The fisheries potential of Marcusenius pongolensis, Oreochromis mossambicus and Schilbe intermedius in Mnjoli dam, Swaziland
- Authors: Khumalo, Nokuthula
- Date: 2006
- Subjects: Mozambique tilapia , Marcusenius pongolensis , Schilbe intermedius , Fishes -- Swaziland , Fisheries -- Swaziland , Tilapia -- Swaziland
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:5283 , http://hdl.handle.net/10962/d1005127 , Mozambique tilapia , Marcusenius pongolensis , Schilbe intermedius , Fishes -- Swaziland , Fisheries -- Swaziland , Tilapia -- Swaziland
- Description: This thesis investigates aspects of the diversity, abundance and biology of the fish species inhabiting Mnjoli Dam, an irrigation dam that is located in the rural lowveld region of Swaziland. Specific objectives for this thesis were: to determine the fish species present and select three principal species based on their abundance; to investigate the relative abundance and distribution of the three selected species; and to describe key population parameters, such as growth, maturity, reproductive seasonality and mortality. The three dominant species were selected on their fisheries potential. These were Marcusenius pongolensis, Oreochromis mossambicus and Schilbe intermedius. Marcusenius pongolensis was the most abundant species in terms of both catch per unit effort and mass. Catch rates were shown to vary according to habitat type. Age and growth parameters were obtained from sectioned otoliths. Marginal zone analysis showed that annulus formation occurred in winter for all three species. The maximum-recorded ages were 8, 6 and 8 for M. pongolensis, O. mossambicus and S. intermedius, respectively. Growth for the three species was best described by the three parameter von Bertalanffy growth model as Lt = 1.88.67(1- exp⁻¹·⁴⁸⁽t⁺°·⁶⁷⁾) mm FL for M. pongolensis; Lt = 226.83(1- exp⁻°·⁴⁽t⁺²·°²⁾) mm TL for O. mossambicus and Lt = 214.59(1- exp⁻°·⁶°⁽t⁺¹·²°⁾) mm FL for S. intermedius. Sexual maturity was estimated for male and female M. pongolensis at 134 mm FL and 119 mm FL, respectively. Oreochromis mossambicus matured at 239 mm TL and S. intermedius at 205 mm FL. Two spawning peaks for M. pongolensis and S. intermedius were observed, one at the onset of summer (November) and the second at the beginning of autumn (February-March). Oreochromis mossambicus also exhibited two spawning peaks, one in spring (September) and the second in autumn (February –March). Mean mortality rate (Z), estimated using catch curve analysis was 0.77 year⁻¹ for M. pongolensis, 0.49 year⁻¹ for O. mossambicus and 0.79 year⁻¹ for S. intermedius. Natural mortality was assumed to be equal to Z since there is no fishery activity at the dam. Size specific selectivity curves were developed for each species. Of all the mesh sizes used, the 44 mm mesh size net had the highest catch rates in terms of numbers for M. pongolensis and S. intermedius. The 75mm mesh caught the highest catch rates for O. mossambicus. . The 44mm and 75 mm meshes seem to be the most suitable mesh sizes for harvesting the candidate species. It is concluded that no commercial fishery should be established on the dam, yet a small subsistence fishery could be a possible option. Management options such as input controls, open and closed seasons and areas and minimum sizes are discussed.
