The ‘Abundant-Centre Hypothesis’ is a well-established but controversial hypothesis stating that ... more The ‘Abundant-Centre Hypothesis’ is a well-established but controversial hypothesis stating that the abundance of a species is highest at the centre of its range and decreases towards the edges, where conditions are unfavourable. As genetic diversity depends on population size, edge populations are expected to show lower intra-population genetic diversity than core populations, while showing high inter-population genetic divergence. Here, the genetic implications of the Abundant-Centre Hypothesis were tested on two coastal mussels from South Africa that disperse by means of planktonic larvae, the native Perna perna and the invasive Mytilus galloprovincialis. Genetic structure was found within P. perna, which, together with evidence from Lagrangian particle simulations, points to significant reductions in gene flow between sites. Despite this, the expected diversity pattern between centre and edge populations was not found for either species. We conclude that the genetic predictions ...
In invertebrates, environmental temperature may induce mothers to invest differently in the early... more In invertebrates, environmental temperature may induce mothers to invest differently in the early development of their offspring. In ectotherms, temperature affects offspring phenotype so that colder mothers produce larger eggs. However, developmental mode and maternal size also contribute to the determination of optimal offspring size. When the maternal experience closely matches the offspring's probable future conditions (e.g., direct developers), it is expected that mothers will produce eggs of similar size within the same brood. While temperature directly affects the size of the eggs (temperature size rule), with potential indirect links to egg number (trade-off between egg size/number), maternal size can be a limiting factor in determining the optimal number of eggs, especially if eggs are brooded. We evaluated the role of temperature in shaping early ontogeny in the sandhopper Talorchestia capensis (Crustacea: Amphipoda), investigating within-brood and among-female variati...
Journal of Experimental Marine Biology and Ecology, 2012
article i nfo Using a multi-disciplinary approach, we evaluated the potential consequences of lon... more article i nfo Using a multi-disciplinary approach, we evaluated the potential consequences of long-term contrasting prey availability on the condition Cape gannets Morus capensis. We compared breeding adults from a decreasing colony on Malgas Island off the west coast of South Africa, where the abundance of small pelagic fish has de- creased, with an increasing colony on Bird Island off the south coast, where pelagic fish are more abundant. We investigated (1) pelagic fish density using data from a hydro-acoustic survey, (2) gannet diet (stomach content analyses, fatty acid and stable isotope analyses), (3) gannet foraging effort and at-sea feeding areas (nest attendance patterns, GPS-tracking and blood haematocrit), (4) chick growth rates, and (5) adult body condition (morphometric measurements and breast muscle thickness). Our data confirmed contrasting prey availability between colonies, although pelagic fish were more abundant on the west coast than in pre- vious years. Gannets exhibit dietary plasticity, feeding on pelagic fish and trawler discards, but favour natural prey when available. Stomach content samples showed that gannets from both islands mainly ate natural prey in 2009, but there were differences in their stable isotope and fatty acid signatures, supporting evidence of long-term diet differences. Nevertheless, chick growth rates and adult body condition were similar at both colonies, which was surprising for breeding adults from the west coast colony that had been feeding exten- sively on energy-poor fishery waste for several years. Breeding gannets' behavioural flexibility seems to have succeeded in maintaining body condition. However, this might have long-term costs, as adult survival at Mal- gas Island has decreased in recent years. Population decreases at this colony are exacerbated by low juvenile survival and perhaps differential recruitment to Bird Island. Higher foraging effort by adult breeding on the large colony of Bird Island compared to birds from the west coast, despite greater pelagic fish abundance on the south coast, probably suggests a greater intra-specific competition there. Monitoring of population trends, population health indices and foraging behaviour, as well as prey availability is necessary to better understand the mechanisms underlying the population trends.
Saldanha Bay forms part of the Benguela upwelling system. Here, the mussel Mytilus galloprovincia... more Saldanha Bay forms part of the Benguela upwelling system. Here, the mussel Mytilus galloprovincialis is farmed by means of floating raft culture. The relationship between the food removed by mussels and the growth, condition, and production of mussels cultured on rafts with two different rope spacings was investigated. The dense culture of mussels on rafts is responsible for local limitation in food supply at the raft scale. Food depletion through a raft increased both with the age of the mussel ropes suspended from it and with decreased rope spacing and was a function of increased feeding and greater retardation of water exchange through rafts with higher mussel mass. Mussel growth rates in summer were 30% greater than in winter, and during summer, growth rates were a further 8% higher on rafts with increased rope spacing. The harvest of marketable mussels was 30% higher from ropes suspended at the increased spacing. Consequently, the harvest of mussels was 9% higher from rafts wit...
Detailed stable isotope data of egg shells and egg membranes for 12 bird species breeding at Mari... more Detailed stable isotope data of egg shells and egg membranes for 12 bird species breeding at Marion Island (Southern Ocean)
The South African coast includes biogeographic provinces which are quite distinct in their abilit... more The South African coast includes biogeographic provinces which are quite distinct in their abilities to sustain exploitation and in the types of exploitation they experience. As a result, no single region can act as a model for the management of living resources around the entire coast. Instead, appropriate management must rest on regionally based research. Past research in marine biology focused on the west coast with much less attention being given to the east and south coasts, where non-commercial, subsistence-level fisheries predominate. A partial decentralisation of research focus is essential. This requires commitment from national-level research bodies and some certainty about at least medium-term funding. This paper addresses the convergence of ecology, economics and politics in South Africa and illustrates the direct relevance that fundamental or basic research can have to socioeconomic problems. Recruitment of the resource; Geographical variation in biological processes; State of research policy; Promise of regulations
The ‘Abundant-Centre Hypothesis’ is a well-established but controversial hypothesis stating that ... more The ‘Abundant-Centre Hypothesis’ is a well-established but controversial hypothesis stating that the abundance of a species is highest at the centre of its range and decreases towards the edges, where conditions are unfavourable. As genetic diversity depends on population size, edge populations are expected to show lower intra-population genetic diversity than core populations, while showing high inter-population genetic divergence. Here, the genetic implications of the Abundant-Centre Hypothesis were tested on two coastal mussels from South Africa that disperse by means of planktonic larvae, the native Perna perna and the invasive Mytilus galloprovincialis. Genetic structure was found within P. perna, which, together with evidence from Lagrangian particle simulations, points to significant reductions in gene flow between sites. Despite this, the expected diversity pattern between centre and edge populations was not found for either species. We conclude that the genetic predictions ...
