Pub Date : 2021-04-03DOI: 10.2989/1814232X.2021.1916591
M. Lipiński, MA Mmethi, D. Yemane, J. Githaiga-Mwicigi, W. Sauer
Temporal trends in the size composition (length frequency) and length–weight (L–W) relationship of chokka squid Loligo reynaudii on the south coast of South Africa were assessed over periods spanning 22 years: length frequencies from 1996 to 2017 (with 15 years represented); and L–W relationships over 9 years between 1994 and 2016. To allow for comparison, identical data selection and processing was adopted for all years considered (i.e. identical period of 60 days in spring–summer; the same depths and areas; chokka with empty stomachs; and squid of the same maturity stage). Although there were no significant long-term temporal trends in the mean lengths, there was a significant short-term drop in the mean lengths over the years 2014–2017 (especially in females), which could not be attributed with certainty to any cause. A tentative explanation is that this drop might be linked to the introduction of an additional closed season in these years. The estimated parameters of the L–W relationship also revealed no trend over the years considered. Investigation of the caecum colour, which indicates the state of starvation (white: 8 h on average after food ingestion; yellow: 6 to 7 h after food ingestion), showed significantly more starving males than starving females. Starvation of males on the spawning grounds might be associated with the spawning behaviour of chokka.
{"title":"Are there long-term temporal trends of size composition and the length– weight relationship? Results for chokka squid Loligo reynaudii during the peak spawning season off the south coast of South Africa","authors":"M. Lipiński, MA Mmethi, D. Yemane, J. Githaiga-Mwicigi, W. Sauer","doi":"10.2989/1814232X.2021.1916591","DOIUrl":"https://doi.org/10.2989/1814232X.2021.1916591","url":null,"abstract":"Temporal trends in the size composition (length frequency) and length–weight (L–W) relationship of chokka squid Loligo reynaudii on the south coast of South Africa were assessed over periods spanning 22 years: length frequencies from 1996 to 2017 (with 15 years represented); and L–W relationships over 9 years between 1994 and 2016. To allow for comparison, identical data selection and processing was adopted for all years considered (i.e. identical period of 60 days in spring–summer; the same depths and areas; chokka with empty stomachs; and squid of the same maturity stage). Although there were no significant long-term temporal trends in the mean lengths, there was a significant short-term drop in the mean lengths over the years 2014–2017 (especially in females), which could not be attributed with certainty to any cause. A tentative explanation is that this drop might be linked to the introduction of an additional closed season in these years. The estimated parameters of the L–W relationship also revealed no trend over the years considered. Investigation of the caecum colour, which indicates the state of starvation (white: 8 h on average after food ingestion; yellow: 6 to 7 h after food ingestion), showed significantly more starving males than starving females. Starvation of males on the spawning grounds might be associated with the spawning behaviour of chokka.","PeriodicalId":7719,"journal":{"name":"African Journal of Marine Science","volume":null,"pages":null},"PeriodicalIF":1.2,"publicationDate":"2021-04-03","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"44817979","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 : 2021-04-03DOI: 10.2989/1814232X.2021.1897039
M. Nunes, D. Lemley, G. Matcher, J. Adams
The occurrence of harmful algal blooms (HABs) is increasing in frequency and intensity in South African estuaries because of eutrophication. This study used DNA metabarcoding to identify the benthic diatom community structure in the eutrophic Sundays Estuary. The recurrent HAB phytoplankton species Heterosigma akashiwo was recorded at bloom concentrations (∼100 µg Chl a l−1) in the brackish, vertically stratified middle reaches of the estuary. In contrast, microphytobenthos biomass increased towards the upper reaches in response to elevated availability of total oxidised nitrogen (NOx) and increased sediment stability. A total of 76 diatom species were identified using the molecular technique. Seven known nutrient-tolerant diatom species indicate a preference for either ammonium or NOx enrichment. Benthic diatom community diversity (H′ < 1) and evenness (J′ < 0.25) were low in the brackish middle reaches of the estuary, where the HABs decreased available light. Heterosigma akashiwo has been found in previous studies to suppress the growth of co-occurring taxa. Future research efforts should focus on verifying this relationship over seasonal timescales given the dominance of H. akashiwo in spring/summer. As the persistently nutrient-enriched state and stable flow conditions of the Sundays Estuary is the key driver of change, management efforts should be geared towards re-establishing the estuary’s natural flow variations and to measures that can mitigate nutrient pollution.
