Surf-zone fish communities and their shifts over time are generally poorly understood. The aim of... more Surf-zone fish communities and their shifts over time are generally poorly understood. The aim of this study was to compare the current surf-zone fish assemblage at King's Beach, South Africa, to a similar study conducted three decades ago, before the collapse of many exploited shore fishes in the region. Beach seine nets (mesh sizes of 10 and 50 mm) were used to target juvenile and adult fishes bimonthly from February to August 2011 over the high tide around sunset. A total of 14 species were recorded in both the 30 m and 100 m seine nets. The catch in these seine nets was dominated by Pomadasys olivaceus and Liza richardsonii, and this was significantly different to three decades ago, when P. olivaceus, Sarpa salpa and Diplodus capensis dominated the catch. Important linefish species belonging to the Sparidae and Sciaenidae families were significantly smaller and less abundant in this study. Two sparids, S. salpa and Lithognathus mormyrus, which made a large contribution to the surf-zone catch three decades ago were absent during this study. Reasons for the significant shifts in the surf-zone fish community, including overexploitation of the linefish and potential habitat modification, are discussed.
... 892 Zooplankton composition, abundance and distribution in selected south and west coast estu... more ... 892 Zooplankton composition, abundance and distribution in selected south and west coast estuaries in South Africa PH Montoya-Maya1 and NA Strydom2* 1 Department ... zooplankton assemblages Page 2. Montoya-Maya and Strydom 148 of ...
There is considerable uncertainty about fish population connectivity and the trajectory for larva... more There is considerable uncertainty about fish population connectivity and the trajectory for larval transport following adult spawning migrations from temperate to subtropical oceanic waters off the south and east coast of South Africa. In particular, the role of the Agulhas Current and local wind drift has not been fully elucidated as drivers for connecting coastal and offshore waters, and the results of a previous study that modelled the movements of plastic nurdles following a spill in Durban Harbour are used to infer dispersal and transport. Planktonic fish eggs and early stage larvae are passive and behave in a similar way to nurdles, and the rapid movement of the nurdles provided a unique opportunity to track coastal dispersal trajectories and provide insights into population connectivity in some migratory fish species. Under the variable wind conditions typical along the coast, the nurdles tended to remain localised but when winds blew consistently for a few days in one direct...
Recruitment of early life stages into estuaries is an integral part of the life cycle of many mar... more Recruitment of early life stages into estuaries is an integral part of the life cycle of many marine fish species. Although estuaries are naturally environmentally dynamic, they also are subject to anthropogenic disturbances, including land use and climate change, which may affect recruitment. Rhabdosargus holubi is an endemic marine-spawning species predominantly associated with freshwater-rich estuaries which serve as nursery areas for postflexion larvae and juveniles. This study assessed the effect of environmental variables on the dynamics of recruitment of R. holubi larvae and juveniles into the Swartkops and Sundays estuaries, South Africa. Over a period of two years, fyke nets were set at each estuary mouth to monitor movement into the estuaries at each tidal phase over a 24 h cycle during two sampling sessions per season. Rhabdosargus holubi larvae recruited into estuaries primarily in summer and autumn and during the ebb tide at night, while juvenile movements showed no pattern. Salinity, turbidity and temperature were significantly important factors affecting R. holubi recruitment, with pH having no significant effect. Turbidity and salinity are affected by rainfall and freshwater abstraction which may thus influence recruitment. Rhabdosargus holubi is dependent on estuaries, so a combination of future changes in turbidity, salinity and temperature due to predicted climate change may place populations at risk.
In total 17 families comprising more than 29 species of larval teleost fishes were recorded in th... more In total 17 families comprising more than 29 species of larval teleost fishes were recorded in the Kromme Estuary during the st udy period. Dominant species included Atherina breviceps, Caffrogobius gilchristi, Diplodus sargus capensis, Gilchristella aestuaria, Glossogobius callidus, Rhabdosargus holubi and Rhabdosargus globiceps. The larval fish catch in the estuary indicated a marine dominance with a relatively high species diversity. The introduction of a regulated freshwater pulse of 2 x 10 6 m3 into the estuary from the Mpofu Dam was monitored. Pre- and post-release collections of fish larvae were made on a weekly basis for a two-month period between October and December 1998. The regulated freshwater release into the estuary resulted in no significant changes to the fish family composition, species diversity or estuarine association of the larval fish assemblage. No significant increa se in total larval fish abundance or recruitment response by estuarine-dependent species was ...