In invertebrates, environmental temperature may induce mothers to invest differently in the early... more In invertebrates, environmental temperature may induce mothers to invest differently in the early development of their offspring. In ectotherms, temperature affects offspring phenotype so that colder mothers produce larger eggs. However, developmental mode and maternal size also contribute to the determination of optimal offspring size. When the maternal experience closely matches the offspring's probable future conditions (e.g., direct developers), it is expected that mothers will produce eggs of similar size within the same brood. While temperature directly affects the size of the eggs (temperature size rule), with potential indirect links to egg number (trade-off between egg size/number), maternal size can be a limiting factor in determining the optimal number of eggs, especially if eggs are brooded. We evaluated the role of temperature in shaping early ontogeny in the sandhopper Talorchestia capensis (Crustacea: Amphipoda), investigating within-brood and among-female variati...
Journal of Experimental Marine Biology and Ecology, 2012
article i nfo Using a multi-disciplinary approach, we evaluated the potential consequences of lon... more article i nfo Using a multi-disciplinary approach, we evaluated the potential consequences of long-term contrasting prey availability on the condition Cape gannets Morus capensis. We compared breeding adults from a decreasing colony on Malgas Island off the west coast of South Africa, where the abundance of small pelagic fish has de- creased, with an increasing colony on Bird Island off the south coast, where pelagic fish are more abundant. We investigated (1) pelagic fish density using data from a hydro-acoustic survey, (2) gannet diet (stomach content analyses, fatty acid and stable isotope analyses), (3) gannet foraging effort and at-sea feeding areas (nest attendance patterns, GPS-tracking and blood haematocrit), (4) chick growth rates, and (5) adult body condition (morphometric measurements and breast muscle thickness). Our data confirmed contrasting prey availability between colonies, although pelagic fish were more abundant on the west coast than in pre- vious years. Gannets exhibit dietary plasticity, feeding on pelagic fish and trawler discards, but favour natural prey when available. Stomach content samples showed that gannets from both islands mainly ate natural prey in 2009, but there were differences in their stable isotope and fatty acid signatures, supporting evidence of long-term diet differences. Nevertheless, chick growth rates and adult body condition were similar at both colonies, which was surprising for breeding adults from the west coast colony that had been feeding exten- sively on energy-poor fishery waste for several years. Breeding gannets' behavioural flexibility seems to have succeeded in maintaining body condition. However, this might have long-term costs, as adult survival at Mal- gas Island has decreased in recent years. Population decreases at this colony are exacerbated by low juvenile survival and perhaps differential recruitment to Bird Island. Higher foraging effort by adult breeding on the large colony of Bird Island compared to birds from the west coast, despite greater pelagic fish abundance on the south coast, probably suggests a greater intra-specific competition there. Monitoring of population trends, population health indices and foraging behaviour, as well as prey availability is necessary to better understand the mechanisms underlying the population trends.
Saldanha Bay forms part of the Benguela upwelling system. Here, the mussel Mytilus galloprovincia... more Saldanha Bay forms part of the Benguela upwelling system. Here, the mussel Mytilus galloprovincialis is farmed by means of floating raft culture. The relationship between the food removed by mussels and the growth, condition, and production of mussels cultured on rafts with two different rope spacings was investigated. The dense culture of mussels on rafts is responsible for local limitation in food supply at the raft scale. Food depletion through a raft increased both with the age of the mussel ropes suspended from it and with decreased rope spacing and was a function of increased feeding and greater retardation of water exchange through rafts with higher mussel mass. Mussel growth rates in summer were 30% greater than in winter, and during summer, growth rates were a further 8% higher on rafts with increased rope spacing. The harvest of marketable mussels was 30% higher from ropes suspended at the increased spacing. Consequently, the harvest of mussels was 9% higher from rafts wit...
Detailed stable isotope data of egg shells and egg membranes for 12 bird species breeding at Mari... more Detailed stable isotope data of egg shells and egg membranes for 12 bird species breeding at Marion Island (Southern Ocean)
The South African coast includes biogeographic provinces which are quite distinct in their abilit... more The South African coast includes biogeographic provinces which are quite distinct in their abilities to sustain exploitation and in the types of exploitation they experience. As a result, no single region can act as a model for the management of living resources around the entire coast. Instead, appropriate management must rest on regionally based research. Past research in marine biology focused on the west coast with much less attention being given to the east and south coasts, where non-commercial, subsistence-level fisheries predominate. A partial decentralisation of research focus is essential. This requires commitment from national-level research bodies and some certainty about at least medium-term funding. This paper addresses the convergence of ecology, economics and politics in South Africa and illustrates the direct relevance that fundamental or basic research can have to socioeconomic problems. Recruitment of the resource; Geographical variation in biological processes; State of research policy; Promise of regulations
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Papers by C. McQuaid