An eco-physiological investigation of fisheries-induced evolution: comparing the resilience of larvae from exploited and unexploited commercial reef fish populations to projected ocean acidification
- Authors: Muller, Cuen
- Date: 2022-04-08
- Subjects: Ocean acidification , Fishes Climatic factors , Fishes Physiology , Fishes Metabolism , Fishes Respiration , Fishes Larvae , Fishery management , Chrysoblephus laticeps (Red roman)
- Language: English
- Type: Doctoral thesis , text
- Identifier: http://hdl.handle.net/10962/232579 , vital:50004 , DOI 10.21504/10962/232579
- Description: It is now accepted that anthropogenic-induced climate change is resulting in unprecedented rates of change to marine environments. Marine organisms are being challenged by rapidly increasing temperatures, acidification, expansion of oxygen dead zones, and higher frequencies and magnitudes of extreme weather events. Exploited fish populations are also undergoing selective harvesting. Certain traits, such as large size, fast growth, and/or bold/active behaviours, are being actively targeted and removed from the population gene pool. This selective removal of individuals may compromise the capacity of fish populations to resist or recover from environmental disturbances and reduce their ability to adapt to a changing environment as many of these traits are heritable. As most marine fishes' embryonic and larval stages represent the period when individuals are most sensitive to environmental disturbances, they are a critical bottleneck to population persistence in the face of exploitation and climate change. This thesis aimed to quantify and compare the metabolic physiology, growth, and development of an exploited and endemic sparid, the roman seabream Chrysoblephus laticeps, during the early larval stages under 1) ocean acidification conditions expected by the year 2100 and 2) from populations experiencing dissimilar rates of exploitation. To quantify and compare the physiology of larvae, adult C. laticeps from an exploited population were captured and field-spawned. Fertilised eggs were placed into either control/present-day conditions (pH = 8.03, pCO2 ≈ 420 μatm) or high-pCO2/hypercapnic treatment conditions (pH = 7.63, pCO2 ≈ 1400 μatm). The metabolic physiology of individual larvae was determined using a novel rolling-regression technique on static respirometry data. Here, estimates of the minimum and maximum oxygen consumption rates (VO2) could be determined with high test-retest reliability. The very early developmental stages (yolk-sac stage) appeared resilient to high pCO2 conditions despite being exposed to treatment conditions throughout the embryonic stage. Preflexion larvae showed sensitivity to treatment conditions by exhibiting reduced metabolic and growth rates, consistent with metabolic depression, associated with environmental stress. However, by the onset of flexion, which coincides with gill development, acid-base regulation, and muscle differentiation, metabolic and growth rates of treatment larvae were significantly greater than that of controls. This suggests that acid-base regulation imposes a high cost to maintain internal pH homeostasis. Importantly, these elevated metabolic costs were likely mediated through increased feeding rates in experimental conditions where food was ad libitum. In natural conditions, where food availability may be varied, high pCO2 conditions could be associated with higher mortality rates. Based on evidence that protected/unexploited populations are more genetically diverse and are composed of individuals representing a greater range of metabolic phenotypes, offspring were collected from a protected population experiencing otherwise similar environmental conditions to the exploited population. Metabolic rates of control larvae were generally similar to those of the exploited population. However, minimum rates of VO2 were typically higher for larvae from the protected population at comparable life stages. Preflexion treatment larvae from the protected population did not appear to undergo a period of reduced metabolism or growth compared to their control counterparts. While metabolic rates at the onset of flexion were significantly higher for treatment larvae, this was not associated with growth differences. Growth over-compensation following periods of growth depression is often associated with deleterious effects, such as organ damage and body or developmental malformations. This suggests somewhat improved resilience to ocean acidification conditions. This thesis found evidence that larval C. laticeps are sensitive to ocean acidification conditions expected by 2100. When this stressor is combined with increasing thermal variability, changing current coastal regimes, and heterogeneous food availability, also expected to occur by 2100, ocean acidification may compromise the population persistence of this species. However, an energetics approach to stress-tolerance suggests that larvae from the protected population may inherently show greater resilience to climate change-related environmental stressors. Evidence that exploitation affects the resilience of fish larvae to climate change highlights the need for an evolutionary approach to fisheries management and the importance of spatial protection in maintaining larger and more resilient populations while providing the raw material essential for adaptation. , Thesis (PhD) -- Faculty of Science, Ichthyology and Fisheries Science, 2022
