Globally, the increasing uncertainty from climate change effects and anthropogenic activities within the watershed areas; and the invasive species are still a concern in conservation of freshwater biodiversity. In Lake Victoria, notable recorded threats includes, flooding events and long durations of the water hyacinth presence in the relatively shallow bay (less than 5m), which is also considered a “water hyacinth hotspot”. In Kisumu bay, water hyacinth is known to have blocked boat transport and hampered artisanal fishing activities during periods of re-appearance and prolonged [1]. There has never been a clear indication of an ecological balance in invaded lake zones, as remnants of the water hyacinth plant still thrive. Therefore, the lake basin and the benthic habitats are continuously shaped by the living and non-living components.As a result of the increasing eutrophication, nuisance algal blooms are a common occurrence in the lake [2-6]. Recent studies document the presence of cyanotoxin producing phytoplankton species such as Microcystis spp., which can cause impairment of the lake water, which serves as a source of domestic water supply within the Kisumu city and peri-urban areas (4,5; 7-12]; and other regions of the L. Victoria basin [3; 13-16). In the Winam gulf, recent molecular studies to identify toxin producing cyanobacterial showed that within a relatively small embayment, composition and toxins synthesis potential of cyanobacterial harmful blooms can vary dramatically [8].

These reports confirm the changing limnological conditions, mainly attributed to watershed changes and unconfirmed impacts from the ongoing fish cage culture. However, there is still a need to ensure sustained fish production and utilization of marine and freshwater resources by prioritizing the potential opportunities in the blue economy. Cage fish culture in L. Victoria is a strategy geared towards increased production of Nile tilapia, and a more efficient utilization of the lake water resources, to ensure nutritional food security and improved livelihoods [17-22].

Embayments and riverine wetland areas within the Winam gulf and other bays of Lake Victoria are recognized fish habitats, and critical nursery and breeding areas for the diverse fish species. However, the close proximity to human settlements and expanding urbanization and other related water uses has meant an increasing threat to the resident biodiversity. Kisumu bay is an important coastal zone for the Kisumu County and the linked hinterland; due to its harbour operations and other maritime related activities and infrastructure. Wild fish production has continued to dwindle, and hence the increasing presence of the new cage aquaculture infrastructure around the Lake Victoria (Kenya), to supplement Nile tilapia production. The endemic Oreochromis variabilis and Labeo victorianus are among the most threatened fish species, with a declining population size [23]. Similar to many aquatic organisms, the distribution and abundance of early life stages of fish are critical determinants of the population size, but the characteristics of nursery areas and spawning habitats are not clearly mapped and protected. Major changes in fish species in Lake Victoria, especially the diverse community of Haplochromines decline have been attributed to Nile perch [24]. In Lake Victoria, the predator Nile perch diet consists of Haplochromines spp., Rastrineobola argentea, tilapia, Nile perch juveniles and other small fish species [25-28], but juvenile Nile perch feeds mostly on Caridina nilotica [27], macroinvertebrates, copepods and fish fry [26]. Therefore, information to guide demarcation of recruitment habitats is certainly important for efficient aquatic conservation in areas surrounded by heavy anthropogenic impacts.

The aquatic micro habitats around the bay are under threats from the increasing impacts from the expanding urban areas; urban runoffs and associated solid and liquid waste discharges into the littoral areas of the lake. Increasing levels of heavy metals, polycyclic aromatic hydrocarbons (PAHs) and polychlorinated biphenyls (PCBs) among other chemical residues; chronically stress fish. Wild freshwater fish could be susceptible to endocrine disruptor chemicals (EDCs), however only in a few cases a causal link between the presence of EDCs in freshwater fish and altered endocrine functions in exposed fish populations been demonstrated [29], but most of the focus on the effects of EDCs has been on effects at individual level. It is therefore hypothesized that there are unknown scales of threats to aquatic biodiversity, distribution and water quality conditions from complex ecological and biochemical stressors besides the increasing effects from local to regional climatic changes. Addressing these issues is in support of the new blue economy (BE) concept or strategy for safeguarding the worlds ocean and water resources. The BE is generally defined as the sustainable use of ocean resources for economic growth, improved livelihoods, and jobs while preserving the health of ocean system [30-31] and is in-line with the United Nations sustainable development goals to reduce unsustainability and promote sustainable development [32].

Kisumu bay waters receives riverine and point discharges, urban runoffs and land sourced materials. Therefore, the study identified Kisumu bay, a more urban impacted zone of Lake Victoria (hereby also considered an “area of concern’’) to try and document the present ecological status in the wake of increasing cage aquaculture and frequent presence of the transient remnants of the floating water hyacinth within the Winam gulf. There are no detailed monitoring studies on the several bays of Winam gulf, apart from recent molecular studies to identify toxin producing cyanobacterial [8]. Shallow lakeshore areas are important fish habitats and serve as nursery grounds, but they are not clearly demarcated. Poor water quality conditions cause increased fish stress and mortality in cage farms, as evidenced by re-occurrence of fish kills that led to economic losses within the Winam gulf bays (near Kisumu and Homa bay, in November 2022 [33]. There are also concerns on potential effects of cyanotoxins in fish but scientific data and evidence is limited. We hypothesize that the fish communities and juvenile growth stages are more affected by the poor water quality in relatively more exposed habitats around the urbanized shorelines besides the use of illegal and unregulated fishing gears. Therefore, the main objective of the study is to provide a zonal assessment of the fish communities and key environmental factors as indicators of ecological stress within the bay. The aims were to determine and monitor changes in key water quality parameters and nutrients elements, and their influence on fish species composition, so as to understand changes in aquatic communities; and to inform on the better use of the water resource and responsible blue economy exploitation for ensuring sustainable fisheries.

2.MATERALS AND METHODS

Study area

Lake Victoria (East Africa) is a large freshwater lake with a lake surface area to catchment area ratio of about 1:2.83. The water surface area, mean and maximum depth, volume and catchment area of Lake Victoria (East Africa) is about 68,800 Km², 40 m, 80 m, 2760 Km³, and 195,000 Km², with about 6% and 43% of the lake water surface area in Kenya and Uganda respectively. This study was conducted in Kisumu Bay (Kisumu County) (Fig.1). The bay forms the extreme eastern end of the Kenyan waters of Lake Victoria (Winam gulf/Nyanza Gulf). Winam gulf is a relatively shallow and narrow basin which widens eastwards forming Homa Bay, Asembo Bay and Nyakach bay. The western section of the Nyanza Gulf deepens and opens to the main Lake Victoria around the Mbita and Rusinga channels. Winam Gulf extends to a length of about 60 km with a variable width of between ca.6 Km (narrow western section) and ca.30 km (Eastern section) and lies between longitude 34° 13′ and 34° 52′ East and 0° 4′ to 0° 32′ South of the equator. Kisumu bay is the waterfront area of Kisumu city, which support a large human population and associated infrastructure in the urban and per-urban areas. Kisumu county has a total surface area of about 2,677 Km² water mass of 567 Km² and a population of 1.16 million people [34-35]. Riparian counties support small-scale fishing as a major economic activity (Owiti et al. 2021) with a fish consumption per capita ranging from 2.3 to 71.9 Kg/capita (average for Kenya s 3.1 Kg/capita). Major urbanized rivers and streams (Kibos, Kisian and Kisat) flow from the neighboring highland areas in the North Eastern areas. The immediate catchment area is more dominated by human settlements and associated urban developments; cropland and grasslands; rice farms, sugarcane plantations and wetland areas.

