The role of ecological processes in structuring reef fish communities in the Agulhas Ecoregion, South Africa
- Authors: Dyer, Alexander
- Date: 2018
- Subjects: Reef fishes -- South Africa -- Agulhas, Cape (Cape) , Reef fishes -- Ecology -- South Africa -- Agulhas, Cape (Cape) , Marine biodiversity -- South Africa -- Agulhas, Cape (Cape) , Biotic communities -- South Africa -- Agulhas, Cape (Cape) , Sparidae -- South Africa -- Agulhas, Cape (Cape) , Reef fishes -- Size -- South Africa -- Agulhas, Cape (Cape) , Baited remote underwater stereo-video systems (stereo-BRUVs)
- Language: English
- Type: text , Thesis , Masters , MSc
- Identifier: http://hdl.handle.net/10962/63899 , vital:28504
- Description: Local, niche-based processes, which arise from the interplay between biotic interactions and abiotic constraints are considered to be important regulators of community structure. However, it is increasingly recognised that patterns of diversity can also be strongly influenced by dispersal-driven processes. While empirical research on the diversity of coral reef fishes on shallow tropical reefs has contributed greatly to the development of general concepts in ecology, there have been considerably fewer studies on the processes which shape the diversity of fish communities on shallow (10 - 30 m) and deep (30 - 75 m) rocky reefs. Consequently, it is less clear at which spatial scales niche partitioning and dispersal limitation contribute most strongly to the structure of reef-associated fish communities within rocky reef ecosystems. To address this caveat, research was conducted at four rocky reef complexes within the warm-temperate Agulhas Ecoregion, South Africa. The diversity of reef-associated fishes was sampled by baited remote underwater stereo-video systems (stereo-BRUVs) to incorporate the range of heterogeneous reef habitat in Tsitsikamma National Park Marine Protected Area (TNP MPA) and Algoa Bay (AB). To examine how niche-based and dispersal-driven processes influence patterns of diversity among species within the dominant family of resident fishes, the sparids (Sparidae), components of diversity were quantified at several spatial scales. Turnover in the number of species which locally co-occurred was found to be largely driven by the limited dispersal of species over hundreds of kilometres. When relative species abundances were taken into account, sparid communities were characterised by higher than expected rates of compositional turnover among local habitat patches separated by hundreds to thousands of metres of contiguous reef. Patterns of compositional turnover were associated with the spatial aggregation of conspecifics, particularly at scales which facilitate the post-settlement dispersal of fishes. Niche-based segregation of species along the depth gradient was found to be the primary driver of compositional turnover among both protected and exploited communities. However, spatial structuring within reefs, which was independent of variation in the environment, suggests that compositional differences among communities are also influenced by the limited post-settlement dispersal of resident fishes to adjacent areas during their ontogeny. Together, the results suggest that the diversity of reef-associated sparids is likely to depend both on an adequate diversity of suitable reef habitat and a sufficient degree of spatial connectivity to facilitate ontogenetic habitat shifts. Taxon-based descriptors of diversity do not adequately account for ecological difference among conspecifics within size-structured populations. To test whether differences in body size facilitated coexistence among sparid fishes, the number of species which coexisted at the local scale was related to variation in the size structure of communities. In communities which have been historically protected from fishing, local coexistence between a greater number of species was promoted by reduced levels of intraspecific variation in size of fishes. This suggests that, among species with similar trophic requirements, further niche segregation along a prey-size and body-size gradient is likely to mitigate the direct impacts of competition for shared food resources. Among exploited communities, size structure did not influence the number of species which coexisted at the local scale. This finding suggests that fishing-induced mortality of larger-bodied fishes is likely to remove some of the constraints to colonisation which arise from asymmetries in competitive fitness between small and large-bodied fishes. Together, these results highlight the importance of post-settlement processes and population size structure to the maintenance of reef-associated fish diversity within contiguous rocky reef habitats.