- Full Text:
- Authors: Muller, Cuen
- Date: 2022-04-08
- Subjects: Ocean acidification , Fishes Climatic factors , Fishes Physiology , Fishes Metabolism , Fishes Respiration , Fishes Larvae , Fishery management , Chrysoblephus laticeps (Red roman)
- Language: English
- Type: Doctoral thesis , text
- Identifier: http://hdl.handle.net/10962/232579 , vital:50004 , DOI 10.21504/10962/232579
- Description: It is now accepted that anthropogenic-induced climate change is resulting in unprecedented rates of change to marine environments. Marine organisms are being challenged by rapidly increasing temperatures, acidification, expansion of oxygen dead zones, and higher frequencies and magnitudes of extreme weather events. Exploited fish populations are also undergoing selective harvesting. Certain traits, such as large size, fast growth, and/or bold/active behaviours, are being actively targeted and removed from the population gene pool. This selective removal of individuals may compromise the capacity of fish populations to resist or recover from environmental disturbances and reduce their ability to adapt to a changing environment as many of these traits are heritable. As most marine fishes' embryonic and larval stages represent the period when individuals are most sensitive to environmental disturbances, they are a critical bottleneck to population persistence in the face of exploitation and climate change. This thesis aimed to quantify and compare the metabolic physiology, growth, and development of an exploited and endemic sparid, the roman seabream Chrysoblephus laticeps, during the early larval stages under 1) ocean acidification conditions expected by the year 2100 and 2) from populations experiencing dissimilar rates of exploitation. To quantify and compare the physiology of larvae, adult C. laticeps from an exploited population were captured and field-spawned. Fertilised eggs were placed into either control/present-day conditions (pH = 8.03, pCO2 ≈ 420 μatm) or high-pCO2/hypercapnic treatment conditions (pH = 7.63, pCO2 ≈ 1400 μatm). The metabolic physiology of individual larvae was determined using a novel rolling-regression technique on static respirometry data. Here, estimates of the minimum and maximum oxygen consumption rates (VO2) could be determined with high test-retest reliability. The very early developmental stages (yolk-sac stage) appeared resilient to high pCO2 conditions despite being exposed to treatment conditions throughout the embryonic stage. Preflexion larvae showed sensitivity to treatment conditions by exhibiting reduced metabolic and growth rates, consistent with metabolic depression, associated with environmental stress. However, by the onset of flexion, which coincides with gill development, acid-base regulation, and muscle differentiation, metabolic and growth rates of treatment larvae were significantly greater than that of controls. This suggests that acid-base regulation imposes a high cost to maintain internal pH homeostasis. Importantly, these elevated metabolic costs were likely mediated through increased feeding rates in experimental conditions where food was ad libitum. In natural conditions, where food availability may be varied, high pCO2 conditions could be associated with higher mortality rates. Based on evidence that protected/unexploited populations are more genetically diverse and are composed of individuals representing a greater range of metabolic phenotypes, offspring were collected from a protected population experiencing otherwise similar environmental conditions to the exploited population. Metabolic rates of control larvae were generally similar to those of the exploited population. However, minimum rates of VO2 were typically higher for larvae from the protected population at comparable life stages. Preflexion treatment larvae from the protected population did not appear to undergo a period of reduced metabolism or growth compared to their control counterparts. While metabolic rates at the onset of flexion were significantly higher for treatment larvae, this was not associated with growth differences. Growth over-compensation following periods of growth depression is often associated with deleterious effects, such as organ damage and body or developmental malformations. This suggests somewhat improved resilience to ocean acidification conditions. This thesis found evidence that larval C. laticeps are sensitive to ocean acidification conditions expected by 2100. When this stressor is combined with increasing thermal variability, changing current coastal regimes, and heterogeneous food availability, also expected to occur by 2100, ocean acidification may compromise the population persistence of this species. However, an energetics approach to stress-tolerance suggests that larvae from the protected population may inherently show greater resilience to climate change-related environmental stressors. Evidence that exploitation affects the resilience of fish larvae to climate change highlights the need for an evolutionary approach to fisheries management and the importance of spatial protection in maintaining larger and more resilient populations while providing the raw material essential for adaptation. , Thesis (PhD) -- Faculty of Science, Ichthyology and Fisheries Science, 2022
