African Journal of Marine Science https://www.ajol.info/index.php/ajms <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" rel="noopener">here</a>.&nbsp;</p> NISC en-US African Journal of Marine Science 1814-232X Copyright for articles published in this journal is retained by the publisher. Effect of rotational harvesting on the size composition of Cape rock oysters <i>Striostrea margaritacea</i> on the east coast of South Africa https://www.ajol.info/index.php/ajms/article/view/261730 <p><strong>A commercial fishery for the Cape rock oyster </strong><strong><em>Striostrea margaritacea </em></strong><strong>along the north coast of KwaZulu-Natal, South Africa (western Indian Ocean), follows a 4-year rotational cycle, with each harvest year followed by 3 fallow years across four harvest zones. We analysed reported harvesting effort and catch information, and fishery-independent oyster size composition data collected over 18 years (Feb 2003–Feb 2020), to investigate the sustainability of rotational harvesting. Total harvesting effort and catches declined over the study period, but on average, the number of oysters collected per outing increased. Fewer outings in recent years were attributed to incomplete reporting and a progressive loss of access to harvest sites. Generalised linear mixed models were used to estimate trends in oyster mean size in relation to fishing method (divers and intertidal collectors), harvest zone, 4-year rotational cycles, and months spent in a zone. Oyster mean size increased from north to south along the coast. Oysters caught by divers on newly exploited deeper reefs were initially larger than those caught by intertidal collectors. Mean oyster size decreased monthly during 1-year harvest periods but recovered to pre-harvest size over 3 fallow years. The results confirmed that the current rotational harvest strategy is well-suited to oyster biology and is sustainable at the present level of effort. Improved reporting on harvesting effort and catch are required to verify longer-term spatiotemporal trends in this fishery. More effective stakeholder communication is needed to resolve potential user conflict.</strong></p> E Steyn JC Groeneveld J Santos Copyright (c) 2024 2024-01-04 2024-01-04 45 4 249 258 Eelgrass <i>Zostera capensis</i> populations in KwaZulu-Natal, South Africa, harbour distinct genomic signals despite limited geographical distance https://www.ajol.info/index.php/ajms/article/view/261731 <p><strong>Seagrasses are threatened by anthropogenic stressors and climate change, with numerous population declines reported. In South Africa, the eelgrass </strong><strong><em>Zostera capensis </em></strong><strong>is restricted to estuarine environments and has a disjunct distribution and declining status. With the majority of the distribution of </strong><strong><em>Z. capensis </em></strong><strong>on the west and southwest coasts of South Africa, the isolated easternmost populations in KwaZulu-Natal Province (KZN) are of particular interest. Following the extirpation of </strong><strong><em>Z. capensis </em></strong><strong>at Durban and St Lucia, only five populations remain, of which three (in the Amatikulu, Mlalazi and Mhlatuze estuaries) are situated &lt;50 km apart. Previous molecular analyses showed strong population structure between </strong><strong><em>Z. capensis </em></strong><strong>populations, but the geographically close populations on the east coast were not included. In this study, using single nucleotide polymorphisms (SNPs) isolated from 31 individual sampled plants collected from three northern KZN estuaries, we provide evidence for distinct population clusters, with unique evolutionary signals. </strong><strong><em>Zostera capensis </em></strong><strong>in the Mlalazi Estuary has a low level of genomic diversity, likely as a result of a small, dynamic population unable to withstand prolonged freshwater exposure. Our results suggest that conservation efforts need to consider unique population signals even among geographically close populations, in particular within the context of restoration, where genomic compatibility may determine the persistence of restored populations.