Coastal pH variability and the eco-physiological and behavioural response of a coastal fish species in light of future ocean acidification
- Authors: Edworthy, Carla
- Date: 2021
- Subjects: Ocean acidification , Diplodus capensis (Blacktail) , Diplodus -- South Africa -- Algoa Bay , Diplodus -- Metabolism , Diplodus -- Food , Diplodus -- Larvae , Marine ecology -- South Africa -- Algoa Bay , Carbon dioxide -- Physiological effect , Respiration -- Measurement
- Language: English
- Type: text , Thesis , Doctoral , PhD
- Identifier: http://hdl.handle.net/10962/176793 , vital:42759 , 10.21504/10962/176793
- Description: Ocean acidification (OA) is a global phenomenon referring to a decrease in ocean pH and a perturbation of the seawater carbonate system due to ever-increasing atmospheric CO2 concentrations. In coastal environments, identifying the impacts of OA is complex due to the multiple contributors to pH variability by coastal processes, such as freshwater inflow, upwelling, hydrodynamic processes, and biological activity. The aim of this PhD study was to quantify the local processes occurring in a temperate coastal embayment, Algoa Bay in South Africa, that contribute to pH and carbonate chemistry variability over time (monthly and 24-hour) and space (~10 km) and examine how this variability impacts a local fish species, Diplodus capensis, also commonly known as ‘blacktail’. Algoa Bay, known for its complex oceanography, is an interesting location in which to quantify carbonate chemistry variability. To assess this variability, monitoring sites were selected to coincide with the Algoa Bay Sentinel Site long-term ecological research (LTER) and continuous monitoring (CMP) programmes. The average pH at offshore sites in the bay was 8.03 ± 0.07 and at inshore sites was 8.04 ± 0.15. High pH variability (~0.55–0.61 pH units) was recorded at both offshore (>10 m depth) and inshore sites (intertidal surf zones). Many sites in the bay, especially the atypical site at Cape Recife, exhibit higher than the average pH levels (>8.04), suggesting that pH variability may be biologically driven. This is further evidenced by high diurnal variability in pH (~0.55 pH units). Although the specific drivers of the high pH variability in Algoa Bay could not be identified, baseline carbonate chemistry conditions were identified, which is necessary information to design and interpret biological experiments. Long-term, continuous monitoring is required to improve understanding of the drivers of pH variability in understudied coastal regions, like Algoa Bay. A local fisheries species, D. capensis, was selected as a model species to assess the impacts of future OA scenarios in Algoa Bay. It was hypothesized that this temperate, coastally distributed species would be adapted to naturally variable pH conditions and thus show some tolerance to low pH, considering that they are exposed to minimum pH levels of 7.77 and fluctuations of up to 0.55 pH units. Laboratory perturbation experiments were used to expose early postflexion stage of D. capensis to a range of pH treatments that were selected based on the measured local variability (~8.0–7.7 pH), as well as future projected OA scenarios (7.6–7.2 pH). Physiological responses were estimated using intermittent flow respirometry by quantifying routine and active metabolic rates as well as relative aerobic scope at each pH treatment. The behavioural responses of the larvae were also assessed at each pH treatment, as activity levels, by measuring swimming distance and speed in video-recording experiments, as well as feeding rates. D. capensis had sufficient physiological capacity to maintain metabolic performance at pH levels as low as 7.27, as evidenced by no changes in any of the measured metabolic rates (routine metabolic rate, active metabolic rate, and relative aerobic scope) after exposure to the range of pH treatments (8.02–7.27). Feeding rates of D. capensis were similarly unaffected by pH treatment. However, it appears that subtle increases in activity level (measured by swimming distance and swimming speed experiments) occur with a decrease in pH. These changes in activity level were a consequence of a change in behaviour rather than metabolic constraints. This study concludes, however, that based on the parameters measured, there is no evidence for survival or fitness related consequences of near future OA on D. capensis. OA research is still in its infancy in South Africa, and the potential impacts of OA to local marine resources has not yet been considered in local policy and resource management strategies. Integrating field monitoring and laboratory perturbation experiments is emerging as best practice in OA research. This is the first known study on the temperate south coast of South Africa to quantify local pH variability and to use this information to evaluate the biological response of a local species using relevant local OA scenarios as treatment levels for current and near future conditions. Research on local conditions in situ and the potential impacts of future OA scenarios on socio-economically valuable species, following the model developed in this study, is necessary to provide national policy makers with relevant scientific data to inform climate change management policies for local resources.
- Full Text:
- Date Issued: 2021
- Authors: Edworthy, Carla
- Date: 2021
- Subjects: Ocean acidification , Diplodus capensis (Blacktail) , Diplodus -- South Africa -- Algoa Bay , Diplodus -- Metabolism , Diplodus -- Food , Diplodus -- Larvae , Marine ecology -- South Africa -- Algoa Bay , Carbon dioxide -- Physiological effect , Respiration -- Measurement
- Language: English
- Type: text , Thesis , Doctoral , PhD
- Identifier: http://hdl.handle.net/10962/176793 , vital:42759 , 10.21504/10962/176793
- Description: Ocean acidification (OA) is a global phenomenon referring to a decrease in ocean pH and a perturbation of the seawater carbonate system due to ever-increasing atmospheric CO2 concentrations. In coastal environments, identifying the impacts of OA is complex due to the multiple contributors to pH variability by coastal processes, such as freshwater inflow, upwelling, hydrodynamic processes, and biological activity. The aim of this PhD study was to quantify the local processes occurring in a temperate coastal embayment, Algoa Bay in South Africa, that contribute to pH and carbonate chemistry variability over time (monthly and 24-hour) and space (~10 km) and examine how this variability impacts a local fish species, Diplodus capensis, also commonly known as ‘blacktail’. Algoa Bay, known for its complex oceanography, is an interesting location in which to quantify carbonate chemistry variability. To assess this variability, monitoring sites were selected to coincide with the Algoa Bay Sentinel Site long-term ecological research (LTER) and continuous monitoring (CMP) programmes. The average pH at offshore sites in the bay was 8.03 ± 0.07 and at inshore sites was 8.04 ± 0.15. High pH variability (~0.55–0.61 pH units) was recorded at both offshore (>10 m depth) and inshore sites (intertidal surf zones). Many sites in the bay, especially the atypical site at Cape Recife, exhibit higher than the average pH levels (>8.04), suggesting that pH variability may be biologically driven. This is further evidenced by high diurnal variability in pH (~0.55 pH units). Although the specific drivers of the high pH variability in Algoa Bay could not be identified, baseline carbonate chemistry conditions were identified, which is necessary information to design and interpret biological experiments. Long-term, continuous monitoring is required to improve understanding of the drivers of pH variability in understudied coastal regions, like Algoa Bay. A local fisheries species, D. capensis, was selected as a model species to assess the impacts of future OA scenarios in Algoa Bay. It was hypothesized that this temperate, coastally distributed species would be adapted to naturally variable pH conditions and thus show some tolerance to low pH, considering that they are exposed to minimum pH levels of 7.77 and fluctuations of up to 0.55 pH units. Laboratory perturbation experiments were used to expose early postflexion stage of D. capensis to a range of pH treatments that were selected based on the measured local variability (~8.0–7.7 pH), as well as future projected OA scenarios (7.6–7.2 pH). Physiological responses were estimated using intermittent flow respirometry by quantifying routine and active metabolic rates as well as relative aerobic scope at each pH treatment. The behavioural responses of the larvae were also assessed at each pH treatment, as activity levels, by measuring swimming distance and speed in video-recording experiments, as well as feeding rates. D. capensis had sufficient physiological capacity to maintain metabolic performance at pH levels as low as 7.27, as evidenced by no changes in any of the measured metabolic rates (routine metabolic rate, active metabolic rate, and relative aerobic scope) after exposure to the range of pH treatments (8.02–7.27). Feeding rates of D. capensis were similarly unaffected by pH treatment. However, it appears that subtle increases in activity level (measured by swimming distance and swimming speed experiments) occur with a decrease in pH. These changes in activity level were a consequence of a change in behaviour rather than metabolic constraints. This study concludes, however, that based on the parameters measured, there is no evidence for survival or fitness related consequences of near future OA on D. capensis. OA research is still in its infancy in South Africa, and the potential impacts of OA to local marine resources has not yet been considered in local policy and resource management strategies. Integrating field monitoring and laboratory perturbation experiments is emerging as best practice in OA research. This is the first known study on the temperate south coast of South Africa to quantify local pH variability and to use this information to evaluate the biological response of a local species using relevant local OA scenarios as treatment levels for current and near future conditions. Research on local conditions in situ and the potential impacts of future OA scenarios on socio-economically valuable species, following the model developed in this study, is necessary to provide national policy makers with relevant scientific data to inform climate change management policies for local resources.
