While there is now strong evidence that many factors can shape dispersal, the mechanisms influenc... more While there is now strong evidence that many factors can shape dispersal, the mechanisms influencing connectivity patterns are species‐specific and remain largely unknown for many species with a high dispersal potential. The rock lobsters Jasus tristani and Jasus paulensis have a long pelagic larval duration (up to 20 months) and inhabit seamounts and islands in the southern Atlantic and Indian Oceans, respectively. We used a multidisciplinary approach to assess the genetic relationships between J. tristani and J. paulensis, investigate historic and contemporary gene flow, and inform fisheries management. Using 17,256 neutral single nucleotide polymorphisms we found low but significant genetic differentiation. We show that patterns of connectivity changed over time in accordance with climatic fluctuations. Historic migration estimates showed stronger connectivity from the Indian to the Atlantic Ocean (influenced by the Agulhas Leakage). In contrast, the individual‐based model couple...
Seamounts have long been recognised as hotspots for pelagic productivity and diversity in the wor... more Seamounts have long been recognised as hotspots for pelagic productivity and diversity in the world’s open ocean habitats. Recent studies have suggested that productivity may vary greatly between different seamounts, depending on complex interactions between the bathymetric features and local oceanography. These processes may enhance local primary production which support elevated biomass at higher trophic levels. In addition to enhancing local biomass, seamounts may also act as aggregative features, attracting pelagic species from the surrounding waters. Such characteristics make seamounts attractive targets for fisheries. However, as these unique habitats are localised and relatively small, they are vulnerable to overexploitation, which may have detrimental impact on the wider region. Mapping and quantitative assessments of the fish biomass at different seamounts are crucial prerequisites to identifying vulnerable seamounts and will aid toward understanding the dynamics of these i...
Quantifying the level of population connectivity within and between geographically separated sing... more Quantifying the level of population connectivity within and between geographically separated single-species deep-water fisheries stocks will be vital for designing effective management plans to preserve such populations. Despite this, stock structure in many fisheries is still poorly described and, at best, subject to precautionary management. Here we use rapidly evolving mitochondrial genes and microsatellite markers to investigate population connectivity patterns in commercially targeted Hyperoglyphe antarctica populations between four seamounts within the Tristan da Cunha Exclusive Economic Zone (EEZ). We find little evidence of population genetic structure between fished populations, with both mtDNA and microsatellite markers showing that there is low genetic population diversity (reflecting substantial gene flow) across the four seamounts. We also find little genetic differentiation between H. antarctica across the wider Southern Hemisphere. Such results support the role for co...
The northeast periphery of the Scotia Sea hosts one of the largest chlorophyll‐a blooms of the So... more The northeast periphery of the Scotia Sea hosts one of the largest chlorophyll‐a blooms of the Southern Ocean. This bloom peaks to the northwest of the island of South Georgia, extending eastward for hundreds of kilometers. Although the Southern Ocean has many islands of similar size, South Georgia is ecologically one of the most significant: It not only sustains one of the Southern Ocean's largest and most diverse ecosystems but also constitutes its single most important region for biological carbon sequestration. While the exceptional nature of South Georgia's blooms has been recognized widely, both the physical processes that contribute to their fertilization and the reasons why these blooms are larger than those of other similar regions (e.g., Kerguelen or Crozet Islands) are poorly understood. We use the results of a high‐resolution ocean model to investigate the physical processes that mediate the entrainment of deep, iron‐rich waters into the surface layers of the Sou...
In the marine environment, understanding the biophysical mechanisms that drive variability in lar... more In the marine environment, understanding the biophysical mechanisms that drive variability in larval dispersal and population connectivity is essential for estimating the potential impacts of climate change on the resilience and genetic structure of populations. Species whose populations are small, isolated and discontinuous in distribution will differ fundamentally in their response and resilience to environmental stress, compared with species that are broadly distributed, abundant and frequently exchange conspecifics. Here, we use an individual-based modelling approach, combined with a population genetics projection model, to consider the impacts of a warming climate on the population connectivity of two contrasting Antarctic fish species, and . Focussing on the Scotia Sea region, sea surface temperatures are predicted to increase significantly by the end of the 21st century, resulting in reduced planktonic duration and increased egg and larval mortality. With shorter planktonic d...