- Full Text:
- Date Issued: 2006
- Authors: Khumalo, Nokuthula
- Date: 2006
- Subjects: Mozambique tilapia , Marcusenius pongolensis , Schilbe intermedius , Fishes -- Swaziland , Fisheries -- Swaziland , Tilapia -- Swaziland
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:5283 , http://hdl.handle.net/10962/d1005127 , Mozambique tilapia , Marcusenius pongolensis , Schilbe intermedius , Fishes -- Swaziland , Fisheries -- Swaziland , Tilapia -- Swaziland
- Description: This thesis investigates aspects of the diversity, abundance and biology of the fish species inhabiting Mnjoli Dam, an irrigation dam that is located in the rural lowveld region of Swaziland. Specific objectives for this thesis were: to determine the fish species present and select three principal species based on their abundance; to investigate the relative abundance and distribution of the three selected species; and to describe key population parameters, such as growth, maturity, reproductive seasonality and mortality. The three dominant species were selected on their fisheries potential. These were Marcusenius pongolensis, Oreochromis mossambicus and Schilbe intermedius. Marcusenius pongolensis was the most abundant species in terms of both catch per unit effort and mass. Catch rates were shown to vary according to habitat type. Age and growth parameters were obtained from sectioned otoliths. Marginal zone analysis showed that annulus formation occurred in winter for all three species. The maximum-recorded ages were 8, 6 and 8 for M. pongolensis, O. mossambicus and S. intermedius, respectively. Growth for the three species was best described by the three parameter von Bertalanffy growth model as Lt = 1.88.67(1- exp⁻¹·⁴⁸⁽t⁺°·⁶⁷⁾) mm FL for M. pongolensis; Lt = 226.83(1- exp⁻°·⁴⁽t⁺²·°²⁾) mm TL for O. mossambicus and Lt = 214.59(1- exp⁻°·⁶°⁽t⁺¹·²°⁾) mm FL for S. intermedius. Sexual maturity was estimated for male and female M. pongolensis at 134 mm FL and 119 mm FL, respectively. Oreochromis mossambicus matured at 239 mm TL and S. intermedius at 205 mm FL. Two spawning peaks for M. pongolensis and S. intermedius were observed, one at the onset of summer (November) and the second at the beginning of autumn (February-March). Oreochromis mossambicus also exhibited two spawning peaks, one in spring (September) and the second in autumn (February –March). Mean mortality rate (Z), estimated using catch curve analysis was 0.77 year⁻¹ for M. pongolensis, 0.49 year⁻¹ for O. mossambicus and 0.79 year⁻¹ for S. intermedius. Natural mortality was assumed to be equal to Z since there is no fishery activity at the dam. Size specific selectivity curves were developed for each species. Of all the mesh sizes used, the 44 mm mesh size net had the highest catch rates in terms of numbers for M. pongolensis and S. intermedius. The 75mm mesh caught the highest catch rates for O. mossambicus. . The 44mm and 75 mm meshes seem to be the most suitable mesh sizes for harvesting the candidate species. It is concluded that no commercial fishery should be established on the dam, yet a small subsistence fishery could be a possible option. Management options such as input controls, open and closed seasons and areas and minimum sizes are discussed.
- Full Text:
- Date Issued: 2006
Age, growth and yield-per-recruit analysis of ndunduma Diplotaxodon limnothrissa (Teleostei: Cichlidae), in the southeastern arm of Lake Malawi
- Authors: Kanyerere, Geoffrey Zantute
- Date: 2004
- Subjects: Fishes -- Nyasa, Lake , Cichlids -- Nyasa, Lake , Fishes -- Growth -- Nyasa, Lake , Fishes -- Age -- Nyasa, Lake , Fishery resources -- Malawi , Fishery management -- Malawi
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:5214 , http://hdl.handle.net/10962/d1005057 , Fishes -- Nyasa, Lake , Cichlids -- Nyasa, Lake , Fishes -- Growth -- Nyasa, Lake , Fishes -- Age -- Nyasa, Lake , Fishery resources -- Malawi , Fishery management -- Malawi
- Description: Diplotaxodon limnothrissa Turner (1995) is a widely distributed species occurring throughout Lake Malawi, extending from the surface to a depth of at least 220m. It is probably the most abundant cichlid in the lake with biomass estimates of around 87 000 tonnes in the pelagic zone alone. The species is exploited commercially in the southern part of the lake but since its inception the fishery has never been assessed. As such this study investigates some aspects of age and growth of the species besides applying a yield-per-recruit analysis to assess the status of the fishery. Analysis of sectioned sagittal otoliths revealed that D. limnothrissa is fast growing and relatively long-lived species, attaining ages in excess of 10 years. Growth in length was rapid in immature fish, with fish attaining almost half of their maximum size within their first year. Le ngth-at-age was described by the von Bertalanffy growth model with combined-sex growth described as Lt = 211.21(1-exp(- 0.24(t+1.36))) mm TL. Total, natural and fishing mortalities were estimated at 0.76 yr⁻¹, 0.31 yr⁻¹ and 0.45 yr⁻¹ respectively. Per-recruit analysis indicated that the D. limnothrissa stock in the southeast arm of the lake is fully exploited as indicated by the current spawner biomass-per-recruit ratios of 31-55% (SB/R)F=0. Modelling indicated that the current age-at-capture (2.67 years) is lower than the age at which yield is optimised (> 5 years) based on the F₀·₁ harvesting strategy. It is, therefore, recommended that the age-at-capture should be increased from 2.67 to 5 years to optimise yield.