- Full Text:
- Date Issued: 2021
Thermal tolerance and the potential effects of climate change on coastal intertidal and estuarine organisms in the Kariega Estuary and adjacent intertitdal coastline, Eastern Cape, South Africa
- Authors: Van der Walt, Kerry-Ann
- Date: 2020
- Subjects: Ectotherms -- Climatic factors , Ectotherms -- Effect of temperature on , Fishes -- Climatic factors , Fishes -- Effect of temperature on , Climatic changes -- South Africa -- Eastern Cape
- Language: English
- Type: text , Thesis , Doctoral , PhD
- Identifier: http://hdl.handle.net/10962/148459 , vital:38741
- Description: Temperature changes due to the effects of climate change are evident on all continents and oceans. As a result, there is a growing concern over how marine ectotherms will respond to extreme or fluctuating environmental temperatures. Temperature changes have strong direct and indirect effects on individual, population, and ecosystem functioning traits. A multi-scale approach determining the thermal tolerance and performance of several marine ectotherms belonging to different coastal habitats is rarely considered in thermal physiology studies but is effective for an integrated ecosystem assessment. As such, for this thesis, I aimed to quantify and compare the thermal tolerance and performance of a range of coastal marine ectotherms (fish and macro-invertebrates) with different biogeographical distributions from estuarine, subtidal and rocky intertidal habitats to available and projected in situ temperature data. This was also undertaken to gauge the local vulnerability of each species across summer and winter in a warm-temperate region of South Africa. This was done using a multi-method physiological approach, which included the dynamic method (CTmax and CTmin), static respirometry and maximum heart rate fHmax). Results of the dynamic method on several fish and macro-invertebrate species indicated that there are differences in thermal tolerance according to taxonomy, biogeography and habitat for both summer and winter. Macro-invertebrate species generally had higher CTmax endpoints, lower CTmin endpoints, higher upper and lower breadths in tolerance, higher upper and lower thermal safety margins and higher thermal scopes than the fish species. This could be a result of the macro-invertebrate species studied being less mobile compared with fish species (which are able to move to more favourable conditions) as well as having broader geographical distributions. In addition, macro-invertebrates from the intertidal rock pool habitat (Palaemon peringueyi; Pernaperna) were more tolerant of high and low temperatures compared with the macro-invertebrates from the estuarine habitat (Clibanarius virescens; Parasesarma catenatum; Upogebia africana). Overall, macro-invertebrates, with the exception of Parechinus angulosus, investigated in this study indicated that current temperatures and projected climate change scenarios across seasons would not have a significant impact on them and that they are highly adaptable to changing temperature regimes. This sign of high tolerance was further supported by the heart rates of P. perna and P. catenatum under an acute increase in temperature (1.0 °C.h-1) which showed individuals of each species physiologically depressing their metabolism until a final Arrhenius breakpoint temperature was reached (TAB). Among the fish species investigated in this study, tropical species (Chaetodon marleyi; Kuhlia mugil) had the highest CTmax and CTmin endpoints when compared with the temperate (Diplodus capensis; Sarpa salpa), warm-water endemic (Chelon dumerili; Rhabdosargus holubi) and cool-water endemic (Chelon richardsonii) fishes. This suggests that due to their lower breadths in tolerance and thermal safety margins being small, tropical species may be less tolerant of cold temperatures and thermal variability, especially in the form of summer upwelling events which are expected to increase in intensity and frequency in this region as a result of anthropogenic climate change effects. On the other hand, however, if a temperature increase of 2.0 - 4.0 °C takes place at the end of the century as predicted by the Intergovernmental Panel on Climate Change (IPCC), it is likely that tropical species such as C. marleyi will become more common. Temperate species such as D. capensis and S. salpa were able to tolerate a wide range of temperatures (wide thermal scope) compared with the other fish species. These findings may suggest that D. capensis and S. salpa are thermally resilient and may be the least vulnerable to climate change effects and temperature variability. When evaluating the different life stages of D. capensis, however, using the dynamic method (juveniles and adults), static respirometry (juveniles) and maximum heart rate (adults), results suggested that juveniles of this temperate species will be more resilient to increases in ocean temperatures compared with the adults because they have a higher thermal tolerance (CTmax/TCRIT) and a greater metabolic scope (TOPT) at higher temperatures. For both juveniles and adults, temperatures beyond 28.0 °C (upper Tpej; Tarr) will have a significant impact on their physiology. Using a multi-scale and multi-method approach thus helped to identify which species or community may be vulnerable to the effects of climate change within shallow coastal environments in this warm-temperate climate change hotspot. Adopting this type of approach will assist policy makers in developing comprehensive climate change management frameworks for coastal ecosystems globally and around South Africa.