The permanently eutrophic Sundays Estuary experiences recurrent harmful algal blooms (HABs) of He... more The permanently eutrophic Sundays Estuary experiences recurrent harmful algal blooms (HABs) of Heterosigma akashiwo (Raphidophyceae). This study aimed to identify the environmental variables shaping phytoplankton community composition and succession patterns during a typical spring/summer harmful algal bloom (HAB) period. Monitoring of abiotic and phytoplankton variables was undertaken over the period of a month in 2016. Surface water salinity corresponding to mesohaline conditions (9 to 12) was a prerequisite for site selection. During the study, two HABs (>550 µg Chl a l−1) of H. akashiwo occurred, each lasting for approximately a week in duration. Analyses highlighted nutrient depletion (i.e. nitrate and phosphate concentrations) as the key constraint on bloom duration. When the density of H. akashiwo decreased, the community composition became more diverse with species belonging to Bacillariophyceae and Dinophyceae becoming more abundant; albeit to a lesser degree (<180 µg Chl a l−1). Dissolved oxygen shifted from super-saturated conditions (>14 mg l−1) during peak HAB conditions, to instances of bottom water oxygen depletion (2–4 mg l−1) during the decay phase. These findings highlight the potential severity of transforming a catchment from natural to one that is highly regulated by agricultural practices, while also emphasising the need for management intervention.
Mangroves in tropical and subtropical regions have been well documented in terms of the advantage... more Mangroves in tropical and subtropical regions have been well documented in terms of the advantages they provide and their role in structuring ichthyofaunal assemblages, but little is known about their warm temperate counterparts. The study aimed to investigate the importance of warm temperate mangroves by comparing the abundance, diversity and distribution of small fishes in mangrove and non-mangrove estuaries in warm temperate South Africa. A 50x2 m (12-mm mesh) seine net was used over three summer seasons to sample small fishes in the Gonubie, Qora, Nahoon and Xhora estuaries (the latter two being mangrove estuaries). Fish abundance and diversity showed little variation among estuaries, despite the presence of mangroves. Estuaries in warm temperate areas are not only at the edge of mangrove distribution, but also offer alternative habitats which lend similar advantages to fish survival. It appears that warm temperate ichthyofauna have not yet evolved a dependence on mangrove syste...
Dispersion processes in the ocean typically involve wind, ocean currents and waves. All these fac... more Dispersion processes in the ocean typically involve wind, ocean currents and waves. All these factors were included in an analysis to model nurdle dispersion from an accidental spill in Durban Harbour, South Africa, in October 2017. Nurdle sightings on beaches by members of the public are used as indicators of the dispersion which extended over 2000 km of the South African coastline in a period of 8 weeks. Using known oceanographic current structures, satellite imagery, wave data and surface wind drift values of between 5% and 8% of wind speed, good agreement was found between the modelled dispersion and nurdle sightings. In particular, it was found that nurdles remained in specific sections of the coast for long periods, and that sporadic wind events were required to move them into new coastal areas. Such results may also contribute to understanding the dispersal behaviours and strategies adopted by larval stages of marine organisms, particularly fishes, that have pelagic larval du...
Estuarine organisms are exposed to hypersaline conditions for prolonged periods during drought co... more Estuarine organisms are exposed to hypersaline conditions for prolonged periods during drought conditions and under severely restricted river flow resulting from freshwater abstraction and impoundments. Consequently, marine estuarine-dependent fish such as Rhabdosargus holubi may be subjected to extreme conditions, such as hypersalinity prevailing for long periods ( > 2 months). Hypersalinity may impact the energetic demands of fish due to osmoregulation leading to compromised growth. This study assessed the impact of high salinity on the growth and skeletal development of R. holubi juveniles. Skeletons of juveniles grown at different salinities in the wild and in aquaria were analysed for anomalies. The impact of hypersaline conditions on juvenile R. holubi growth was also determined in aquaria. Aquarium experiments indicated that hypersalinity of 50 did not significantly impact growth rates over two months. Overall, anomalies were rare and vertebral-related anomalies specifical...