由于富营养化，南非河口有害藻华的发生频率和强度都在增加。本研究利用DNA代谢编码技术对富营养化Sundays河口的底栖硅藻群落结构进行了鉴定。在河口半咸水、垂直分层的中游地区，记录到赤潮浓度为（～100µg Chl a l−1）的赤潮浮游植物物种Heterosigma akashiwo。相比之下，由于总氧化氮（NOx）的可用性提高和沉积物稳定性增加，向上游方向的微型底栖生物生物生物量增加。利用分子技术共鉴定出76种硅藻。七种已知的耐营养硅藻物种表明它们更喜欢富集铵或NOx。河口半咸水中游底栖硅藻群落多样性（H′<1）和均匀度（J′<0.025）较低，赤潮降低了可用光。在以前的研究中已经发现赤尾异西格玛可以抑制共存类群的生长。鉴于H.akashiwo在春季/夏季占主导地位，未来的研究工作应侧重于在季节性时间尺度上验证这种关系。由于Sundays河口持续的营养丰富状态和稳定的水流条件是变化的关键驱动力，管理工作应致力于重建河口的自然流量变化，并采取措施减轻营养污染。
{"title":"The influence of estuary eutrophication on the benthic diatom community: a molecular approach","authors":"M. Nunes, D. Lemley, G. Matcher, J. Adams","doi":"10.2989/1814232X.2021.1897039","DOIUrl":"https://doi.org/10.2989/1814232X.2021.1897039","url":null,"abstract":"The occurrence of harmful algal blooms (HABs) is increasing in frequency and intensity in South African estuaries because of eutrophication. This study used DNA metabarcoding to identify the benthic diatom community structure in the eutrophic Sundays Estuary. The recurrent HAB phytoplankton species Heterosigma akashiwo was recorded at bloom concentrations (∼100 µg Chl a l−1) in the brackish, vertically stratified middle reaches of the estuary. In contrast, microphytobenthos biomass increased towards the upper reaches in response to elevated availability of total oxidised nitrogen (NOx) and increased sediment stability. A total of 76 diatom species were identified using the molecular technique. Seven known nutrient-tolerant diatom species indicate a preference for either ammonium or NOx enrichment. Benthic diatom community diversity (H′ < 1) and evenness (J′ < 0.25) were low in the brackish middle reaches of the estuary, where the HABs decreased available light. Heterosigma akashiwo has been found in previous studies to suppress the growth of co-occurring taxa. Future research efforts should focus on verifying this relationship over seasonal timescales given the dominance of H. akashiwo in spring/summer. As the persistently nutrient-enriched state and stable flow conditions of the Sundays Estuary is the key driver of change, management efforts should be geared towards re-establishing the estuary’s natural flow variations and to measures that can mitigate nutrient pollution.","PeriodicalId":7719,"journal":{"name":"African Journal of Marine Science","volume":null,"pages":null},"PeriodicalIF":1.2,"publicationDate":"2021-04-03","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"43570471","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 : 2021-04-03DOI: 10.2989/1814232X.2021.1922501
C. Allison, C. Muller, A. Childs, W. Froneman, L. Bailey, W. Potts
Recent investigations suggest that global warming is likely to alter temperature regimes along the southeastern coastline of South Africa through the increased frequency of upwelling events. Identifying thermal thresholds is fundamental in predicting the response of marine ectotherms to rapidly changing ocean temperatures. The aim of this study was to determine the thermal tolerance of the endemic sparid Boopsoidea inornata. To achieve this, 20 wild fish were captured from near Noordhoek in Gqeberha (Port Elizabeth). The fish were exposed to laboratory-simulated upwelling and heat-plume conditions until sublethal endpoints were identified to estimate the critical lower (CTmin) and critical upper (CTmax) thermal limits, respectively. During the simulated cooling or heating events, the opercular beat (OB) rates were recorded, their sublethal endpoints (loss of equilibrium) were identified, and their CTmin and CTmax were estimated. Breakpoint analyses of the OB rates identified the lower and upper thermal stress limits to occur at an average of 9 °C and 25 °C, respectively. The CTmin was estimated to be 7.8 °C and the CTmax 30 °C. When compared with in situ temperatures, these limits suggest that B. inornata is susceptible to small reductions in the minimum temperature. Given that the frequency and magnitude of upwelling events are expected to increase in response to global warming, this may have significant consequences for this and other sympatric, resident species.