- Full Text:
- Date Issued: 2020
- Authors: Van der Walt, Kerry-Ann
- Date: 2020
- Subjects: Ectotherms -- Climatic factors , Ectotherms -- Effect of temperature on , Fishes -- Climatic factors , Fishes -- Effect of temperature on , Climatic changes -- South Africa -- Eastern Cape
- Language: English
- Type: text , Thesis , Doctoral , PhD
- Identifier: http://hdl.handle.net/10962/148459 , vital:38741
- Description: Temperature changes due to the effects of climate change are evident on all continents and oceans. As a result, there is a growing concern over how marine ectotherms will respond to extreme or fluctuating environmental temperatures. Temperature changes have strong direct and indirect effects on individual, population, and ecosystem functioning traits. A multi-scale approach determining the thermal tolerance and performance of several marine ectotherms belonging to different coastal habitats is rarely considered in thermal physiology studies but is effective for an integrated ecosystem assessment. As such, for this thesis, I aimed to quantify and compare the thermal tolerance and performance of a range of coastal marine ectotherms (fish and macro-invertebrates) with different biogeographical distributions from estuarine, subtidal and rocky intertidal habitats to available and projected in situ temperature data. This was also undertaken to gauge the local vulnerability of each species across summer and winter in a warm-temperate region of South Africa. This was done using a multi-method physiological approach, which included the dynamic method (CTmax and CTmin), static respirometry and maximum heart rate fHmax). Results of the dynamic method on several fish and macro-invertebrate species indicated that there are differences in thermal tolerance according to taxonomy, biogeography and habitat for both summer and winter. Macro-invertebrate species generally had higher CTmax endpoints, lower CTmin endpoints, higher upper and lower breadths in tolerance, higher upper and lower thermal safety margins and higher thermal scopes than the fish species. This could be a result of the macro-invertebrate species studied being less mobile compared with fish species (which are able to move to more favourable conditions) as well as having broader geographical distributions. In addition, macro-invertebrates from the intertidal rock pool habitat (Palaemon peringueyi; Pernaperna) were more tolerant of high and low temperatures compared with the macro-invertebrates from the estuarine habitat (Clibanarius virescens; Parasesarma catenatum; Upogebia africana). Overall, macro-invertebrates, with the exception of Parechinus angulosus, investigated in this study indicated that current temperatures and projected climate change scenarios across seasons would not have a significant impact on them and that they are highly adaptable to changing temperature regimes. This sign of high tolerance was further supported by the heart rates of P. perna and P. catenatum under an acute increase in temperature (1.0 °C.h-1) which showed individuals of each species physiologically depressing their metabolism until a final Arrhenius breakpoint temperature was reached (TAB). Among the fish species investigated in this study, tropical species (Chaetodon marleyi; Kuhlia mugil) had the highest CTmax and CTmin endpoints when compared with the temperate (Diplodus capensis; Sarpa salpa), warm-water endemic (Chelon dumerili; Rhabdosargus holubi) and cool-water endemic (Chelon richardsonii) fishes. This suggests that due to their lower breadths in tolerance and thermal safety margins being small, tropical species may be less tolerant of cold temperatures and thermal variability, especially in the form of summer upwelling events which are expected to increase in intensity and frequency in this region as a result of anthropogenic climate change effects. On the other hand, however, if a temperature increase of 2.0 - 4.0 °C takes place at the end of the century as predicted by the Intergovernmental Panel on Climate Change (IPCC), it is likely that tropical species such as C. marleyi will become more common. Temperate species such as D. capensis and S. salpa were able to tolerate a wide range of temperatures (wide thermal scope) compared with the other fish species. These findings may suggest that D. capensis and S. salpa are thermally resilient and may be the least vulnerable to climate change effects and temperature variability. When evaluating the different life stages of D. capensis, however, using the dynamic method (juveniles and adults), static respirometry (juveniles) and maximum heart rate (adults), results suggested that juveniles of this temperate species will be more resilient to increases in ocean temperatures compared with the adults because they have a higher thermal tolerance (CTmax/TCRIT) and a greater metabolic scope (TOPT) at higher temperatures. For both juveniles and adults, temperatures beyond 28.0 °C (upper Tpej; Tarr) will have a significant impact on their physiology. Using a multi-scale and multi-method approach thus helped to identify which species or community may be vulnerable to the effects of climate change within shallow coastal environments in this warm-temperate climate change hotspot. Adopting this type of approach will assist policy makers in developing comprehensive climate change management frameworks for coastal ecosystems globally and around South Africa.
- Full Text:
- Date Issued: 2020
Understanding a West African recreational fishery as a complex social-ecological system – a case study of the fishery for giant African threadfin Polydactylus quadrifilis (Cuvier, 1829) in the Kwanza Estuary, Angola
- Authors: Butler, Edward C
- Date: 2020
- Subjects: Cuanza River (Angola) , Fishing -- Angola , Polynemidae , Fishes -- Angola -- Ecology
- Language: English
- Type: text , Thesis , Doctoral , PhD
- Identifier: http://hdl.handle.net/10962/146688 , vital:38548
- Description: Despite increasing global recognition of the importance of recreational fisheries, their management largely remains poor. This is because they exhibit unique human-nature relationships and are nested within complex social-ecological systems (SESs). Recreational fisheries in the developing world have large potential for socio-economic development, but are generally underappreciated, in terms of their value and their impact, and are poorly governed. This is particularly concerning as they are highly complex and often compete for resources with dependent artisanal and subsistence fisheries. Developing world recreational fishery SESs are not well understood and present an important research gap for improved governance. The general aim of this thesis was to explore the recreational fishery targeting Polydactylus quadrifilis on the Kwanza Estuary, Angola, and provide context for how managers should approach recreational fisheries in the developing world and in Africa. To do this, the recreational fishery SES was explored using a combination of methodologies including those characteristic of traditional fisheries science, and new methods involving biology, sociology, and economics. The thesis contains an introductory chapter, a chapter describing the background, study area and study species, five data chapters and a discussion chapter. Chapter 3 aimed to investigate the reproductive style of P. quadrifilis. Results identified the species as a protandrous hermaphrodite. Evidence to suggest this included degenerating testicular tissue and the presence of early developing ovarian tissues in transitional individuals. Early-stage oocytes were commonly found in the outer area of male regions and residual late-stage spermatids and spermatozoa were found in the luminal space of ovarian regions, suggesting a process of sex change from the outside inwards. Owing to the species’ reliance on large highly fecund females for reproduction, it is likely that P. quadrifilis will be sensitive to fisheries that target larger individuals, such as trophy recreational fisheries and line fisheries within other sectors. Chapter 4 aimed to investigate alternative methods for adequately describing the growth of P. quadrifilis individuals belonging to either one of two distinct hypothetical life-history pathways: pathway I (‘changers’) – initial maturation as a primary male followed by a sex change to female; pathway II (‘non-changers’) – initial maturation as a male fish with no subsequent sex change, using von Bertalanffy Growth Functions (VBGFs). Other specific objectives included determining the size- and age-at-maturity and size- and age-at-sex-change for P. quadrifilis. Otolith aging revealed rapid growth and early maturation (L50 = 399.2 mm FL, A50 = 1.50 years) and sex change occurred over a wide size (790–1125 mm FL) and age (3–8 years) range. There was strong evidence for partial protandry in P. quadrifilis with several extremely old male fish (up to 22 years) observed in the population. When compared to the conventional model produced for the entire