Study design

Several past surveys have documented the socio-economic effects and impacts from water hyacinth (after initial invasion in L. Victoria in 1988); and the changing face of the Winam gulf and the bay surface waters between June 1995 and November 1999 [36-39]. There are also increasing concerns on the presence of the algal blooms and associated exposure risks of hepatotoxins (e.g Microcystins) to communities abstracting and using water directly from the lake bays [4-5, 9, 40]. Again, Nile tilapia cage culture is the fastest growing aquaculture production technology in Kenya with 3,696 cages (2018) located along the shores of the five Lake Victoria riparian counties [18-19, 41]. Around the bay the cages are located on the outer areas of the bay near Dunga and Maboko Islands [18, 42-43], to avoid the heavy navigational traffic into the Kisumu bay and due to the bathymetry limitations. Therefore, the shallow bay is exposed to increasing environmental stressors and require frequent monitoring of the water quality, and development of clear early warning systems to avoid the commonly occurring fish kills.

Physico-chemical parameters and nutrients

Sixteen lake sampling stations were sampled to represent the inner and outer zones of the Kisumu bay. which experiences both the long and short rains between the months of March – June and October- December respectively. The inner zone sites are representative of the urban impacted and discharge zones (KSB2 – KSB6, KSB8 – KSB9 and KSB15 – KSB16) and the impact gradient is expected to decrease towards the outer zone sites (KSB-1, KSB7 and KSB10 – KSB14). However, due to logistical challenges, the sampling and setting of fish nets was done within the inner bay zone. In-situ measurements of the vertical profile of water temperature, water depth, pH, dissolved oxygen (D.O), total dissolved solids (TDS), salinity, conductivity and redox potential (ORP) were made using the multi-parameter water quality monitoring meter (YSI – 650 multi-parameter, Professional Series handheld meter; YSI Incorporated, USA). Water samples were collected at 10cm below the water surface at each sampling site and kept in a cooler box for later laboratory analysis of nutrients (Nitrate –nitrogen (NO₃-N); Nitrite –nitrogen (NO₂-N), Ammonium- nitrogen (NH₄-N), total nitrogen (TN), total phosphorus (TP), soluble reactive phosphorus (SRP) and soluble reactive silicon (SRSi)). Dissolved nutrient species were analysed in filtered water samples and the unfiltered portion of the sample was used to analyse TN and TP. The standard methods for the analysis of water and wastewater [44] were employed for the preservation and storage of samples in the field, and laboratory analysis.

Sampling for fish

Fish surveys were conducted within the bay during different months between 2021 upto August 2023) (Fig. 1b), using different monofilament nets of mesh sizes ranging from 0.5″, 0.75″, 1.0″, 1.25″, 1.5″, 1.75″, 2.0″, 2.5″, 3.0″, and 3.5″. The caught fish samples were sorted, and individual fish species identified. The fish length (Total length, fork length and standard length were measured in cm), fresh weights (g), sex ratios, maturity stages were determined according to the standard operating procedures (SOPs) manual [45]. Fish species diversity was determined using the species richness and equitability indices [46-48].

Data analysis

The data exploration and analysis was conducted using R Core Team [49] software package. The assumption of normality distribution of data (homogeneity of variance) was tested and confirmed using the using the Kolmogorov – Smirnov (KS) and Shapiro – Wilk statistical tests. The significant differences in water quality variables over the years and between zones was evaluated by analysis of variance (ANOVA) and comparison tests using 0.05 as the level of significance. The non-normal distributed data was analysed using the non-parametric statistical tests. Significant relationships between variables were assessed using Spearman’s rho correlation two-tailed test for non-parametric data. The significantly different pairwise variations in the mean values were separated using Wilcoxon test and T – test (for normally distributed variables). Fish Environmental influences on the fish size composition and diversity were determined.

3. RESULTS

The surface waters of Kisumu bay exhibited variations in water quality (Table 1a & 1b) with an overall mean (± SD) water depth, transparency, total alkalinity, hardness, and elevation of 3.7± 1.2 m, 0.53±0.2 m, 58±8 mgL^-1, 50±8 mgL^-1 and 1142±4 m. a. s. l; whereas the bay water column temperature, conductivity, TDS, turbidity, salinity and chlorophyll-a showed an overall mean (± SD) of 27.23±0.99 °C, 0.187± 0.019 mScm^-1, 0.119± 0.012 gL^-1, 14.10±10.18 NTU, 0.09± 0.01 gL^-1 and 45. 88±61.60 µgL^-1 respectively. The in-situ profiles of water temperature, conductivity, TDS, turbidity, salinity and chlorophyll-a at each site (2023-2024 study) revealed significant variations between the surface and bottom water layers with respect to TDS, salinity and temperature but the variations of turbidity and chlorophyll-a were not significantly different. Seasonal differences were also evident for TDS, salinity and temperature (Table 1a). The water column exhibited significant spatial variations (p < 0.05) in the mean water temperature, conductivity, TDS, turbidity and chlorophyll-a except, the Secchi depth and total water depth. Salinity was uniform and varied slightly at sites KSB 15 (Off Kichinjio) and KSB 9 (Off Kisat RM). Similarly, a comparison between the outer and inner bay areas showed a significant difference between all the variables (p < 0.05). The water column temperature, turbidity, conductivity, TDS and phytoplankton biomass was much higher within the inner bay waters. The mean (±SD) temperature ranged from 25.56±0.08 ºC (KSB13) to 28.33±0.64 ºC (KSB9). The mean water conductivity and TDS varied from 0.171±0.04 mScm^-1 (KSB14) to 0.222±0.00 mScm^-1 (KSB15) and 0.120±0.001gL^-1 (KSB13) to 0.137±0.001 gL^-1 (KSB15) between the stations respectively. The mean chlorophyll-a concentration (proxy for phytoplankton biomass) and water turbidity varied from 18.83±5.83 µgL^-1 (KSB11) to 94.90±11.1 µgL^-1 (KSB5) and 8.4±1.9 NTU (KSB11) to 42.3±0.1 NTU (KSB15) respectively. We observed significant relationships between the water quality variables (Table 2a & 2b) except for the sampling depth versus conductivity, salinity chlorophyll-a and turbidity. The secchi depth and total water depth revealed a significant positive relationship, whereas all the rest of the parameters had a variable negative and significant associations with both the total depth and the light transparency.

The trophic state index (TSI) was calculated based on Carlson [50] equations of the total phosphorus, secchi depth and chlorophyll-a concentration.  The bay TSI _{(Chlorophyll-a)}, TSI _{(Secchi depth)} and TSI _(TP) values ranged from 34.60 to 94.97 (mean = 66.96±10.31), 61.50 to 83.19 (mean = 69.80±4.88) and 51.30 to 88.01 (mean = 77.55±11.75) respectively, with a mean TSI for the bay (TSI _(bay)) of 68.91±7.04 (range 53.40 – 89.63). The minimum and maximum mean TSI _(bay) of the bay was 61.41±2.98 (KSB 11) and 73.28±5.30 (KSB 9) respectively. The inner bay waters showed significantly higher values (p < 0.05) than the outer zone, but overall, all the indices reflected a eutrophic to hypereutrophic status of the bay waters based on the total phosphorus, phytoplankton biomass and secchi depth.