Surf-zone fish communities and their shifts over time are generally poorly understood. The aim of... more Surf-zone fish communities and their shifts over time are generally poorly understood. The aim of this study was to compare the current surf-zone fish assemblage at King's Beach, South Africa, to a similar study conducted three decades ago, before the collapse of many exploited shore fishes in the region. Beach seine nets (mesh sizes of 10 and 50 mm) were used to target juvenile and adult fishes bimonthly from February to August 2011 over the high tide around sunset. A total of 14 species were recorded in both the 30 m and 100 m seine nets. The catch in these seine nets was dominated by Pomadasys olivaceus and Liza richardsonii, and this was significantly different to three decades ago, when P. olivaceus, Sarpa salpa and Diplodus capensis dominated the catch. Important linefish species belonging to the Sparidae and Sciaenidae families were significantly smaller and less abundant in this study. Two sparids, S. salpa and Lithognathus mormyrus, which made a large contribution to the surf-zone catch three decades ago were absent during this study. Reasons for the significant shifts in the surf-zone fish community, including overexploitation of the linefish and potential habitat modification, are discussed.
... 892 Zooplankton composition, abundance and distribution in selected south and west coast estu... more ... 892 Zooplankton composition, abundance and distribution in selected south and west coast estuaries in South Africa PH Montoya-Maya1 and NA Strydom2* 1 Department ... zooplankton assemblages Page 2. Montoya-Maya and Strydom 148 of ...
There is considerable uncertainty about fish population connectivity and the trajectory for larva... more There is considerable uncertainty about fish population connectivity and the trajectory for larval transport following adult spawning migrations from temperate to subtropical oceanic waters off the south and east coast of South Africa. In particular, the role of the Agulhas Current and local wind drift has not been fully elucidated as drivers for connecting coastal and offshore waters, and the results of a previous study that modelled the movements of plastic nurdles following a spill in Durban Harbour are used to infer dispersal and transport. Planktonic fish eggs and early stage larvae are passive and behave in a similar way to nurdles, and the rapid movement of the nurdles provided a unique opportunity to track coastal dispersal trajectories and provide insights into population connectivity in some migratory fish species. Under the variable wind conditions typical along the coast, the nurdles tended to remain localised but when winds blew consistently for a few days in one direct...
Recruitment of early life stages into estuaries is an integral part of the life cycle of many mar... more Recruitment of early life stages into estuaries is an integral part of the life cycle of many marine fish species. Although estuaries are naturally environmentally dynamic, they also are subject to anthropogenic disturbances, including land use and climate change, which may affect recruitment. Rhabdosargus holubi is an endemic marine-spawning species predominantly associated with freshwater-rich estuaries which serve as nursery areas for postflexion larvae and juveniles. This study assessed the effect of environmental variables on the dynamics of recruitment of R. holubi larvae and juveniles into the Swartkops and Sundays estuaries, South Africa. Over a period of two years, fyke nets were set at each estuary mouth to monitor movement into the estuaries at each tidal phase over a 24 h cycle during two sampling sessions per season. Rhabdosargus holubi larvae recruited into estuaries primarily in summer and autumn and during the ebb tide at night, while juvenile movements showed no pattern. Salinity, turbidity and temperature were significantly important factors affecting R. holubi recruitment, with pH having no significant effect. Turbidity and salinity are affected by rainfall and freshwater abstraction which may thus influence recruitment. Rhabdosargus holubi is dependent on estuaries, so a combination of future changes in turbidity, salinity and temperature due to predicted climate change may place populations at risk.
In total 17 families comprising more than 29 species of larval teleost fishes were recorded in th... more In total 17 families comprising more than 29 species of larval teleost fishes were recorded in the Kromme Estuary during the st udy period. Dominant species included Atherina breviceps, Caffrogobius gilchristi, Diplodus sargus capensis, Gilchristella aestuaria, Glossogobius callidus, Rhabdosargus holubi and Rhabdosargus globiceps. The larval fish catch in the estuary indicated a marine dominance with a relatively high species diversity. The introduction of a regulated freshwater pulse of 2 x 10 6 m3 into the estuary from the Mpofu Dam was monitored. Pre- and post-release collections of fish larvae were made on a weekly basis for a two-month period between October and December 1998. The regulated freshwater release into the estuary resulted in no significant changes to the fish family composition, species diversity or estuarine association of the larval fish assemblage. No significant increa se in total larval fish abundance or recruitment response by estuarine-dependent species was ...