population, there were significant differences in the models for the ‘non-changers’ (LRT, p < 0.01) and their parameters L∞ (full model = 130.8, ‘non-changers’ = 113.3, p < 0.01), k (0.32, 0.44, p < 0.01) and t0 (0.23, 0.43, p = 0.03) in the first approach and the models (LRT, p < 0.01) and their L∞ (‘changers’ = 113.7, p < 0.01) values in the second approach. This suggests that utilising conventional modelling techniques may be inappropriate for the stock assessment and management of P. quadrifilis and, potentially, other sequentially hermaphroditic fishery species. Chapter 5 aimed to assess the sensitivity of P. quadrifilis to recreational C&R within the foreign recreational fishery using a rapid assessment approach. To do this, a number of C&R variables including fight time, air exposure, hook placement, hooking injury, total time of the stress event, river depth and angling method were measured and related to two indicators of fish health and survival – the physiological stress indicators blood glucose and blood lactate concentration and reflex action mortality predictors (RAMPs). Air exposure was identified as a major contributor to motor impairment (Cumulative Link Model: p < 0.01) and fight time was an important contributor to motor impairment via its interaction with air exposure (Cumulative Link Model: p = 0.02). Handling practices appear to be particularly important for larger individuals as fish size was positively correlated with air exposure (Pearson’s r coefficient = 0.41, p < 0.01) and fight times (0.88, p < 0.01). The findings suggest that recreational C&R may result in mortalities directly, via C&R, and indirectly, via predation, and several recommendations were made for best practice. Chapter 6 aimed to assess the direct economic contribution of the recreational fishery for Polydactylus quadrifilis on the Kwanza Estuary. Results indicated that the recreational fishery for contributed significantly to the economy of an area that would otherwise likely receive little external input ($282 054 per four-month fishing season). However, high rates of economic leakage from the study area were identified (58.7%–92.9% of locally spent revenue) and were attributed to the sourcing of lodge supplies, services and staff outside of the local area and the repatriation of profit by foreign business owners. Capacity building within the local community is likely required to develop ‘linkages’ between the local community and the recreational fishery. Greater community involvement in the fishery is suggested to incentivise the protection of recreationally important fishery species and their associated ecosystems. Chapter 7 aimed to investigate the resource user groups involved within the SES. Results illustrated that artisanal and domestic recreational anglers are well-established and are characterised by long histories of participation. The artisanal fishery was highly valued as a source of livelihoods for the local community. Artisanal fishers were eager for involvement in the recreational sector, through the chartering of their vessels, due to the attractiveness of extra earnings. Both recreational and artisanal fishers reported recent decreases in P. quadrifilis catch and anticipated further declines. Domestic recreational anglers appeared to be highly consumptive in their use of the fishery and C&R angling was uncommon. User conflict may be problematic for future management as recreational anglers perceived the artisanal gill-net fishery to be a threat towards P. quadrifilis stocks. In conclusion, the open-access nature of the fishery was identified as the most pertinent threat to its sustainability and likely needs to be addressed. Potential solutions involve offering users the opportunity to purchase access rights (e.g. day permits), thus initiating the concept that users must pay for their use of public resources. Management should aim to protect large female fish due to their increased reproductive value and worth as trophy fish. Thus, C&R angling is likely to be an important interaction between users and the resource. However, angler behaviour will need to be manipulated to promote C&R and minimise C&R-related mortalities. Solutions include angler educational drives and interventions and the implementation of competitive C&R-only angling. Foreign recreational fisheries, although touted as potential ecotourism ventures, will only succeed in improving the lives of local people if they fully integrate the community into the operation of the fishery.
- Full Text:
- Date Issued: 2020
- Authors: Butler, Edward C
- Date: 2020
- Subjects: Cuanza River (Angola) , Fishing -- Angola , Polynemidae , Fishes -- Angola -- Ecology
- Language: English
- Type: text , Thesis , Doctoral , PhD
- Identifier: http://hdl.handle.net/10962/146688 , vital:38548
- Description: Despite increasing global recognition of the importance of recreational fisheries, their management largely remains poor. This is because they exhibit unique human-nature relationships and are nested within complex social-ecological systems (SESs). Recreational fisheries in the developing world have large potential for socio-economic development, but are generally underappreciated, in terms of their value and their impact, and are poorly governed. This is particularly concerning as they are highly complex and often compete for resources with dependent artisanal and subsistence fisheries. Developing world recreational fishery SESs are not well understood and present an important research gap for improved governance. The general aim of this thesis was to explore the recreational fishery targeting Polydactylus quadrifilis on the Kwanza Estuary, Angola, and provide context for how managers should approach recreational fisheries in the developing world and in Africa. To do this, the recreational fishery SES was explored using a combination of methodologies including those characteristic of traditional fisheries science, and new methods involving biology, sociology, and economics. The thesis contains an introductory chapter, a chapter describing the background, study area and study species, five data chapters and a discussion chapter. Chapter 3 aimed to investigate the reproductive style of P. quadrifilis. Results identified the species as a protandrous hermaphrodite. Evidence to suggest this included degenerating testicular tissue and the presence of early developing ovarian tissues in transitional individuals. Early-stage oocytes were commonly found in the outer area of male regions and residual late-stage spermatids and spermatozoa were found in the luminal space of ovarian regions, suggesting a process of sex change from the outside inwards. Owing to the species’ reliance on large highly fecund females for reproduction, it is likely that P. quadrifilis will be sensitive to fisheries that target larger individuals, such as trophy recreational fisheries and line fisheries within other sectors. Chapter 4 aimed to investigate alternative methods for adequately describing the growth of P. quadrifilis individuals belonging to either one of two distinct hypothetical life-history pathways: pathway I (‘changers’) – initial maturation as a primary male followed by a sex change to female; pathway II (‘non-changers’) – initial maturation as a male fish with no subsequent sex change, using von Bertalanffy Growth Functions (VBGFs). Other specific objectives included determining the size- and age-at-maturity and size- and age-at-sex-change for P. quadrifilis. Otolith aging revealed rapid growth and early maturation (L50 = 399.2 mm FL, A50 = 1.50 years) and sex change occurred over a wide size (790–1125 mm FL) and age (3–8 years) range. There was strong evidence for partial protandry in P. quadrifilis with several extremely old male fish (up to 22 years) observed in the population. When compared to the conventional model produced for the entire population, there were significant differences in the models for the ‘non-changers’ (LRT, p < 0.01) and their parameters L∞ (full model = 130.8, ‘non-changers’ = 113.3, p < 0.01), k (0.32, 0.44, p < 0.01) and t0 (0.23, 0.43, p = 0.03) in the first approach and the models (LRT, p < 0.01) and their L∞ (‘changers’ = 113.7, p < 0.01) values in the second approach. This suggests that utilising conventional modelling techniques may be inappropriate for the stock assessment and management of P. quadrifilis and, potentially, other sequentially hermaphroditic fishery species. Chapter 5 aimed to assess the sensitivity of P. quadrifilis to recreational C&R within the foreign recreational fishery using a rapid assessment approach. To do this, a number of C&R variables including fight time, air exposure, hook placement, hooking injury, total time of the stress event, river depth and angling method were measured and related to two indicators of fish health and survival – the physiological stress indicators blood glucose and blood lactate concentration and reflex action mortality predictors (RAMPs). Air exposure was identified as a major