- Full Text:
- Date Issued: 2004
- Authors: Kanyerere, Geoffrey Zantute
- Date: 2004
- Subjects: Fishes -- Nyasa, Lake , Cichlids -- Nyasa, Lake , Fishes -- Growth -- Nyasa, Lake , Fishes -- Age -- Nyasa, Lake , Fishery resources -- Malawi , Fishery management -- Malawi
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:5214 , http://hdl.handle.net/10962/d1005057 , Fishes -- Nyasa, Lake , Cichlids -- Nyasa, Lake , Fishes -- Growth -- Nyasa, Lake , Fishes -- Age -- Nyasa, Lake , Fishery resources -- Malawi , Fishery management -- Malawi
- Description: Diplotaxodon limnothrissa Turner (1995) is a widely distributed species occurring throughout Lake Malawi, extending from the surface to a depth of at least 220m. It is probably the most abundant cichlid in the lake with biomass estimates of around 87 000 tonnes in the pelagic zone alone. The species is exploited commercially in the southern part of the lake but since its inception the fishery has never been assessed. As such this study investigates some aspects of age and growth of the species besides applying a yield-per-recruit analysis to assess the status of the fishery. Analysis of sectioned sagittal otoliths revealed that D. limnothrissa is fast growing and relatively long-lived species, attaining ages in excess of 10 years. Growth in length was rapid in immature fish, with fish attaining almost half of their maximum size within their first year. Le ngth-at-age was described by the von Bertalanffy growth model with combined-sex growth described as Lt = 211.21(1-exp(- 0.24(t+1.36))) mm TL. Total, natural and fishing mortalities were estimated at 0.76 yr⁻¹, 0.31 yr⁻¹ and 0.45 yr⁻¹ respectively. Per-recruit analysis indicated that the D. limnothrissa stock in the southeast arm of the lake is fully exploited as indicated by the current spawner biomass-per-recruit ratios of 31-55% (SB/R)F=0. Modelling indicated that the current age-at-capture (2.67 years) is lower than the age at which yield is optimised (> 5 years) based on the F₀·₁ harvesting strategy. It is, therefore, recommended that the age-at-capture should be increased from 2.67 to 5 years to optimise yield.
- Full Text:
- Date Issued: 2004
An assessment of the monkfish resource of Namibia
- Authors: Maartens, Lima
- Date: 2000
- Subjects: Lophius -- Namibia Lophius
- Language: English
- Type: Thesis , Doctoral , PhD
- Identifier: vital:5231 , http://hdl.handle.net/10962/d1005074
- Description: Two monkfish species are caught off the coast of Namibia. These are Lophius vomerinus Valenciennes (1837) and L. vaillanti Regan (1903). L. vomerinus is the more abundant of the two and contributes approximately 99% to the total Namibian monkfish landings. The Namibian monkfish fishery is one of the largest and most valuable of its kind in the world. Two fishing fleets harvest the resource, viz. the monkfish and sole fleet and the hake directed fleet. In 1998 and 1999, monkfish were also harvested by one gillnet vessel that had an experimental right. In this study, the biology, abundance and exploitation patterns of monkfish were investigated and used as inputs into a stock assessment framework as a contribution towards the development of a management procedure for this species. Ground and sectioned otoliths and sectioned illicia were used to age Lophius vomerinus. The illicia provided more precise estimates of age, and growth was subsequently modelled using these age estimates. Growth in length and weight was best described by the threeparameter specialised von Bertalanffy model as Lt (cm TL) = 72.29 (1-e⁻°·¹⁴⁽t⁺°·³°⁾) and Wt (kg) = 3.96 (1-e⁻°·¹⁸⁽t⁺°·¹°⁾)²·⁹⁹ for males, and Lt (cm TL) = 111.98 (1-e⁻°·°⁸⁽t⁺°·³⁶⁾) and Wt (kg) = 6.92 (1-e⁻°·¹⁸⁽t⁻°·⁶⁴⁾)³·°³ for females. L. vomerinus is a slow-growing species with a life span in excess of 10 years. Reproductive activity was shown to occur throughout the year, with a slight peak during the winter months. The adult sex ratio of fish > 37 cm TL was biased towards females. Length-at-50% sexual maturity was attained at 32.1 cm TL for females and 23.7 cm TL for