- Full Text:
- Date Issued: 2018
- Authors: Dyer, Alexander
- Date: 2018
- Subjects: Reef fishes -- South Africa -- Agulhas, Cape (Cape) , Reef fishes -- Ecology -- South Africa -- Agulhas, Cape (Cape) , Marine biodiversity -- South Africa -- Agulhas, Cape (Cape) , Biotic communities -- South Africa -- Agulhas, Cape (Cape) , Sparidae -- South Africa -- Agulhas, Cape (Cape) , Reef fishes -- Size -- South Africa -- Agulhas, Cape (Cape) , Baited remote underwater stereo-video systems (stereo-BRUVs)
- Language: English
- Type: text , Thesis , Masters , MSc
- Identifier: http://hdl.handle.net/10962/63899 , vital:28504
- Description: Local, niche-based processes, which arise from the interplay between biotic interactions and abiotic constraints are considered to be important regulators of community structure. However, it is increasingly recognised that patterns of diversity can also be strongly influenced by dispersal-driven processes. While empirical research on the diversity of coral reef fishes on shallow tropical reefs has contributed greatly to the development of general concepts in ecology, there have been considerably fewer studies on the processes which shape the diversity of fish communities on shallow (10 - 30 m) and deep (30 - 75 m) rocky reefs. Consequently, it is less clear at which spatial scales niche partitioning and dispersal limitation contribute most strongly to the structure of reef-associated fish communities within rocky reef ecosystems. To address this caveat, research was conducted at four rocky reef complexes within the warm-temperate Agulhas Ecoregion, South Africa. The diversity of reef-associated fishes was sampled by baited remote underwater stereo-video systems (stereo-BRUVs) to incorporate the range of heterogeneous reef habitat in Tsitsikamma National Park Marine Protected Area (TNP MPA) and Algoa Bay (AB). To examine how niche-based and dispersal-driven processes influence patterns of diversity among species within the dominant family of resident fishes, the sparids (Sparidae), components of diversity were quantified at several spatial scales. Turnover in the number of species which locally co-occurred was found to be largely driven by the limited dispersal of species over hundreds of kilometres. When relative species abundances were taken into account, sparid communities were characterised by higher than expected rates of compositional turnover among local habitat patches separated by hundreds to thousands of metres of contiguous reef. Patterns of compositional turnover were associated with the spatial aggregation of conspecifics, particularly at scales which facilitate the post-settlement dispersal of fishes. Niche-based segregation of species along the depth gradient was found to be the primary driver of compositional turnover among both protected and exploited communities. However, spatial structuring within reefs, which was independent of variation in the environment, suggests that compositional differences among communities are also influenced by the limited post-settlement dispersal of resident fishes to adjacent areas during their ontogeny. Together, the results suggest that the diversity of reef-associated sparids is likely to depend both on an adequate diversity of suitable reef habitat and a sufficient degree of spatial connectivity to facilitate ontogenetic habitat shifts. Taxon-based descriptors of diversity do not adequately account for ecological difference among conspecifics within size-structured populations. To test whether differences in body size facilitated coexistence among sparid fishes, the number of species which coexisted at the local scale was related to variation in the size structure of communities. In communities which have been historically protected from fishing, local coexistence between a greater number of species was promoted by reduced levels of intraspecific variation in size of fishes. This suggests that, among species with similar trophic requirements, further niche segregation along a prey-size and body-size gradient is likely to mitigate the direct impacts of competition for shared food resources. Among exploited communities, size structure did not influence the number of species which coexisted at the local scale. This finding suggests that fishing-induced mortality of larger-bodied fishes is likely to remove some of the constraints to colonisation which arise from asymmetries in competitive fitness between small and large-bodied fishes. Together, these results highlight the importance of post-settlement processes and population size structure to the maintenance of reef-associated fish diversity within contiguous rocky reef habitats.
- Full Text:
- Date Issued: 2018
Towards standardised reef fish monitoring: an assessment of stereo-video techniques to sample shallow and deep reef fish assemblages
- Authors: Halse, Sarah Jane
- Date: 2017
- Subjects: Reef fishes -- South Africa -- Agulhas, Cape (Cape) , Reef fishes -- Ecology -- South Africa -- Agulhas, Cape (Cape) , Marine biodiversity -- South Africa -- Agulhas, Cape (Cape) , Biotic communities -- South Africa -- Agulhas, Cape (Cape) , Sparidae -- South Africa -- Agulhas, Cape (Cape) , Reef fishes -- Size -- South Africa -- Agulhas, Cape (Cape) , Underwater videography in wildlife monitoring -- South Africa -- Tsitsikamma , Baited remote underwater stereo-video systems (stereo-BRUVs)
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: http://hdl.handle.net/10962/4635 , vital:20702
- Description: Baited remote underwater stereo-video systems (stereo-BRUVs) were developed to determine fish abundance and size structure in a more unbiased, and relatively non-invasive manner across a broader range of depths and habitats than conventional sampling methods achieve. These characteristics make stereo-BRUVs particularly useful for research inside marine protected areas (MPAs) where researchers are required to minimize disturbance to the protected environment. Although stereo-BRUVs have been widely used around the world, they are novel to South Africa and standardised protocols for their application are required. As such, this study aimed to address i) the effect of different bait types, namely pilchard (Sardinops sagax), squid (Loligo reynaudi) and bivalves (Crassostrea gigas and Perna perna) on the observed reef fish assemblage structure and ii) the effect of artificial illumination (lighting) on the observed reef fish assemblage structure sampled with stereo-BRUVs. One key component of the stereo-BRUVs methodology is the application of bait to attract fish into the field of view. Globally, pilchard is the most often used bait type for stereo- BRUVs, however, its suitability has never been tested comparatively against other bait types for South African conditions. Significant differences in fish abundance and assemblage structure were recorded between stereo-BRUVs deployments when applying the different bait types. The bivalve baits, oyster and mussel, were typically poor when sampling the reef fish assemblages of the region. Pilchard and squid were sampled in similar communities and were able to attract the majority of the reef fish from the region, suggesting they are suitable bait types for stereo-BRUVs in the Agulhas ecoregion. However, a power analysis indicated that roughly twice the amount of samples are required to effectively sample the same abundances of Chrysoblephus laticeps (roman) and Cheilodactylidae (fingerfins) species when sampling with squid as opposed to pilchard. Both bait types can be locally sourced, however, squid is considerably more expensive than pilchard. Considering the difference in cost, together with the fact that twice as much squid bait is required, squid is not a cost-effective option relative to pilchard. As such this study recommends that pilchard is the most appropriate bait for the Agulhas ecoregion of South Africa. Due to low light levels, artificial lighting is required when conducting remote video research in the sub-photic zone. Blue light has a shorter wavelength than other colour lights and attenuates slower through water. In addition, blue lighting has no recorded effect on fish behaviour and has proven to be a successful light colour for underwater sampling. The use of lighting is unavoidable for sub-photic stereo-BRUVs sampling and is recommended for all stereo-BRUVs sampling for comparability of data from environments characterised by different light levels. A study was conducted to test the necessity of artificial lighting when sampling from environments characterised by ambient light levels adequate for stereo- BRUVs sampling, a comparative to measure the effect of light on the fish assemblage structure. Of the more commonly detected fish species (those observed in >50% of the samples), most were seen at similar abundances in samples using lighting and in those without lighting. No significant differences were observed in fish length data between samples collected with and samples collected without lighting. This suggests that standardised stereo-BRUVs sampling across photic zones on the continental shelf of South Africa can be collected without blue lighting when ambient light levels are sufficient to see the survey area.