{"title":"When cooling is worse than warming: investigations into the thermal tolerance of an endemic reef fish, Boopsoidea inornata","authors":"C. Allison, C. Muller, A. Childs, W. Froneman, L. Bailey, W. Potts","doi":"10.2989/1814232X.2021.1922501","DOIUrl":"https://doi.org/10.2989/1814232X.2021.1922501","url":null,"abstract":"Recent investigations suggest that global warming is likely to alter temperature regimes along the southeastern coastline of South Africa through the increased frequency of upwelling events. Identifying thermal thresholds is fundamental in predicting the response of marine ectotherms to rapidly changing ocean temperatures. The aim of this study was to determine the thermal tolerance of the endemic sparid Boopsoidea inornata. To achieve this, 20 wild fish were captured from near Noordhoek in Gqeberha (Port Elizabeth). The fish were exposed to laboratory-simulated upwelling and heat-plume conditions until sublethal endpoints were identified to estimate the critical lower (CTmin) and critical upper (CTmax) thermal limits, respectively. During the simulated cooling or heating events, the opercular beat (OB) rates were recorded, their sublethal endpoints (loss of equilibrium) were identified, and their CTmin and CTmax were estimated. Breakpoint analyses of the OB rates identified the lower and upper thermal stress limits to occur at an average of 9 °C and 25 °C, respectively. The CTmin was estimated to be 7.8 °C and the CTmax 30 °C. When compared with in situ temperatures, these limits suggest that B. inornata is susceptible to small reductions in the minimum temperature. Given that the frequency and magnitude of upwelling events are expected to increase in response to global warming, this may have significant consequences for this and other sympatric, resident species.","PeriodicalId":7719,"journal":{"name":"African Journal of Marine Science","volume":null,"pages":null},"PeriodicalIF":1.2,"publicationDate":"2021-04-03","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"48849944","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 : 2021-04-03DOI: 10.2989/1814232X.2021.1906318
G. Jordaan, B. Mann, R. Daly, S. Dunlop, P. Cowley
Information on the movement ecology of endangered species is critical for the implementation of effective conservation measures. This study made use of a long-term dart tagging dataset to reveal the movement patterns and growth rates of two size classes of the Critically Endangered whitespotted wedgefish Rhynchobatus djiddensis within its southern African distribution, which can have important implications for fisheries management. A total of 4 768 individuals were tagged with 340 recaptures recorded, ranging from 1 to 2 639 days (7.2 years) at liberty. Most of the tag releases and recaptures occurred within the KwaZulu-Natal central region in South Africa, with catches increasing significantly during summer (October to March). Most recaptures (43%) were recorded within 5 km of the tagging (release) site. Tagged adults recorded significantly greater distances moved than juveniles (p < 0.002) but there was no significant difference between juveniles or adults in terms of their direction of movement (p > 0.30). A Francis growth model showed that smaller individuals had a substantially faster growth rate (198.69 [SE 21.75] mm year−1) compared with larger individuals (57.41 [SE 27.83] mm year−1) confirming that the species is relatively slow-growing. Ultimately, this study identified important knowledge gaps in the broadscale movement patterns of R. djiddensis and provided new information on the growth rate of this Critically Endangered species. Filling in these knowledge gaps will aid in conservation measures for two important size classes of the R. djiddensis population as the species faces increasing targeted fishing pressure.
