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.     

Lake Victoria fisheries: Outlook and management

The past subsistence of the Lake Victoria fishery was dominated by rich, diverse haplochromine cichlids. This multispecies fishery has undergone a decline over the past four decades, evolving into a commercial fishery consisting mainly of Nile perch (Lates niloticus), Nile tilapia (Oreochromis niloticus) and cyprinid (Rastrineobola argentea species). To better understand Lake Victoria fisheries, studies dating as far back as the 1920s have been carried out to assess the status of the fish stocks. These past studies indicated the lake fisheries were declining because of numerous major challenges, including intense fishing, invasive species, loss of biodiversity, ecological alterations, climate change, inadequate information to inform management and unharmonized policies. Numerous policies and regulations have been developed and implemented over the years to address these issues and manage the fisheries sustainably. Most of the interventions have been sectorial, disjointed and unharmonized and have not reduced the declining fish catch rates. With reestablishment of the East Africa Community (EAC) with several institutions in 1994, the Lake Victoria riparian states initiated an ecosystem approach to manage the Lake Victoria fishery resources in a sustainable manner. This study reviews the development of the Lake Victoria fisheries, outlines major past and present management challenges and provides a set of new strategies to manage the lake's fisheries resources, with emphasis on an ecosystem approach.     

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.     

The responses of Nile tilapia Oreochromis niloticus (Linnaeus, 1758) in Lake Wamala (Uganda) to changing climatic conditions

Changes in the catches of Nile tilapia, Oreochromis niloticus (Linnaeus, 1758), in Lake Wamala (Uganda) have been observed since its introduction. The factors contributing to these changes, however, are not well understood. This study examined changes in species composition, size structure, size at first maturity, length–weight relationship and condition factor of Nile tilapia in Lake Wamala, in relation to changes in temperature, rainfall and lake depth, to provide a better understanding of the possible role of changing climatic conditions. There was an increase in the minimum, maximum and average temperatures since 1980, but only the minimum (0.021 °C year^?1) and average temperatures (0.018 °C year^?1) exhibited a significant trend (P < 0.05). Rainfall increased by 8.25 mm year^?1 since 1950 and accounted for 79.5% of the water input into the lake during the period 2011–2013, while evaporation accounted for 86.2% of the water loss from the lake. The lake depth was above 4 m during the years when the rainfall exceeded the average of 1180 mm, except after 2000. The contribution of Nile tilapia to total fish catch and catch per unit effort (CPUE) increased with rainfall and lake depth up to the year 2000, after which they decreased, despite an increased rainfall level. The lake depth was positively correlated with the average total length and length at 50% maturity (r = 0.991 and 0.726, respectively), while the slopes of the length–weight relationships differed significantly between high and low lake depths [t₍₆₎ = 3.225, P < 0.05]. Nile tilapia shifted from an algal‐dominated diet during the wet season to include more insects during the dry season. The results of this study indicate Nile tilapia in Lake Wamala displays a typical r‐selected reproductive strategy, by growing to a small size, maturing faster and feeding on different food types, in order to survive high mortality rates under unfavourable conditions attributable to higher temperatures, low rainfall and low lake water levels.     

Management of the Nile tilapia (Oreochromis niloticus (L.)) fishery in the Kenyan portion of Lake Victoria, in light of changes in its life history and ecology

This study reports on the population parameters, catch distribution and feeding ecology of Nile tilapia (Oreochromis niloticus) from bottom trawls and commercial catches obtained in the Kenyan portion of Lake Victoria during 1997–2006. The population parameters were analysed using the FAO‐ICLARM stock assessment tool (FISAT). The fish biomass and the food ingested by the fish were estimated using the swept area and point methods, respectively. Immature fish comprised ≈70% of the total fish population. The asymptotic length, maximum weight, maximum age, exploitation rate (E) and length at 50% maturity of Nile tilapia have decreased, whereas the growth curvature and fishing mortality have increased. The commercial catches increased from 13.93 t in 1997, to 23.70 t in 1999, decreasing thereafter to 18.73 t in 2005. The bottom trawl catches increased from 46.90 kg ha^?1 in 1997, to 401.48 kg ha^?1 in 2000, decreasing thereafter to 15.57 kg ha^?1 in 2006. The major food items ingested by the fish were algae, insects and other fish. Population parameters, and the catch and diet of O. niloticus, have changed over the years in Lake Victoria. The population characteristics suggest a population under stress, attributable to intense catch exploitation. Even under intense exploitation (E = 0.68), however, the mature fish constituted ≈30% of the population. The commercial catches are still high, indicating a very resilient fishery. Nevertheless, despite this resilience, the future of Oreochromis fishery is threatened by increased fishing capacity in the lake, and there is need to re‐evaluate the effectiveness of current fishery management measures, with the goal of possibly adopting new measures. Enactment of new fishery policies also should provide for co‐management to enhance the management process. Furthermore, there is a need to reduce fishing capacity and illegal fishing methods, and to seek alternative livelihoods for lake fishers and other stakeholders.     