The permanently eutrophic Sundays Estuary experiences recurrent harmful algal blooms (HABs) of He... more The permanently eutrophic Sundays Estuary experiences recurrent harmful algal blooms (HABs) of Heterosigma akashiwo (Raphidophyceae). This study aimed to identify the environmental variables shaping phytoplankton community composition and succession patterns during a typical spring/summer harmful algal bloom (HAB) period. Monitoring of abiotic and phytoplankton variables was undertaken over the period of a month in 2016. Surface water salinity corresponding to mesohaline conditions (9 to 12) was a prerequisite for site selection. During the study, two HABs (>550 µg Chl a l−1) of H. akashiwo occurred, each lasting for approximately a week in duration. Analyses highlighted nutrient depletion (i.e. nitrate and phosphate concentrations) as the key constraint on bloom duration. When the density of H. akashiwo decreased, the community composition became more diverse with species belonging to Bacillariophyceae and Dinophyceae becoming more abundant; albeit to a lesser degree (<180 µg Chl a l−1). Dissolved oxygen shifted from super-saturated conditions (>14 mg l−1) during peak HAB conditions, to instances of bottom water oxygen depletion (2–4 mg l−1) during the decay phase. These findings highlight the potential severity of transforming a catchment from natural to one that is highly regulated by agricultural practices, while also emphasising the need for management intervention.
Mangroves in tropical and subtropical regions have been well documented in terms of the advantage... more Mangroves in tropical and subtropical regions have been well documented in terms of the advantages they provide and their role in structuring ichthyofaunal assemblages, but little is known about their warm temperate counterparts. The study aimed to investigate the importance of warm temperate mangroves by comparing the abundance, diversity and distribution of small fishes in mangrove and non-mangrove estuaries in warm temperate South Africa. A 50x2 m (12-mm mesh) seine net was used over three summer seasons to sample small fishes in the Gonubie, Qora, Nahoon and Xhora estuaries (the latter two being mangrove estuaries). Fish abundance and diversity showed little variation among estuaries, despite the presence of mangroves. Estuaries in warm temperate areas are not only at the edge of mangrove distribution, but also offer alternative habitats which lend similar advantages to fish survival. It appears that warm temperate ichthyofauna have not yet evolved a dependence on mangrove syste...
Dispersion processes in the ocean typically involve wind, ocean currents and waves. All these fac... more Dispersion processes in the ocean typically involve wind, ocean currents and waves. All these factors were included in an analysis to model nurdle dispersion from an accidental spill in Durban Harbour, South Africa, in October 2017. Nurdle sightings on beaches by members of the public are used as indicators of the dispersion which extended over 2000 km of the South African coastline in a period of 8 weeks. Using known oceanographic current structures, satellite imagery, wave data and surface wind drift values of between 5% and 8% of wind speed, good agreement was found between the modelled dispersion and nurdle sightings. In particular, it was found that nurdles remained in specific sections of the coast for long periods, and that sporadic wind events were required to move them into new coastal areas. Such results may also contribute to understanding the dispersal behaviours and strategies adopted by larval stages of marine organisms, particularly fishes, that have pelagic larval du...
Estuarine organisms are exposed to hypersaline conditions for prolonged periods during drought co... more Estuarine organisms are exposed to hypersaline conditions for prolonged periods during drought conditions and under severely restricted river flow resulting from freshwater abstraction and impoundments. Consequently, marine estuarine-dependent fish such as Rhabdosargus holubi may be subjected to extreme conditions, such as hypersalinity prevailing for long periods ( > 2 months). Hypersalinity may impact the energetic demands of fish due to osmoregulation leading to compromised growth. This study assessed the impact of high salinity on the growth and skeletal development of R. holubi juveniles. Skeletons of juveniles grown at different salinities in the wild and in aquaria were analysed for anomalies. The impact of hypersaline conditions on juvenile R. holubi growth was also determined in aquaria. Aquarium experiments indicated that hypersalinity of 50 did not significantly impact growth rates over two months. Overall, anomalies were rare and vertebral-related anomalies specifical...
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