关于濒危物种运动生态学的信息对于实施有效的保护措施至关重要。本研究利用长期飞镖标记数据，揭示了非洲南部分布的极度濒危白斑楔鱼(Rhynchobatus djiddensis)的两种大小类别的运动模式和增长率，这对渔业管理具有重要意义。共标记了4768只个体，记录了340次重新捕获，自由时间从1天到2639天(7.2年)不等。大多数释放标签和重新捕获发生在南非夸祖鲁-纳塔尔省中部地区，在夏季(10月至3月)捕获量显著增加。大多数捕获(43%)记录在标记(释放)地点5公里范围内。标记成虫的移动距离明显大于幼虫(p < 0.002)，但幼虫和成虫在移动方向上无显著差异(p < 0.30)。Francis生长模型表明，小个体的生长速率(198.69 [SE 21.75] mm /年)明显快于大个体(57.41 [SE 27.83] mm /年)，表明该物种生长相对缓慢。最终，本研究确定了吉毛鼠大规模迁移模式的重要知识空白，并为这一极危物种的生长速度提供了新的信息。填补这些知识空白将有助于采取保护措施，以保护两种重要大小类别的吉齿鼠种群，因为该物种面临越来越大的目标捕捞压力。
{"title":"Movement patterns and growth rate of the whitespotted wedgefish Rhynchobatus djiddensis in southern Africa based on tag-recapture data","authors":"G. Jordaan, B. Mann, R. Daly, S. Dunlop, P. Cowley","doi":"10.2989/1814232X.2021.1906318","DOIUrl":"https://doi.org/10.2989/1814232X.2021.1906318","url":null,"abstract":"Information on the movement ecology of endangered species is critical for the implementation of effective conservation measures. This study made use of a long-term dart tagging dataset to reveal the movement patterns and growth rates of two size classes of the Critically Endangered whitespotted wedgefish Rhynchobatus djiddensis within its southern African distribution, which can have important implications for fisheries management. A total of 4 768 individuals were tagged with 340 recaptures recorded, ranging from 1 to 2 639 days (7.2 years) at liberty. Most of the tag releases and recaptures occurred within the KwaZulu-Natal central region in South Africa, with catches increasing significantly during summer (October to March). Most recaptures (43%) were recorded within 5 km of the tagging (release) site. Tagged adults recorded significantly greater distances moved than juveniles (p < 0.002) but there was no significant difference between juveniles or adults in terms of their direction of movement (p > 0.30). A Francis growth model showed that smaller individuals had a substantially faster growth rate (198.69 [SE 21.75] mm year−1) compared with larger individuals (57.41 [SE 27.83] mm year−1) confirming that the species is relatively slow-growing. Ultimately, this study identified important knowledge gaps in the broadscale movement patterns of R. djiddensis and provided new information on the growth rate of this Critically Endangered species. Filling in these knowledge gaps will aid in conservation measures for two important size classes of the R. djiddensis population as the species faces increasing targeted fishing pressure.","PeriodicalId":7719,"journal":{"name":"African Journal of Marine Science","volume":null,"pages":null},"PeriodicalIF":1.2,"publicationDate":"2021-04-03","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"43349218","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 : 2021-04-03DOI: 10.2989/1814232X.2021.1902855
G. van der Heever, L. Atkinson, Ceo von der Meden
Sediment particle size is an important environmental parameter that influences benthic species composition and is frequently used in habitat and ecosystem classification systems. In South Africa, benthic grabs are regularly used to collect seafloor sediment samples for particle size analysis. However, grab sample failures are a common, time-consuming and costly exercise, particularly in deep-sea work. This study compares replicate sediment samples collected using a custom-designed cone dredge and a conventional Van Veen grab, deployed at nine stations, ranging from 355 to 508 m in depth, in the Southern Benguela Sandy Shelf Edge ecosystem off the west coast of South Africa. No significant differences were detected between sediment properties of the samples collected by the different devices (all grain size categories [particles <3.9 to >2 000 μm]: D = 0.101, p = 0.693; fine grain size categories [particles ≤63 μm]: D = 0.177, p = 0.480). This finding suggests that, despite its different mode of operation, the cone dredge collected an appropriate sediment sample for a particle size analysis of this ecosystem, and thus the device might be usefully applied in other areas of a similar nature. The cone dredge can be considered a cost-effective and efficient alternative for sampling seafloor sediment at depth. However, further comparative analyses are required for different substrates and ecosystem types.