contributor to motor impairment (Cumulative Link Model: p < 0.01) and fight time was an important contributor to motor impairment via its interaction with air exposure (Cumulative Link Model: p = 0.02). Handling practices appear to be particularly important for larger individuals as fish size was positively correlated with air exposure (Pearson’s r coefficient = 0.41, p < 0.01) and fight times (0.88, p < 0.01). The findings suggest that recreational C&R may result in mortalities directly, via C&R, and indirectly, via predation, and several recommendations were made for best practice. Chapter 6 aimed to assess the direct economic contribution of the recreational fishery for Polydactylus quadrifilis on the Kwanza Estuary. Results indicated that the recreational fishery for contributed significantly to the economy of an area that would otherwise likely receive little external input ($282 054 per four-month fishing season). However, high rates of economic leakage from the study area were identified (58.7%–92.9% of locally spent revenue) and were attributed to the sourcing of lodge supplies, services and staff outside of the local area and the repatriation of profit by foreign business owners. Capacity building within the local community is likely required to develop ‘linkages’ between the local community and the recreational fishery. Greater community involvement in the fishery is suggested to incentivise the protection of recreationally important fishery species and their associated ecosystems. Chapter 7 aimed to investigate the resource user groups involved within the SES. Results illustrated that artisanal and domestic recreational anglers are well-established and are characterised by long histories of participation. The artisanal fishery was highly valued as a source of livelihoods for the local community. Artisanal fishers were eager for involvement in the recreational sector, through the chartering of their vessels, due to the attractiveness of extra earnings. Both recreational and artisanal fishers reported recent decreases in P. quadrifilis catch and anticipated further declines. Domestic recreational anglers appeared to be highly consumptive in their use of the fishery and C&R angling was uncommon. User conflict may be problematic for future management as recreational anglers perceived the artisanal gill-net fishery to be a threat towards P. quadrifilis stocks. In conclusion, the open-access nature of the fishery was identified as the most pertinent threat to its sustainability and likely needs to be addressed. Potential solutions involve offering users the opportunity to purchase access rights (e.g. day permits), thus initiating the concept that users must pay for their use of public resources. Management should aim to protect large female fish due to their increased reproductive value and worth as trophy fish. Thus, C&R angling is likely to be an important interaction between users and the resource. However, angler behaviour will need to be manipulated to promote C&R and minimise C&R-related mortalities. Solutions include angler educational drives and interventions and the implementation of competitive C&R-only angling. Foreign recreational fisheries, although touted as potential ecotourism ventures, will only succeed in improving the lives of local people if they fully integrate the community into the operation of the fishery.
- Full Text:
- Date Issued: 2020
A physiological study on a commercial reef fish to quantify the relationship between exploitation and climate change resilience
- Authors: Duncan, Murray Ian
- Date: 2019
- Subjects: Chrysoblephus laticeps -- Climatic factors , Chrysoblephus laticeps -- Physiology , Sparidae -- South Africa -- Climatic factors , Reef fishes -- South Africa -- Climatic factors , Fish populations -- South Africa -- Climatic factors , Fish populations -- Measurement , Fish populations -- Monitoring , Fisheries -- South Africa -- Environmental aspects , Ocean temperature -- Physiological effect -- South Africa
- Language: English
- Type: text , Thesis , Doctoral , PhD
- Identifier: http://hdl.handle.net/10962/76541 , vital:30599
- Description: The persistence of harvested fish populations in the Anthropocene will be determined, above all, by how they respond to the interacting effects of climate change and fisheries exploitation. Predicting how populations will respond to both these threats is essential for any adaptive and sustainable management strategy. The response of fish populations to climate change is underpinned by physiological rates and tolerances, and emerging evidence suggests there may be physiological-based selection in capture fisheries. By quantifying important physiological rates of a model species, the endemic seabream, Chrysoblephus laticeps, across ecologically relevant thermal gradients and from populations subjected to varying intensities of commercial exploitation, this thesis aimed to 1) provide the first physiologically grounded climate resilience assessment for a South African linefish species, and 2) elucidate whether exploitation can drive populations to less physiologically resilient states in response to climate change. To identify physiologically limiting sea temperatures and to determine if exploitation alters physiological trait distributions, an intermittent flow respirometry experiment was used to test the metabolic response of spatially protected and exploited populations of C. laticeps to acute thermal variability. Exploited populations showed reduced metabolic phenotype diversity, fewer high-performance aerobic scope phenotypes, and a significantly lower aerobic scope curve across all test temperatures. Although both populations maintained a relatively high aerobic scope across a wide thermal range, their metabolic rates were compromised when extreme cold events were simulated (8 °C), suggesting that predicted future increases in upwelling frequency and intensity may be the primary limiting factor in a more thermally variable future ocean. The increment widths of annuli in the otoliths of C. laticeps from contemporary and historic collections were measured, as a proxy for the annual growth rate of exploited and protected populations. Hierarchical mixed models were used to partition growth variation within and among individuals and ascribe growth to intrinsic and extrinsic effects. The best model for the protected population indicated that the growth response of C. laticeps was poorer during years characterised by a high cumulative upwelling intensity, and better during years characterised by higher mean autumn sea surface temperatures. The exploited population growth chronology was too short to identify an extrinsic growth driver. The growth results again highlight the role of thermal variability in modulating the response of C. laticeps to its ambient environment and indicate that the predicted increases in upwelling frequency and intensity may constrain future growth rates of this species. A metabolic index (ϕ), representing the ratio of O2 supply to demand at various temperatures and oxygen concentrations, was estimated for exploited and protected populations of C. laticeps and used to predict future distribution responses. There was no difference in the laboratory calibrations of ϕ between populations, and all data was subsequently combined into a single piecewise (12 °C) calibrated ϕ model. To predict the distribution of C. laticeps, ϕ was projected across a high-resolution ocean model of the South African coastal zone, and a species distribution model implemented using the random forest algorithm and C. laticeps occurrence points. The future distribution of C. laticeps was estimated by predicting trained models across ocean model projections up to 2100. The best predictor of C. laticeps’ current distribution was minimum monthly ϕ and future predictions indicated only a slight range contraction on either edge of C. laticeps’ distribution by 2100. In order to provide policy makers, currently developing climate change management frameworks for South Africa’s ocean, with a usable output, the results of all research chapters were combined into a marine spatial model. The spatial model identified areas where C. laticeps is predicted to be resilient to climate change in terms of physiology, growth and distribution responses, which can then be prioritised for adaptation measures, such as spatial protection from exploitation. While these results are specific to C. laticeps, the methodology developed to identify areas of climate resilience has broad applications across taxa. From a global perspective, perhaps the most salient points to consider from this case study are the evidence of selective exploitation on physiological traits and the importance of environmental variability, rather than long-term mean climate changes, in affecting organism performance. These ideas are congruent with the current paradigm shift in how we think of the ocean, selective fisheries, and how they relate to organism climate resilience.