The concentration of nitrogen species (µM) during the study (2023-2024) period ranged from 0.05 – 0.69 (0.36±0.16), 0.0 – 0.52 (0.14±0.13), 1.38 – 58.16 (10.06±13.62) and 2.21 – 57.01 (15.77±14.33) for NO₃-N, NO₂-N, NH₄-N and TN respectively (Table 1b). The levels of ammonium -N were the highest among the dissolved N species then followed by the oxidized forms. Much higher mean (± SD) levels of NH₄-N were observed at the extreme inner zone sites at KSB15, KSB3 and KSB9. The mean minimum and maximum levels of both the nitrate and nitrite forms of N were highest at inner zone sites KSB13 and KSB15 and KSB6 and KSB2 respectively. The mean TN levels were similarly much higher within the inner sites and ranged from a mean minimum value of 6.57 µM (KSB14) to 31.93 µM (KSB3). There were no significant zonal variations between the N species, except for the significant temporal variations in nitrate – N and ammonium – N concentrations. The levels of dissolved silicon, soluble P and total phosphorus in surface lake water varied from 0.07 – 0.60 mM(0.35 ± 0.16 mM), 0.03 – 5.79 µM (0.93 ± 1.24 µM) and0.85 – 13.92 µM (4.20 ± 3.56 µM) respectively. The inner bay areas recorded much higher mean levels of the soluble and total P nutrient elements as compared to the outer zone areas (Fig. 2), but only the zonal mean levels of TP and SRP were significantly different. All the measured mean nutrient elements were significantly different among the months (except for SRP, TP and nitrite – N levels), and also amongst the years (except for TP, nitrite – N, SRP and silica-Si). The total nitrogen and total phosphorus molar ratio ranged from 0.46 to 24.63 with an overall mean (±SD) value of 5.28 ±4.51. The mean N/P ratio ranged from a minimum of 2.18±1.12 (KSB7) to a maximum value of 10.44±12.38 (KSB8) within the outer and inner zone stations respectively. However, the means values were not significantly different among the two zones. The ratios depicts the established nitrogen deficiency for the phytoplankton production in the waters of Winam gulf of Lake Victoria (Kenya) as reported earlier in different studies.

The surface water concentrations of total P and N species were strongly and moderately correlated (p < 0.05) with soluble ammonium nitrogen (Fig 2 & Table 2a &2b). Similarly, the dissolved oxidized forms of nitrogen were positively and moderately associated, whereas TP was positively correlated (p < 0.05) with levels of soluble reactive silica and SRP. All the nutrient elements (except silica-Si) revealed a moderate but significant relationship with turbidity and chlorophyll-a. The turbidity and phytoplankton biomass proxy (chlorophyll- a) in surface water showed a high significant positive (TP-turbidity) and negative (Chlorophyll-a – TP) correlation (p < 0.05) with total phosphorus in surface water Surface water temperature was only significantly and positively correlated with TP and dissolved silica Si (p < 0.05). Other significant relationships were observed between the water conductivity and TP and dissolved ammonium-N. Water depth and transparency were mostly negatively associated with measured nutrients (p < 0.05) except silica Si. The TSI also exhibited moderate negative (secchi and total depth, water hardness and total depth) and positive relationships (p < 0.05) with most of the environmental variables (TN, N/P ratio, nitrate, chlorophyll-a, turbidity, conductivity).

The inter annual spatial and temporal trends in water quality in the past 7 years

We explored the changes in environmental conditions using data collected between 2018 to 2022 (Table 1c) which covered both the dry and wet season months. Over the seven years, the mean annual water temperature, turbidity, salinity, total dissolved solids, conductivity, D.O, ORP, pH and secchi depth revealed variations between the years, sampling months and stations (Fig. 2). Overally, between 2018 – 2024, the highest zonal means were observed for water column salinity, temperature, pH and turbidity within the inner zone, whereas the highest water D.O, ORP and Secchi depth was recorded in the outer zone. There were significant (p < 0.05) annual (n = 588) and seasonal (n = 316) differences in values of the measured parameters except for the D.O levels between the seasons.

The annual mean water pH and D.O fluctuated from a low value of 7.77±0.48 (2020) and 4.17±1.37 mgL^-1 (2020), to a maxima of 7.99±0.39 (2019) and 5.91±1.17 mgL^-1 (2018) respectively. The overall mean D.O, pH and ORP were 4.60±1.64, 7.88±0.43 and -164.34±120.63 respectively. Trends of the D.O levels showed a general decline across the years from 2018 to 2020 followed by a wider variation and increasing trend in 2021. Over the seven years significant associations emerged between the surface water quality parameters (total water depth, turbidity, chlorophyll – a and transparency) and nutrient elements in Kisumu Bay (Table 2a and 2b)..

However, the D.O levels were not significantly different among the seasons. The monthly mean D.O levels in the bay revealed well oxygenated waters, but with relatively lower levels around the months of January – July when compared to September – December. Hypoxic D.O levels (below 2.0 mgL^-1) were only observed during the months of April (2019), January, February, November and October (2021). The water pH varied slightly and was near neutral (mean range of 7.77±0.4 – 7.99±0.39) with an ORP value reflecting reducing conditions.

Zonal water temperature variations were more pronounced than seasonal differences. However, a wider variation appeared within the inner zone and wet season months. Water temperature across the seven years ranged from 22.42 °C to 30.0 °C (mean range of 26.64±0.55 – 27.56±0.88 °C) with values lower than 24.0 °C recorded during February 2019 September 2021 and much higher levels between 27.0 °C to 29.0 °C recorded from 2021 to 2024. Over the seven years, the overall mean turbidity, secchi depth and temperature were 41.36±23.7 NTU, 0.37±0.1m and 26.72±1.1 °C respectively. The annual mean water temperature fluctuated from a low value of 26.11±1.18 °C (2021) to a maxima of 28.10 °C (2023. Mean inter-annual turbidity peaked at 95.75±11.6 NTU and decreased to a low mean value of 16.6±11.4 and 9.17±3.74 in 2023 and 2024 respectively. This was reflected in the unsteady increase in water transparency, which was within a narrow mean range of 0.29±0.07 to 0.53±0.18m. However, the water turbidity level was above the 5 Nephelometric turbidity units, a level recommended for drinking water quality (WHO 1998). Monthly light transparency remained above 0.1m up-to 2020 and increased from May-June 2021 to 2024. Monthly turbidity declined progressively over the years with slight peaks over the wet months. A lower mean turbidity and higher conductivity and light transparency was recorded in 2023 than previous years in areas which are within the inner zone of the bay. (Fig. 4a-b). Significant spatial and temporal variations (p < 0.05) were evident for the measured turbidity, secchi depth and temperature (except for the zonal temperature). The water conductivity and TDS declined over the seven years with overall mean values of 0.177±0.026 mScm^-1 and 0.10±0.019 gL^-1 respectively. The highest values of conductivity, salinity and TDS were associated with areas around the Kisat river mouth (inner bay) and revealed significant variations (p < 0.05) between the years, stations, seasons and zones.

Nutrient elements, phytoplankton biomass, total alkalinity and hardness were not measured from 2018 to 2021 (Table 1c). Over the three years (2022-2024) total N, ammonium – N and silica revealed significant variation across the seasons and months, except for SRP, TP and oxidized N forms. The levels of nutrient elements also varied across the years (except for silica), zones (except for TN, ammonium – N and silica) and stations (except for TN and silica). The levels of SRP and silicon varied between the sampling months. Over the three years, the phytoplankton biomass measured in the water column varied significantly (p < 0.05) across the stations and bay zones (Figure 6). The chlorophyll – a levels ranged from 2.32 to 715.1 ugL^-1 with an overall mean of 91.83±138.96 ugL^-1 (Table 1b) over the three years. The maximum annual mean of 127.79±176.40 ugL^-1 was recorded in 2022 with much lower mean levels from September 2023 to February 2024. Of the total N, the oxidized N forms ranged from 0.39 % to 15.88 %, with much higher levels of ammonium – N (mean = 7.11±10.70 µM). The mean (±SD) dissolved P, total P, NO₃-N, NO₂-N and SiO₂-Si concentration (2022 – 2024) in Kisumu bay was 1.71±266 µM, 5.52±4.80 µM, 0.75±0.89 µM, 0.31±0.38 uM, 0.36±0.19 mM respectively (TN was not measured in 2022). Water hardness and total alkalinity ranged 26 – 92 mgL^-1 (mean = 40±11 mgL^-1) and 42- 128 mgL^-1 (mean = 56±15 mgL^-1) with an overall three years mean of 44±10 mgL^-1 and 57±12 mgL^-1 respectively. These concentrations both varied significantly between years, zones (except alkalinity) and months.