Understanding the key drivers of population connectivity in the marine environment is essential f... more Understanding the key drivers of population connectivity in the marine environment is essential for the effective management of natural resources. Although several different approaches to evaluating connectivity have been used, they are rarely integrated quantitatively. Here, we use a 'seascape genetics' approach, by combining oceanographic modelling and microsatellite analyses, to understand the dominant influences on the population genetic structure of two Antarctic fishes with contrasting life histories, Champsocephalus gunnari and Notothenia rossii. The close accord between the model projections and empirical genetic structure demonstrated that passive dispersal during the planktonic early life stages is the dominant influence on patterns and extent of genetic structuring in both species. The shorter planktonic phase of C. gunnari restricts direct transport of larvae between distant populations, leading to stronger regional differentiation. By contrast, geographic distance did not affect differentiation in N. rossii, whose longer larval period promotes long-distance dispersal. Interannual variability in oceanographic flows strongly influenced the projected genetic structure, suggesting that shifts in circulation patterns due to climate change are likely to impact future genetic connectivity and opportunities for local adaptation, resilience and recovery from perturbations. Further development of realistic climate models is required to fully assess such potential impacts.
ABSTRACT Population connectivity in the marine environment has different meanings and implication... more ABSTRACT Population connectivity in the marine environment has different meanings and implications depending on whether the context is primarily ecological or evolutionary. From an ecological perspective the focus is typically on the exchange of individuals among geographically separated subpopulations – encompassing the dispersal phase from reproduction to the completion of the settlement process (including habitat choice and metamorphosis). Such life history events have a strong influence on processes such as population dynamics and recruitment, coupling of ecosystem processes and the nature and rate of response to environmental perturbations. The key in such ecological aspects of connectivity is the importance of within and between generational events across short time-scales on distribution, recruitment and persistence. Genetic estimates that relate in some way to gene flow derive from processes across a fundamentally different time-scale: it is the mean rate of exchange and interbreeding across many generations that drives patterns of population genetic differentiation. Even rare and sporadic gene flow can have a disproportionate impact on genetic differentiation, events that are often difficult to track or detect in real time, especially in the marine environment. Moreover, our ability to generate ecologically meaningful predictions of population connectivity derived from essentially oceanographic modelling with its inherent local, regional and global stochasticity present a formidable challenge: integrating the ecological and evolutionary perspectives requires careful estimation of the spatial and temporal scale of events at the demographic and genetic levels. Coincident with the requirement to capture processes at different scales is the diversity of tools available for assessing connectivity, including among others, oceanography, genetics and otoliths. Here, we illustrate challenges and insights by exploring briefly the relationship between the methodologies for assessing larval connectivity and the range of spatial and temporal scales pertinent to ecological and evolutionary events, with reference to projected patterns of larval dispersal, retention, recruitment and mortality.
Deep Sea Research Part Ii Topical Studies in Oceanography, Jul 1, 2011
The marine ecosystem on the shelf and open ocean adjacent to South Georgia is extraordinarily ric... more The marine ecosystem on the shelf and open ocean adjacent to South Georgia is extraordinarily rich, with a history of commercial exploitation. Although much progress has been made, attempts at modelling (and hence better understanding) this system have consistently been hampered by the poor representation of key physical processes in global or regional ocean general circulation models. Here we present the development of a high-resolution 3D hydrodynamic model of the South Georgia shelf and the adjacent open ocean, including a novel method for prescribing freshwater fluxes. The ability of the model to reproduce the observed oceanography of the region is quantified by comparisons with data from tide gauges at South Georgia, with satellite-derived sea surface temperatures, and with an extensive CTD dataset collected during January-April 1995. Predicted cotidal charts for the diurnal tides O 1 and K 1 show a periodic amplification in both the current and elevation fields at the shelf edge, suggesting the presence of a diurnally forced continental shelf wave. This could have important implications for processes such as larval transport and retention. The comparison with CTD data reveals mean and root mean square errors in temperature (salinity) of -0.27 °C (-0.07) and 0.64 °C (0.23), respectively. Vertical profiles of potential temperature and salinity on the shelf agree acceptably well with observations, but there is a tendency for the model to under-predict the density contrast between surface and bottom waters. The main limitation on model accuracy is found to be the large-scale forcing. Releasing a passive tracer into the model, transport and retention pathways are identified, including a prevalence for tracer export from the shelf to the west of South Georgia, and a transport pathway linking South Georgia and Shag Rocks. Significantly, the model suggests this to be a unidirectional link, from South Georgia to Shag Rocks, with possible significance for fisheries management. The implications of these results in the context of the South Georgia ecosystem are discussed briefly, demonstrating the usefulness of this new tool for interdisciplinary studies of the region.