- Full Text:
Demersal fish distribution in the shallow marine nearshore and estuarine seascape of Algoa Bay: Nursery areas and the effect of environmental drivers
- Authors: Nodo, Phakama
- Date: 2022-04-08
- Subjects: Groundfishes South Africa Algoa Bay , Estuarine fishes South Africa Algoa Bay , Marine nurseries South Africa Algoa Bay , Estuarine fishes Habitat South Africa Algoa Bay , Estuarine fishes Physiology South Africa Algoa Bay , Estuarine fishes Effect of pollution on South Africa Algoa Bay , Estuarine fishes Larvae South Africa Algoa Bay , Estuarine fishes Effect of human beings on South Africa Algoa Bay
- Language: English
- Type: Doctoral thesis , text
- Identifier: http://hdl.handle.net/10962/232399 , vital:49988 , DOI 10.21504/10962/232399
- Description: Estuaries and shallow marine nearshore areas are highly productive and valuable ecosystems, which provide numerous habitats for fish and support fundamental ecological links with other environments. Assessing fish distribution across estuarine and marine nearshore habitats is important to identify ecologically important habitats and develop effective management strategies for coastal fishes, many of which are important fishery species. Despite this, only a few studies have focussed on fish community patterns across an estuary and marine nearshore gradient concurrently, particularly including early life history stages, to determine the nursery value of both environments, and to examine whether these two coastal environments have distinct fish assemblages in relation to physical factors. The main aim of this study was to assess the environmental drivers of demersal fish communities in soft-bottom benthic habitats in two permanently open estuaries and adjacent marine nearshore areas (5 – 10 m) of Algoa Bay, South Africa, as well as to assess the relative roles of these two habitats as settlement and nursery areas for demersal fish species. A 1.5 m, conical shoeless beam trawl net was used to sample the demersal fish community concurrently in each habitat between July 2017 and September 2019. Sampling was conducted in July 2017, February, March, May, July, August, October and November 2019 and February, April and September 2019. DNA barcoding was used to verify identification of the early life history stages of fish caught in the estuarine and marine nearshore areas of Algoa Bay. In addition, since the two estuaries (Swartkops and Sundays) are heavily polluted, the effect of low dissolved oxygen and hypoxia and associated shifts in spatial distribution of demersal species was investigated. The two sampled estuaries had a higher abundance of demersal fishes, with a total of 6437 fishes (28 species) caught (3752 and 2685 individuals with 24 and 20 species recorded in the Sundays and Swartkops estuaries, respectively). Species richness was higher in the marine nearshore of Algoa Bay, with 29 species (797 individuals) caught. Of the 7234 individuals caught, the identification of 100 specimens, in either a larval or early juvenile phase, were uncertain and therefore DNA barcoding was used to verify their identification. Of these 100 individuals, 86 were positively identified to species level using COI sequences. Fourteen failed to amplify by PCR and could only be identified morphologically. The marine nearshore sites were dominated by species which spawn in the marine environment and are not dependent on estuaries (marine species and marine estuary-opportunists), whilst the estuaries were dominated by estuarine spawners or marine spawners dependent on estuaries to some degree. Two discrete demersal fish assemblages were identified representing the marine nearshore and the estuary, with no significant differences observed between the two estuaries (Sundays and Swartkops). The differences observed between the marine nearshore and estuary were mostly driven by salinity, turbidity, silt and organic content of the sediment. These distinct fish assemblages might be considered as indicators for the respective environments they inhabit. Both habitats were dominated by early life history stages (larvae to juveniles), indicating the nursery function of both habitats. Early life stages collectively comprised 97% of the catch in the marine nearshore and 68% in the estuary. Young-of-the-year (YOY) juveniles (< 1-year-old juveniles) and transformation stages (when changes in body shape and pigment pattern occur) dominated the total catch in the marine nearshore, while YOY juveniles dominated the estuarine fish assemblages. Ariidae Galeichthys feliceps, Haemulidae Pomadasys olivaceus, Sciaenidae Argyrosomus inodorus and Cynoglossidae Cynoglossus zanzibarensis, comprised the largest proportion of YOY juveniles in the marine nearshore. The transformation