African Journal of Marine Science 2021-05-28T12:22:06+00:00 Publishing Manager 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="" target="_blank">here</a>. </p> Stock separation of the shallow-water hake Merluccius capensis in the Benguela ecosystem using otolith shape analysis 2021-05-28T11:17:48+00:00 E.N.G. Shoopala M.R. Wilhelm S.C. Paulus <p>The fishing industry is an important economic sector in Namibia and South Africa, with the shallow-water hake <em>Merluccius capensis</em> being an important target species. Recent genetic studies of <em>M. capensis</em> found two stocks in the Benguela Current Large Marine Ecosystem, one in the north (17–29° S) and one in the south (29–36° S), and a proposed mixed stock in the Orange River area (around 29° S). The present study investigated the use of otolith shape analysis for purposes of stock-structure analysis of <em>M. capensis</em>.&nbsp; <em>Merluccius capensis</em> otolith samples were collected during demersal-trawl surveys along the Benguela, for the years 1992, 2004 and 2005. Different years were selected to investigate temporal stability in otolith shape in the northern Benguela. A total of 1 628 otolith<br>images were analysed using the shapeR library in <em>R. Otolith</em> shape was analysed using wavelet transformation, and ANOVA-like permutation tests indicated no significant differences between the northern (17°31′–25°29′ S) and central (25°30′–29°05′ S) Benguela for all years but showed significant differences between the northern and southern (29°05′–35°50′ S) Benguela. This study therefore demonstrated that otolith shape could be used for stock discrimination of <em>M. capensis</em>. It confirmed one stock of<em> M. capensis</em> in the northern and central Benguela and another in the southern Benguela, which supports the current, separate management approach for this species. It also showed some differences in otolith shape from the 1990s to the 2000s, which could be explained by increased movement of the southern Benguela stock to the northern Benguela and increased hybridisation in the later years.</p> <p><strong>Keywords</strong>: fishery management units, imaging techniques, otolith morphometrics, package ‘shapeR’, stock identification, stock&nbsp; structure, wavelet method </p> 2021-05-28T00:00:00+00:00 Copyright (c) Evaluating Kenya’s coastal gillnet fishery: trade-offs in recommended mesh-size regulations 2021-05-28T11:21:36+00:00 K. Osuka J.A. Kawaka M.A. Samoilys <p>Gillnets are a widely used fishing gear in Kenya’s artisanal fisheries, yet their mesh sizes are inadequately monitored or regulated. This study evaluated the impacts of gillnets of seven stretched-mesh sizes, through comparative analysis of species-related metrics and catch per unit effort (CPUE), to inform Kenya’s small-scale fisheries regulations. Data were collected from June 2014 to May 2015. Three mesh-size groups were identified from catch composition data using non-metric multidimensional scaling and comprised small (1.3, 5.1 and 7.6 cm), medium (10.2 and 15.2 cm) and large (20.3 and 25.4 cm) mesh. The dominant species (and their mean lengths) that were caught in the small, medium and large mesh sizes, respectively, were whitespotted rabbitfish Siganus sutor (21.7 cm [SD 5.3]), mackerel tuna Euthynnus affinis (40.8 cm [SD 9.1]) and honeycomb stingray Himantura uarnak (87.3 cm [SD 37.4]). Values of length at first capture (L50) for S. sutor and E. affinis caught with the small and medium mesh sizes were below length at maturity (Lm). Catches of juveniles were proportionately high in the small meshes (61.3–74.2%) and lower in the medium (38.3–50.9%) and large (9.1–36.2%) mesh sizes. Catches with small mesh tended to be species categorised as Least Concern on the IUCN Red List, in contrast to catches with large mesh which tended to be Near Threatened or Vulnerable species. Biomass CPUE differed between mesh-size groups, with<br>the small sizes recording low CPUE. The medium sizes caught mid- to high-trophic-level species with high-income returns, displayed moderate CPUE, and had the lowest juvenile retention and capture of threatened species. Medium mesh sizes are therefore recommended for artisanal fisheries, given low trade-offs between ecological impact and fishery returns.</p> <p><strong>Keywords</strong>: adaptive co-management, artisanal fisheries, conservation status, juvenile capture, multispecies catches, size selectivity, small-scale fisheries </p> 2021-05-28T00:00:00+00:00 Copyright (c) Demersal fish communities in KwaZulu-Natal, South Africa, indicate partial congruence with proposed conservation biozones 2021-05-28T11:26:39+00:00 W.N. Dalton S.N. Porter T-C. Livingstone B.Q. Mann <p>The KwaZulu-Natal shelf, on the east coast of South Africa, is inhabited by diverse communities of demersal fishes; however, previous studies deeper than 30 m have largely investigated these with extractive techniques. Using baited remote underwater videos (BRUVs), this study quantified how depth and substrate type correlated with the composition of demersal fish communities between 35 and 100 m, and assessed whether the communities displayed congruence with three benthic biozones, which were modelled using abiotic surrogates for purposes of conservation planning: Biozone 1 (warm; coarse sediment dominant; low seafloor oxygen, phosphate and organic carbon concentrations), Biozone 2 (flat; coarse sediment dominant; high