Pub Date : 2022-01-02DOI: 10.2989/1814232X.2022.2042383
Ecv Butler, A. Childs, Mks Smith, R. Foster, W. Potts
Knowledge of the location and timing of spawning events is critical for fisheries management. As is the case for many southern African fishes, the spotted grunter Pomadasys commersonnii was historically thought to reproduce in the coastal waters off KwaZulu-Natal Province (KZN), South Africa, with subsequent egg and larval dispersal towards the Eastern Cape (EC) and Western Cape (WC) provinces facilitated by the Agulhas Current. More recently, year-round residency within areas of the EC and the observation of reproductively mature individuals in certain WC estuaries has provided some support for spawning events southwest of KZN. This study reports empirical evidence of active spawning in the sheltered marine section of the Knysna estuarine bay, WC. Observations of spawning behaviour were noted during large aggregations of adult fish over shallow sandbanks in February 2017. A single male and female were collected, and their reproductive organs were macroscopically staged as ‘spawning.’ Gonad histological examination verified active spawning of the female via the presence of hydrated oocytes, migratory nucleus oocytes and post-ovulatory follicles. Future research should focus on the identification and conservation of critical spawning events and investigate the potential role of large marine-dominated estuarine systems in the life history of this and other marine estuarine-dependent species. Lastly, the results of this study contribute towards a knowledge base that challenges the traditional theory of northeastward spawning migrations as the sole life-history strategy for numerous South African fishery species.
{"title":"Spawning observations of Pomadasys commersonnii in the marine section of the Knysna estuarine bay, Western Cape, South Africa","authors":"Ecv Butler, A. Childs, Mks Smith, R. Foster, W. Potts","doi":"10.2989/1814232X.2022.2042383","DOIUrl":"https://doi.org/10.2989/1814232X.2022.2042383","url":null,"abstract":"Knowledge of the location and timing of spawning events is critical for fisheries management. As is the case for many southern African fishes, the spotted grunter Pomadasys commersonnii was historically thought to reproduce in the coastal waters off KwaZulu-Natal Province (KZN), South Africa, with subsequent egg and larval dispersal towards the Eastern Cape (EC) and Western Cape (WC) provinces facilitated by the Agulhas Current. More recently, year-round residency within areas of the EC and the observation of reproductively mature individuals in certain WC estuaries has provided some support for spawning events southwest of KZN. This study reports empirical evidence of active spawning in the sheltered marine section of the Knysna estuarine bay, WC. Observations of spawning behaviour were noted during large aggregations of adult fish over shallow sandbanks in February 2017. A single male and female were collected, and their reproductive organs were macroscopically staged as ‘spawning.’ Gonad histological examination verified active spawning of the female via the presence of hydrated oocytes, migratory nucleus oocytes and post-ovulatory follicles. Future research should focus on the identification and conservation of critical spawning events and investigate the potential role of large marine-dominated estuarine systems in the life history of this and other marine estuarine-dependent species. Lastly, the results of this study contribute towards a knowledge base that challenges the traditional theory of northeastward spawning migrations as the sole life-history strategy for numerous South African fishery species.","PeriodicalId":7719,"journal":{"name":"African Journal of Marine Science","volume":"44 1","pages":"101 - 106"},"PeriodicalIF":1.2,"publicationDate":"2022-01-02","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"42741998","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"生物学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2022-01-02DOI: 10.2989/1814232X.2022.2036234
HJ Truter, L. Atkinson, Ceo von der Meden, D. Bailey, W. Goschen, A. Lombard
Unconsolidated marine sediment habitats spatially make up the majority of global ocean seabed, yet benthic faunal patterns and their abiotic drivers remain poorly understood. Benthic research in Algoa Bay, on the south coast of South Africa, has largely focused on rocky reefs, while the dominant unconsolidated sediment habitats have been poorly studied. This study describes epibenthic assemblages associated with unconsolidated sediment in Algoa Bay, at between 30 and 100 m depth, and investigates the relationship between biotic patterns and physical drivers. Epibenthic abundance data were quantified from benthic imagery and tested against the long-term means and coefficients of variation of 12 abiotic factors, including depth, mean grain size and bottom temperature. Multivariate analyses revealed two statistically distinct epibenthic communities. This pattern was largely explained by depth, mean grain size, mean bottom temperature and mean current speed (cumulative variation of 52.49%). To a lesser extent, the long-term variability of bottom temperature, current speed and dissolved oxygen also influenced the community (cumulative variation of 34.44%). Visual classification of the substrates indicated that a mixed substrate type (i.e. sand and a low percentage of rock) significantly influences the benthic community. The findings suggest that a combination of depth and substrate type are largely responsible for the epibenthic assemblages observed.