Marine organisms with pelagic larvae are generally assumed to experience high gene flow and low l... more Marine organisms with pelagic larvae are generally assumed to experience high gene flow and low levels of population differentiation. However, variability in life history and environmental characteristics, in particular oceanographic flow fields, can significantly influence dispersal, and their relative effects are frequently unclear. Our research examines the influence of oceanographic and life history variability on gene flow in two species of Antarctic fish: Champsocephalus gunnari and Notothenia rossii. These species are broadly sympatric in their distribution, but differ in aspects of life history that are expected to strongly affect their dispersal capabilities. Our research has used two complementary techniques. Genetic analyses, specifically mtDNA and microsatellite markers, have been used to examine historic and contemporary gene flow and thus describe patterns of population differentiation at the circumpolar scale. These analyses have been compared with predicted larval transport from a global oceanographic model (OCCAM) combined with individual based particle tracking models. In using these complementary techniques, the relative influences of early life history and oceanographic variability can be elucidated. Here we present the key findings of our research, including evidence for inter-specific variation in mitochondrial gene flow at the circumpolar level and a limited degree of genetic structuring within the Scotia Sea.
While there is now strong evidence that many factors can shape dispersal, the mechanisms influenc... more While there is now strong evidence that many factors can shape dispersal, the mechanisms influencing connectivity patterns are species‐specific and remain largely unknown for many species with a high dispersal potential. The rock lobsters Jasus tristani and Jasus paulensis have a long pelagic larval duration (up to 20 months) and inhabit seamounts and islands in the southern Atlantic and Indian Oceans, respectively. We used a multidisciplinary approach to assess the genetic relationships between J. tristani and J. paulensis, investigate historic and contemporary gene flow, and inform fisheries management. Using 17,256 neutral single nucleotide polymorphisms we found low but significant genetic differentiation. We show that patterns of connectivity changed over time in accordance with climatic fluctuations. Historic migration estimates showed stronger connectivity from the Indian to the Atlantic Ocean (influenced by the Agulhas Leakage). In contrast, the individual‐based model couple...
Seamounts have long been recognised as hotspots for pelagic productivity and diversity in the wor... more Seamounts have long been recognised as hotspots for pelagic productivity and diversity in the world’s open ocean habitats. Recent studies have suggested that productivity may vary greatly between different seamounts, depending on complex interactions between the bathymetric features and local oceanography. These processes may enhance local primary production which support elevated biomass at higher trophic levels. In addition to enhancing local biomass, seamounts may also act as aggregative features, attracting pelagic species from the surrounding waters. Such characteristics make seamounts attractive targets for fisheries. However, as these unique habitats are localised and relatively small, they are vulnerable to overexploitation, which may have detrimental impact on the wider region. Mapping and quantitative assessments of the fish biomass at different seamounts are crucial prerequisites to identifying vulnerable seamounts and will aid toward understanding the dynamics of these i...