</strong></p> SE Smit R Henriques M Jackson R Taylor L Vivier S von der Heyden Copyright (c) 2024 2024-01-04 2024-01-04 45 4 259 264 Macrobenthic fauna of the Agulhas Bank shelf edge https://www.ajol.info/index.php/ajms/article/view/261732 <p><strong>The deep sea (&gt;200 m depth) off South Africa is largely unexplored, with most benthic macrofauna samples limited to depths shallower than 100 m. The benthic infaunal diversity of the Agulhas Bank shelf edge has not yet been studied. We analysed seven grab samples that were opportunistically collected along the shelf edge at depths of 290‒533 m. A total of 136 macrofauna items representing 75 species were identified, and the first species list of the infauna of the shelf edge was compiled. Polychaetes comprised 48% of total abundance, and the sites had an average similarity of 7% based on species abundance. Spearman rank correlation showed that depth and sediment particle size are important environmental drivers of community abundance and biomass variation on the shelf edge. Habitat heterogeneity is likely high along the Agulhas Bank shelf edge. Greater sampling effort in habitats for which data are scarce provides crucial information for offshore biodiversity assessment and management.</strong></p> S Brandt N Karenyi K Sink Copyright (c) 2024 2024-01-04 2024-01-04 45 4 265 272 Network analysis of the endemic spotted gully shark <i>Triakis megalopterus</i> reveals spatial vulnerability to exploitation in the Western Cape, South Africa https://www.ajol.info/index.php/ajms/article/view/261733 <p><strong>The spotted gully shark </strong><strong><em>Triakis megalopterus </em></strong><strong>(Triakidae) is a mesopredatory species endemic to southern Africa. It is currently listed as Least Concern on the IUCN Red List in accordance with an estimated increase in population size, general release by recreational linefishers and incidental catches in the commercial linefisheries. Previous research suggests this species to be resident, and as such it is likely to receive protection in coastal marine protected areas (MPAs). However, its ecology and movement behaviour remain poorly studied. This study employed acoustic telemetry to provide information on the species’ movements along the coast of the Western Cape Province, South Africa. We used network analyses to investigate movement randomness, associations between individuals, sexual segregation, and the effectiveness of MPAs. Our findings reveal non-random movements as well as patterns of co-occurrence between individuals. Spatial network analysis suggested sexual segregation, because areas of high use (Walker Bay and De Hoop) differed between males and females. Co-occurrences were observed exclusively in Walker Bay, chiefly between males, with no co-occurrence found between females. The tagged spotted gully sharks were not detected extensively within existing MPA boundaries, though there was no significant difference between their movements inside and outside protected areas for both sexes.</strong></p> E Cottrant NJ Drobniewska TL Johnson LG Underhill TS Murray N Hammerschlag PS Albano C Elston ME McCord PD Cowley C Fallows TG Paulet Copyright (c) 2024 2024-01-04 2024-01-04 45 4 273 283 Geographical and seasonal patterns in the diet of Cape fur seals <i>Arctocephalus pusillus pusillus</i> in Namibia, based on extensive scat analyses https://www.ajol.info/index.php/ajms/article/view/261734 <p><strong>Namibia’s population of Cape fur seals </strong><strong><em>Arctocephalus pusillus pusillus </em></strong><strong>is a major constituent of top predator biomass in the northern Benguela upwelling system. Understanding their diet is key to comprehending their role in the ecosystem and potential drivers of recent population changes, such as a northward extension of the population’s range. Using prey remnants retrieved from 1 413 scat samples collected monthly at 12 Cape fur seal colonies along the Namibian coast over 28 years (1994‒2021), we assessed seasonal and geographical differences in their diet. We include the first comparison of diet between the growing colonies in the far northern area of Namibia and the colonies to the south, many of which have been in decline. Four fish species dominated in the diet. The bearded goby </strong><strong><em>Sufflogobius bibarbatus</em></strong><strong>, which is ubiquitous throughout the northern Benguela ecosystem, was the most abundant prey item in the diet. However, Cape fur seals appeared to show a preference for the more-nutritious fish species Cape horse mackerel </strong><strong><em>Trachurus capensis </em></strong><strong>and the lanternfish </strong><strong><em>Lampanyctodes hectoris </em></strong><strong>by switching to those prey during autumn and spring in the north and south, respectively, coinciding with the greater availability of those species during upwelling periods. The availability of Cape horse mackerel, especially during a critical period for the growth and development of pups before the onset of winter, may be key to the greater productivity of Cape fur seal colonies in the north relative to those in the south of Namibia, and warrants further investigation. Juveniles of Cape horse mackerel and Cape hake </strong><strong><em>Merluccius capensis </em></strong><strong>were the only fishery species of commercial importance that were relatively abundant in the seals’ diet, while the sardine </strong><strong><em>Sardinops sagax, </em></strong><strong>which was historically dominant, was found in negligible proportions.</strong></p> DN Mwaala MR Wilhelm SP Kirkman J-P Roux Copyright (c) 2024 2024-01-04 2024-01-04 45 4 285 294 Marine macroalgae of the Agulhas Marine Province of South Africa: Biodiversity, exploitation and potential for aquaculture https://www.ajol.info/index.php/ajms/article/view/261727 <p><strong>The Agulhas Marine Province (AMP) extends along most of the south coast of South Africa, from Cape Agulhas to the vicinity of the Mbashe River mouth in the Eastern Cape Province, and has its own distinctive warm-temperate seaweed flora, with high levels of diversity and endemism. Seaweed floras are excellent indicators of seawater temperature regimes and therefore very useful in studies of climate change. We provide new analyses of seawater temperatures for 50-km sections along this coast and discuss them in relation to upwelling mechanisms, with particular focus on Algoa Bay. The biogeography of the South African south coast/AMP seaweed flora is placed in a southern African context, and patterns of endemism are discussed. Commercial harvesting has involved only intertidal collection of </strong><strong><em>Gelidium pristoides</em></strong><strong>, but other species may have commercial potential. Seaweed aquaculture on this coast is currently limited to shore-based cultivation as abalone feed, but the potential for sea-based seaweed cultivation is discussed, with particular reference to Algoa Bay and the declared Algoa Bay Aquaculture Development Zone.</strong></p> JJ Bolton CD McQuaid Copyright (c) 2024 2024-01-04 2024-01-04 45 4 221 234 The changing status of important marine fishery species in selected South African estuaries https://www.ajol.info/index.php/ajms/article/view/261729 <p><strong>This review examines the changing status of 10 estuary-dependent marine fish species in 10 South African estuarine systems, ranging from the Kosi Estuary in the northeast to the Berg Estuary in the southwest. In all of these systems, the selected fish species were found to be in population decline, but the causes of the declines varied from one system to another. Recreational and small-scale/subsistence fishing was a common pressure on fish stocks in most of the reviewed estuaries, but environmental degradation and pollution were the prime drivers for major population declines in the St Lucia and uMhlanga estuaries, respectively. Of six primary linefish species that have been well studied, two species are categorised as having an overexploited stock status and four species have reached a collapsed level where the spawner biomass per recruit (SBR) is now &lt;25% of the original unimpacted level. Furthermore, two of those species (dusky kob </strong><strong><em>Argyrosomus japonicus </em></strong><strong>and white steenbras </strong><strong><em>Lithognathus lithognathus</em></strong><strong>) are in the 4‒6% SBR range and have been officially categorised as Endangered on the IUCN Red List. Unless definite steps are taken to reduce fishing pressure on the species discussed in this review, and successful policies put in place to promote healthy estuarine environments around the coast, estuary-dependent fish stocks will continue to decline—to the detriment of the fish populations and the people who depend on fish for food, recreation and/or employment.</strong></p> AK Whitfield BQ Mann Copyright (c) 2024 2024-01-04 2024-01-04 45 4 235 248 Allan Heydorn (1930–2023): Marine biologist and conservationist https://www.ajol.info/index.php/ajms/article/view/261736 <p>No abstract.</p> Rudy van der Elst George M Branch Copyright (c) 2024 2024-01-04 2024-01-04 45 4 295 296