{"title":"Characterising epibenthic diversity and physical drivers in unconsolidated marine habitats of Algoa Bay, South Africa","authors":"HJ Truter, L. Atkinson, Ceo von der Meden, D. Bailey, W. Goschen, A. Lombard","doi":"10.2989/1814232X.2022.2036234","DOIUrl":"https://doi.org/10.2989/1814232X.2022.2036234","url":null,"abstract":"Unconsolidated marine sediment habitats spatially make up the majority of global ocean seabed, yet benthic faunal patterns and their abiotic drivers remain poorly understood. Benthic research in Algoa Bay, on the south coast of South Africa, has largely focused on rocky reefs, while the dominant unconsolidated sediment habitats have been poorly studied. This study describes epibenthic assemblages associated with unconsolidated sediment in Algoa Bay, at between 30 and 100 m depth, and investigates the relationship between biotic patterns and physical drivers. Epibenthic abundance data were quantified from benthic imagery and tested against the long-term means and coefficients of variation of 12 abiotic factors, including depth, mean grain size and bottom temperature. Multivariate analyses revealed two statistically distinct epibenthic communities. This pattern was largely explained by depth, mean grain size, mean bottom temperature and mean current speed (cumulative variation of 52.49%). To a lesser extent, the long-term variability of bottom temperature, current speed and dissolved oxygen also influenced the community (cumulative variation of 34.44%). Visual classification of the substrates indicated that a mixed substrate type (i.e. sand and a low percentage of rock) significantly influences the benthic community. The findings suggest that a combination of depth and substrate type are largely responsible for the epibenthic assemblages observed.","PeriodicalId":7719,"journal":{"name":"African Journal of Marine Science","volume":"44 1","pages":"69 - 81"},"PeriodicalIF":1.2,"publicationDate":"2022-01-02","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"42827940","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"生物学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2022-01-02DOI: 10.2989/1814232X.2022.2030797
E. Steyn, J. Groeneveld, J. Santos, XI Mselegu
The brown mussel Perna perna is the dominant indigenous mussel along the east coast of South Africa, where it is harvested by recreational and subsistence fishers. High fishing pressure near urban areas led to declining abundance and consequently to the closure of some reefs to fishing in 1998. We estimated trends in mussel population dynamics at exploited and unexploited sites, along fixed transects, over a 27-year period (1993–2019). Trends in recreational fishing effort were inferred from yearly permit sales and existing catch statistics. At high levels of fishing effort, short-term trends in mussel cover and densities were inversely related to fishing effort at three of the four sites considered, while the fourth site was influenced by intermittent breaching of a nearby estuary. Mussel size was inversely related to population densities. The effects of longer-term harvesting bans were partially obscured by sharp declines in fishing effort across the entire recreational fishery. Seasonal and interannual patterns in cover and density were partially synchronised among sites, indicating environmental forcing at similar time-scales. The long-term dataset was invaluable in disentangling the relative effects of fishing and environmental factors on mussel population dynamics and should be continued as a baseline for assessing future climate-induced effects on rocky-shore biota.
