in both less and more polluted areas for O. niloticus (0.77 ± 0.08 µg/L), L. niloticus (0.73 ± 0.09 µg/L) and P. aethiopicus (0.55 ± 0.06 µg/L). Liver tissue showed lesions such as vacuolations, cellular degeneration, sinusoidal dilation, focal necrosis, increased Küpffer cells and congestion of sinusoids. The prevalence of liver tissue alteration showed normal lesion (19.9%, n = 73), slight (8.2%, n = 30), moderate (41.5%, n = 152), severe (18.6%, n = 68) alterations and irreparable damage (11.8%, n = 43). Severe liver alterations in O. niloticus, L. niloticus and P. aethiopicus were higher in more polluted compared to less polluted areas. Chemical contamination of Lake Victoria caused liver lesions and other changes in fishes, possibly leading to adverse effects on the lake’s fisheries resources. Overtime, such chemical contamination could lead to negative impacts on the consumers of fish if actions are not taken to mitigate the risks.     

Population dynamics and exploitation patterns of Oreochromis niloticus in Lake Tana,northwest Ethiopia

Oreochromis niloticus is the dominant commercial fish in the Lake Tana region. However, its fishery is progressively declining over time. Little or no updated information exists on the population dynamics and exploitation patterns of the species, which is crucial to guide its sustainable management. Accordingly, the purpose of the present study was to generate essential biological parameters on the growth, mortality and stock status of O. niloticus, using length‐frequency data collected monthly from the commercial fish catches of 1 year (2014–2015). The total mortality coefficient (Z) was derived from the length‐converted catch curve. Biological reference points were predicted from relative yield‐ and biomass‐per‐recruit analyses. The estimated values of the von Bertalanffy growth parameters were L_∞ = 44.1 cm, K = 0.44/year, and t₀ = ?0.34/year, and the growth performance index (Φ′) was 2.93. The total mortality (Z), natural (M) and fishing mortality (F) rates were 2.37, 0.98 and 1.39 per year, respectively. The current fishery exploitation rate of 0.59 exceeds the estimated biological reference points of E_max (0.52), confirming the stock of O. niloticus in the lake is being overexploited above optimum levels. Size indicators of the catches further illustrate 31% of the landed fish are harvested before reaching sexual maturity, with mega‐spawners comprising only 15%. This indicates the stock is suffering from both growth and recruitment overfishing. The logistic selection model indicated 50% of the fish vulnerable to capture was at 18.14 cm TL. The fish exhibited a year‐round recruitment pattern, with a major peak during May and June. Sustainably managing the fishery, therefore, requires increasing the fish size at first capture (L_c) towards L_opt.     

Some aspects of the biology of Nile perch,Lates niloticus,in the open waters of Lake Victoria,Kenya

Samples (6404) of Nile perch were collected monthly from commercial catches between June 2014 and June 2015. The total length (cm), weight (g) and sex of fish were all determined in the field, with 3771 (59%) being male and 2059 (32%) being female fish, for an overall sex ratio of 1.83:1.00 (male: female). The average length and weight for all fish were 55.38 ± 0.14 cm TL and 2355.10 ± 22.30 g, respectively. The size of male fish ranged from 15.5 to 128.5 cm TL and weighed between 190 and 25 000 g, whereas that of female ranged from 21.0 to 130.0 cm TL and weighed between 124 and 25 800 g. The value of the regression slope b of the length–weight relationship was equal to 3.04. The mean (±SD) condition factor for all fish was 1.23 ± 0.13. The condition factor was different in the length groups, with the highest (1.32 ± 0.13) and lowest (1.12 ± 0.20) values recorded in length groups 100–110 and <30 cm TL, respectively. The highest and lowest condition factors were recorded between January and March, respectively. The length–weight relationship and condition of Nile perch in Lake Victoria have deteriorated greatly, this finding being linked to the reduction of its prey species in the lake.     