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.     

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.     

Testing for hybridization between Nile tilapia (Oreochromis niloticus) and blue spotted tilapia (Oreochromis leucostictus) in the Lake Edward system

Oreochromis is the most important tropical freshwater fish genus for aquaculture purposes, with various introduction and translocation events having played an integral part of its management. Some Oreochromis species such as the Nile tilapia, O. niloticus, are thought to be superior for aquacultural purposes when compared to other species. Hence, understanding the admixed nature of the species within a native system is of key importance for the development of future broodstock. Using a combination of mitochondrial (COI) and nuclear (microsatellites) DNA, we tested for the presence of O. niloticus and O. leucostictus hybrids within the Lake Edward-George system. The present study revealed that, despite their natural overlapping ranges, hybridization between O. niloticus and O. leucostictus appears to be rare within the Lake Edward-George system, suggesting that the native populations of these two species may serve as suitable broodstock to develop future aquaculture strains. Moreover, our study provides the first COI sequences for O. leucostictus from its native range and adds to the growing database of O. niloticus sequences.     

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 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.     

An assessment of the purse-seine (winch-net) fishery in Lake Volta, Ghana

An assessment has been made of the stock conditions of five major component fishery species (Chrysichthys nigrodigitatus, Chrysichthys auratus, Hemisynodontis membranaceus, Oreochromis niloticus and Schilbe intermedius) exploited with illegal purse‐seine (winch‐net) gear in the Yeji sector of Lake Volta, Ghana. Chrysichthys nigrodigitatus was clearly found to be overexploited, but the others were being exploited near to, or under, optimum harvest levels. All five species were harvested at an inappropriate age‐at‐first‐capture of ≈ 6 months, suggesting overfishing of young fishes. Consequently, the computed L_m:L_∞ ratios were > 0.64. For most fish species, such a ratio implies their exploitation when at early maturity and at very small sizes. The estimated range of mesh sizes in the cod‐ends was far lower (5–8 mm) than the legal mesh size of 25 mm, calling for urgent enforcement of rules and regulations. Single‐species yield‐per‐recruit analyses indicated that yields could be improved by the reduction in the current fishing effort. As the fishery of the lake is not segmented to allow fish to be taken by the illegal small‐mesh purse seines, studies on the interactions between the different fishery sectors need to be undertaken, alongside the proposed ban of the gear as enshrined in the Fisheries Law of Ghana.     

Temporal patterns in phytoplankton,zooplankton and fish composition,abundance and biomass in Shirati Bay,Lake Victoria,Tanzania

Shirati Bay is among the important breeding and nursery sites for major fish species in Lake Victoria. Weekly samplings were conducted to assess the temporal patterns in phytoplankton, zooplankton and fish composition, abundance and biomass in relation to prevailing water quality parameters. The study also determined the influences of plankton dynamics and water quality on the fish catch composition and biomass. It was hypothesized that temporal patterns in the composition, abundance and biomass in the plankton in the bay are controlled by water quality parameters that, in turn, affect the composition and biomass of fish catches. The phytoplankton comprised mainly cyanophytes and bacillariophytes, while the zooplankton were dominated by copepods. The heavy rain season exhibited a significantly higher plankton abundance and biomass than the dry season. The plankton abundances in both seasons exhibited significant positive correlations with water temperature and transparency. The phytoplankton community was controlled by calanoid and cyclopoid species. At higher trophic levels, Lates niloticus juveniles, Oreochromis niloticus juveniles and haplochromines controlled Cladocera and Cyclopoid copepods, while Tilapia rendalli juveniles controlled the Rotifera. This study revealed that Cyanophyta and Bacillariophyta are the dominant phytoplankton, whereas cyclopoids dominate the zooplankton species in the bay. These dominant plankton groups are partly controlled by rainfall, water temperature and transparency. Fish biomass, zooplankton and phytoplankton exhibit a typical predator–prey inverse relationship. Thus, evaluation of the plankton composition, abundance and biomass should be mandatory during fisheries stock assessments to effectively manage the fishery resources in the bay.     