- Full Text:
- Date Issued: 2019
- Authors: Duncan, Murray Ian
- Date: 2019
- Subjects: Chrysoblephus laticeps -- Climatic factors , Chrysoblephus laticeps -- Physiology , Sparidae -- South Africa -- Climatic factors , Reef fishes -- South Africa -- Climatic factors , Fish populations -- South Africa -- Climatic factors , Fish populations -- Measurement , Fish populations -- Monitoring , Fisheries -- South Africa -- Environmental aspects , Ocean temperature -- Physiological effect -- South Africa
- Language: English
- Type: text , Thesis , Doctoral , PhD
- Identifier: http://hdl.handle.net/10962/76541 , vital:30599
- Description: The persistence of harvested fish populations in the Anthropocene will be determined, above all, by how they respond to the interacting effects of climate change and fisheries exploitation. Predicting how populations will respond to both these threats is essential for any adaptive and sustainable management strategy. The response of fish populations to climate change is underpinned by physiological rates and tolerances, and emerging evidence suggests there may be physiological-based selection in capture fisheries. By quantifying important physiological rates of a model species, the endemic seabream, Chrysoblephus laticeps, across ecologically relevant thermal gradients and from populations subjected to varying intensities of commercial exploitation, this thesis aimed to 1) provide the first physiologically grounded climate resilience assessment for a South African linefish species, and 2) elucidate whether exploitation can drive populations to less physiologically resilient states in response to climate change. To identify physiologically limiting sea temperatures and to determine if exploitation alters physiological trait distributions, an intermittent flow respirometry experiment was used to test the metabolic response of spatially protected and exploited populations of C. laticeps to acute thermal variability. Exploited populations showed reduced metabolic phenotype diversity, fewer high-performance aerobic scope phenotypes, and a significantly lower aerobic scope curve across all test temperatures. Although both populations maintained a relatively high aerobic scope across a wide thermal range, their metabolic rates were compromised when extreme cold events were simulated (8 °C), suggesting that predicted future increases in upwelling frequency and intensity may be the primary limiting factor in a more thermally variable future ocean. The increment widths of annuli in the otoliths of C. laticeps from contemporary and historic collections were measured, as a proxy for the annual growth rate of exploited and protected populations. Hierarchical mixed models were used to partition growth variation within and among individuals and ascribe growth to intrinsic and extrinsic effects. The best model for the protected population indicated that the growth response of C. laticeps was poorer during years characterised by a high cumulative upwelling intensity, and better during years characterised by higher mean autumn sea surface temperatures. The exploited population growth chronology was too short to identify an extrinsic growth driver. The growth results again highlight the role of thermal variability in modulating the response of C. laticeps to its ambient environment and indicate that the predicted increases in upwelling frequency and intensity may constrain future growth rates of this species. A metabolic index (ϕ), representing the ratio of O2 supply to demand at various temperatures and oxygen concentrations, was estimated for exploited and protected populations of C. laticeps and used to predict future distribution responses. There was no difference in the laboratory calibrations of ϕ between populations, and all data was subsequently combined into a single piecewise (12 °C) calibrated ϕ model. To predict the distribution of C. laticeps, ϕ was projected across a high-resolution ocean model of the South African coastal zone, and a species distribution model implemented using the random forest algorithm and C. laticeps occurrence points. The future distribution of C. laticeps was estimated by predicting trained models across ocean model projections up to 2100. The best predictor of C. laticeps’ current distribution was minimum monthly ϕ and future predictions indicated only a slight range contraction on either edge of C. laticeps’ distribution by 2100. In order to provide policy makers, currently developing climate change management frameworks for South Africa’s ocean, with a usable output, the results of all research chapters were combined into a marine spatial model. The spatial model identified areas where C. laticeps is predicted to be resilient to climate change in terms of physiology, growth and distribution responses, which can then be prioritised for adaptation measures, such as spatial protection from exploitation. While these results are specific to C. laticeps, the methodology developed to identify areas of climate resilience has broad applications across taxa. From a global perspective, perhaps the most salient points to consider from this case study are the evidence of selective exploitation on physiological traits and the importance of environmental variability, rather than long-term mean climate changes, in affecting organism performance. These ideas are congruent with the current paradigm shift in how we think of the ocean, selective fisheries, and how they relate to organism climate resilience.