Quantifying the level of population connectivity within and between geographically separated sing... more Quantifying the level of population connectivity within and between geographically separated single-species deep-water fisheries stocks will be vital for designing effective management plans to preserve such populations. Despite this, stock structure in many fisheries is still poorly described and, at best, subject to precautionary management. Here we use rapidly evolving mitochondrial genes and microsatellite markers to investigate population connectivity patterns in commercially targeted Hyperoglyphe antarctica populations between four seamounts within the Tristan da Cunha Exclusive Economic Zone (EEZ). We find little evidence of population genetic structure between fished populations, with both mtDNA and microsatellite markers showing that there is low genetic population diversity (reflecting substantial gene flow) across the four seamounts. We also find little genetic differentiation between H. antarctica across the wider Southern Hemisphere. Such results support the role for co...
The northeast periphery of the Scotia Sea hosts one of the largest chlorophyll‐a blooms of the So... more The northeast periphery of the Scotia Sea hosts one of the largest chlorophyll‐a blooms of the Southern Ocean. This bloom peaks to the northwest of the island of South Georgia, extending eastward for hundreds of kilometers. Although the Southern Ocean has many islands of similar size, South Georgia is ecologically one of the most significant: It not only sustains one of the Southern Ocean's largest and most diverse ecosystems but also constitutes its single most important region for biological carbon sequestration. While the exceptional nature of South Georgia's blooms has been recognized widely, both the physical processes that contribute to their fertilization and the reasons why these blooms are larger than those of other similar regions (e.g., Kerguelen or Crozet Islands) are poorly understood. We use the results of a high‐resolution ocean model to investigate the physical processes that mediate the entrainment of deep, iron‐rich waters into the surface layers of the Sou...
In the marine environment, understanding the biophysical mechanisms that drive variability in lar... more In the marine environment, understanding the biophysical mechanisms that drive variability in larval dispersal and population connectivity is essential for estimating the potential impacts of climate change on the resilience and genetic structure of populations. Species whose populations are small, isolated and discontinuous in distribution will differ fundamentally in their response and resilience to environmental stress, compared with species that are broadly distributed, abundant and frequently exchange conspecifics. Here, we use an individual-based modelling approach, combined with a population genetics projection model, to consider the impacts of a warming climate on the population connectivity of two contrasting Antarctic fish species, and . Focussing on the Scotia Sea region, sea surface temperatures are predicted to increase significantly by the end of the 21st century, resulting in reduced planktonic duration and increased egg and larval mortality. With shorter planktonic d...
Understanding the key drivers of population connectivity in the marine environment is essential f... more Understanding the key drivers of population connectivity in the marine environment is essential for the effective management of natural resources. Although several different approaches to evaluating connectivity have been used, they are rarely integrated quantitatively. Here, we use a 'seascape genetics' approach, by combining oceanographic modelling and microsatellite analyses, to understand the dominant influences on the population genetic structure of two Antarctic fishes with contrasting life histories, Champsocephalus gunnari and Notothenia rossii. The close accord between the model projections and empirical genetic structure demonstrated that passive dispersal during the planktonic early life stages is the dominant influence on patterns and extent of genetic structuring in both species. The shorter planktonic phase of C. gunnari restricts direct transport of larvae between distant populations, leading to stronger regional differentiation. By contrast, geographic distance did not affect differentiation in N. rossii, whose longer larval period promotes long-distance dispersal. Interannual variability in oceanographic flows strongly influenced the projected genetic structure, suggesting that shifts in circulation patterns due to climate change are likely to impact future genetic connectivity and opportunities for local adaptation, resilience and recovery from perturbations. Further development of realistic climate models is required to fully assess such potential impacts.
