https://www.ajol.info/index.php/ajms/issue/feed African Journal of Marine Science 2022-05-21T16:54:48+00:00 Publishing Manager publishing@nisc.co.za Open Journal Systems <p>The <em>African</em> (formerly <em>South African</em>) <em>Journal of Marine Science</em> provides an international forum for the publication of original scientific contributions or critical reviews, involving oceanic, shelf or estuarine waters, inclusive of oceanography, studies of organisms and their habitats, and aquaculture. Papers on the conservation and management of living resources, relevant social science and governance, or new techniques, are all welcomed, as are those that integrate different disciplines. Priority will be given to rigorous, question-driven research, rather than descriptive research. Contributions from African waters, including the Southern Ocean, are particularly encouraged, although not to the exclusion of those from elsewhere that have relevance to the African context. Submissions may take the form of a paper or a short communication. The journal aims to achieve a balanced representation of subject areas but also publishes proceedings of symposia in dedicated issues, as well as guest-edited suites on thematic topics in regular issues.</p><p class="MsoNormal">The <em>African Journal of Marine Science</em> is available full text online and more information can be accessed <a href="http://www.nisc.co.za/products/3/journals/african-journal-of-marine-science" target="_blank">here</a>. </p> https://www.ajol.info/index.php/ajms/article/view/225499 Predicted future changes in ocean temperature and pH do not affect prey selection by the girdled dogwhelk <i>Trochia cingulata</i> 2022-05-21T16:24:36+00:00 N. Martin trobins@sun.ac.za S. Clusella-Trullas trobins@sun.ac.za TB Robinson trobins@sun.ac.za <p>Predator–prey relationships can drive community dynamics in marine systems, but it remains unclear how future changes in seawater temperatures and pH will influence these relationships. This study assessed the effect of predicted future temperatures and pH on the prey choice of the girdled dogwhelk <em>Trochia cingulata</em> (family Muricidae) when offered native (<em>Aulacomya atra, Choromytilus meridionalis</em>) and alien (<em>Semimytilus algosus</em>) mussels. Whelks were exposed to three pH levels: 8.0 (current), 7.7 (intermediate) and 7.5 (extreme), at each of three temperatures: 9 °C (cooling), 13 °C (current) and 17 °C (warming) for 6 weeks. Thereafter, the prey preference and predation rate were compared among treatments. Within two weeks, 98% of whelks exposed to warming died, precluding assessment of how warming affects their prey preference. Despite high mortality, the highest predation rates were recorded at 17 °C regardless of the pH level, likely reflecting increased energy costs and ingestion rates associated with warming. In the remaining treatments whelks preferred <em>S. algosus</em> irrespective of the levels of seawater cooling or acidification. These results align with previous work that demonstrated a preference by <em>T. cingulata</em> for <em>S. algosus</em> and suggest that the predator–prey relationship between this whelk and its mussel prey is unlikely to be disrupted under future marine conditions.</p> 2022-05-21T00:00:00+00:00 Copyright (c) https://www.ajol.info/index.php/ajms/article/view/225500 Reproductive biology of the blue swimming crab <i>Portunus segnis</i> (Forskål, 1775) (Brachyura: Portunidae) in the Gulf of Gabes (southeastern Tunisia, central Mediterranean Sea) 2022-05-21T16:32:22+00:00 O Ben Abdallah-Ben Hadj Hamida olfa.benabdallah@instm.rnrt.tn N Ben Hadj Hamida olfa.benabdallah@instm.rnrt.tn H Chaouch olfa.benabdallah@instm.rnrt.tn B Nafkha lfa.benabdallah@instm.rnrt.tn N Ben Ali olfa.benabdallah@instm.rnrt.tn D Abidi olfa.benabdallah@instm.rnrt.tn H Missaoui olfa.benabdallah@instm.rnrt.tn <p>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<br>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.<br>Keywords: fecundity, Lessepsian migrant, maturity, ovarian maturation, reproduction, sex rati</p> 2022-05-20T00:00:00+00:00 Copyright (c) https://www.ajol.info/index.php/ajms/article/view/225501 Trends in mussel cover, density and size at exploited and unexploited intertidal reefs in eastern South Africa 2022-05-21T16:36:56+00:00 E Steyn erika@ori.org.za JC Groeneveld erika@ori.org.za J Santos erika@ori.org.za XI Mselegu erika@ori.org.za <p>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.</p> 2022-05-20T00:00:00+00:00 Copyright (c) https://www.ajol.info/index.php/ajms/article/view/225509 Movement patterns and catch trends of the diamond ray <i>Gymnura natalensis</i> (Dasyatidae) in South African waters 2022-05-21T16:43:40+00:00 R Daly rdaly@ori.org.za GL Jordaan rdaly@ori.org.za D Parker rdaly@ori.org.za G Cliff rdaly@ori.org.za N Nkabi rdaly@ori.org.za R Kyle rdaly@ori.org.za ST Fennessy rdaly@ori.org.za BQ Mann rdaly@ori.org.za <p>The diamond ray <em>Gymnura natalensis</em> is endemic to southern Africa where its preference for shallow coastal habitats makes it vulnerable to recreational shore-based angling. Although it makes up approximately 1% of the shore-based tag numbers, little is known about its movements, reproduction or population status in South Africa. This study used three independent long-term (34–41 years) datasets, including tagging by recreational anglers, competitive shore angling catches and shark net catches, to investigate the species’ movements, catch