{"title":"Comparison of the particle size distributions of sediment collected from sandy seafloor using a Van Veen grab and cone dredge","authors":"G. van der Heever, L. Atkinson, Ceo von der Meden","doi":"10.2989/1814232X.2021.1902855","DOIUrl":"https://doi.org/10.2989/1814232X.2021.1902855","url":null,"abstract":"Sediment particle size is an important environmental parameter that influences benthic species composition and is frequently used in habitat and ecosystem classification systems. In South Africa, benthic grabs are regularly used to collect seafloor sediment samples for particle size analysis. However, grab sample failures are a common, time-consuming and costly exercise, particularly in deep-sea work. This study compares replicate sediment samples collected using a custom-designed cone dredge and a conventional Van Veen grab, deployed at nine stations, ranging from 355 to 508 m in depth, in the Southern Benguela Sandy Shelf Edge ecosystem off the west coast of South Africa. No significant differences were detected between sediment properties of the samples collected by the different devices (all grain size categories [particles <3.9 to >2 000 μm]: D = 0.101, p = 0.693; fine grain size categories [particles ≤63 μm]: D = 0.177, p = 0.480). This finding suggests that, despite its different mode of operation, the cone dredge collected an appropriate sediment sample for a particle size analysis of this ecosystem, and thus the device might be usefully applied in other areas of a similar nature. The cone dredge can be considered a cost-effective and efficient alternative for sampling seafloor sediment at depth. However, further comparative analyses are required for different substrates and ecosystem types.","PeriodicalId":7719,"journal":{"name":"African Journal of Marine Science","volume":null,"pages":null},"PeriodicalIF":1.2,"publicationDate":"2021-04-03","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"49134289","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 : 2021-04-03DOI: 10.2989/1814232x.2021.1945798
K. Osuka, J. Kawaka, M. Samoilys
{"title":"Evaluating Kenya’s coastal gillnet fishery: trade-offs in recommended mesh-size regulations","authors":"K. Osuka, J. Kawaka, M. Samoilys","doi":"10.2989/1814232x.2021.1945798","DOIUrl":"https://doi.org/10.2989/1814232x.2021.1945798","url":null,"abstract":"","PeriodicalId":7719,"journal":{"name":"African Journal of Marine Science","volume":null,"pages":null},"PeriodicalIF":1.2,"publicationDate":"2021-04-03","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"43063073","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 : 2021-04-03DOI: 10.2989/1814232X.2021.1919759
S. Singh, J. Groeneveld, J. Huggett, D. Naidoo, R. Cedras, S. Willows‐Munro
Metabarcoding is an emerging method in which DNA barcoding is combined with next-generation sequencing to determine the biodiversity of taxonomically complex samples. We assessed the current state of DNA barcode reference databases for marine zooplankton in South Africa and undertook a metabarcoding analysis to determine the species composition of samples collected with plankton tow nets. Analysis of DNA sequences mined from the literature and in online barcode reference databases revealed incomplete records for all taxa examined. Barcode records were dominated by meroplanktonic species with commercially important life-history phases (fishes and decapod crustaceans) and by species occurring in easily accessible nearshore habitats. Holoplanktonic species were underrepresented, despite making up the bulk of zooplankton biodiversity, including most potential indicator species. Metabarcoding analysis of plankton samples could identify 45% of amplicon sequence variants to species level based on BOLD databases (123 species) and similar numbers using GenBank and the MIDORI COI classifier. Morphological analysis of samples could not achieve comparable resolution at species level, but with some exceptions it recovered similar classes of organisms to those found by metabarcoding. The need for integrative molecular/morphological studies to increase and validate barcode reference databases of key zooplankton taxa is recognised. Metabarcoding of marine zooplankton in South Africa has now been successfully undertaken and the methodology is expected to facilitate high-resolution monitoring of zooplankton biodiversity in pelagic ecosystems and accelerate the discovery of new species.