Fish species

A total of 18 fish species among the Telestoi class were recorded from seven stations around the inner and outer areas of Kisumu bay during the wet and dry months of 2021 – 2023, and consisted of 12 genera, 8 families and 6 orders. The species were dominated by Haplochromines, Oreochromis niloticus, Lates niloticus and Synodontis victoriae (Table 3-4). There were significant differences in the mean sizes of the dominant individual fish species caught. Kisat, Pier and Kichinjio recorded a higher species diversity although only 2 sites (Kisat and Pier) were sampled in 2021. The stations maximum Shannon Weiner fish diversity was 1.380, 1.133 and 1.305 at Kichinjio, Kisat and Pier stations (Fig. 4). The samples of Lates niloticus (2021 survey were dominated by immature fish (> 93%). Sexed Lates niloticus ratio was 1:0.5. The regression coefficient or slope from the log tansformed fish body weight and total length was used to understand the growth performance of fish species and general wellbeing of the population. The coefficient of determination (L. niloticus) was above 0.97 for both seasons, but the slope (b) of the relationship between TL and weight was found to be significantly lower (p < 0.05) than the isometric value for both seasons, indicating allometric fish regrowth (Fig. 5a-b). The relative condition factor [51] ranged from 0.93 – 1.18 (dry) and 0.28-1.12 (wet) with a mean (±SD) of 1.0±0.07 and 0.99(±0.10) respectively.

The size frequency distribution of the Haplochromines shows a modal class length of 4.6 – 5 8 cm (TL) with the spatial mean range of 6.4±1.26 to 8.7±1.42 cm. Lates niloticus modal class length was 5.0 – 8.6 cm (TL) with the spatial mean range of 8.56±4.41 to 14.75±3.4 cm (TL) and an individual body weight ranging from 12.14±21.71g to 38.95±23.18g. The modal class total length of Oreochromis niloticus and Synodontis victoriae were 8 – 12 cm and 14.5 to 15.9 cm respectively. The spatial total length mean ranged from 7.01±2.37 to 16 cm (Oreochromis niloticus) and 13.72±1.03 cm to 15.2±2.08 cm (Synodontis victoriae). The male: female sex ratio ranged from 1:0.73 (Haplochromines);1:0.36 (Oreochromis niloticus); 1:0.5 (Lates niloticus); 1:0.64 (Synodontis victoriae); 1:0.56 (Clarias gariepinus); 1:2.33 (E. profundus and 1:0.6 (Brycinus sadleri);1:1.22(Brycinus jacksonii). A much higher number of individual Haplochromines Lates niloticus nd Synodontis victoriae were caught in the 0.5″-1.75″ 0.5″-1.5″ and 0.75″-1.5″ mesh size nets during the 2021 to 2023 fish gill netting surveys. (Table 5).

4.DISCUSSIONS

4.1 Water quality

Studies on specific ecologies of the bay areas are few and are mostly part of the larger gulf ecological studies. However, there is significant evidence that most of the sheltered bays are “areas of concern” due to their close proximity to increasing urbanization and maritime activities. The limnological conditions influence to a greater extent the normal functioning and productivity of lake ecosystems. Within the shallow lake bays of Lake Victoria, water conditions reflect the high littoral exchanges and the influx of different materials from surrounding areas (including both the diffuse and point sources of polluting substances). The addition of untreated effluents may alter the ambient water quality and ecological communities in receiving waters. Increased nutrients and elevated temperatures increase primary producer growth rates and biomass [52], and in turn result in an increased BOD and turbidity. These changes can cause increased fish mortality and blooms of toxic cyanobacteria [53]. High total ammonia – N concentrations (greater than 2mg/L chronic exposure and 17 mg/L acute exposure at pH = 7 and at 20°C [54] are particularly influential in reducing growth rates and reproductive success or causing direct mortality in aquatic invertebrates and fish [55]. In rivers and streams sensitive invertebrate taxa are less favored in such conditions than the more tolerant worms and true flies (Chironomus), and this may be similar to the littoral shoreline areas. The lake bays have been shown to suffer from frequent periods of excessive growth of phytoplankton, especially the cyanobacteria (blue green algae) [4,14, 56-57]. This study focused on the inner and outer bay areas as water surfaces for potential utilization for the blue economy. Within the bay, water quality was found to differ significantly between the inner shallow zones and the outer exposed areas, which define a general increase in water depth outwards the bay area. In particular, a higher algal productivity, higher temperatures and increased water turbidity characterized the inner areas of the bay. This is consistent with previous gradients along the whole gulf [58] when moving away from the littoral zones. In the gulf, from the Ndere Islands the euphotic depth ranges from 2.1 m to 5.8m, with a much higher value of 8.5m in the main lake.

The most recent data on the chlorophyll-a concentrations depicts increasing levels with a range of in the open waters of the Winam Gulf. Urban storm drainage and watershed contribute to the increased loading of suspended materials around the bay, however this may become apparent when seasonal effects are also considered. Such effects of urban runoffs can override the true seasonal effects around the shores and discharge areas of urban fed streams Kisian, Kisat and Kibos are all urban related streams and transport and discharge suspended loads nearby the bay waters. Water quality changes noted in urban fed streams includes high TSS and nutrients [59-60] and the downstream reaches are characterized by depleted DO levels, but may vary depending on the flow volumes which can be highly variable.

The lake water was found to exhibit eutrophic to hypereutrophic conditions (Table 7) based on the algal biomass and light transparency values. Environmental conditions associated with a high potential for cyanobacterial biomass are TP (>50 – 100 µgL^-1,) high water residence time, pH (>6), secchi depth (<1m) and temperature of ≥ 25 ºC. The shallow bay is restricted to the hydrological exchanges and hence the water quality conditions exhibited of high turbidity and algal biomass. Whole gulf trends (from 2016 – 2022) of phytoplankton biomass maps from Sentinel -2 imagery [2] depicts increasing monthly average levels of chlorophyll- a towards the shore and bays (18 – 362 µgL^-1) from the Rusinga channel areas. Similar high concentrations of chlorophyll-a and total phytoplankton biovolume were observed between 2011 and January 2012 at a site in the bay

32                          

[4]. Concentrations ranged from 19-50 µgL^-1 (surface water), 10-30 µgL^-1 (integrated water samples), 274 to 1038 µgL^-1 (patch water samples) and 18 to 1737 µgL^-1 (shore water samples) with a corresponding high biovolume of phytoplankton cells in patch and shore areas [4]. A lake-wide beach sites study (September-November 2015) by Orina [61] found significant variations in chlorophyll-a levels among the sites and the distance from the lakeshore (0m to 100m distance), with a high concentration at Kichinjio site (of 66.9±0.97 ugL^-1). This study recorded lower chlorophyll-a levels, possibly due to the temporal variations reported earlier, but still there were signs of algal bloom development during the sampling time. Periodic influxes of suspended solids from adjoining terrestrial surfaces and urban rivers discharge contribute particulate material loading to the surface water, effectively reducing the light availability, but increasing available nutrients for primary production. This selectively favours specialized algal species such as Microcystis which are capable of remaining near the water surface through use of gas vesicles [62]. Buoyant cyanobacteria such as Anabaena and Microcystis may float upward when mixing is weak and accumulate in dense surface blooms, while others such as Cylindrospermopsis and Planktothrix stay dispersed, but can reach very high cell densities, resulting in high turbidity. Toxic cyanobacterial species can result in production of high levels of intracellular and extracellular cyanotoxins such as microcystins [3,9,63, 64-66] in water which is a potential health risk. The localized nature of such blooms around the bays may mean a high exposure risk to microcystins to resident fish and caged tilapia.