composition and population status in South Africa. Of the 3 739 individuals tagged (1984–2018), only 30 (1%) were recaptured after an average of 487 days at liberty. The majority (60%) of the recaptures occurred within 10 km of the release site, while 7% had moved more than 1 000 km along the coast. The longest recorded movements (1 577 and 1 756 km) were undertaken by adult rays tagged in the Western Cape Province moving to KwaZulu-Natal Province (KZN). The competitive shore angling catch (1977–2018; <em>n</em> = 9 150) from KZN was dominated by adult rays caught north of Durban, while the shark net catch in KZN (1981–2018; <em>n</em> = 584) was dominated by juvenile rays primarily from the central beaches of Durban. All the datasets exhibited strong seasonal trends with most catches taking place in summer. A risk assessment confirmed a stable to increasing population trend over four generations, suggesting that the population sampled along the east coast of South Africa should be classified as Least Concern.</p> 2022-05-21T00:00:00+00:00 Copyright (c) https://www.ajol.info/index.php/ajms/article/view/225510 The ecology of coastal wetland ponds created by diamond mining in southern Namibia. 1. Physical conditions 2022-05-21T10:45:53+00:00 L Maritz deena.pillay@uct.ac.za D Pillay deena.pillay@uct.ac.za GM Branch deena.pillay@uct.ac.za <p>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<sup>−1</sup>. 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.</p> 2022-05-21T00:00:00+00:00 Copyright (c) https://www.ajol.info/index.php/ajms/article/view/225511 The ecology of coastal wetland ponds created by diamond mining in southern Namibia. 2. Saltmarsh vegetation 2022-05-21T16:49:01+00:00 L Maritz deena.pillay@uct.ac.za D Pillay deena.pillay@uct.ac.za GM Branch deena.pillay@uct.ac.za <p>Diamond mining on the southern Namibian coastline has created multiple large coastal ponds of up to 380 000 m<sup>2</sup> 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, <em>Salicornia natalensis</em>. 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,<br>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<em> S. natalensis</em> along the coast.</p> 2022-05-21T00:00:00+00:00 Copyright (c) https://www.ajol.info/index.php/ajms/article/view/225512 Characterising epibenthic diversity and physical drivers in unconsolidated marine habitats of Algoa Bay, South Africa 2022-05-21T16:52:02+00:00 HJ Truter hannahtrut3r@gmail.com LJ Atkinson hannahtrut3r@gmail.com CEO von der Meden hannahtrut3r@gmail.com D Bailey hannahtrut3r@gmail.com W Goschen hannahtrut3r@gmail.com AT Lombard hannahtrut3r@gmail.com <p>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.</p> 2022-05-21T00:00:00+00:00 Copyright (c) https://www.ajol.info/index.php/ajms/article/view/225513 Revisiting ‘<i>A monograph on the Polychaeta of southern Africa</i>’: establishing taxonomic research priorities in southern Africa 2022-05-21T15:47:13+00:00 CA Simon csimon@sun.ac.za J Kara csimon@sun.ac.za DT Clarke csimon@sun.ac.za S Sedick csimon@sun.ac.za <p>Originally published in 1967, John H Day’s work ‘<em>A monograph on the Polychaeta of southern Africa</em>’ is still used widely to identify polychaetes. However, ongoing taxonomic revisions have revealed that several putative cosmopolitan or locally widespread taxa contained in the monograph are complexes of species with discrete distributions, globally and locally. This study therefore aimed to develop lists of taxa, including unresolved cosmopolitan and widespread indigenous species, that should be prioritised for revision to unlock their hidden<br>diversity. A total of 609 species (56 families and 316 genera) were scored according to their time since description, global and local distribution, availability of genetic data and vouchers, alien status and economic importance, and then ranked. At least half the taxa reported locally are unresolved cosmopolitan complexes, and a quarter have wide local distributions, probably hiding cryptic diversity. Accordingly, we estimate that approximately 500<br>polychaete species are still undescribed in southern Africa. The four highest-scoring families (<em>Syllidae, Nereididae, Spionidae and Eunicidae</em>) comprise 25% of the species and 53–85% of the unresolved cosmopolitans, while multiple species are considered pests, used as bait or possible aliens. Prioritised genera (e.g. <em>Eunice, Syllis, Nereis,</em><br><em>Prionospio, Dipolydora</em>) and species (e.g. Pseudonereis variegata) are usually members of prioritised families, but some species are not (e.g. <em>Sabella cf. pavonina, Fimbriosthenelais zetlandica, Paleanotus chrysolepis, Gunnarea </em><em>gaimardi, Capitella capitata</em>). All taxon levels should therefore be considered to ensure that all species most in need<br>of revision are identified. Ways to facilitate revisions are discussed.</p> 2022-05-21T00:00:00+00:00 Copyright (c) https://www.ajol.info/index.php/ajms/article/view/225514 Spawning observations of <i>Pomadasys commersonnii</i> in the marine section of the Knysna estuarine bay, Western Cape, South Africa 2022-05-21T16:54:17+00:00 EC Butler e.butler@ru.ac.za AR Childs e.butler@ru.ac.za MKS Smith e.butler@ru.ac.za RM Foster e.butler@ru.ac.za WM Potts e.butler@ru.ac.za <p>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 <em>Pomadasys commersonnii</em> 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,<br>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.</p> 2022-05-21T00:00:00+00:00 Copyright (c)