{"title":"Metabarcoding of marine zooplankton in South Africa","authors":"S. Singh, J. Groeneveld, J. Huggett, D. Naidoo, R. Cedras, S. Willows‐Munro","doi":"10.2989/1814232X.2021.1919759","DOIUrl":"https://doi.org/10.2989/1814232X.2021.1919759","url":null,"abstract":"Metabarcoding is an emerging method in which DNA barcoding is combined with next-generation sequencing to determine the biodiversity of taxonomically complex samples. We assessed the current state of DNA barcode reference databases for marine zooplankton in South Africa and undertook a metabarcoding analysis to determine the species composition of samples collected with plankton tow nets. Analysis of DNA sequences mined from the literature and in online barcode reference databases revealed incomplete records for all taxa examined. Barcode records were dominated by meroplanktonic species with commercially important life-history phases (fishes and decapod crustaceans) and by species occurring in easily accessible nearshore habitats. Holoplanktonic species were underrepresented, despite making up the bulk of zooplankton biodiversity, including most potential indicator species. Metabarcoding analysis of plankton samples could identify 45% of amplicon sequence variants to species level based on BOLD databases (123 species) and similar numbers using GenBank and the MIDORI COI classifier. Morphological analysis of samples could not achieve comparable resolution at species level, but with some exceptions it recovered similar classes of organisms to those found by metabarcoding. The need for integrative molecular/morphological studies to increase and validate barcode reference databases of key zooplankton taxa is recognised. Metabarcoding of marine zooplankton in South Africa has now been successfully undertaken and the methodology is expected to facilitate high-resolution monitoring of zooplankton biodiversity in pelagic ecosystems and accelerate the discovery of new species.","PeriodicalId":7719,"journal":{"name":"African Journal of Marine Science","volume":null,"pages":null},"PeriodicalIF":1.2,"publicationDate":"2021-04-03","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"49388329","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 : 2021-04-03DOI: 10.2989/1814232X.2021.1927179
A. Whitfield
The St Lucia estuarine system on the east coast of South Africa is a declared World Heritage Site and Ramsar Site of International Importance. A major ecological feature of St Lucia during the last century was the annual spawning migration of the flathead mullet Mugil cephalus down the system in the first half of each year. Top predators, such as the African fish eagle Haliaeetus vocifer, white pelican Pelecanus onocrotalus, Nile crocodile Crocodylus niloticus and Zambezi shark Carcharhinus leucas, have made extensive use of adult M. cephalus as a food resource. With the advent of prolonged closure of the St Lucia mouth in the first decade of the 21st century, caused by the lack of St Lucia system connectivity with the Mfolozi River and a prolonged drought, this spawning migration has ceased to exist. The almost complete disappearance of M. cephalus was reinforced in the second decade of this century by a continued lack of any estuarine–marine connectivity. This loss of connectivity between Lake St Lucia and the sea for more than 12 years is longer than the normal life cycle of M. cephalus, and the possibility exists that the putative subpopulation of this species that occupied the system prior to the turn of the century may have been rendered locally extinct. In January 2021, the berm at the mouth of the estuary was artificially breached and the outflow of St Lucia estuarine waters into the sea occurred for the first time since 2002. However, it remains to be seen whether the recovery of the M. cephalus population to pre-2000 levels will occur over the short term (year) or longer term (decade). It is strongly recommended that an adaptive management strategy, rather than a fixed management approach, be adopted for the sake of future connectivity of the St Lucia system to the marine environment.
