Department of Aquaculture and Fisheries Management

Permanent URI for this collectionhttps://dspace.njala.edu.sl/handle/20.500.144402/158

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Start date: 2020Has files: No

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Now showing 1 - 9 of 9
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    Influence of net cages on water quality and trophic status of Lake Victoria, Kenya: The case of Kadimu Bay
    (Wiley, 2023) Mawundu, Sellu
    Water quality is a critical component regulating ecosystem functioning in aquatic habitats, requiring regular monitoring for sustainable ecosystem services. Cage fish farming has the potential to affect water quality because of its rapid increase in many African waterbodies in response to dwindling wild fish stocks. Thus, there is a need for more studies to guide sustainable cage aquaculture in African lakes and reservoirs. This study evaluated the possible effects of cage farming of Nile tilapia (Oreochromis niloticus) on water quality parameters and the trophic state of Kadimu Bay, Lake Victoria, Kenya. Sampling for physicochemical and biological variables, including nutrient load, was conducted from January to October 2021, at five fish cage sites and a control site within the bay. In situ measurements of physical variables were undertaken in the field, while analysis of water samples for nutrient loads, biological and chemical variables was undertaken in the laboratory, following the methods described in APHA (American public health association standard methods for the examination of water and waste water. APHA‐AWWA‐WEF, 2005). The Carlson's Trophic State Index (CTSI) was used to classify the trophic state of the cage sites, while the total nitrogen:total phosphorus (TN:TP) ratio was used to determine the primary productivity limiting nutrient in the bay. The study results indicated electrical conductivity was significantly lower at the control (97.53 ± 4.17 μS/cm), compared to cage sites (105.42 ± 5.32 μS/cm at the Utonga cage site to 112.84 ± 1.94 μS/cm at the Oele cage site), indicating water of relatively lower quality at the cage sites. Similarly, the nitrite concentrations were higher at cage sites (6.35 ± .96 μg/L at the Uwaria cage site to 3.16 ± 2.25 μg/L at the Utonga cage site), and lower at the control site (2.68 ± 1.39 μg/L). In all, 14 physicochemical variables did not vary significantly between the cage and control sites, with nine variables (temperature, turbidity, electrical conductivity, total suspended solids, particulate organic matter, chlorophyll‐a, TP, nitrate and TN) being within the recommended thresholds for aquatic life processes. The bay was evaluated as being in a light eutrophic state, indicating moderate influence of the fish cages on the trophic state of the sites. There was a moderate relationship between chlorophyll‐a and TP concentration at the sampling sites (R2 = .50), compared to a stronger relationship with (R2 = .78). The TN:TP ratios were <10 at the sampling sites, indicating nitrogen was the limiting factor for primary production in the bay. The calculated CTSI suggests that the bay exhibited a light eutrophic state. Overall, although the results of this study showed cage aquaculture is not a current challenge to the water quality of the bay, regular monitoring is nevertheless recommended to inform sustainable aquaculture development in the bay and lake.
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    Sex ratio, gonad maturity and size-structure of Pagelus bellotti in Sierra Leone
    (2021-02-16) Mansaray, Alie Hindolo Dembeh
    A total of 135 specimens of Pagellus bellotti were randomly collected from artisanal boats in August 2018. Study of sex ratio, gonad maturity and size-structure of Pagellus bellottii is meagre in Sierra Leone, and forms the basis of the present study. The study will contribute to current knowledge and promote rational exploitation of the stock. Length and weight were measured using a fish measuring board (cm) and digital weight scale (g) respectively. Sex and gonad maturity were macroscopically assessed. Results showed dominant males (71.9%) and significant difference observed (1:2.6) and expected (1:1) female: male sex ratios (p 0.9). Indexes of condition were greater than 1.0 (K > 1) for all sex categories. Results portrayed a heterogeneous gender population of Pagelus bellottii and vast similarity in reproductive strategies by both sexes. The fish comprised of one size group and males may be growing larger than the females. Growth pattern was near-isometric for females and negative allometric for the males, and both sexes were in better condition (K >1.0). An annual study could provide further insight.
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    Harnessing light: innovative photodynamic therapy for controlling Aeromonas hydrophila in aquaculture
    (Springer Science+Business Media, 2025-02-28) Ayana, Gelana Urgesa
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    Chemical interactions between filter-feeding mussels and Ulva prolifera: The role of dissolved organic matter and secondary metabolites in growth promotion and competition inhibition of algal species
    (Elsevier BV, 2025-09-06) Sumana, Sahr Lamin