The of environmental gradients were assessed using PCA of the measured physical and chemical parameters (Fig. 8 a-b). The surface water quality variations were explained by more than two dimensions (over 70% of the variations). Within the water column, PC1 explained about 50.2% of the variations and was positively and well correlated with all the parameters except the sampling depth, whereas PC2 explained about 21.3 % of the variation and was positively correlated with the sampling depth. Similarly, when only the surface water quality variations were analysed, both PC1 (37.8 – 34.5%) and PC2 (13.4 – 15. 8 %) explained about 51.2% of the variation. All the variables were well correlated with PC1. The total depth and light transparency were all negatively correlated with PC1, whereas water temperature, turbidity, conductivity and TDS were more positively correlated with PC1 (except for the sampling depth) compared to salinity and chlorophyll-a. Principal component 2 separated the outer zone stations from the inner zone; and appeared to represent the effects of bathymetry on the microhabitat. PC1 represents the effects of the increasing littoral or shoreline exchanges within the inner zone; and contribution of the frequent re-suspension and redistribution of the unsettled externally derived particulate and dissolved materials influx due to the different anthropogenic impacts.

Shallow lakes have a pronounced impact from wind driven mixing and bottom sediments resuspension. Surface mixing dynamics affect the transport of both dissolved substances, such as nutrients, and of particulates, such as phytoplankton, near the lake surface. There is limited littoral vegetation which is a critical component along the bay as it buffers the lake from particulate materials influx. Rooted macrophytes (vascular plants), once established, can reduce turbulent mixing, increase sedimentation rates and reduce sediment resuspension in shallow water bodies [67].

Sediments resuspended during dredging operations pose a variety of water quality and ecological concerns. The sediment plume in the immediate vicinity of the dredging could influence the behavior of fish and impact the health of less mobile aquatic vertebrates and invertebrates. Resettling of suspended particulates could also impact bottom-dwelling organisms. Resuspension can also result in higher concentrations of particulate-associated contaminants in the water column, drive increases of dissolved contaminant concentrations (a contaminant “release” pathway), and disperse contaminants wider in the aquatic environment. Upon settling, resuspended sediments become generated residuals [68]. Recent dredging operations in the bay may have influenced water column nutrients. This may also contribute to increased contaminant exposure and risk to biota [68] due to post dredging residuals

Spatio – temporal trends in water quality within Kisumu bay

Gulf-wide gradients in the total ionic concentrations depict a deccreasing electrical conductivity towards the mainlake. Within the bay area, the variatons in conductivity emerged between the inner and outer zones and between seasons. The bay is a narrow  zone consstng of submerged, floatng and emergent shore vegetaton compared to the open waters moving outwards into the Winam gulf gulf. This reflects ncreased effects from onland washout and waste water discharge into the bay zone as the surroundng area s manly an urban and industral zone wth agrcultural actvtes located in the peri-urban areas. The open bay sedments mainly consists of soft silty mud but more sandy bottoms appear on the rver nlet areas n the southern and northern zones. Prevous studies show lack of contnous published data within the bay and other lake embayments of Lake Victoria (Kenya) from 2003 to 2008 and 2011 to 2016; but the Kisumu bay area has been sampled as a single site in many of the past lake surveys (Table 8). More recently, investigations conducted by Guya [ 69] in November 2017 using littoral and river mouth sites within the  southern Winam gulf revealed the persistent eutrophcation of the relatively shallow gulf. The eastern section of the Winam gulf recorded much higher levels of SRP (mean range  = 1.9 – 2.7 µM), TP (mean range = 4.1 – 6.0 µM), TN (mean range = 29.5 – 39.6 µM) and SiO₂ (0.2 – 0.6 mM) than the western section [69] as most of the sites were more repesented by river mouth areas. Five littoral North-South transects across the Winam gulf 70], but located outside the Kisumu bay recorded surface water SRP and TP concentratons ranging from 0.74 – 4.61 µM and 1.84 – 8.95 µM respectvely. Mean and median levels of TP (5.52 ±4.80 and 3.88 µM) and SRP (1.71 ±2.66 and 0.94 µM) found in ths study (2022 – 2024 data) were much lower than the range values reported for the transect sites located near the urbanised areas of Kisumu and Homabay [70] which receive treated wastewaters. The difference between TP and SRP in water above the sediment – water interface showed a decreasing pattern from the inner to the outer transects across the Winam gulf [71] with a concentration range of 3.50 – 6.91µM (SRP) and 6.45 – 17.21 µM (TP) during July 2007 sampling. River Nyando is among the inflowng rivers into the Eastern section of Winam gulf found to exihibit apparently high SRP (range = 1.11 – 3.90 µM) and TP (2.21 – 27.95 µM) in surface water [72] durng both the low and hgh flow regmes between July 2015 and March 2016.

The total concentration of phosphorus found in lake waters (overall mean of 5.52±4.80 µM and an annual mean range of 7.03±5.58 µM to 3.05±2.10µM) within the extreme eastern edge of the Winam gulf (Kenya) is way above the levels reported for mesotrophic and oligotrophic basins of a large lake [73] which ranged from 5.93 – 59.33 µgL^-1 (43 years monitoring data 1970-2013) and 2.73±2.05 to 7.96±6.55 µgL^-1 (18 years monitoring data 1996-2014) of total phosphorus and an accompanying low range of the phytoplankton biomass (0.67 – 16.7 µgL^-1) and a very high secchi depth of 1.17m to 10.84 m. These lakes are characterized as P – limited as compared to the N limitation of phytoplankton growth within most of the Lake Victoria (Kenya) bays. The percentage of the dissolved P in surface water ranged from 2.11 – 66.45% (outer bay) and 1.34 – 89.65 % (Inner bay); implying readily available soluble P within the inner bay and increasing availability of organic and particulate phosphorus towards the outer bay. The appreciably high levels of N sources and available P with optimal temperatures and light conditions are important drivers of the common harmful cyanobacterial blooms frequently encountered in many of the bays within Winam gulf which exposes the water users to associated risks of exposure to cyanotoxins.

Water quality of urban streams and rivers draining the bay were investigated by Omondi et al. [59] and Kobingi et al. [60] who reported the increasing impacts in the relatively small river channels and waterways from the urban human activities. Treist et al. [74] also used epilithic diatoms assemblage and observed a decline downstream of the algal species in rivers with conductivity, alkalinity, turbidity, dissolved oxygen and silicate as the most important water quality variables which influenced the species distribution in studied rivers. The high nutrient loading within the small waterways into the bay are of significance in management of the bay eutrophication. Therefore, there is need to consider rehabilitation plans for the impacted urban rivers and streams into the bay.

4.2 Fish composition and diversity

Three fish species (Haplochromines spp. Lates niloticus and Synodontis victoriae) dominated (percentage by numbers in 2021) the experimental catch survey at KPC/Pipeline pier and Kisat, between March and December 2021 with the rest of the species present in very low numbers. Spatially, more sites were sampled during 2022 and 2023 compared to 2021 monthly surveys. During the survey, the lowest number of species were recorded during the month of July (2021). In 2022, a high species richness was observed at Kichinjio and Kisat, compared to the rest of the sites, which recorded between 1 and 3 fish species. Very low number of fish species (Lates niloticus, Haplochomine spp. Oreochromis leucostictus and Enteromius profundus) and abundances were recorded in 2023 from 3 sites.