{"title":"Trends in mussel cover, density and size at exploited and unexploited intertidal reefs in eastern South Africa","authors":"E. Steyn, J. Groeneveld, J. Santos, XI Mselegu","doi":"10.2989/1814232X.2022.2030797","DOIUrl":"https://doi.org/10.2989/1814232X.2022.2030797","url":null,"abstract":"The brown mussel Perna perna is the dominant indigenous mussel along the east coast of South Africa, where it is harvested by recreational and subsistence fishers. High fishing pressure near urban areas led to declining abundance and consequently to the closure of some reefs to fishing in 1998. We estimated trends in mussel population dynamics at exploited and unexploited sites, along fixed transects, over a 27-year period (1993–2019). Trends in recreational fishing effort were inferred from yearly permit sales and existing catch statistics. At high levels of fishing effort, short-term trends in mussel cover and densities were inversely related to fishing effort at three of the four sites considered, while the fourth site was influenced by intermittent breaching of a nearby estuary. Mussel size was inversely related to population densities. The effects of longer-term harvesting bans were partially obscured by sharp declines in fishing effort across the entire recreational fishery. Seasonal and interannual patterns in cover and density were partially synchronised among sites, indicating environmental forcing at similar time-scales. The long-term dataset was invaluable in disentangling the relative effects of fishing and environmental factors on mussel population dynamics and should be continued as a baseline for assessing future climate-induced effects on rocky-shore biota.","PeriodicalId":7719,"journal":{"name":"African Journal of Marine Science","volume":"44 1","pages":"21 - 33"},"PeriodicalIF":1.2,"publicationDate":"2022-01-02","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"43014262","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"生物学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2022-01-02DOI: 10.2989/1814232X.2022.2033321
L. Maritz, D. Pillay, GM Branch
Coastal diamond mining in southern Namibia involves constructing seawalls to hold the sea at bay, and seaward accretion of the shoreline by up to 800 m opens what was previously the surf zone for excavation and extraction of bedrock alluvial diamonds. This has created large coastal wetland ponds of up to 380 000 m2 as the sea overtops the seawalls or seeps into the excavated areas. The ages of these ponds span 1–38 years. We investigated physical conditions in the ponds to determine whether they can function as saline wetlands equivalent to blind estuaries. Water temperatures were 6–10 °C higher than in the sea, as expected of shallow enclosed waterbodies. Dissolved oxygen was 82–137%, peaking at midday owing to photosynthesis, and the ponds were never hypoxic. Correlated with oxygen levels, pH values spanned 7.7–8.3, and always exceeded the pH of seawater. Chlorophyll a concentrations matched or exceeded the levels in seawater, reaching 76 µg l−1. The southern and central ponds had salinities close to those of seawater, but the salinity of northern ponds exceeded 80 after ∼15 years, thus limiting their capacity to support wetland communities. Apart from this, these ponds are viable habitat that can support flora and fauna typical of saline wetlands, a habitat that is scarce along this arid coastline.
纳米比亚南部的沿海钻石开采需要建造海堤来阻挡海水，海岸线向海增加了800米，使以前的冲浪区成为挖掘和提取基岩冲积钻石的场所。这创造了多达38万平方米的大型沿海湿地池塘，因为海水超过海堤或渗入挖掘区域。这些池塘的年龄跨度为1-38年。我们调查了池塘的物理条件，以确定它们是否可以作为相当于盲河口的盐水湿地。水温比海中温度高6-10°C，与预期的浅封闭水体一致。溶解氧为82-137%，由于光合作用，溶解氧在中午达到峰值，池塘从不缺氧。与氧含量相关，pH值在7.7-8.3之间，并且总是超过海水的pH值。叶绿素a浓度达到76 μ g l−1，与海水中的水平相当或超过了海水中的水平。南部和中部池塘的盐度接近海水，但北部池塘的盐度在约15年后超过80，从而限制了其支持湿地群落的能力。除此之外，这些池塘是可行的栖息地，可以支持典型的含盐湿地的动植物，这是干旱海岸线上稀缺的栖息地。
{"title":"The ecology of coastal wetland ponds created by diamond mining in southern Namibia. 1. Physical conditions","authors":"L. Maritz, D. Pillay, GM Branch","doi":"10.2989/1814232X.2022.2033321","DOIUrl":"https://doi.org/10.2989/1814232X.2022.2033321","url":null,"abstract":"Coastal diamond mining in southern Namibia involves constructing seawalls to hold the sea at bay, and seaward accretion of the shoreline by up to 800 m opens what was previously the surf zone for excavation and extraction of bedrock alluvial diamonds. This has created large coastal wetland ponds of up to 380 000 m2 as the sea overtops the seawalls or seeps into the excavated areas. The ages of these ponds span 1–38 years. We investigated physical conditions in the ponds to determine whether they can function as saline wetlands equivalent to blind estuaries. Water temperatures were 6–10 °C higher than in the sea, as expected of shallow enclosed waterbodies. Dissolved oxygen was 82–137%, peaking at midday owing to photosynthesis, and the ponds were never hypoxic. Correlated with oxygen levels, pH values spanned 7.7–8.3, and always exceeded the pH of seawater. Chlorophyll a concentrations matched or exceeded the levels