Stomach contents and stable isotopes confirm ontogenetic diet shifts of Nile perch, Lates niloticus, in southern Lake Victoria

Nile perch (Lates niloticus), a main target for Lake Victoria's fisheries, shifts its diet from invertebrates to fish during its ontogeny. We combined stomach contents with stable isotopes to study these diet shifts in Mwanza Gulf, southern Lake Victoria. During four seasons between 2009 and 2011 we collected and analysed 6697 stomachs and 516 stable isotope samples of Nile perch. Caridina shrimp and haplochromine cichlids were the major food items in Nile perch of 5–40?cm. Based on stomach contents, the shift of dominance of Caridina towards haplochromines in the diet varied from 6.7?cm length in the southern part of the gulf, to >20?cm in the northern part, possibly related to the higher abundance of Caridina in the north. In general isotope-based and stomach-content-based estimates of Nile perch diets agreed. We observed only two discrepancies: in Nile perch >20?cm, the isotope-based estimates of the contribution of haplochromines to the diet were much lower than based on stomach contents and the contribution of juvenile Nile perch to the diet were much higher in the isotope-based estimates than in the stomach based-based estimates. This discrepancy may be explained by e.g. different timespan-coverages between isotopes and stomach contents, by movements of Nile perch between the Gulf and the rest of Lake Victoria, or by the overlapping isotopic profiles of haplochromines and juvenile Nile perch. With the recent recovery of haplochromines in Lake Victoria, Nile perch appears to rely more on this prey again, as it did in the 1980s.     

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.     

Egg Thiamine Status of Lake Ontario Salmonines 1995–2004 with Emphasis on Lake Trout

Alewives (Alosa pseudoharengus), the major prey fish for Lake Ontario, contain thiaminase. They are associated with development of a thiamine deficiency in salmonines which greatly increases the potential for developing an early mortality syndrome (EMS). To assess the possible effects of thiamine deficiency on salmonine reproduction we measured egg thiamine concentrations for five species of Lake Ontario salmonines. From this we estimated the proportion of families susceptible to EMS based on whether they were below the ED20, the egg thiamine concentration associated with 20% mortality due to EMS. The ED20s were 1.52, 2.63, and 2.99 nmol/g egg for Chinook salmon (Oncorhynchus tshawytscha), lake trout (Salvelinus namaycush), and coho salmon (Oncorhynchus kisutch), respectively. Based on the proportion of fish having egg thiamine concentrations falling below the ED20, the risk of developing EMS in Lake Ontario was highest for lake trout, followed by coho (O. kisutch), and Chinook salmon, with the least risk for rainbow trout (O. mykiss). For lake trout from western Lake Ontario, mean egg thiamine concentration showed significant annual variability during 1994 to 2003, when the proportion of lake trout at risk of developing EMS based on ED20 ranged between 77 and 100%. Variation in the annual mean egg thiamine concentration for western Lake Ontario lake trout was positively related (p < 0.001, r² = 0.94) with indices of annual adult alewife biomass. While suggesting the possible involvement of density-dependent changes in alewives, the changes are small relative to egg thiamine concentrations when alewife are not part of the diet and are of insufficient magnitude to allow for natural reproduction by lake trout.     

Bacterial abundance on the skin,gills and intestines of Cyprinus carpio in Lake Naivasha,Kenya: Implications for public health and fish quality

Cyprinus carpio is the most important fish species in the Lake Naivasha fishery, comprising 51% of the total catch in the lake. Microflora, especially enteric bacteria of human or animal origin, are the causative agent for fish contamination and spoilage. Poor sanitation standards and poor sewage treatment and disposal methods within Lake Naivasha and its catchment pose a great threat for degrading the quality of C. carpio. The potential impact is rejection of the fish in the local, regional and international markets, risking the collapse of the Lake Naivasha fishery. This study determined the bacterial quality of water and C. carpio from three different sites within Lake Naivasha, namely Malewa River mouth, sewage discharge point and a mid‐lake site, based on plate count techniques. Physicochemical parameters characterizing the lake water also were also measured in situ. This study results indicated that both the fish and water in Lake Naivasha exhibited poor bacterial quality. All the physicochemical parameters were within the recommended range for fish culture, although they also were conducive to the proliferation of bacteria. Most of the sampling sites exhibited significant spatial variation in their bacterial abundance (P < 0.05). The sewage discharge sampling site exhibited the highest mean density values for bacterial densities and clearly degrade the quality of the fish in the lake. Proper sewage treatment, and the installation of modern sanitation facilities, is recommended to improve the bacterial quality of the fish.     