The sex ratio (M:F) of Haplochromine spp., Synodontis victoriae and Oreochromis niloticus was 1:0.73, 1: 0.64 and 1:0.36 respectively. Stage 3 and 4 (O. niloticus) and stage 3 -5 (S. victoriae). Haplochromne spp. were dominated by stages 3 – 6 (Males and females). Fish surveys sites were more concentrated in the inner zone compared to the outer zone and this may not allow making detailed comparisons in diversity results and spatial variations. Shallow littoral waters are ecologically important aquatic habitats and support a number of fish species and other aquatic organisms. These fishing areas are often exploited by artisanal fishermen who often are capable of using illegal gears. Around the lake, the management of small -scale inland fisheries has outlawed the use of small size fishing gears in Lake Victoria [75]; which eventually contribute to undersize catches of commercially regulated and exploited fish species. However, from the experimental gears employed in the survey; low strict enforcement of this regulation can result in exploitation of juveniles. The few rocky lakeshores on the southern and sandy bottoms in the north are suitable spawning areas for Nile tilapia within the gulf, but are under threat from increasing human disturbances. The sampled Nile tilapia fish caught were all below the length at 50% maturity for Nile tilapia population. Previously, the recorded size at 50% maturity for Nile tilapia in Lake Victoria (Kenya) was 31cm TL (females) and 35 cm TL (males) [76] which was higher than 27.5 cm TL (females) and 31.5 cm TL (males) recorded by Ogari and Asila [77].  Most of the Nile perch were below 30 cm TL, a sign of good recruitment which usually follows the rainy months. The age groups entering the fishery [78] in Tanzanian waters coincides with the months of November and December; and July to September in Uganda. The length at which 50% of the caught individuals mature in Nyanza gulf decreased from was at 74 cm TL (males) and 102 cm (females) (in 1978 – 1984) to 55 cm TL (males) and 70 cm TL (females) in 1990s. [77,79]. The shallow nearshore areas are important spawning and nursery areas for small Nile perch.

Nile perch introduction into Lake Victoria has played a key role in shaping the trophic relationships which saw the decline (1980s) and suspect recovery of the haplochromine fish species. Lates niloticus prefer three key species (Haplochromines, Rastrineobola argentea Caridina nilotica) but changes its diet depending on the availability of the food items and growth stage. In Mwanza gulf [26] shrimps were the main diet for Nile perch of between 5 and 30 cm TL. At a size of 3 to 4cm Nile perch shifted from size-selective predation on the largest cyclopoids to predation on the largest, less abundant, calanoids and zooplanktivory ended at a size of ca. 5cm.

The study area is an important habitat providing refugia zones for prey species and juvenile fish species. Larval studies show that major nursery grounds for Nile perch, Nile tilapia and dagaa are the shallow inshore areas [76, 80]. The zones are highly productive compared to the outer areas, hence delivering a rich food source from the remaining patchy macrophytes along the vegetated littoral areas, river mouths and a wetland zone; with frequent influx of scattered water hyacinth and other floating macrophytes in the bay. In the study area, benthic macroinvertebrates were found dominated by the tolerant groups. The Chaoborid and Chironomid larvae are major food source of insectivorous and zoo-planktivorous haplochromines and Odonata nymphs are the food of Lates niloticus [81-83]; and therefore can influences the fish distribution besides the seasonal rainfall influences on the fish species reproduction cycles. Nile tilapia spawns throughout the year and in Kenya, the breeding peaks are in March to May and October to December [76, 84]. Besides this, juvenile Clarias sp., Haplochromine spp., Synodontis and Shilbe are among the indigenous bait fish species used in the hook and line fishery in Lake Victoria. Therefore, continued use of small mesh size gear (illegal gear) in fish breeding and nursery areas may result in risks to biodiversity conservation. More awareness and actions to protect the diverse fish microhabitats is key to a more sustainable blue economy.

The mean TL for O. niloticus, caught by all experimental gears was less than 16 cm. Nile tilapia are known to be herbivorous, and feeds mostly on algae but was also found to consume diverse food items which included insects, algae and plant materials. The major diet of fish <5 cm TL was zooplankton whereas bigger fish included a wider range of food items in their diet. Mean total length (±SD) at first maturity from trawl and beach seine data in Nyanza gulf (1998 to 2000) was 30.81±0.09cm for females and 34.5 6 0.48cm for males [84]. Several recent investigations on the biology of major species have reported changes in Nile perch, Nile tilapia and Rastrineobola argentea (2014-2015) fish sizes [85-88] in the Kenyan part of L. Victoria compare to previous study data and recommend enforcement of the fishery regulations (Nile perch slot size of 50-85 cm TL; and illegal gears for R. argentea and O. niloticus fishery). The declining fish abundance has been reported previously and was manly attributed to over-exploitation use of illegal fishing gears and deteriorating water quality from increasing pollution. Within the Winam gulf during the period 2012 -2016 [89] there was a difference between the turbid and eutrophic waters and clear open waters; and 13 fish species were recorded from the bottom and mid-water trawl surveys. The bottom-dwelling fish (Clarias gariepinus, Synodontis victoriae, Bagrus docmak, Protopterus aethiopicus and Schilbe intermedius were prevalent [89] in zones generally characterized by a relatively low dissolved oxygen; and high TN and TP, turbidity and fecal coliforms levels. The type of fish gear and areas sampled can influences the fish catch composition greatly. This is evident from the study as more untrawlable littoral and shallower waters (less than 5m) were fished. The bay area consists of diverse fish habitat types (wetlands and river mouth areas) which are considered refugia and breeding areas for fish species and are recognized as important critical areas for fish breeding and nursery grounds (hotspot areas for biodiversity). This explains the high number of fish species encountered during the survey. Within the Winam gulf, there was observed an increasing light transparency and decreasing water turbidity, which was also evident in the study area over the past six years. This could be explained by increased water exchange after the causeway at Mbita was opened from 2017. The shallow bays are flushed more frequently and even the water hyacinth does not permanently reside in the bay. Scattered water hyacinth mats are seen flushed in and out of the bay faster than before allowing uptake of nutrients; and not causing much effects on the underwater environmental conditions. The relatively clear water facilitates co-existence of many fish species, with great influences on the catch rates of haplochromines reported n northern Lake Victoria [90-91]. The bay and other shallow water zones are critical fish habitats and need to be protected from threats of expanding urban and industrial establishments and uncontrolled artisanal and commercial fisheries activities.

The loss of biodiversity of fish in Lake Victoria is mainly due to habitat degradation and loss, eutrophication, predation and competition from introduced non-native fish species (Nile perch and Nile tilapia) and, in some cases, the unsustainable use of the lake from overfishing or the use of improper fishing gears [88]. As cage aquaculture expands within the Lake bays, good water quality is central towards achieving a sustainable tilapia cage aquaculture in Lake Victoria, and hence the need for such continuous long-term monitoring data. A number of fish health management practices and biosecurity measures have been recommended [92-93] as fish are susceptible to a variety of infections and diseases caused by microbial pathogens and parasites [93]. Cage sites outside the bay were initially prone to fish kills due to poor cage sitting and sudden water deoxygenation events. This explains the relocation of cages to deeper lake areas (> 4 m deep) where natural water mixing occurs and farmers are assured of sustainable investments with application of good aquaculture practices and enforcement of developed guidelines.

