Growth,mortality and recruitment of Nile perch (Lates niloticus) in Lake Victoria,Kenya

This study investigated the growth, mortality and recruitment of Lates niloticus in Lake Victoria basis on length–frequency data collected during the period 2014‐2015. The asymptotic length (L _∞ ) had a value of 124 cm TL , growth curvature (K ) of 0.22 year^?1, total mortality (Z ) of 0.96 year^?1, a natural mortality (M ) of 0.42 year^?1, a fishing mortality (F ) of 0.54 year^?1, an exploitation rate (E ) of 0.57 and a growth performance index () of 3.53. Logistic selection model showed that 50% of fish of 46.09 cm TL encountering the gear are retained. There were two peak recruitment periods, a minor one in March and a major one in July, accounting for 12.04% and 22.04%, respectively, of the total fish catch. The Beverton and Holt's relative yield‐per‐recruit model indicated the indices for sustainable yields are 0.32 for optimum sustainable yield (E _0.5), 0.60 for maximum sustainable yield (E _max) and 0.51 for economic yield (E _0.1). Compared to previous findings, there is a great decline in the sizes of Nile perch stocks in Lake Victoria. Thus, managing the fishery requires strict adherence to the slot size of 50–85 cm TL , and restrictions on illegal gear and methods, by the devolved governments through monitoring, control and surveillance in liaison with the Beach Management Units (BMU s).     

Shifts in the food of Nile perch (Lates niloticus) in Lake Victoria

The study investigated diet of Nile perch in Lake Victoria and compared results with past data from the same lake in order to analyse diet shifts over time. Fish samples (2020) were collected by seining and trawling from 2012 to 2016. In overall, Caridina nilotica (59%) was the dominant food item in the diet, while Rastrineobola argentea (5%) contributed the least. The diet of Nile perch of 1 cm was copepods, cladocerans and rotifers. The diet changed to only copepods and cladocerans at 2 cm. The proportion of the relatively large calanoids increased with Nile perch size in 1988/89, comprising between 35 and 80% of the diet of fishes of 3–4 cm. Nile perch of 6–25 cm TL fed more on C. nilotica in 2012/2016, compared to 1988/1989 and 2006/08. Haplochromines were fed on more in 2006/2008 than in 2012/2016 by Nile perch of 6–30 cm TL as fish size increased. The frequencies of occurrence of R. argentea in the diet were highest in 1988/1989, and lowest in 2006/2008, for fish size of up to 30 cm TL. The proportion of Odonata in the diet of size class 16–20 and 21–30 cm TL were highest in 2012/2016 and 1988/1989, respectively. Thus, shifts in Nile perch diet was observed from zooplankton to C. nilotica, then to haplochromines and finally to fish prey as size increased. Nile perch preferred haplochromines with C. nilotica as the substitute food. Cannibalism was observed to have decreased, only being evidenced in Nile perch above 35 cm TL, which could actually have a positive impact on the Nile perch fishery. The information on shifts in diet of Nile perch in Lake Victoria is of considerable ecological importance.     

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.     

Growth,mortality and recruitment of silver cyprinid (Rastrineobola argentea) in the open waters of Lake Victoria,Kenya

This study investigated the growth, mortality and recruitment of Rastrineobola argentea in Lake Victoria on the basis of length–frequency data collected during the period 2014–2015. The asymptotic length (L _∞ ) had a mean (±SE ) value of 53.50 ± 0.50 mm SL , growth curvature (K ) of 0.81 ± 0.06 year^?1, total mortality (Z ) of 2.96 ± 0.12 year^?1, a natural mortality (M ) of 1.23 ± 0.06 year^?1, a fishing mortality (F ) of 1.74 ± 0.07 year^?1, an exploitation rate (E ) of 0.59 ± 0.01, a growth performance index (?′ ) of 3.36 ± 0.02 and a length at 50% capture (L ₅₀) of 28.25 ± 0.43 mm SL . The fish exhibited a peak breeding during the months of May and August. The Beverton and Holt relative yield‐per‐recruit model indicated mean (±SE ) indices as 0.37 ± 0.01 for optimum sustainable yield (E _0.5), 0.76 ± 0.01 for maximum sustainable yield (E _max) and 0.66 ± 0.01 for economic yield (E _0.1). Compared with previous studies, there is a great decline in the sizes of R. argentea stocks in Lake Victoria. Thus, management measures should include restriction on illegal seine nets <10 mm mesh size and re‐enforcement of a fishing ban or a closed season.     

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.     

Population biology of common carp,Cyprinus carpio (Linnaeus, 1758), in Lake Naivasha,Kenya

The common carp (Cyprinus carpio) is an exotic fish, being one of the commercially important species in Lake Naivasha. The present study investigated its growth, mortality, maturity, condition and exploitation. Sampling was done monthly from January to December 2017 with gillnets of 2–5 inches mesh size. Each individual fish was measured (cm) and weighed (g) in the field. The specimens were preserved in ice and transported to the laboratory for analysis of maturity. The fish lengths ranged from 12.0 to 69.0 cm TL, and weighed between 32 and 6,840 g. The overall population sex ratio was 1:1.66 (female:male). The length at first maturity (L_m50) was calculated to be 54.0 and 49.0 cm for females and males, respectively. The length at first capture (23.9 cm) was lower than its length at first maturity, suggesting the capture of immature fish. The mean condition factor for all fish was 1.05. The peak condition values were observed during April (1.35), September (1.59) and October (1.80). The highest (1.377) mean condition was observed for size class 56–60 and the lowest (0.933) in class 26–30. The asymptotic length (L∞) was 73.5 cm, and the growth curvature (K) was 1.05 year^‐1, with a growth performance index (Ф) of 3.72. The total (Z), natural (M) and fishing (F) mortality coefficients were 2.85, 1.34 and 1.78 year^‐1, respectively. The exploitation rate (E) of 0.57 year^‐1 was higher than the optimum sustainable yield (E_0.5) of 0.32, but was not different from the maximum sustainable yields (E_max) 0.54 and economic yield (E_0.1) 0.46. The results of the present study provide parameters of fish biology and population that should prove useful in managing the fishery of Lake Naivasha.     

Ontogenic Changes in Prey Ingested by Nile perch (Lates niloticus) Caught in Nyanza Gulf of Lake Victoria,Kenya

A total of 289 fish were analysed in this study, with Caradina nilotica (44%) being the dominant food item in their diet, while tilapia (8%) contributed the least. Haplochromines, unidentified fish prey and juvenile Nile perch also were observed. An ontogenic shift was observed, with C. nilotica contributing highest (61.1%) of the diet of fingerlings, but decreasing to 21% in the diet of adults. Nile perch contributed 35.7% of the diet in the adults and 8.1% in the fingerlings. This study indicates Nile perch is a predatory fish, feeding mainly on C. nilotica, haplochromines, tilapia, Nile perch and other fish materials.     

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.     

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.     

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.