ABSTRACT Population connectivity in the marine environment has different meanings and implication... more ABSTRACT Population connectivity in the marine environment has different meanings and implications depending on whether the context is primarily ecological or evolutionary. From an ecological perspective the focus is typically on the exchange of individuals among geographically separated subpopulations – encompassing the dispersal phase from reproduction to the completion of the settlement process (including habitat choice and metamorphosis). Such life history events have a strong influence on processes such as population dynamics and recruitment, coupling of ecosystem processes and the nature and rate of response to environmental perturbations. The key in such ecological aspects of connectivity is the importance of within and between generational events across short time-scales on distribution, recruitment and persistence. Genetic estimates that relate in some way to gene flow derive from processes across a fundamentally different time-scale: it is the mean rate of exchange and interbreeding across many generations that drives patterns of population genetic differentiation. Even rare and sporadic gene flow can have a disproportionate impact on genetic differentiation, events that are often difficult to track or detect in real time, especially in the marine environment. Moreover, our ability to generate ecologically meaningful predictions of population connectivity derived from essentially oceanographic modelling with its inherent local, regional and global stochasticity present a formidable challenge: integrating the ecological and evolutionary perspectives requires careful estimation of the spatial and temporal scale of events at the demographic and genetic levels. Coincident with the requirement to capture processes at different scales is the diversity of tools available for assessing connectivity, including among others, oceanography, genetics and otoliths. Here, we illustrate challenges and insights by exploring briefly the relationship between the methodologies for assessing larval connectivity and the range of spatial and temporal scales pertinent to ecological and evolutionary events, with reference to projected patterns of larval dispersal, retention, recruitment and mortality.
Deep Sea Research Part Ii Topical Studies in Oceanography, Jul 1, 2011
The marine ecosystem on the shelf and open ocean adjacent to South Georgia is extraordinarily ric... more The marine ecosystem on the shelf and open ocean adjacent to South Georgia is extraordinarily rich, with a history of commercial exploitation. Although much progress has been made, attempts at modelling (and hence better understanding) this system have consistently been hampered by the poor representation of key physical processes in global or regional ocean general circulation models. Here we present the development of a high-resolution 3D hydrodynamic model of the South Georgia shelf and the adjacent open ocean, including a novel method for prescribing freshwater fluxes. The ability of the model to reproduce the observed oceanography of the region is quantified by comparisons with data from tide gauges at South Georgia, with satellite-derived sea surface temperatures, and with an extensive CTD dataset collected during January-April 1995. Predicted cotidal charts for the diurnal tides O 1 and K 1 show a periodic amplification in both the current and elevation fields at the shelf edge, suggesting the presence of a diurnally forced continental shelf wave. This could have important implications for processes such as larval transport and retention. The comparison with CTD data reveals mean and root mean square errors in temperature (salinity) of -0.27 °C (-0.07) and 0.64 °C (0.23), respectively. Vertical profiles of potential temperature and salinity on the shelf agree acceptably well with observations, but there is a tendency for the model to under-predict the density contrast between surface and bottom waters. The main limitation on model accuracy is found to be the large-scale forcing. Releasing a passive tracer into the model, transport and retention pathways are identified, including a prevalence for tracer export from the shelf to the west of South Georgia, and a transport pathway linking South Georgia and Shag Rocks. Significantly, the model suggests this to be a unidirectional link, from South Georgia to Shag Rocks, with possible significance for fisheries management. The implications of these results in the context of the South Georgia ecosystem are discussed briefly, demonstrating the usefulness of this new tool for interdisciplinary studies of the region.
Marine organisms with pelagic larvae are generally assumed to experience high gene flow and low l... more Marine organisms with pelagic larvae are generally assumed to experience high gene flow and low levels of population differentiation. However, variability in life history and environmental characteristics, in particular oceanographic flow fields, can significantly influence dispersal, and their relative effects are frequently unclear. Our research examines the influence of oceanographic and life history variability on gene flow in two species of Antarctic fish: Champsocephalus gunnari and Notothenia rossii. These species are broadly sympatric in their distribution, but differ in aspects of life history that are expected to strongly affect their dispersal capabilities. Our research has used two complementary techniques. Genetic analyses, specifically mtDNA and microsatellite markers, have been used to examine historic and contemporary gene flow and thus describe patterns of population differentiation at the circumpolar scale. These analyses have been compared with predicted larval transport from a global oceanographic model (OCCAM) combined with individual based particle tracking models. In using these complementary techniques, the relative influences of early life history and oceanographic variability can be elucidated. Here we present the key findings of our research, including evidence for inter-specific variation in mitochondrial gene flow at the circumpolar level and a limited degree of genetic structuring within the Scotia Sea.
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