4.4 The Blue economy concept and Kisumu bay ecology

Maritime, fisheries, freshwater aquaculture, mariculture and bio-prospecting and mining activities are among the key components of the blue economic growth. The blue economy concept promotes economic growth, social inclusion and improved livelihoods at the same time as ensuring the environmental sustainability of oceans and seas [94]. The FAO flagship initiative (blue growth initiative) aims at supporting more productive responsible and sustainable fisheries and aquaculture sectors by improving the governance and management of the aquatic ecosystems by conserving biodiversity and habitats and by empowering communities [95]. However human activities and types of landuse have an increasing impact on the water resources. They affect both the quality and quantity of available safe water for the different uses; and hence the need of integrated approaches in the exploitation of both the marine and freshwater resources.

In Kenya, Lake Victoria dominates the inland waters capture fishery production (94,349 metric tonnes in 2021) which is about 71% of country’s total fish production [96], but globally there is still unmet demand for fish. Lake fish production is both of nutritional and commercial benefits to the riparian countries communities. The unique combination of high quality protein and important micronutrients (Iodine, calcium, zinc, iron, vitamin A, D and B12, polyunsaturated fatty acids such as eicosapentaenoic acid and docosahexaenoic acid) in fish plays a significant role in combating the triple burden of hunger micronutrient deficiencies and non-communicable diseases [97-98].

Fisheries and aquaculture production is projected to increase (by over 60%) by 2050 in order to feed the world population [99]. Aquaculture development is therefore set to provide the shortfall from capture production. Nile tilapia fish cage culture in Lake Victoria (Kenya) has attracted rapid investments since 2015 [41;43,100], with different types of designs and cage sizes (ca. 3,696 to 4,357 total cages during the 2018 survey). Fish cages in Lake Victoria are normally deployed in areas of suitable depth (inshore and offshore) and away from navigational routes, to allow free water circulation and to ensure they meet the acceptable water quality criteria. Using a profitability analysis model, it is suggested that improvement in economic profitability of Nile tilapia (Oreochromis niloticus) cage culture by changing from current small volume cages to large cages is more likely to attract formal financial institutions to cage culture financing and hence advancing the blue economy concept [101]. Hence the implementation of a spatial plan on the use of lake water for other integrated new developments juxtaposition the normal fisheries and maritime activities.

There is a growing body of evidence showing that biodiversity is important for generating and stabilizing ecosystem functions, and thus ensures the provisioning of numerous ecosystem services to society [102]. The inner and outer zones of the Kisumu Bay area surveyed has an approximate water surface area of about 26 Km² and is mainly dominated by urban and maritime related human activities. However, it is hypothesized that invasion and presence of water hyacinth caused a significant change to under water biodiversity especially periphyton, microbial and macroinvertebrates communities although there is less matching scientific evidence of data due to a lack of long term investigation during periods on extensive coverage and difficulties in accessing areas under water hyacinth (Fig. 7). Again, urbanization or expansion of cities to meet the demands of the population and economic growth has a significant local impact on water resources. Habitat alteration due to urbanization can be both drastic and increasingly widespread. Large parcels of land are devegetated, paved and dramatically modified in ways that often greatly exceed habitat changes that occur from logging, traditional farming and many other land uses [103]. Also, land modifications during urban growth are usually long-term and indeed often intensify with time so that there is no opportunity for successional recovery [104]. Therefore, the preservation and restoration of local indigenous species biodiversity must be emphasized. Restoration ecologists, land managers, and urban planners can help maintain native birds in fragmented landscapes by a combination of short- and long-term actions designed to restore ecological function [103] (e. g the increase coverage of urban parks; preservation of wetland areas and restoration of vegetated riparian lands islands and shorelines). Macrophytes and vegetated littoral areas constitute important aquatic biodiversity refugia zones and; fish nursery and breeding areas.

Persistent pollution problems and ecological threats

Plastic pollution and associated ecological threats to both the freshwater and marine ecosystems s still perceived as a serious challenge to the blue economy developments due to the worldwide production and utilization of plastic materials and products. Already this has elicited a series of studies to quantify amounts in the different ecosystems and create the required awareness amongst the various stakeholders to ensure eradication of many types of plastics materials in surface waters. Rainfall variability has been observed over Africa between the years 1900-2000. Seasonal rainfall has also been established to be sensitive to El Niño climate variability [105]. An extremely wet category of rainfall which was recorded between 1997 and1998 coincided with the 1997 El Niño episode experienced in Eastern Africa. The extremely dry season experienced between the years 2002 and 2006 would be attributed to teleconnection of El Niño and La Niña in the region [106]. According to recorded water level trends [107] the Lake Victoria water levels rose steadily from 2008 to 2021 causing backflow flooding. The 2020 floods displaced hundreds to thousands of people and disrupted transportation, drinking water, sanitation, and power systems. The trends (1987-2016) of Kenyan lake basin wide precipitation record high rainfall events in March April and May (long rains) and October November and December (short rains) [108]. The high variability and intensity of precipitation may exacerbate such problems due to increased flushing of uncollected and undisposed or poorly disposed solid wastes around the urbanized bay catchments especially after flooding events around the low-lying riparian areas. Conversion of forest lands and human settlements on riparian areas, including the growth of towns, is changing water quality and quantity, organic matter and nutrient inputs, and the diversity and composition of aquatic communities [109]. Natural buffer vegetation and wetland areas contribute to the maintenance of the aquatic habitat integrity and hence there is need to ensure their protection around the bay. Therefore, it is important for the Blue economy investments to ensure they adopt clear actions on reducing plastic pollution sources n surface waters (both marine and freshwater) juxtaposition the fast urban areas expansion and rapid population growth, especially around the major surface water bodies (lakes rivers and reservoirs). Lake Victoria basin has one of the highest population densities in Africa [14]. The demand for land and its associated resources and ecosystem services, such as agriculture, grazing land, urbanization, rural and urban settlements is still increasing in much of East Africa. This has driven human-related change over short and long-term scales [110].

Cage Aquaculture

The national and county governments and multilateral development projects and programs have supported cage aquaculture development in the lake (the space of the Kenyan lake water surface suitable for cage aquaculture is about 362 Km² or 9 percent); with a multiplier effect on the associated infrastructure for feed and seed production countywide. International donors and development agencies’ role should not be limited to technology transfer, and capacity building, but they should collaborate with the national and county governments in developing innovative financial models that favor sustainable aquaculture enterprises [111]. More collaborative research should be devoted to design and construction of climate smart culture systems, developing new species to guarantee supply of high-quality products; develop and scale low cost and highly nutritious fish feeds based on novel ingredients; and to enhance resilient livelihoods through innovative aquaculture practices and market linkages to create employment opportunities for youths and women [111]. Even though cage culture holds great promise for boosting output, generating employment opportunities, and enhancing the economic wellbeing of rural communities, site suitability for installing cage is still poorly regulated [111]. Over 45% of cage installations are made within 200 meters of fish breeding grounds, which may put other lake users in conflict [42]. To sustainably utilize the blue economy potential on cage aquaculture the National and County governments should provide cage culture investment and management guidelines to cater for any water use conflicts with other maritime activities; wild capture fisheries and demarcated fish protection areas. There is also a need to develop a long-term inter county lake surface spatial plan for a sustainable lake basin resources utilization.

Most of the tropical running waters studied, commonly exhibit longitudinal variations in water quality. Some of the medium-sized rivers drainage basins show downstream decrease compositional loads and concentrations of solutes due to the different landuse characteristics. The downstream total N and P concentrations decreased (1740±417 to 654±144 µgL^-1) and increased (223±35 to 51±2 µgL^-1) respectively, along the Awach-Kibuon river [112]. However, the water quality of the more urbanized but small streams was evidently of poor quality based on some of the measured physico-chemical variables and total N and P (s). This is attributed to the use of urbanized streams as dilutants and conduits of treated effluents and other sold and liquid surface waste loads before entry into the bay waters.