in seawater, reaching 76 µg l−1. The southern and central ponds had salinities close to those of seawater, but the salinity of northern ponds exceeded 80 after ∼15 years, thus limiting their capacity to support wetland communities. Apart from this, these ponds are viable habitat that can support flora and fauna typical of saline wetlands, a habitat that is scarce along this arid coastline.","PeriodicalId":7719,"journal":{"name":"African Journal of Marine Science","volume":"44 1","pages":"49 - 60"},"PeriodicalIF":1.2,"publicationDate":"2022-01-02","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"48335481","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"生物学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2022-01-02DOI: 10.2989/1814232X.2022.2035818
L. Maritz, D. Pillay, GM Branch
Diamond mining on the southern Namibian coastline has created multiple large coastal ponds of up to 380 000 m2 adjacent to the coastline, as the sea overtops erected seawalls or seeps into excavated areas. These ponds span ages of 1–38 years. We investigated whether the ponds offer an environment for the establishment, growth and dispersal of saltmarsh vegetation along the coast, which is otherwise devoid of natural wetlands apart from at the Orange River estuary and Lüderitz Bay. Most ponds supported saltmarshes, but they comprised only a single species, Salicornia natalensis. The abundance of this succulent, mat-forming, salt-tolerant plant was greatest around old ponds, but its health decreased with increasing age and hence salinity of the ponds. The orientation of saltmarshes around the ponds was correlated with prevailing wind direction, suggesting that wind determines dispersal of this plant along the coast. However, any saltmarsh communities that have developed will be disturbed by possible future mining activities. In addition, once mining ends, the saltmarshes will become stressed owing to rising salinities as ponds age. Nevertheless, the ponds are capable of supporting saltmarshes for up to 15 years, and new ponds will be created as mining progresses; this offers an ongoing opportunity for the ponds to serve as ‘stepping stones’ in the dispersal and establishment of S. natalensis along the coast.
{"title":"The ecology of coastal wetland ponds created by diamond mining in southern Namibia. 2. Saltmarsh vegetation","authors":"L. Maritz, D. Pillay, GM Branch","doi":"10.2989/1814232X.2022.2035818","DOIUrl":"https://doi.org/10.2989/1814232X.2022.2035818","url":null,"abstract":"Diamond mining on the southern Namibian coastline has created multiple large coastal ponds of up to 380 000 m2 adjacent to the coastline, as the sea overtops erected seawalls or seeps into excavated areas. These ponds span ages of 1–38 years. We investigated whether the ponds offer an environment for the establishment, growth and dispersal of saltmarsh vegetation along the coast, which is otherwise devoid of natural wetlands apart from at the Orange River estuary and Lüderitz Bay. Most ponds supported saltmarshes, but they comprised only a single species, Salicornia natalensis. The abundance of this succulent, mat-forming, salt-tolerant plant was greatest around old ponds, but its health decreased with increasing age and hence salinity of the ponds. The orientation of saltmarshes around the ponds was correlated with prevailing wind direction, suggesting that wind determines dispersal of this plant along the coast. However, any saltmarsh communities that have developed will be disturbed by possible future mining activities. In addition, once mining ends, the saltmarshes will become stressed owing to rising salinities as ponds age. Nevertheless, the ponds are capable of supporting saltmarshes for up to 15 years, and new ponds will be created as mining progresses; this offers an ongoing opportunity for the ponds to serve as ‘stepping stones’ in the dispersal and establishment of S. natalensis along the coast.","PeriodicalId":7719,"journal":{"name":"African Journal of Marine Science","volume":"44 1","pages":"61 - 68"},"PeriodicalIF":1.2,"publicationDate":"2022-01-02","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"45787585","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"生物学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2022-01-02DOI: 10.2989/1814232X.2022.2030796
O. B. A. Hadj Hamida, N. B. Hadj Hamida, H. Chaouch, B. Nafkha, N. Ben Ali, D. Abidi, H. Missaoui