stage in the marine nearshore was numerically dominated by P. olivaceus and G. feliceps. In the estuarine environment, YOY juveniles were mostly dominated by Sparidae Rhabdosargus holubi, Soleidae Heteromycteris capensis and Gobiidae Caffrogobius gilchristi. The greatest abundance of early life stage fishes was observed in the lower reaches of the Sundays Estuary and the upper reaches of the Swartkops Estuary, as well as nearshore sites located in close proximity to the estuary mouths, particularly during spring and summer. Despite the fact that these coastal ecosystems are important nursery areas, they are threatened by a number of factors, including habitat loss and modification due to urban development, intensification of agriculture and subsequent eutrophication, climate change, and overfishing, all of which reduce ecosystem functioning and reduce the ecological and economic value of these habitats around the world. Hypoxia is one of the major threats to the functioning of coastal ecosystems, particularly estuaries. The Sundays and Swartkops estuaries both experience persistent eutrophic conditions, with frequent phytoplankton blooms (> 20 μg Chl-a l-1) that result in instances of bottom water oxygen depletion (< 4 mg/l). During the present study in the Sundays Estuary, low oxygen waters were recorded in the middle reaches (Site S5) mostly during summer (four months of low DO conditions). In the Swartkops Estuary, low dissolved oxygen was recorded in the upper reaches during spring. The lowest dissolved oxygen concentration recorded was 0.5 mg/l and 2.4 mg/l in the bottom waters of the Sundays and Swartkops estuaries, respectively. Selected dominant species were only absent from areas where dissolved oxygen was < 1 mg/l and present in the adjacent sites (for example Site S4, and S3) where DO was higher mostly during January 2019. As such, the low dissolved oxygen concentrations recorded in the Swartkops Estuary did not have a noticeable impact on fish distribution, although the total abundance of species did show a slight decline when dissolved oxygen was < 3 mg/l. This study demonstrates the importance of concurrently examining estuarine and nearshore marine habitats in order to identify ecologically important habitats, which has important implications for the development of effective management strategies for coastal fish populations, particularly in the light of anthropogenic change. In addition, in order to identify nursery hotspots it is crucial to correctly identify all the species occupying these areas. As such, this study confirms the importance of also using DNA barcoding for fish identification, particularly for the early life history stages of cryptic species (e.g. Argyrosomus inodorus and Argyrosomus japonicus). , Thesis (PhD) -- Faculty of Science, Ichthyology & Fisheries Sciences, 2022
- Full Text:
- Authors: Nodo, Phakama
- Date: 2022-04-08
- Subjects: Groundfishes South Africa Algoa Bay , Estuarine fishes South Africa Algoa Bay , Marine nurseries South Africa Algoa Bay , Estuarine fishes Habitat South Africa Algoa Bay , Estuarine fishes Physiology South Africa Algoa Bay , Estuarine fishes Effect of pollution on South Africa Algoa Bay , Estuarine fishes Larvae South Africa Algoa Bay , Estuarine fishes Effect of human beings on South Africa Algoa Bay
- Language: English
- Type: Doctoral thesis , text
- Identifier: http://hdl.handle.net/10962/232399 , vital:49988 , DOI 10.21504/10962/232399
- Description: Estuaries and shallow marine nearshore areas are highly productive and valuable ecosystems, which provide numerous habitats for fish and support fundamental ecological links with other environments. Assessing fish distribution across estuarine and marine nearshore habitats is important to identify ecologically important habitats and develop effective management strategies for coastal fishes, many of which are important fishery species. Despite this, only a few studies have focussed on fish community patterns across an estuary and marine nearshore gradient concurrently, particularly including early life history stages, to determine the nursery value of both environments, and to examine whether these two coastal environments have distinct fish assemblages in relation to physical factors. The main aim of this study was to assess the environmental drivers of demersal fish communities in soft-bottom benthic habitats in two permanently open estuaries and adjacent marine nearshore areas (5 – 10 m) of Algoa Bay, South Africa, as well as to assess the relative roles of these two habitats as settlement and nursery areas for demersal fish species. A 1.5 m, conical shoeless beam trawl net was used to sample the demersal fish community concurrently in each habitat between July 2017 and September 2019. Sampling was conducted in July 2017, February, March, May, July, August, October and November 2019 and February, April and September 2019. DNA barcoding was used to verify identification of the early life history stages of fish caught in the estuarine and marine nearshore areas of Algoa Bay. In addition, since the two estuaries (Swartkops and Sundays) are heavily polluted, the effect of low dissolved oxygen