Climatic changes can cause small to large variations in key environmental variables of the freshwater aquatic ecosystems, and influence the productivity and distribution patterns of the different aquatic communities and at varying scale. The study area is a hotspot area and is exposed to several anthropogenic impacts compared to other bay areas with less urbanized catchments. We therefore support the continuous monitoring of such key embayment areas, which are under increasing urbanization, human settlements and population, for the long-term effective strategy for habitat protection and ecosystem management.

CONCLUSIONS

The dominant fish species were represented by the two introduced species (Lates niloticus and Oreochromis niloticus) and the endemic Haplochromines and Synodontis victoriae. Other rare endemic fish species caught were O. esculentus and O. variabilis and the introduced Coptodon zilli and O. leucostictus. The bay area is among the suitable shallow sheltered zones for spawning and nursery ground for most fish species. Haplochromines are omnivores exploiting a wide range of food resources (algae molluscs insects plant material detritus and zooplankton) and this explains their dominance and wide occurrences within the bay. Conditions encountered within the bay favour zooplankton growth and hence the zooplanktivores exploit the available food item. Also pronounced seasonality effects on available nutrients ensures sustained phytoplankton community commonly found dominated by cyanobacteria within major bays of the Winam gulf. The decline in fish production mainly results from the use of illegal undersize nets that harvest the juvenile fish species; unprotected breeding and nursery areas; beach seining and increasing fishing effort. Therefore, the fisheries managers and community BMUs need to ensure measures to reduce the fish decline are enforced. Embayments and beach sites around urbanizing lake areas need to be supported with improved sanitation facilities waste collection and infrastructure to reduce the direct influx of both surface waste discharges and solid wastes into the surrounding aquatic habitats. Also the rehabilitation of urban impacted streams and maintenance of the littoral wetland areas and vegetation will ensure the sustenance of the natural buffer zones and reduce silt deposition and sedimentation into the bay.

In conclusion, this study provided a more localised and specific investigation of the ecology of an “area of concern”, experiencing increasing population and economic growth, as the focus on the blue economy opportunities continue to emerge and the gulf-wide need to accommodate cage aquaculture in the same surface water space. Overall, there appears to be ecologically important effects of the spatial extent of the different urban land uses on adjoining water quality. The bay water was found to exhibit eutrophic (based on algal biomass) to hypereutrophic conditions (based on the and light transparency values and total phosphorus concentration). However, based on the mean TSI of these water quality variables (Secchi depth, TP and chlorophyll-a), the lake is still of a high ecological productivity (eutrophic state).

Recommendations to inform management

• Lake riparian counties attract different investments and developments and should therefore incorporate strategies to minimize destruction of wetland areas; improve sanitation services, enhance watershed management and reduce eutrophication sources; and improved solid waste management plans.
• Investments in eco-tourisms activities can create more awareness on the need to protect the biodiversity and critical aquatic habitats.

There is need to reduce any potential conflicts between water users and maritime activities as cage aquaculture expands; and more importantly improve capability and preparedness for emergencies such as the recent cage-related fish kills in Lake Victoria, through strengthening of research, training, extension, and advisory services

DECLARATION

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

FUNDING

There was no funding.

AUTHOR CONTRIBUTIONS

JM: Investigation; Writing -original draft; Writing-review and editing; conceptualization; Formal analysis; Project administration; supervision; methodology visualisation and validation; CAM and CSN: Investigation; Writing-review and editing; Project administration; supervision; methodology visualisation and validation; EA; SM; JM; JA; GO: Investigation Writing -original draft; Writing-review and editing; conceptualization; Formal analysis; methodology visualisation and validation

ACKNOWLEDGEMENTS

This work was supported by Kenya Marine and Fisheries Research Institute (KMFRI). We would like to acknowledge the support of KMFRI in conducting the field activities and development of this report. Much appreciation to the scientific, mercantile staff and technical teams at KMFRI who were involved in the data collection and analysis.

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Table captions

Table 1a. The mean (±SD) values of the water quality variables (n = 195 for all variables, except turbidity and chlorophyll- a where n = 193 during the dry (2023 and 2024 n= 133) and wet (2023) season months across the sampling sites (Sample size for nutrient elements (2023 to 2024) in surface water = 32 (Dry) and 16 (Wet); Significantly different values (p < 0.05) between the seasons are indicated with a different alphabetical letter).

Table 1b. A comparison of the dissolved nutrient concentrations, total alkalinity, hardness and phytoplankton biomass between 2022 and 2024 (p = Kruskal-Wallis test p – value).

Table 1c. The annual mean (±SD) values of the water quality variables between 2018 to February 2024 in Kisumu bay.

Table 2a. The Spearman’s correlation coefficients (significant = bolded) of the relationships between the water quality parameters in Kisumu Bay (Sample size = 651 for data collected from 2018 to February 2024; except for D.O = 472; pH = 450; ORP = 458; Turbidity =601 Secchi depth =504; Alkalinity and hardness = 68; TSI_TP; TSI_Chla TSI _Secchi & TSI_Bay = 83; Chlorophyll-a = 193; total depth =44 Total nitrogen = 42; N:P ratio = 42). The lower value represents the exact p value).

Table 2b. The Spearman’s correlation coefficients (significant = bolded) of the relationships between the surface water quality parameters and nutrient elements in Kisumu Bay (Sample size = 73; n = 89 for nutrient elements) The lower value represents the exact p value).

Table 3. Fish species recorded and overall percentage by numbers and weight during the 2021 – 2023 monthly surveys in Kisumu bay.

Table 4. The mean total length (cm) and weight (g) of the dominant fish species (a different superscript letter indicates significantly different means across the stations).

Table 5. The overall monthly percentage composition of fish species according to the numbers and weight.

Table 6. Percentage composition of Haplochromine spp. (a) Synodontis victoriae (b) Lates niloticus (c) and Oreochromis nilotcus (d) according the mesh sizes for the weight and total length classes.

Table 7. Recorded fish species maturity stages indicated as a percentage of sexed fish species (Imm = immature).

Table 8. Carlson’s trophic state index (TSI) and categories of lake productivity.

Table 9. A comparison of the long-term water quality data showing the mean (±SD) and range values of some of the physico-chemical variables determined within Winam gulf and Kisumu bay between 2003 and 2024.

Figure captions

Figure 1. Map of Kisumu Bay showing the sampling stations for water quality measurements (a) and fish sampling (b).

Figure 2 (a-t). The spatial mean (±SD) nutrient concentrations (µM) of the Total P and N, dissolved P, Si (mM), nitrate – N and nitrite – N in lake surface water and the significant variations among the zones, seasons and sampling years from 2022 to 2024.

Figure 3. Boxplots showing the water column variations of the D.O, pH, ORP, temperature, conductivity, TDS, salinity, chlorophyll-a, turbidity and water transparency (surface), between the stations, years (a), seasons (b) and bay zones(c) within Kisumu bay between 2018 to 2024.

Figure 4. The fish diversity indices (Shannon-Weiner diversity H’, Pielou’s evenness E, Simpson dominance D, Simpson index of diversity 1-D, Simpson reciprocal 1/D and Margalef richness d) according to seasons (Wet and dry) for each station surveyed between 2021 – 2023.

Figure 5(a-b). The relationship between total length (cm) and weight (g) of Lates niloticus caught in 2021 (a) Synodontis victoriae and Clarias gariepinus (b) and Oreochromis niloticus (c)caught in 2021 to 2023 during the dry and wet seasons.

Figure 6a – b. PCA ordination plot of the sampling stations and measured water quality variables in the surface water (a) and the water column (b) during 2023and 2024 surveys grouped by seasons and bay zones.

Figure 7. The changing cover of Kisumu bay water surface during water hyacinth coverage and other maritime activities.