The blue swimming crab Portunus segnis (Forskål, 1775) (family Portunidae) is one of the earliest Lessepsian invaders of the Mediterranean Sea and has been recorded for several decades in various Mediterranean areas. However, its presence on the southeastern Tunisian coast is very recent. This study describes the reproductive biology of the species in the Gulf of Gabes, including sex ratio, ovarian maturation, size at sexual maturity, spawning season and fecundity. Samples for biological investigation were collected from the commercial catches of trawlers and artisanal fishing units, from January to December in 2018. A total of 2 762 specimens, ranging from 19 to 158 mm carapace width (CW) and 0.638 to 356.109 g body weight (BW), were analysed. Females outnumbered males by 1.3 to 1 (1 581 vs 1 181 individuals). Sexual maturity was classified into five stages for females and three stages for males, based on visual observation of the colour and shape of the gonads. Spawning occurred three times during the year, with the first peak in May, the second in July, and the third—the most intensive peak—in October–November. Size at sexual maturity was 93.1 mm CW for males, and 93.6 mm CW for females. Females carried 142 242–2 640 080 eggs on their abdomen, with a positive linear relationship between fecundity and CW. The data presented in this study should be useful not only to detect variations in the reproductive cycle of P. segnis between regions but also to ensure sustainable management of the new fishery for the species in Tunisian waters.
{"title":"Reproductive biology of the blue swimming crab Portunus segnis (Forskål, 1775) (Brachyura: Portunidae) in the Gulf of Gabes (southeastern Tunisia, central Mediterranean Sea)","authors":"O. B. A. Hadj Hamida, N. B. Hadj Hamida, H. Chaouch, B. Nafkha, N. Ben Ali, D. Abidi, H. Missaoui","doi":"10.2989/1814232X.2022.2030796","DOIUrl":"https://doi.org/10.2989/1814232X.2022.2030796","url":null,"abstract":"The blue swimming crab Portunus segnis (Forskål, 1775) (family Portunidae) is one of the earliest Lessepsian invaders of the Mediterranean Sea and has been recorded for several decades in various Mediterranean areas. However, its presence on the southeastern Tunisian coast is very recent. This study describes the reproductive biology of the species in the Gulf of Gabes, including sex ratio, ovarian maturation, size at sexual maturity, spawning season and fecundity. Samples for biological investigation were collected from the commercial catches of trawlers and artisanal fishing units, from January to December in 2018. A total of 2 762 specimens, ranging from 19 to 158 mm carapace width (CW) and 0.638 to 356.109 g body weight (BW), were analysed. Females outnumbered males by 1.3 to 1 (1 581 vs 1 181 individuals). Sexual maturity was classified into five stages for females and three stages for males, based on visual observation of the colour and shape of the gonads. Spawning occurred three times during the year, with the first peak in May, the second in July, and the third—the most intensive peak—in October–November. Size at sexual maturity was 93.1 mm CW for males, and 93.6 mm CW for females. Females carried 142 242–2 640 080 eggs on their abdomen, with a positive linear relationship between fecundity and CW. The data presented in this study should be useful not only to detect variations in the reproductive cycle of P. segnis between regions but also to ensure sustainable management of the new fishery for the species in Tunisian waters.","PeriodicalId":7719,"journal":{"name":"African Journal of Marine Science","volume":"44 1","pages":"11 - 20"},"PeriodicalIF":1.2,"publicationDate":"2022-01-02","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"47420953","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"生物学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2021-10-02DOI: 10.2989/1814232X.2021.1979096
Candice Jansen van Rensburg, A. Robbins, C. Griffiths
The only previous study on the macrobiota of South African boulder shores showed this assemblage to be more diverse than on nearby rocky platforms, but the drivers of that difference remain unresolved. We recorded water and air temperatures under intertidal boulders and on adjacent exposed rocky platforms, at both the upper- and lower-shore level, at Kommetjie, South Africa, over 2-week tidal cycles, repeating this during summer and winter. The macrobiota in each location was also sampled during diurnal low tides. The most-extreme temperatures, and greatest ranges, occurred during summer on upper-shore open rock (13.9–37.7 °C, range difference of 23.8 °C), while under upper-shore boulders the maximum temperature was lower at 28.9 °C and the range spanned only 15.5 °C. In winter, maximum temperatures and ranges were lower at all locations. Maximum temperatures did not coincide with spring tides, but occurred several days later, when low tides happened during the heat of the day. Closer analysis of daily fluctuations revealed influence of weather on temperatures attained and showed that durations of elevated temperatures were much shorter in the lower shore. The upper shore exhibited low biodiversity, supporting only 9 species, with 8 found only under boulders during diurnal low tide; the lower shore was more diverse, with 11 species on open rock and 29 under boulders. Biomass and abundance were also higher under boulders at both levels, especially on the upper shore. More-benign under-boulder conditions thus facilitate more abundant and diverse biota sheltering there than occurs on adjacent exposed rocks, particularly on the upper shore.