males. The rate of natural mortality was estimated at 0.15 year⁻¹, while fishing mortality rates during the 1970s, 1980s and 1990s were estimated at 0.01 year⁻¹, 0.04 year⁻¹ and 0.14 year⁻¹, respectively. Gillnetting for monkfish (300 mm stretched mesh) was highly efficient with a moderate bycatch of around 20% during the two years of operation. The main bycatch species were red crab, spider crab, squalid sharks, rays and Cape and Deep-water hake. The mean length of the monkfish caught in gillnets (67 cm TL) was significantly larger than the monkfish landed by the trawlers (38 cm TL) and less than 1% of immature fish were landed. Gillnet catch-per-unit-effort for monkfish fluctuated between 0.03 and 0.67 kg.day⁻¹.50 m net panel⁻¹, with a soak time of between one and sixteen days. More than 50% (by weight) of monkfish landed by monkfish and sole trawlers, consisted of fish below 36 cm TL. There was a significant increase in catches of juvenile monkfish during 1997 and 1998 in comparison to the period 1994 to 1996. Various types of rigid sorting grids were tested to release juvenile monkfish below 32 cm TL. Five grid designs were tested. These included an “Ex-it” grid with horizontal bars spaced at 55 mm, single grids with vertical and horizontal bars spaced at 55 mm and grids with circular openings of 110 and 168 mm in diameter. The most efficient design was the grid with circular openings of 110 cm in diameter, which ensured the escape of 66% of monkfish smaller than 31 cm TL. However, studies need to be undertaken to quantify the survival of released fish and to test the feasibility of using grid sorters on commercial monkfish and sole trawling gear. The monkfish resource was assessed by means of length cohort analyses, the Thompson and Bell predictive model and by way of a deterministic age-structured production modelling approach. The length cohort analysis models were sensitive to the rate of natural mortality and insensitive to changes in the terminal fishing mortality rate. These biases may, however, not be serious provided that estimates of abundance are used to reflect relative changes. Fish ranging between 26 and 59 cm TL are the most heavily exploited. The Thompson and Bell model predicted that the monkfish resource is exploited above MSY -levels and a reduction of approximately 40% in fishing effort would provide a higher yield. Yield-per-recruit ranged between 10 000 and 14 000 tonnes. Results should, however, be treated with caution, as the condition of steady state was not satisfied. The age-structured production model was tuned using trends in catch-per-unit-effort data, estimated by Generalised Linear Modeling, as well as relative abundance indices calculated from hake biomass surveys. The model was found to be sensitive to both the ‘steepness’ parameter h and estimates of natural mortality. The ‘depletion’ level of the monkfish resource is currently estimated to be 49%. Estimated coefficients of variation were high (> 63%) due to the lack of a consistent trend within the abundance indices to tune the model. Overall productivity of the monkfish resource was estimated to be approximately 16%, similar to other southern African demersal resources. Results of the risk analyses suggest that catches in excess of 7 000 tonnes may be unsustainable and that catches of 5 000 or 6 000 tonnes would decrease the risk of stock collapse and possibly lead to a recovery in the stock. Monkfish management strategies were reviewed and these were considered in relation to the results of this study. The following management recommendations were made: to follow the precautionary approach and implement a total allowable catch for monkfish; to implement rigid sorting grids as these would be the most appropriate way in which to reduce catches of juvenile monkfish; to restrict soak time, depth of operation and implement means to reduce ‘ghost fishing’ by gillnetting and finally, to develop a management procedure for Namibian monkfish with the main objective being the sustainable exploitation of the resource.