Impacts of the Victoria Nile Bujagali hydropower dam on fisheries and livelihoods

The Upper Victoria Nile (UVN) flows from Lake Victoria into Lake Kyoga, spanning 117 km and supports a diverse aquatic fauna that sustains livelihoods of riverside communities. The UVN is habitat to critically endangered (Labeo victorianus), near threatened (Oreochromis variabilis and Oreochromis esculentus), and rare species (Neochromis simotes). Four hydropower dams were built on the UVN to provide energy for industrial and socioeconomic development, notwithstanding consequential environmental and socioeconomic impacts. The impacts of Bujagali hydropower (BHP) dam on fisheries and livelihoods were assessed biannually (April and September) from 2006 to 2019, using fishing effort, species abundance, catch composition, and economic beach revenue at upstream, mid-reservoir, and downstream transects. The fishing boats increased from 31 in 2009 to 293 in 2019 and fishers from 83 to 500 over the monitoring period. Maximum annual catch of 461.4 t was recorded in 2014 and lowest of 54.2 t in 2009. In all transects, Protopterus aethiopicus and Labeo victorianus were least in the commercial catches and the use of Mormyrus kannume juveniles as bait for Nile perch fishery corresponded with increased catches from 3.3 t in 2009 to 148.2 t in 2019. Women were mostly engaged in post-harvest activities such as fish drying, smoking, and food vending. These observations suggest coupling effects of the hydropower dam and ineffective fisheries management. The harvest of M. kannume wild stocks for bait should be banned and research in possible domestication of the species undertaken. There is a need to strengthen fisheries enforcement to curb illegal effort and overexploitation and to implement conservation actions to mitigate potential biodiversity impacts from the hydropower dam operations.     

Some aspects of reproduction and feeding habits of Nile tilapia (Oreochromis niloticus) in three dams in Uasin Gishu County,Kenya

Knowledge of the fisheries status of dams within Uasin Gishu County was needed prior to the government's plan to introduce fish and fisheries in the area. The dams were constructed in the 1950s and stocked with tilapia for local consumption, recreation and control of macrophytes. The Nile tilapia (Oreochromis niloticus) was selected for the present study due to its establishment success and popularity in the Kenyan market. Water samples were collected at subsurface levels for phytoplankton analysis and compared with the phytoplankton found in the stomachs of O. niloticus, revealing the food preference of the fish in a natural environment. Fish samples were collected with gillnets and beach seines. The results of the present study identified the most important food items for the fish were Chlorophyceae (green algae), Bacillariophyceae (diatoms) and Cyanophyceae (blue‐green algae). The fish exhibited a relative condition factor of about 1.00, indicating their robustness or well‐being in the dams. The LM₅₀was reached at 18–20 cm class interval, which coincides with the most critical breeding biomass needing some kind of protection for sustainable management of the fishery.     

Some aspects of the biology and life‐history strategies of Oreochromis variabilis (Boulenger 1906) in the Lake Victoria Basin

Oreochromis variabilis (Boulenger), a fish species endemic to Lake Victoria, was abundant, forming an important component of the indigenous fisheries stocks before and up to the late‐1950s. Catches declined drastically thereafter, and only sporadic catches are currently found in Lake Victoria. Remnants population of the species, however, are found in several small waterbodies (SWBs) within the lake basin. The life‐history characteristics of O. variabilis in Lake Victoria, including, sex ratio, reproduction and length–weight relationship, were compared to those in selected three SWBs in the lake basin. Fish samples were collected by monofilament gillnets of 30–255 mm between 2001 and 2005. Males predominated over females from all the sampled sites (sex ratio 1.00:0.33). Length at first maturity (Lm₅₀) had mean (±SE) of 18.48 ± 1.50 cm TL for males, and 16.87 ± 0.95 cm TL for females, and did not exhibit any significant differences between habitats. Fecundity ranged between 73 and 14 800 eggs for fish of 13.5–18.6 cm TL, respectively. Absolute fecundity of O. variabilis was proportional to the body weight, but nearly proportional to the cube of the fish length. Egg diameter varied from 0.3 to 5.19 mm, with a mean (±SE) of 3.44 ± 0.08 mm. Growth was allometric in both male and female, being significantly different from the expected value of 3 (P < 0.05). The life‐history strategy of O. variabilis is discussed within the context of changes in the lake and the SWBs.     

Population characteristics of five major commercial fish species of the Tono Reservoir in northern Ghana

Five major commercial fish species, comprising Sarotherodon galilaeus, Oreochromis niloticus, Coptodon zillii, Clarias gariepinus and Auchenoglanis occidentalis of the Tono Reservoir fishery, were studied from January, 2015 to December, 2016 using length‐based models to establish the state of the stocks. Routines in the FAO‐ICLARM Stock Assessment Tools II (FISAT II) were used to determine the growth and mortality parameters, exploitation rates (E) and recruitment patterns of the length–frequency data generated from commercial fish landings and experimental catches. The growth coefficients (K) of the five fish species ranged 0.48–4.89 per year; asymptotic lengths (L_∞) ranged from 18.38 to 76.16 cm; and hypothetical ages (t₀) ranged between 0.03 and 0.28 years. All five fish species were exploited above the optimal exploitation rate (E_opt = 0.5), indicating overfishing of the species. There was a year‐round recruitment, with major and minor peaks corresponding with the major and minor spawning seasons. Fishing gear regulation and community‐based co‐management system were some management options recommended to enhance sustainable exploitation and management of the fishery.     