{"title":"Temperature cycles beneath, and adjacent to, intertidal boulders and associated differences in biotic composition","authors":"Candice Jansen van Rensburg, A. Robbins, C. Griffiths","doi":"10.2989/1814232X.2021.1979096","DOIUrl":"https://doi.org/10.2989/1814232X.2021.1979096","url":null,"abstract":"The only previous study on the macrobiota of South African boulder shores showed this assemblage to be more diverse than on nearby rocky platforms, but the drivers of that difference remain unresolved. We recorded water and air temperatures under intertidal boulders and on adjacent exposed rocky platforms, at both the upper- and lower-shore level, at Kommetjie, South Africa, over 2-week tidal cycles, repeating this during summer and winter. The macrobiota in each location was also sampled during diurnal low tides. The most-extreme temperatures, and greatest ranges, occurred during summer on upper-shore open rock (13.9–37.7 °C, range difference of 23.8 °C), while under upper-shore boulders the maximum temperature was lower at 28.9 °C and the range spanned only 15.5 °C. In winter, maximum temperatures and ranges were lower at all locations. Maximum temperatures did not coincide with spring tides, but occurred several days later, when low tides happened during the heat of the day. Closer analysis of daily fluctuations revealed influence of weather on temperatures attained and showed that durations of elevated temperatures were much shorter in the lower shore. The upper shore exhibited low biodiversity, supporting only 9 species, with 8 found only under boulders during diurnal low tide; the lower shore was more diverse, with 11 species on open rock and 29 under boulders. Biomass and abundance were also higher under boulders at both levels, especially on the upper shore. More-benign under-boulder conditions thus facilitate more abundant and diverse biota sheltering there than occurs on adjacent exposed rocks, particularly on the upper shore.","PeriodicalId":7719,"journal":{"name":"African Journal of Marine Science","volume":"43 1","pages":"435 - 441"},"PeriodicalIF":1.2,"publicationDate":"2021-10-02","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"45388396","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"生物学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2021-10-02DOI: 10.2989/1814232X.2021.1995493
J. Mbega, O. Sadio, J. Liwouwou, Jean Hervé Mve Beh, I. Bamba, A. Eleng Spanian, M-L Yedi, F. Cardiec, F. Le Loc’h
The lesser African threadfin Galeoides decadactylus (family Polynemidae) is one of the most captured marine fish species in Central Africa. This study examines aspects of the reproductive biology of G. decadactylus in the Libreville area of Gabon. Fish caught with encircling gillnets and bottom gillnets were collected from May 2017 to May 2018 from artisanal fishermen. A total of 776 specimens were studied, comprising 401 females (14–36 cm total length [TL]), 347 males (13–28 cm TL), and 28 individuals of indeterminate sex (12–16 cm TL). Monthly monitoring of gonadosomatic ratio, condition factor and sexual maturity stages revealed that G. decadactylus reproduces continuously but has two slight peak periods: one in the long rainy season and the other in the short rainy season. The species is protandrous, with sizes at first sexual maturity of 17.7 cm TL for males and 18.7 cm for females. Mature individuals largely dominated the catches of small-scale fishers in Gabon. Mean absolute fecundity of females was 179 447 (SD 107 240) oocytes, and mean relative fecundity was 848 (SD 323) oocytes g–1. This study provides fisheries managers with crucial knowledge, such as size at sexual maturity, that could be used as a basis for sustainable management of G. decadactylus stocks in Gabon using minimum size limits.