and hypoxia and associated shifts in spatial distribution of demersal species was investigated. The two sampled estuaries had a higher abundance of demersal fishes, with a total of 6437 fishes (28 species) caught (3752 and 2685 individuals with 24 and 20 species recorded in the Sundays and Swartkops estuaries, respectively). Species richness was higher in the marine nearshore of Algoa Bay, with 29 species (797 individuals) caught. Of the 7234 individuals caught, the identification of 100 specimens, in either a larval or early juvenile phase, were uncertain and therefore DNA barcoding was used to verify their identification. Of these 100 individuals, 86 were positively identified to species level using COI sequences. Fourteen failed to amplify by PCR and could only be identified morphologically. The marine nearshore sites were dominated by species which spawn in the marine environment and are not dependent on estuaries (marine species and marine estuary-opportunists), whilst the estuaries were dominated by estuarine spawners or marine spawners dependent on estuaries to some degree. Two discrete demersal fish assemblages were identified representing the marine nearshore and the estuary, with no significant differences observed between the two estuaries (Sundays and Swartkops). The differences observed between the marine nearshore and estuary were mostly driven by salinity, turbidity, silt and organic content of the sediment. These distinct fish assemblages might be considered as indicators for the respective environments they inhabit. Both habitats were dominated by early life history stages (larvae to juveniles), indicating the nursery function of both habitats. Early life stages collectively comprised 97% of the catch in the marine nearshore and 68% in the estuary. Young-of-the-year (YOY) juveniles (< 1-year-old juveniles) and transformation stages (when changes in body shape and pigment pattern occur) dominated the total catch in the marine nearshore, while YOY juveniles dominated the estuarine fish assemblages. Ariidae Galeichthys feliceps, Haemulidae Pomadasys olivaceus, Sciaenidae Argyrosomus inodorus and Cynoglossidae Cynoglossus zanzibarensis, comprised the largest proportion of YOY juveniles in the marine nearshore. The transformation stage in the marine nearshore was numerically dominated by P. olivaceus and G. feliceps. In the estuarine environment, YOY juveniles were mostly dominated by Sparidae Rhabdosargus holubi, Soleidae Heteromycteris capensis and Gobiidae Caffrogobius gilchristi. The greatest abundance of early life stage fishes was observed in the lower reaches of the Sundays Estuary and the upper reaches of the Swartkops Estuary, as well as nearshore sites located in close proximity to the estuary mouths, particularly during spring and summer. Despite the fact that these coastal ecosystems are important nursery areas, they are threatened by a number of factors, including habitat loss and modification due to urban development, intensification of agriculture and subsequent eutrophication, climate change, and overfishing, all of which reduce ecosystem functioning and reduce the ecological and economic value of these habitats around the world. Hypoxia is one of the major threats to the functioning of coastal ecosystems, particularly estuaries. The Sundays and Swartkops estuaries both experience persistent eutrophic conditions, with frequent phytoplankton blooms (> 20 μg Chl-a l-1) that result in instances of bottom water oxygen depletion (< 4 mg/l). During the present study in the Sundays Estuary, low oxygen waters were recorded in the middle reaches (Site S5) mostly during summer (four months of low DO conditions). In the Swartkops Estuary, low dissolved oxygen was recorded in the upper reaches during spring. The lowest dissolved oxygen concentration recorded was 0.5 mg/l and 2.4 mg/l in the bottom waters of the Sundays and Swartkops estuaries, respectively. Selected dominant species were only absent from areas where dissolved oxygen was < 1 mg/l and present in the adjacent sites (for example Site S4, and S3) where DO was higher mostly during January 2019. As such, the low dissolved oxygen concentrations recorded in the Swartkops Estuary did not have a noticeable impact on fish distribution, although the total abundance of species did show a slight decline when dissolved oxygen was < 3 mg/l. This study demonstrates the importance of concurrently examining estuarine and nearshore marine habitats in order to identify ecologically important habitats, which has important implications for the development of effective management strategies for coastal fish populations, particularly in the light of anthropogenic change. In addition, in order to identify nursery hotspots it is crucial to correctly identify all the species occupying these areas. As such, this study confirms the importance of also using DNA barcoding for fish identification, particularly for the early life history stages of cryptic species (e.g. Argyrosomus inodorus and Argyrosomus japonicus). , Thesis (PhD) -- Faculty of Science, Ichthyology & Fisheries Sciences, 2022
- Full Text:
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