- Full Text:
- Date Issued: 2019
Towards a norm of compliance in recreational fisheries
- Authors: Bova, Christopher S
- Date: 2019
- Subjects: Fishing -- Management -- South Africa , Fishery law and legislation -- South Africa
- Language: English
- Type: text , Thesis , Doctoral , PhD
- Identifier: http://hdl.handle.net/10962/95909 , vital:31213
- Description: The activity of fishing can be traced back to prehistoric times. However, only in the last century has there been a focus on the management of fisheries. Fishery regulations are tools used by resource managers with the aim of protecting the long-term sustainability of fishery resources. Although there is an overwhelming amount of evidence demonstrating the decline of fisheries, non-compliance with these regulations by fishers continues to manifest, which can exacerbate the negative ecological impacts of fisheries. Popular methods towards the measurement of noncompliance in fisheries derived from previous human dimensions literature may be flawed. Theories on improving compliance behaviour have typically relied on theory, which has at times proved paradoxical. Addressing the issues of non-compliance within a fishery of interest requires measuring the levels of non-compliance within the fishery and determining the relevant sociopsychological drivers behind the non-compliant behaviour. The data collection methods used during these assessments are limited in human dimensions research and are often case and context specific, requiring researchers to identify which approach is most practical for the specific fishery of interest. By identifying relevant behavioural drivers of non-compliance, a more effective approach aimed at improving compliance can be tailored. The recreational marine-based shore fishery (MBSF) in South Africa is not impervious to noncomplaint behaviour. In fact, it has been estimated to have relatively high rates of non-compliance. This high level of non-compliance makes the South African MBSF a unique and optimal context in which to undertake research that aims to formulate a framework towards compliance assessments and that develops a suitable approach for improving compliance rates. Using surveys to obtain compliance data can provide a range of details about violators, however they are susceptible to social desirability bias (SDB). Choosing the best method for controlling SDB required an assessment of existing methods for doing so. In this first part of the study, only fishers who were covertly observed breaking the rules were surveyed, using one of three methods for reducing SDB, to ground-truth the responses. Ground-truthing was done to determine which method would be most effective for a large-scale study within the same fishery. Of the methods used, which include the direct questioning method (DQM), the random response technique (RRT) and the ballot box method (BBM), all contained some level of SDB. However, the BBM provided a significantly higher level of response accuracy (79.6% ± 11.9) than the DQM (46.5% ± 14.9) and the RRT (44.3% ± 12.5). Random-stratified roving creel compliance surveys that employed the BBM were then undertaken at various locations along South Africa’s coastline to estimate current rates of non-compliance, and the face-to-face results were compared to results from an identical online survey to determine the suitability of online surveys as a replacement. The results indicated that online surveys only represent a subgroup of the fisher population within the MBSF, suggesting that face-to-face survey methods are required to obtain a more comprehensive sample and a more robust estimate of noncompliance. The results, based on 453 face-to-face surveys, showed a high level of overall self-reported noncompliance (48.3%) within the fishery. Responses to Likert scale survey questions on various aspects of the fishery, including angler motivations for fishing, were then modelled to test the relationship between the anglers’ responses and their compliance behaviour. In the South African MBSF context, the most significant behavioural drivers behind non-compliance related to normative concepts. Specifically, the poor perceptions of management and value-based legitimacy as well as low levels of moral obligation to adhere to the regulations appeared to contribute most to the observed non-compliant behaviour. Angler motivations for fishing also played a significant role in determining the compliance behaviour of anglers, with those fishing for food being more likely to violate regulations. In most countries, regardless of economic context, interventions to improve recreational fishery compliance have been developed around the instrumental concept. However, these findings suggest that for recreational fisheries, managers would do well to evaluate the impact of normative concepts on compliance and to design interventions aimed at addressing these. In the case of the South African MBSF, interventions that address angler perceptions of legitimacy and aim to correct misperceptions about social norms of compliance may provide a more practical and cost- effective method for improving poor compliance behaviour.
- Full Text:
- Date Issued: 2019
- Authors: Bova, Christopher S
- Date: 2019
- Subjects: Fishing -- Management -- South Africa , Fishery law and legislation -- South Africa
- Language: English
- Type: text , Thesis , Doctoral , PhD
- Identifier: http://hdl.handle.net/10962/95909 , vital:31213
- Description: The activity of fishing can be traced back to prehistoric times. However, only in the last century has there been a focus on the management of fisheries. Fishery regulations are tools used by resource managers with the aim of protecting the long-term sustainability of fishery resources. Although there is an overwhelming amount of evidence demonstrating the decline of fisheries, non-compliance with these regulations by fishers continues to manifest, which can exacerbate the negative ecological impacts of fisheries. Popular methods towards the measurement of noncompliance in fisheries derived from previous human dimensions literature may be flawed. Theories on improving compliance behaviour have typically relied on theory, which has at times proved paradoxical. Addressing the issues of non-compliance within a fishery of interest requires measuring the levels of non-compliance within the fishery and determining the relevant sociopsychological drivers behind the non-compliant behaviour. The data collection methods used during these assessments are limited in human dimensions research and are often case and context specific, requiring researchers to identify which approach is most practical for the specific fishery of interest. By identifying relevant behavioural drivers of non-compliance, a more effective approach aimed at improving compliance can be tailored. The recreational marine-based shore fishery (MBSF) in South Africa is not impervious to noncomplaint behaviour. In fact, it has been estimated to have relatively high rates of non-compliance. This high level of non-compliance makes the South African MBSF a unique and optimal context in which to undertake research that aims to formulate a framework towards compliance assessments and that develops a suitable approach for improving compliance rates. Using surveys to obtain compliance data can provide a range of details about violators, however they are susceptible to social desirability bias (SDB). Choosing the best method for controlling SDB required an assessment of existing methods for doing so. In this first part of the study, only fishers who were covertly observed breaking the rules were surveyed, using one of three methods for reducing SDB, to ground-truth the responses. Ground-truthing was done to determine which method would be most effective for a large-scale study within the same fishery. Of the methods used, which include the direct questioning method (DQM), the random response technique (RRT) and the ballot box method (BBM), all contained some level of SDB. However, the BBM provided a significantly higher level of response accuracy (79.6% ± 11.9) than the DQM (46.5% ± 14.9) and the RRT (44.3% ± 12.5). Random-stratified roving creel compliance surveys that employed the BBM were then undertaken at various locations along South Africa’s coastline to estimate current rates of non-compliance, and the face-to-face results were compared to results from an identical online survey to determine the suitability of online surveys as a replacement. The results indicated that online surveys only represent a subgroup of the fisher population within the MBSF, suggesting that face-to-face survey methods are required to obtain a more comprehensive sample and a more robust estimate of noncompliance. The results, based on 453 face-to-face surveys, showed a high level of overall self-reported noncompliance (48.3%) within the fishery. Responses to Likert scale survey questions on various aspects of the fishery, including angler motivations for fishing, were then modelled to test the relationship between the anglers’ responses and their compliance behaviour. In the South African MBSF context, the most significant behavioural drivers behind non-compliance related to normative concepts. Specifically, the poor perceptions of management and value-based legitimacy as well as low levels of moral obligation to adhere to the regulations appeared to contribute most to the observed non-compliant behaviour. Angler motivations for fishing also played a significant role in determining the compliance behaviour of anglers, with those fishing for food being more likely to violate regulations. In most countries, regardless of economic context, interventions to improve recreational fishery compliance have been developed around the instrumental concept. However, these findings suggest that for recreational fisheries, managers would do well to evaluate the impact of normative concepts on compliance and to design interventions aimed at addressing these. In the case of the South African MBSF, interventions that address angler perceptions of legitimacy and aim to correct misperceptions about social norms of compliance may provide a more practical and cost- effective method for improving poor compliance behaviour.