{"title":"Reproductive biology of the lesser African threadfin Galeoides decadactylus in Gabon, Gulf of Guinea","authors":"J. Mbega, O. Sadio, J. Liwouwou, Jean Hervé Mve Beh, I. Bamba, A. Eleng Spanian, M-L Yedi, F. Cardiec, F. Le Loc’h","doi":"10.2989/1814232X.2021.1995493","DOIUrl":"https://doi.org/10.2989/1814232X.2021.1995493","url":null,"abstract":"The lesser African threadfin Galeoides decadactylus (family Polynemidae) is one of the most captured marine fish species in Central Africa. This study examines aspects of the reproductive biology of G. decadactylus in the Libreville area of Gabon. Fish caught with encircling gillnets and bottom gillnets were collected from May 2017 to May 2018 from artisanal fishermen. A total of 776 specimens were studied, comprising 401 females (14–36 cm total length [TL]), 347 males (13–28 cm TL), and 28 individuals of indeterminate sex (12–16 cm TL). Monthly monitoring of gonadosomatic ratio, condition factor and sexual maturity stages revealed that G. decadactylus reproduces continuously but has two slight peak periods: one in the long rainy season and the other in the short rainy season. The species is protandrous, with sizes at first sexual maturity of 17.7 cm TL for males and 18.7 cm for females. Mature individuals largely dominated the catches of small-scale fishers in Gabon. Mean absolute fecundity of females was 179 447 (SD 107 240) oocytes, and mean relative fecundity was 848 (SD 323) oocytes g–1. This study provides fisheries managers with crucial knowledge, such as size at sexual maturity, that could be used as a basis for sustainable management of G. decadactylus stocks in Gabon using minimum size limits.","PeriodicalId":7719,"journal":{"name":"African Journal of Marine Science","volume":"43 1","pages":"499 - 509"},"PeriodicalIF":1.2,"publicationDate":"2021-10-02","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"45500584","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"生物学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2021-10-02DOI: 10.2989/1814232X.2021.1974557
DC van den Berg, K. Sethebe, G. Maneveldt
Following a long-term herbivore-exclusion study (2003-2008) in the mid-eulittoral zone at Kalk Bay, Cape Town, South Africa, the post-herbivore-exclusion effect (12 years later) of the limpet Cymbula oculus, the dominant herbivore in the community, was examined after it had recolonised the area. Despite the limpet having recruited into all plots (previous exclusion plots and control plots), the species richness, percentage cover of sessile organisms, invertebrate densities and invertebrate biomass were all significantly higher in previous exclusion plots compared with in control plots. Cymbula oculus density was higher in previous exclusion plots (11.11 [SD 12.54] ind. m−2) than in control plots (9.26 [SD 7.97] ind. m−2) (p = 0.038); however, its biomass was similar between the previous exclusion plots and control plots (366.39 [SD 421.63] vs 367.96 [SD 285.44] g m−2, p = 0.179), indicating that limpets outside exclusion plots were generally larger. This was likely because of younger C. oculus individuals, heightened interspecific competition, or insufficient space in previous exclusion plots. The observed differences might be attributed to a combination of factors, including the persistence of species that have achieved a size refuge from grazing and bulldozing by C. oculus, as well as the establishment of invasive mussels Mytilus galloprovincialis which offer secondary substrates and microhabitats for the establishment of macroalgae and other invertebrates. The recruitment of M. galloprovincialis into the previous exclusion plots is likely to be the cause of the alternative community state observed. Unless some extreme disturbance event occurs, the community from which C. oculus was previously excluded is unlikely to revert to its pre-C. oculus exclusion state.