    This review examines the chemical and ecological interactions between filter-feeding mussels and the green macroalga Ulva prolifera in integrated multi-trophic aquaculture (IMTA) systems. Mussels are crucial for nutrient recycling, as they filter water and release bioavailable compounds such as ammonium (NH 4 + ), urea (CO(NH 2 ) 2 ), and dissolved organic matter (DOM). These compounds promote Ulva growth and enhance microbial activity. In turn, U. prolifera produces sulfated polysaccharides, phenolics, and halogenated metabolites that can influence microbial communities, suppress competitors, and potentially affect mussel physiology at high concentrations. The review emphasizes the interconnectedness of nutrient exchange, DOM cycling, and microbial genes such as pmoA and mcrA, highlighting the novelty of integrating microbial ecology with biogeochemical cycles and ecosystem outcomes, illustrating both the synergies and risks present in co-culture systems. While moderate production of metabolites helps control biofouling and maintain ecosystem stability, excessive DOM or allelochemical accumulation can hinder mussel filtration and lead to hypoxia. Maintaining specific thresholds, such as DOM concentrations below 5 mg C/L and labile-to-refractory DOM ratios above 1:1, is essential for balance. This synthesis integrates microbial ecology, metabolite feedbacks, and biogeochemical processes to provide a framework for resilient IMTA design. It also emphasizes practical strategies like adjusting stocking densities, optimizing water exchange, and employing microbial monitoring tools to promote sustainable IMTA practices. By linking molecular interactions to ecosystem-scale outcomes, the review offers guidance for sustainable aquaculture systems that enhance productivity, minimize environmental risks, and improve resilience in the face of climate stress.
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    Overexploitation and urgent need for precautionary management of Bonga Shad ( Ethmalosa fimbriata ) in West African waters: A multi‐model Bayesian assessment
    (Wiley, 2026-07-30) Konoyima, Komba Jossie
    Ethmalosa fimbriata, a key small pelagic species underpinning food security and livelihoods along the West African coast, warrants a robust stock assessment due to concerns of overexploitation. This study synthesizes multi-fleet catch records (1970-2021) and unstandardized catch per unit effort (CPUE) data (1995-2017) from the Eastern Central Atlantic (ECAO) to evaluate the northern stock using four surplus production models: the catch-only (CMSY) and abundance-only (AMSY) maximum sustainable yield models, the Bayesian state-space model (BSM) and the Just Another Bayesian Biomass Assessment (JABBA) model implementing Schaefer, Fox and Pella-Tomlinson functions. The objective was to evaluate model predictive accuracy and stock status to provide evidence-based management advice. From the results, CMSY indicated 2021 biomass (B) was marginally above the sustainable limit (B/BMSY ≈ 1.03) with current fishing mortality (F) near FMSY, while AMSY and BSM both indicated ongoing depletion (B/BMSY ≈ 0.56-0.81) and overfishing (F/FMSY >1.0). JABBA's Schaefer model, demonstrating superior fit (root-mean-square error = 16.9%, Deviance Information Criterion = -382.7) over Fox and Pella-Tomlinson models, estimated 2021 biomass at 9% below BMSY and fishing mortality at 59% above FMSY. Fox and Pella-Tomlinson models yielded similar biomass trajectories but were less precautionary. Projections indicate that only Schaefer-based total allowable catches (TACs) below 100,000 t would stabilize biomass; higher quotas risk continued decline. Despite limitations including incomplete catch data and reliance on unstandardized indices, results underscore the urgency for precautionary catch limits, regional coordination and improved compliance to ensure sustainability. This study offers a transferable assessment framework for data-limited fisheries and supports evidence-based policy for the conservation of E. fimbriata and food-security resilience in West Africa.
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    How Grouping Data over Time Can Hide Signs of Stock Status: A Case Study Using LBSPR on Frigate Tuna (Auxis thazard, Lacépède, 1800) in the Northeast Atlantic Ocean
    (Multidisciplinary Digital Publishing Institute, 2026-01-23) Bayon, Mustapha Sly
    Length-based stock assessment methods are widely applied in data-limited fisheries, yet the effects of how length-frequency data are temporally grouped prior to analysis remain poorly examined. Temporal grouping is routinely used to increase sample size and approximate equilibrium conditions, but it may also alter the size structure presented to assessment models and bias inference. In this study, we evaluate how alternative temporal grouping schemes influence stock status inference within a single length-based framework, using the length-based spawning potential ratio (LBSPR) model as a diagnostic tool. Using a 30-year length-frequency dataset from a tropical purse seine fishery in the Northeast Atlantic as an illustrative case, we applied LBSPR under four practice-relevant temporal grouping schemes: full-period pooling, a broad regime-based scheme, decadal blocks, and five-year blocks. Life history parameters and model settings were held constant across schemes to isolate the effect of temporal grouping. A sensitivity analysis of biological parameters was conducted for the finest temporal scheme to contextualise robustness. Results show that temporal grouping alone can substantially alter the inferred status of the illustrative case. The fully pooled scheme produced an apparently favourable status signal, whereas finer temporal groupings revealed extended periods of inferred reproductive depletion, followed by a more recent recovery. Sensitivity analyses indicate that, while biological parameter uncertainty influences the magnitude of estimates, it does not overturn the dominant effect of temporal grouping on inferred status patterns. This study demonstrates that temporal grouping is not a neutral preprocessing step but a structural decision with the potential to conceal or reveal exploitation signals in length-based assessments. We argue that temporal grouping should be treated as an explicit sensitivity dimension in data-limited assessment workflows. By shifting attention from stock-specific outcomes to data-structuring choices, this work provides practical guidance for improving transparency and robustness in length-based stock status inference.
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