Trophic interactions among algal blooms,macroinvertebrates, and brown trout: Implications for trout recovery in a restored river

Positive correlation between trout abundance and dissolved metal concentrations along the Upper Clark Fork River (UCFR; Montana, USA) have forced restoration practitioners to seek underlying causes of reduced fish density beyond heavy metal contamination. Throughout the river, nutrient enrichment and summer algal blooms may be hindering full recovery of trout populations. In this study, we evaluated the community structure and metal body burdens of benthic invertebrates and characterized existing trophic linkages between brown trout and dominant invertebrate taxa before and during summer algal blooms in a downstream reach of the UCFR where fish densities are low (20–30 trout/km), and where metal contamination is relevant but minimal compared with upstream. In spring, estimated invertebrate abundance was 1,727 ± 217 individuals/m² and dominated by Ephemerellidae and Baetidae families. During summer algal bloom, invertebrate abundance increased 15‐fold (20,580 ± 3,510 individuals/m²) mostly due to greater abundance of Chironomidae, Hydropsychidae, and Simulidae. Copper body burdens (130 ± 42 ppm) were higher than any other heavy metal regardless of season, but detectable concentrations of arsenic, cadmium, and lead were also found. A Bayesian mixing model combining metal burdens and stable isotopes showed that in the spring, trout of average size (355 ± 65 g) relied mostly on epibenthic taxa (Ephemerellidae and Hydropsychidae), contrasting with small (<100 g) and large (>400 g) trout relying heavily on Baetidae, a major component of invertebrate drift. Foraging segregation related to trout size did not occur during summer algal blooms, which may reflect increasing influence of benthic algal proliferation or indicate the indiscriminate use of pool habitats as thermal refugia over summer conditions by trout of different ages.     

Growth and population parameters of Nile tilapia,Oreochromis niloticus (L.) in the open waters of Lake Victoria,Kenya

The Nile tilapia (Oreochromis niloticus) was introduced into Lake Victoria in the early 1950s and 1960s and has since become the dominant tilapiine in the lake. This study investigated the growth and population parameters of O. niloticus in Lake Victoria on the basis of length–frequency data collected during the period June 2014 and June 2015. The asymptotic length (L_∞) had a mean (±SE) value of 46.24 ± 0.04 cm TL, growth curvature (K) of 0.69 ± 0.25 year^?1, total mortality (Z) of 2.18 ± 0.80 year^?1, a natural mortality (M) of 1.14 ± 0.28 year^?1, a fishing mortality (F) of 1.05 ± 0.53 year^?1, an exploitation rate (E) of 0.46 ± 0.08, a growth performance index (?) of 3.14 ± 0.17 and a length at first capture (L_C50) of 20.31 ± 0.40 cm TL. Comparing the results of this study with previous studies indicates the parameters K, Z and M have increased, whereas ?, F, E and L_C50 have decreased. Changes in these parameters could be attributed to the existing high fishing capacity, and changing lake conditions. Thus, management measures should include continued restriction on illegal fishing methods and gears, such as the use of undersized gillnets (<5 in. mesh size) and beach seines. More attention also should be directed to the implementation of measures to control pollution of the lake from its various sources.     

Exotic introductions to the fishery of Lake Victoria: What are the management options?

Lake Victoria is Africa's most important source of inland fishery production, exhibiting annual catches of ≈ 400 000 mt. The predatory Nile perch, Lates niloticus, and the herbivorous tilapiines, Oreochromis niloticus, Oreochromis leucostictus, Tilapia zillii and Tilapia rendalii, were introduced in Lake Victoria in the 1950s and 1960s. Nile perch were introduced to convert the abundant, but bony, haplochromines to fish flesh, while the tilapiines were introduced to boost the declining fishery. Since that time, the fisheries of Lake Victoria have undergone dramatic social and ecological changes. The catches increased tremendously, changing the fishery from artisanal to commercial, in turn increasing fisher income and employment opportunities. However, there was a decline and, in some cases, the disappearance of many indigenous fish species, especially the haplochromine cichlids. This reduction was attributed to overexploitation, predation, and competition and hybridization with the introduced species. The decline of the native fish species has had impacts on the trophic and ecological status of the lake. Nile perch now dominate the formerly complex food web. The loss of phytoplanktivorous haplochromines has contributed to an increase in algal blooms, reduction in water quality and occasional fish kills. Water hyacinth, Eichhornia crassipes, invaded Lake Victoria in 1988, with high rates of infestation in shallow waters and bays, which are breeding and nursery grounds for most fish species. Catches of Nile perch decreased following the infestation, while those of Nile tilapia, lung fish (Protopterus aethiopicus) and mud fish (Clarias gariepinus) increased. Haplochromines species also showed some signs of recovery. In view of all these changes, the future of the Lake Victoria fishery is uncertain.     