{"title":"Determining the post-herbivore-exclusion effect on an intertidal community by the recovery response of a known dominant herbivore","authors":"DC van den Berg, K. Sethebe, G. Maneveldt","doi":"10.2989/1814232X.2021.1974557","DOIUrl":"https://doi.org/10.2989/1814232X.2021.1974557","url":null,"abstract":"Following a long-term herbivore-exclusion study (2003-2008) in the mid-eulittoral zone at Kalk Bay, Cape Town, South Africa, the post-herbivore-exclusion effect (12 years later) of the limpet Cymbula oculus, the dominant herbivore in the community, was examined after it had recolonised the area. Despite the limpet having recruited into all plots (previous exclusion plots and control plots), the species richness, percentage cover of sessile organisms, invertebrate densities and invertebrate biomass were all significantly higher in previous exclusion plots compared with in control plots. Cymbula oculus density was higher in previous exclusion plots (11.11 [SD 12.54] ind. m−2) than in control plots (9.26 [SD 7.97] ind. m−2) (p = 0.038); however, its biomass was similar between the previous exclusion plots and control plots (366.39 [SD 421.63] vs 367.96 [SD 285.44] g m−2, p = 0.179), indicating that limpets outside exclusion plots were generally larger. This was likely because of younger C. oculus individuals, heightened interspecific competition, or insufficient space in previous exclusion plots. The observed differences might be attributed to a combination of factors, including the persistence of species that have achieved a size refuge from grazing and bulldozing by C. oculus, as well as the establishment of invasive mussels Mytilus galloprovincialis which offer secondary substrates and microhabitats for the establishment of macroalgae and other invertebrates. The recruitment of M. galloprovincialis into the previous exclusion plots is likely to be the cause of the alternative community state observed. Unless some extreme disturbance event occurs, the community from which C. oculus was previously excluded is unlikely to revert to its pre-C. oculus exclusion state.","PeriodicalId":7719,"journal":{"name":"African Journal of Marine Science","volume":"43 1","pages":"423 - 434"},"PeriodicalIF":1.2,"publicationDate":"2021-10-02","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"46702351","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"生物学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2021-10-02DOI: 10.2989/1814232X.2021.1989488
TG Loureiro, K. Peters, T. Robinson
Biological invasions pose a major threat to biodiversity and significant investment is required to prevent the introduction of alien species, contain introduced populations and mitigate associated impacts. The implementation of standardised long-term monitoring programmes enables the early identification of new alien species, the tracking of spread, the assessment of the effectiveness of management interventions, and an understanding of temporal and spatial trends. Globally, fouling species are known to cause economic losses through accumulation on vessel hulls and port infrastructure and via the disruption of aquaculture activities. Additionally, fouling taxa can have ecological impacts in recipient systems, most often driven by their dominance in biological interactions. In South Africa, this group accounts for the majority of marine alien taxa. Accordingly, this study tested an approach for monitoring fouling biota using open and caged PVC settlement plates deployed in marinas. After 16 weeks, plates were removed, biota identified, and relative abundance captured as percentage cover. This method proved to be effective, logistically simple and cost-efficient. Twenty-four taxa were recorded, of which 58% were invasive species. It is recommended that this monitoring approach be implemented around the South African coast as a first step to providing key information to inform management and research.
{"title":"Dropping plates to pick up aliens: towards a standardised approach for monitoring alien fouling species","authors":"TG Loureiro, K. Peters, T. Robinson","doi":"10.2989/1814232X.2021.1989488","DOIUrl":"https://doi.org/10.2989/1814232X.2021.1989488","url":null,"abstract":"Biological invasions pose a major threat to biodiversity and significant investment is required to prevent the introduction of alien species, contain introduced populations and mitigate associated impacts. The implementation of standardised long-term monitoring programmes enables the early identification of new alien species, the tracking of spread, the assessment of the effectiveness of management interventions, and an understanding of temporal and spatial trends. Globally, fouling species are known to cause economic losses through accumulation on vessel hulls and port infrastructure and via the disruption of aquaculture activities. Additionally, fouling taxa can have ecological impacts in recipient systems, most often driven by their dominance in biological interactions. In South Africa, this group accounts for the majority of marine alien taxa. Accordingly, this study tested an approach for monitoring fouling biota using open and caged PVC settlement plates deployed in marinas. After 16 weeks, plates were removed, biota identified, and relative abundance captured as percentage cover. This method proved to be effective, logistically simple and cost-efficient. Twenty-four taxa were recorded, of which 58% were invasive species. It is recommended that this monitoring approach be implemented around the South African coast as a first step to providing key information to inform management and research.","PeriodicalId":7719,"journal":{"name":"African Journal of Marine Science","volume":"43 1","pages":"483 - 497"},"PeriodicalIF":1.2,"publicationDate":"2021-10-02","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"47951935","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"生物学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
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