- Full Text:
- Date Issued: 2017
- Authors: Halse, Sarah Jane
- Date: 2017
- Subjects: Reef fishes -- South Africa -- Agulhas, Cape (Cape) , Reef fishes -- Ecology -- South Africa -- Agulhas, Cape (Cape) , Marine biodiversity -- South Africa -- Agulhas, Cape (Cape) , Biotic communities -- South Africa -- Agulhas, Cape (Cape) , Sparidae -- South Africa -- Agulhas, Cape (Cape) , Reef fishes -- Size -- South Africa -- Agulhas, Cape (Cape) , Underwater videography in wildlife monitoring -- South Africa -- Tsitsikamma , Baited remote underwater stereo-video systems (stereo-BRUVs)
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: http://hdl.handle.net/10962/4635 , vital:20702
- Description: Baited remote underwater stereo-video systems (stereo-BRUVs) were developed to determine fish abundance and size structure in a more unbiased, and relatively non-invasive manner across a broader range of depths and habitats than conventional sampling methods achieve. These characteristics make stereo-BRUVs particularly useful for research inside marine protected areas (MPAs) where researchers are required to minimize disturbance to the protected environment. Although stereo-BRUVs have been widely used around the world, they are novel to South Africa and standardised protocols for their application are required. As such, this study aimed to address i) the effect of different bait types, namely pilchard (Sardinops sagax), squid (Loligo reynaudi) and bivalves (Crassostrea gigas and Perna perna) on the observed reef fish assemblage structure and ii) the effect of artificial illumination (lighting) on the observed reef fish assemblage structure sampled with stereo-BRUVs. One key component of the stereo-BRUVs methodology is the application of bait to attract fish into the field of view. Globally, pilchard is the most often used bait type for stereo- BRUVs, however, its suitability has never been tested comparatively against other bait types for South African conditions. Significant differences in fish abundance and assemblage structure were recorded between stereo-BRUVs deployments when applying the different bait types. The bivalve baits, oyster and mussel, were typically poor when sampling the reef fish assemblages of the region. Pilchard and squid were sampled in similar communities and were able to attract the majority of the reef fish from the region, suggesting they are suitable bait types for stereo-BRUVs in the Agulhas ecoregion. However, a power analysis indicated that roughly twice the amount of samples are required to effectively sample the same abundances of Chrysoblephus laticeps (roman) and Cheilodactylidae (fingerfins) species when sampling with squid as opposed to pilchard. Both bait types can be locally sourced, however, squid is considerably more expensive than pilchard. Considering the difference in cost, together with the fact that twice as much squid bait is required, squid is not a cost-effective option relative to pilchard. As such this study recommends that pilchard is the most appropriate bait for the Agulhas ecoregion of South Africa. Due to low light levels, artificial lighting is required when conducting remote video research in the sub-photic zone. Blue light has a shorter wavelength than other colour lights and attenuates slower through water. In addition, blue lighting has no recorded effect on fish behaviour and has proven to be a successful light colour for underwater sampling. The use of lighting is unavoidable for sub-photic stereo-BRUVs sampling and is recommended for all stereo-BRUVs sampling for comparability of data from environments characterised by different light levels. A study was conducted to test the necessity of artificial lighting when sampling from environments characterised by ambient light levels adequate for stereo- BRUVs sampling, a comparative to measure the effect of light on the fish assemblage structure. Of the more commonly detected fish species (those observed in >50% of the samples), most were seen at similar abundances in samples using lighting and in those without lighting. No significant differences were observed in fish length data between samples collected with and samples collected without lighting. This suggests that standardised stereo-BRUVs sampling across photic zones on the continental shelf of South Africa can be collected without blue lighting when ambient light levels are sufficient to see the survey area.
- Full Text:
- Date Issued: 2017
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