Climate change: A potential risk to cage culture and genetic structure of native fisheries in fish cage‐installed inland lakes

Fish cage aquaculture (FCA) in lakes is a viable climate change (CC) mitigation approach; however, deficiencies in monitoring, early warning and risk planning could threaten its sustainability. Thus, the present study focused on examining the experiences with CC and its risks to cage fish culture, and phenotypic structure of a typical endemic fishery in fish cage‐installed (FCI) and fish cage‐free (FCF) lakes. Using a structured questionnaire, fisherfolks of typical FCI and FCF lakes in southwestern Nigeria were consulted for their local experiences regarding CC variables, effects on cage aquaculture and the occurrence of caged fish in their fish catches. For phenotypic structure analysis, size cohorts of cage‐cultured (CCU) and wild‐caught (WC) Oreochromis niloticus from FCI (CC_UFCI and WCFCI), and WC from FCF (WCFCF) were obtained and analysed for phenotypic differences (p < .05) and mix‐up through cluster/linkage analysis. Respondents experienced unpredictable climatic conditions, increased rainfall fluctuations and declining rainfall (86.4; 47.1%, 50.0; 64.7%, 100.0; 88.2%, in FCI and FCF lakes, respectively), indicating that CC affects cage culture, induces cage damage and floods, and results in caged fish escapes (40.9; 35.3%, 81.8; 11.8%, 95.5; 41.2% in FCI and FCF lakes, respectively), while caged fishes occur in fisherfolks’ catches (63.6% FCI). Phenotypically, WCFCF and WCFCI were similar, but differed from CC_UFCI for pre‐dorsal length. WCFCF and WCFCI differed for caudal peduncle lengths. In addition, some individuals of CC_UFCI clustered with WCFCI and WCFCF, rather than their population cohorts. Climate change impacts on FCA lakes have implications regarding caged fish escapes. Phenotypic structures indicated a possible mix‐up of caged and wild O. niloticus populations. Thus, monitoring, control and surveillance (MCS) of cage culture in lakes is necessary, and development of genetic markers for endemic species would also facilitate MCS.     

Food‐web patterns and diversity in tropical fish communities

The food webs for three Sri Lankan reservoirs, Minneriya (ancient and shallow), Udawalawe (young and shallow) and Victoria (young and deep), were compared. The species richness of the fish communities was highest in Minneriya (30 species), intermediate in Udawalawe (21 species) and lowest in Victoria (18 species). The fish species belonged to 11 families, with Cyprinidae dominating the community in terms of both abundance and species richness. The daily quantity of food consumed per fish species was used to distinguish weak from strong trophic interactions in these food webs. The fish community consumption pattern was characterized by a few strong interactions and many weak ones. The number of major consumers (i.e. contributing >5% of the total fish community consumption) for each reservoir was small and similar for all three reservoirs. Dawkinsia singhala and Amblypharyngodon melettinus were the two major consumers in all three reservoir food webs. Puntius chola was a major consumer in Minneriya and Victoria, although not in Udawalawe, where it fed less on detritus than for the other two food webs. The fish community fed mainly at the bottom of the food web, primarily on algae, macrophytes and detritus in all three reservoirs, with very little piscivory occurring. The average food‐web length was measured as the mean trophic index weighted for the consumption rates of the various fish populations that together comprised the fish community. The average food‐web length in these three Sri Lankan reservoirs relatively short, compared with most other tropical lake/reservoir food webs in Asia and Africa for which relevant data were available. Furthermore, traditionally unexploited species (Oreochromis mossambicus; O. niloticus), in contrast to species of commercial fisheries interest, are major role players in the trophic dynamics of reservoir ecosystems. Thus, based on this study, the potential of their exploitation should be considered in the management of reservoir fisheries.     