{"title":"When the flathead mullet left St Lucia","authors":"A. Whitfield","doi":"10.2989/1814232X.2021.1927179","DOIUrl":"https://doi.org/10.2989/1814232X.2021.1927179","url":null,"abstract":"The St Lucia estuarine system on the east coast of South Africa is a declared World Heritage Site and Ramsar Site of International Importance. A major ecological feature of St Lucia during the last century was the annual spawning migration of the flathead mullet Mugil cephalus down the system in the first half of each year. Top predators, such as the African fish eagle Haliaeetus vocifer, white pelican Pelecanus onocrotalus, Nile crocodile Crocodylus niloticus and Zambezi shark Carcharhinus leucas, have made extensive use of adult M. cephalus as a food resource. With the advent of prolonged closure of the St Lucia mouth in the first decade of the 21st century, caused by the lack of St Lucia system connectivity with the Mfolozi River and a prolonged drought, this spawning migration has ceased to exist. The almost complete disappearance of M. cephalus was reinforced in the second decade of this century by a continued lack of any estuarine–marine connectivity. This loss of connectivity between Lake St Lucia and the sea for more than 12 years is longer than the normal life cycle of M. cephalus, and the possibility exists that the putative subpopulation of this species that occupied the system prior to the turn of the century may have been rendered locally extinct. In January 2021, the berm at the mouth of the estuary was artificially breached and the outflow of St Lucia estuarine waters into the sea occurred for the first time since 2002. However, it remains to be seen whether the recovery of the M. cephalus population to pre-2000 levels will occur over the short term (year) or longer term (decade). It is strongly recommended that an adaptive management strategy, rather than a fixed management approach, be adopted for the sake of future connectivity of the St Lucia system to the marine environment.","PeriodicalId":7719,"journal":{"name":"African Journal of Marine Science","volume":null,"pages":null},"PeriodicalIF":1.2,"publicationDate":"2021-04-03","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"45646602","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 : 2021-04-03DOI: 10.2989/1814232X.2021.1906319
T. Riddin, J. Adams
Salt marshes protect estuary banks from erosion by acting as buffers between marine and terrestrial environments. Residents living near the Breede River estuary, Western Cape Province, South Africa, raised concerns about ongoing erosion evident at Groenpunt, the main salt marsh. This study aimed to determine how long erosion has been taking place, the rate and possible causes thereof. Aerial images and environmental data were assessed for the years 2002–2020. Erosion was first evident in 2003. By 2020, the marsh edge had been eroded into a series of micro-bays, incised horizontally up to 7 m, with a scarp height of 0.7 m, corresponding to a loss of 1 313 m2 of salt marsh and a bank retreat rate of 0.66 (SE 0.44) m year−1. Over the study period, there was a regular pattern of high-frequency gale-force winds (>8 on the Beaufort scale), with significantly more winds of this magnitude occurring in 2002 than in other years. The wind wave fetch adjacent to the marsh is up to 1 km in the direction of the predominant wind, and it is likely that the cumulative effects of constant wind-generated waves drove the erosion process. Estuary water and tidal levels over the period reflected normal seasonal fluctuation patterns. The bank supporting Groenpunt salt marsh is eroding at a rate that could possibly see it disappear within the next 60 years, reducing biodiversity and ecosystem services in the estuary. In the face of increasing climatic variability predicted in the future, similar salt marsh erosion is likely to become more prevalent.
{"title":"Salt marsh erosion in a microtidal estuary","authors":"T. Riddin, J. Adams","doi":"10.2989/1814232X.2021.1906319","DOIUrl":"https://doi.org/10.2989/1814232X.2021.1906319","url":null,"abstract":"Salt marshes protect estuary banks from erosion by acting as buffers between marine and terrestrial environments. Residents living near the Breede River estuary, Western Cape Province, South Africa, raised concerns about ongoing erosion evident at Groenpunt, the main salt marsh. This study aimed to determine how long erosion has been taking place, the rate and possible causes thereof. Aerial images and environmental data were assessed for the years 2002–2020. Erosion was first evident in 2003. By 2020, the marsh edge had been eroded into a series of micro-bays, incised horizontally up to 7 m, with a scarp height of 0.7 m, corresponding to a loss of 1 313 m2 of salt marsh and a bank retreat rate of 0.66 (SE 0.44) m year−1. Over the study period, there was a regular pattern of high-frequency gale-force winds (>8 on the Beaufort scale), with significantly more winds of this magnitude occurring in 2002 than in other years. The wind wave fetch adjacent to the marsh is up to 1 km in the direction of the predominant wind, and it is likely that the cumulative effects of constant wind-generated waves drove the erosion process. Estuary water and tidal levels over the period reflected normal seasonal fluctuation patterns. The bank supporting Groenpunt salt marsh is eroding at a rate that could possibly see it disappear within the next 60 years, reducing biodiversity and ecosystem services in the estuary. In the face of increasing climatic variability predicted in the future, similar salt marsh erosion is likely to become more prevalent.","PeriodicalId":7719,"journal":{"name":"African Journal of Marine Science","volume":null,"pages":null},"PeriodicalIF":1.2,"publicationDate":"2021-04-03","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"44374298","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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