- Full Text:
- Date Issued: 2000
- Authors: Maartens, Lima
- Date: 2000
- Subjects: Lophius -- Namibia Lophius
- Language: English
- Type: Thesis , Doctoral , PhD
- Identifier: vital:5231 , http://hdl.handle.net/10962/d1005074
- Description: Two monkfish species are caught off the coast of Namibia. These are Lophius vomerinus Valenciennes (1837) and L. vaillanti Regan (1903). L. vomerinus is the more abundant of the two and contributes approximately 99% to the total Namibian monkfish landings. The Namibian monkfish fishery is one of the largest and most valuable of its kind in the world. Two fishing fleets harvest the resource, viz. the monkfish and sole fleet and the hake directed fleet. In 1998 and 1999, monkfish were also harvested by one gillnet vessel that had an experimental right. In this study, the biology, abundance and exploitation patterns of monkfish were investigated and used as inputs into a stock assessment framework as a contribution towards the development of a management procedure for this species. Ground and sectioned otoliths and sectioned illicia were used to age Lophius vomerinus. The illicia provided more precise estimates of age, and growth was subsequently modelled using these age estimates. Growth in length and weight was best described by the threeparameter specialised von Bertalanffy model as Lt (cm TL) = 72.29 (1-e⁻°·¹⁴⁽t⁺°·³°⁾) and Wt (kg) = 3.96 (1-e⁻°·¹⁸⁽t⁺°·¹°⁾)²·⁹⁹ for males, and Lt (cm TL) = 111.98 (1-e⁻°·°⁸⁽t⁺°·³⁶⁾) and Wt (kg) = 6.92 (1-e⁻°·¹⁸⁽t⁻°·⁶⁴⁾)³·°³ for females. L. vomerinus is a slow-growing species with a life span in excess of 10 years. Reproductive activity was shown to occur throughout the year, with a slight peak during the winter months. The adult sex ratio of fish > 37 cm TL was biased towards females. Length-at-50% sexual maturity was attained at 32.1 cm TL for females and 23.7 cm TL for males. The rate of natural mortality was estimated at 0.15 year⁻¹, while fishing mortality rates during the 1970s, 1980s and 1990s were estimated at 0.01 year⁻¹, 0.04 year⁻¹ and 0.14 year⁻¹, respectively. Gillnetting for monkfish (300 mm stretched mesh) was highly efficient with a moderate bycatch of around 20% during the two years of operation. The main bycatch species were red crab, spider crab, squalid sharks, rays and Cape and Deep-water hake. The mean length of the monkfish caught in gillnets (67 cm TL) was significantly larger than the monkfish landed by the trawlers (38 cm TL) and less than 1% of immature fish were landed. Gillnet catch-per-unit-effort for monkfish fluctuated between 0.03 and 0.67 kg.day⁻¹.50 m net panel⁻¹, with a soak time of between one and sixteen days. More than 50% (by weight) of monkfish landed by monkfish and sole trawlers, consisted of fish below 36 cm TL. There was a significant increase in catches of juvenile monkfish during 1997 and 1998 in comparison to the period 1994 to 1996. Various types of rigid sorting grids were tested to release juvenile monkfish below 32 cm TL. Five grid designs were tested. These included an “Ex-it” grid with horizontal bars spaced at 55 mm, single grids with vertical and horizontal bars spaced at 55 mm and grids with circular openings of 110 and 168 mm in diameter. The most efficient design was the grid with circular openings of 110 cm in diameter, which ensured the escape of 66% of monkfish smaller than 31 cm TL. However, studies need to be undertaken to quantify the survival of released fish and to test the feasibility of using grid sorters on commercial monkfish and sole trawling gear. The monkfish resource was assessed by means of length cohort analyses, the Thompson and Bell predictive model and by way of a deterministic age-structured production modelling approach. The length cohort analysis models were sensitive to the rate of natural mortality and insensitive to changes in the terminal fishing mortality rate. These biases may, however, not be serious provided that estimates of abundance are used to reflect relative changes. Fish ranging between 26 and 59 cm TL are the most heavily exploited. The Thompson and Bell model predicted that the monkfish resource is exploited above MSY -levels and a reduction of approximately 40% in fishing effort would provide a higher yield. Yield-per-recruit ranged between 10 000 and 14 000 tonnes. Results should, however, be treated with caution, as the condition of steady state was not satisfied. The age-structured production model was tuned using trends in catch-per-unit-effort data, estimated by Generalised Linear Modeling, as well as relative abundance indices calculated from hake biomass surveys. The model was found to be sensitive to both the ‘steepness’ parameter h and estimates of natural mortality. The ‘depletion’ level of the monkfish resource is currently estimated to be 49%. Estimated coefficients of variation were high (> 63%) due to the lack of a consistent trend within the abundance indices to tune the model. Overall productivity of the monkfish resource was estimated to be approximately 16%, similar to other southern African demersal resources. Results of the risk analyses suggest that catches in excess of 7 000 tonnes may be unsustainable and that catches of 5 000 or 6 000 tonnes would decrease the risk of stock collapse and possibly lead to a recovery in the stock. Monkfish management strategies were reviewed and these were considered in relation to the results of this study. The following management recommendations were made: to follow the precautionary approach and implement a total allowable catch for monkfish; to implement rigid sorting grids as these would be the most appropriate way in which to reduce catches of juvenile monkfish; to restrict soak time, depth of operation and implement means to reduce ‘ghost fishing’ by gillnetting and finally, to develop a management procedure for Namibian monkfish with the main objective being the sustainable exploitation of the resource.
- Full Text:
- Date Issued: 2000
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