Aspects of the biology of Labeo cylindricus (Pisces: cyprinidae) in Lake Baringo,Kenya

Aspects of the biology of Labeo cylindricus from Lake Baringo were investigated, based on experimental beach seining and gillnetting between August and October 2007. The length–weight relationship indicated the species exhibited positive allometric growth (b = 3.7083), with a condition factor (K) of 0.84 ± 0.0298 S.D. Males dominated fish catches, with an adult sex ratio of males/females of 1:1.7. The fish length‐at‐50% maturity (Lm50) was 127.7 and 126.1 mm total length (TL) for males and females, respectively. Fish age and growth were determined from sagittal otoliths considered to be from the young‐of‐the‐year. Clear circuli observed in the otoliths were used to determine fish age in days. Validation by oxytetracycline marking indicated that a single growth increment (1.1 ± 0.1 SE) is formed daily on fish otoliths. Fish length (TL mm) was correlated significantly with age (in days), TL = 1.398 Age + 26.523 (r² = 0.91). Fish growth was subsequently estimated to be 1.398 mm day^?1. The weights and ages (in days) of fish exhibited a significant (P < 0.05) power relationship defined by the equation: W = 0.0003 Age^2.5804 (r² = 0.88). The results of this study provide parameters that, if appropriately monitored, can be used to predict responses of fish populations in Lake Baringo and elsewhere to human interventions (exploitation) and natural environmental change.     

Changes in some biological parameters of the silver cyprinid,Rastrineobola argentea (Pellegrin, 1904) in the Nyanza Gulf of Lake Victoria,Kenya

The study examined length–weight relationship, condition factor, size at 50% maturity and fecundity of Rastrineobola argentea in the Nyanza Gulf of Lake Victoria, Kenya. The 500 fish examined had a mean length of 39.4 ± 0.3 mm SL, ranging from 17 to 55 mm SL and weighed between 0.08 and 2.44 g. The length–frequency analysis indicated a unimodal distribution between 35 and 40 mm SL, with a modal length at 37 mm SL. The fish exhibited a positive allometric growth pattern (b = 3.4). The condition factor for all fish was 1.02. Male fish showed a decreasing trend in condition factor with increasing body length, while females exhibited an increasing trend. The length at 50% maturity was estimated at 43 and 44 mm SL for males and females, respectively. The smallest mature male was 41 mm SL and 42 mm SL for females, with 100% maturity already exhibited at 49 mm SL. Fecundity varied from 235 eggs for 33 mm SL to 2,309 eggs for 54 mm SL fish. positive linear relationships were found between fecundity against standard length and ovary weight. Comparisons with previous studies in this lake over time indicated a decrease in length at maturity and an increase in fecundity, which are strategies to maximize reproductive success. The findings of this study provide useful information for managing the fishery.     

Broad screening of toxic and bioactive metabolites in cyanobacterial and harmful algal blooms in Lake of the Woods (Canada and USA), 2016–2019

Cyanobacteria produce toxic/bioactive metabolites that affect human and ecosystem health via inhibition of proteases, carboxypeptidases, or phosphatases. From surface water samples collected between 2016 and 2019 inclusive (n = 78), we report intracellular concentrations of more than 30 such compounds in Lake of the Woods (LOW), a large lake system of multinational importance. Although most surface biomass blooms were dominated by the cyanobacteria Dolichospermum (max. ～50 mg/L) and Aphanizomenon (max. ～60 mg/L), there was also significant biomass of other cyanobacteria (Gloeotrichia echinulate along shoreline, max. ～40,000 mg/L; Woronichinia naegeliana max. ～100 mg/L; Lyngbya max. ～10 mg/L; Planktothrix agardhii max. ～6 mg/L) and phytoplankton (Stephanodiscus niagarae max. ～6 mg/L). Microcystins (MCs) were present in ～80 % of samples, peaking at ～33 µg/L (MC variant-LR). Approximately 50 % of samples contained MC concentrations below 0.1 µg/L. Alkaloid metabolites detected included neurotoxin anatoxin-a (～60 % < 0.1 µg/L) and cytotoxic cylindrospermopsins (～30 % < 0.1 µg/L). Anabaenopeptins A/B/F were prevalent (>96 %) and exceeded MCs by >100 fold (max. >4,000 µg/L). Aeruginosamide B was present in ～75 % of samples (max. ～10 µg/L). Cyanopeptolin 1040 MB, microginin 690 methyl ester, and oscillaginin A were not detected however, traces of micropeptin 1106 were detected. Microcystin and anabaenopeptin genes were also detected in the majority of samples (78 % and 79 %, respectively), while saxitoxin, cylindrospermopsin, and anatoxin genes occurred at lower frequencies with 59 %, 36 %, and 38 % detection, respectively. These results will aid assessment of risk from cyanobacterial blooms in LOW and inform ongoing binational lake management and policy development.