- Full Text:
- Date Issued: 2019
Using a multi-method approach to understand the movement patterns and the associated environmental correlates of an iconic West African recreational fish
- Authors: Winkler, Alexander Claus
- Date: 2019
- Subjects: Carangidae fishing , Carangidae -- Migration , Carangidae -- Namibia , Carangidae -- Angola , Fish tagging , Carangidae -- Benguela Current , Underwater acoustic telemetry , Ocean temperature -- Physiological effect , Fishes -- Effect of temperature on
- Language: English
- Type: text , Thesis , Doctoral , PhD
- Identifier: http://hdl.handle.net/10962/76530 , vital:30597
- Description: The leerfish (Lichia amia), is a large, primarily coastal recreational fish species with a distribution extending from Portugal down the west coast of African to southern Mozambique. Owing to its large size (30 kg), strong fighting abilities and habit of taking surface artificial lures, this species has taken on an iconic stature among shore-based recreational anglers. Its reputation has made it an important angling tourism species that makes an important contribution to the economy of developing countries. For example, the species brought US$243 per harvested kilogramme into the local southern Angola economy. Despite its high value, little is known about its movement patterns in the northern Benguela coastal region, a region which includes southern Angola and northern Namibia. While much is known about the migratory patterns of the South African stock of L. amia, recent molecular studies have shown that the northern Benguela stock of L. amia has been isolated from the South African population for at least two million years, a consequence of the development of the cold Lüderitz upwelling cell in southern Namibia. Although the global population of L. amia is considered a single species, prominent biogeographic barriers within its distribution and subtle morphological differences between specimens captured within its tropical versus warm-temperate distribution suggest otherwise. A multi-method approach incorporating passive acoustic telemetry (PAT), recreational catch-per-unit-effort (CPUE) and conventional tagging (CT) in southern Angola, as well as recreational fisher-ecological knowledge (FEK) from Namibia, was used to investigate the large-scale movement patterns of L. amia within the northern Benguela coastal region. While each method had its own associated limitations, the combination provided a holistic picture of the population's seasonal migratory patterns. Furthermore, PAT successfully identified partial migration with 25% vs 75% of monitored fish exhibiting resident (movements < 100 km) or migratory (movements > 100 km) behaviour, respectively. Further behavioural diversity was observed with ‘resident’, ‘roaming’ and ‘embayment’ contingents identified based on varying levels of affinity to certain habitats. The presence of both resident and migratory individuals within the northernmost study during June and July, combined with available biological information, suggested that area-specific spawning may take place. While PAT, CPUE and CT largely aligned in determining area specific high-area use, results from network analyses and mixed effects models conducted on the PAT data supported the spawning hypothesis, with anomalous behaviour around specific receivers during the spawning season. All fish, regardless of behavioural contingent, displayed similar movement behaviour during the spawning season and this was driven by factors generally associated with reproduction, such as lunar illumination. Interestingly, these drivers were different from those that determined the area specific use of individuals outside of the spawning season. The environmental drivers of longshore migration into the northern study site were identified as a decline in water temperature and shorter day lengths. The results of this study highlight the importance of using a multi-method approach in determining migratory movement behaviour, area specific area use, and stock structure of key fisheries species. The identification of different behavioural contingents highlights the importance of acknowledging individual variation in movement and habitat-use patterns. This is particularly relevant as future climate change and spatiotemporal variation in fishing effort may artificially skew natural selection processes to favour certain behavioural groups. This study also highlighted the importance of scientists forming relationships with resource-users, such as recreational angling lodges in areas where limited research has been conducted. This is particularly relevant within the West African context where little is known about many of the fish species that are being increasingly targeted by tourism angling ventures.
- Full Text:
- Date Issued: 2019
- Authors: Winkler, Alexander Claus
- Date: 2019
- Subjects: Carangidae fishing , Carangidae -- Migration , Carangidae -- Namibia , Carangidae -- Angola , Fish tagging , Carangidae -- Benguela Current , Underwater acoustic telemetry , Ocean temperature -- Physiological effect , Fishes -- Effect of temperature on
- Language: English
- Type: text , Thesis , Doctoral , PhD
- Identifier: http://hdl.handle.net/10962/76530 , vital:30597
- Description: The leerfish (Lichia amia), is a large, primarily coastal recreational fish species with a distribution extending from Portugal down the west coast of African to southern Mozambique. Owing to its large size (30 kg), strong fighting abilities and habit of taking surface artificial lures, this species has taken on an iconic stature among shore-based recreational anglers. Its reputation has made it an important angling tourism species that makes an important contribution to the economy of developing countries. For example, the species brought US$243 per harvested kilogramme into the local southern Angola economy. Despite its high value, little is known about its movement patterns in the northern Benguela coastal region, a region which includes southern Angola and northern Namibia. While much is known about the migratory patterns of the South African stock of L. amia, recent molecular studies have shown that the northern Benguela stock of L. amia has been isolated from the South African population for at least two million years, a consequence of the development of the cold Lüderitz upwelling cell in southern Namibia. Although the global population of L. amia is considered a single species, prominent biogeographic barriers within its distribution and subtle morphological differences between specimens captured within its tropical versus warm-temperate distribution suggest otherwise. A multi-method approach incorporating passive acoustic telemetry (PAT), recreational catch-per-unit-effort (CPUE) and conventional tagging (CT) in southern Angola, as well as recreational fisher-ecological knowledge (FEK) from Namibia, was used to investigate the large-scale movement patterns of L. amia within the northern Benguela coastal region. While each method had its own associated limitations, the combination provided a holistic picture of the population's seasonal migratory patterns. Furthermore, PAT successfully identified partial migration with 25% vs 75% of monitored fish exhibiting resident (movements < 100 km) or migratory (movements > 100 km) behaviour, respectively. Further behavioural diversity was observed with ‘resident’, ‘roaming’ and ‘embayment’ contingents identified based on varying levels of affinity to certain habitats. The presence of both resident and migratory individuals within the northernmost study during June and July, combined with available biological information, suggested that area-specific spawning may take place. While PAT, CPUE and CT largely aligned in determining area specific high-area use, results from network analyses and mixed effects models conducted on the PAT data supported the spawning hypothesis, with anomalous behaviour around specific receivers during the spawning season. All fish, regardless of behavioural contingent, displayed similar movement behaviour during the spawning season and this was driven by factors generally associated with reproduction, such as lunar illumination. Interestingly, these drivers were different from those that determined the area specific use of individuals outside of the spawning season. The environmental drivers of longshore migration into the northern study site were identified as a decline in water temperature and shorter day lengths. The results of this study highlight the importance of using a multi-method approach in determining migratory movement behaviour, area specific area use, and stock structure of key fisheries species. The identification of different behavioural contingents highlights the importance of acknowledging individual variation in movement and habitat-use patterns. This is particularly relevant as future climate change and spatiotemporal variation in fishing effort may artificially skew natural selection processes to favour certain behavioural groups. This study also highlighted the importance of scientists forming relationships with resource-users, such as recreational angling lodges in areas where limited research has been conducted. This is particularly relevant within the West African context where little is known about many of the fish species that are being increasingly targeted by tourism angling ventures.
- Full Text:
- Date Issued: 2019
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