Comparing echosounder efficiency using field observations

Time series, using different echosounders or large‐scale multiship acoustic surveys, can be criticized because equipment changes might affect the final results. This criticism was addressed previously by comparing the results from different vessels using echo integration on the target species. The acoustically estimated standing stock of Nile perch (Lates niloticus) in Lake Victoria, East Africa, declined to 50% between successive surveys six months apart in 2007, prompting the criticism that a change in echosounder was responsible for this observation. This concern has been addressed, using data from the same four small localities around the lake, Emin Pasha Gulf, Nyanza Gulf, Speke Gulf and the vicinity of the Sesse Islands, from six surveys, spanning the time when the change in echosounder occurred. For three of the locations, echo integration and single target detections within the first bottom echo indicated no significant differences in echosounder performance. Results from the fourth location, Sesse Islands, showed very low backscatter, possibly due to a layer of detritus on the lake bed. It is concluded that all data are equally comparable, providing echosounders are correctly calibrated with the vessel being stationary, although there may still be differences under operational conditions. Characteristics of intercalibration sites are discussed in this study. The results also show changes in substrate, likely attributable to local environmental changes between surveys.     

The role of the Yala swamp lakes in the conservation of Lake Victoria region haplochromine cichlids: Evidence from genetic and trophic ecology studies

Lake Kanyaboli, an isolated satellite lake of Lake Victoria, has been suggested as a potential refugium for haplochromine cichlids that have gone extinct in the main basin of Lake Victoria. Mitochondrial DNA (mtDNA) molecular markers, as well as feeding ecology studies, were employed in this study to re‐evaluate the evolutionary and ecological significance of six common Lake Kanyaboli haplochromines. The mtDNA marker revealed high genetic variability within four of the six haplochromine cichlids. Five haplotypes were discerned in Astatoreochromis alluaudi (n = 27), seven in Lipochromis maxillaris (n = 29), five in Astatotilapia nubila (n = 12) and 11 in the endangered Xystichromis phytophagus (n = 205). A haplotype genealogy suggests that Lake Kanyaboli harbours mtDNA haplotypes that could have been lost or not sampled in Lake Victoria, or could have arisen in situ. Lipochromis maxillaris appears to have undergone a recent demographic expansion. The pairwise F_STs indicated that only the comparison between X. phytophagus and A. nubila led to a non‐significant F_ST value. All other comparisons were significant at the 0.01 level, indicating the genetic distinctiveness of the haplochromines in the satellite lake. This could suggest that the lake harbours ‘pure’ relict populations of the haplochromines and therefore that Lake Kanyaboli can be considered a ‘genetic reservoir’. Gut content analysis of the six haplochromine species revealed that eight different food items were consumed. No single species fed exclusively on a single food item, but certain food items contributed higher proportions of the fish diet for each fish species. Resource partitioning therefore could be discerned within this haplochromine community. Thus, Lake Kanyaboli and similar satellite lakes provide an opportunity for conservation of both genetic and trophic diversity threatened by introduction of exotics in the Lake Victoria basin. Lake Kanyaboli should be recognized and conserved as important evolutionary significant units for Lake Victoria region haplochromine species.     

The history and future of Lake Champlain's fishes and fisheries

In the last two centuries, physical, chemical, and biological alterations of Lake Champlain have resulted in the loss of two species, addition of 15 fish species, and listing of 16 species as endangered, threatened or of special concern. The lake currently supports 72 native fish species; lake trout (Salvelinus namaycush) and Atlantic salmon (Salmo salar) were extirpated by 1900, American eel (Anguilla rostrata) and lake sturgeon (Acipenser fulvescens) populations are extremely low, and walleye (Sander vitreum) are declining. Dams on several rivers, and ten causeways constructed in the mid 1800s to early 1900s, cut off access to critical spawning areas and may have limited fish movements. Siltation and sediment loading from agricultural activity and urban growth have degraded substrates and led to noxious algal blooms in some bays. A commercial fishery targeting spawning grounds of lake whitefish (Coregonus clupeaformis), lake trout, and walleye probably reduced numbers of these species prior to its closure in 1912. Non-native species introductions have had ecosystem-wide impacts. Sea lamprey (Petromyzon marinus) populations were very high prior to successful control, possibly as a consequence of ecological imbalance and habitat changes. A paucity of historic survey data or accurate species accounts limits our understanding of the causes of current fish population trends and status; in particular, the effects of habitat fragmentation within the lake and between the lake and its watershed are poorly understood. Holistic, ecosystem management, including pollution reduction and examination of habitat impacts, is necessary to restore the general structure of native biological assemblages.