seafloor oxygen and low phosphate and organic carbon concentrations) and Biozone 3 (shallow; warm; fine sediment dominant; low seafloor oxygen and higher organic carbon concentrations). A total of 118 fish taxa were recorded from 200 BRUV deployments conducted on different substrate types. All diversity indices differed significantly among substrate types, whereas depth correlated significantly with only the number of fish species per site and Margalef’s diversity index. Multivariate analyses determined that substrate type and depth explained significant amounts of the variation in fish communities. A significant difference was also detected between the fish communities of Biozones 1 and 3, with Biozone 3 being the most distinct of the three biozones investigated. Overall congruence between fish communities and their associated biozones ranged between 41% and 45%. This suggests that, at a broad spatial scale, the demersal fish communities could be adequately represented by Biozone 3 and a second biozone comprising Biozones 1 and 2 combined.</p> <p><strong>Keywords</strong>: baited remote underwater video, biodiversity surrogates, depth, fish community composition, KwaZulu-Natal shelf,&nbsp; substrate type, systematic conservation planning </p> 2021-05-28T00:00:00+00:00 Copyright (c) The effect of fishmeal replacement with terrestrial protein sources on growth, body condition and intestinal microbiota of juvenile dusky kob <i>Argyrosomus japonicus</i> 2021-05-28T11:32:29+00:00 C.L.W. Jones A. Nel A. Adesola T. Shipton H. Kaiser <p>A balanced combination of protein sources to partially replace fishmeal in the diets of cultured carnivorous fish can promote optimal fish health and production performance. In the present study, the growth, body condition and gut microbiota of juvenile dusky kob Argyrosomus japonicus fed fishmeal-substituted diets comprising different blends of animal and plant protein sources, with crystalline amino acid supplementation, were compared. The fish (15.7 [SE 0.12] g fish<sup>−1</sup>) were fed one of three isonitrogenous (41% protein) and isoenergetic (13 kJ g<sup>−1</sup>) diets for 60 days. Fish fed a diet with no fishmeal (D0; blood meal and soybean protein sources only) displayed the lowest specific growth rate (p &lt; 0.001). Longer body lengths were achieved in fish fed a 50% fishmeal diet (D50; supplemented with poultry meal, bloodmeal, canola meal and soybean) (13.2 [SE 0.4] cm) compared with those fed either a 100% fishmeal diet (D100) (12.8 [SE 0.4] cm) or a diet with no fishmeal (D0) (12.1 [SE 0.08] cm) (p &lt; 0.001). There were no differences in condition factor, hepatosomatic index (HSI) and feed conversion ratio (FCR) between fish fed diets D50 and D100; whereas fish fed diet D0 had a lower HSI and a higher FCR when compared with the other treatments (p &lt; 0.01). The dietary protein source did not alter the gut microbiota of these fish. In conclusion, 50% replacement of dietary fishmeal with a favourable balance of meals processed from poultry, blood, canola and soybean improved length gain in juvenile dusky kob compared with those fed a diet with no fishmeal replacement.</p> <p><strong>Keywords:</strong> blood meal, canola meal, crystalline amino acids, gut bacteria, hepatosomatic index, poultry meal, soybean meal </p> 2021-05-28T00:00:00+00:00 Copyright (c) Abundance and distribution of Antarctic blue whales <i>Balaenoptera musculus intermedia</i> off the Queen Maud Land coast of Antarctica 2021-05-28T11:39:08+00:00 S. Paarman E. Vermeulen E. Seyboth M. Thornton K. Findlay <p>The Antarctic blue whale Balaenoptera musculus intermedia was hunted to near extinction in the twentieth century. Current data on the abundance and distribution of the species are lacking owing to the difficulty and expense of surveys under adverse weather conditions in open-ocean habitats, and to the small population size. The most recently accepted global abundance estimate, based on the middle survey (1997/1998) of three circumpolar Antarctic surveys conducted between 1991/1992 and 2003/2004, was less than 1% of the original pre-whaling population size. The present study used a visual line-transect survey off the Queen Maud Land coast of Antarctica, in an area between 0° and 18° E and south of 67° S, in January 2014, to estimate the abundance of Antarctic blue whales in this area. Effort-accounted densities of sightings averaged 13.3 individuals per 1 000 nautical miles of survey effort (CV = 0.26) and reinforce recent findings that the area has significantly higher densities than averaged in circumpolar surveys (0.17–1.48 per 1 000 nautical miles). Distance sampling resulted in a population density estimate of 0.019 whales nautical-mile-2 (CV = 0.24) and an estimated abundance of 1 026 Antarctic blue whales (CV = 0.20, 95% CI 632–1 450) in the surveyed area. Obtaining such current estimates of abundance is crucial for assessment of the conservation status of the Antarctic blue whale population and for monitoring its recovery.</p> <p><strong>Keywords:</strong> baleen whale, distance sampling, endangered species, Southern Ocean, vessel survey, whale conservation </p> 2021-05-28T00:00:00+00:00 Copyright (c) Characterising the seasonal cycle of wind forcing, surface circulation and temperature around the sub-Antarctic Prince Edward Islands 2021-05-28T11:44:42+00:00 T. Toolsee T. Lamont M. Rouault I. Ansorge <p>Located between the sub-Antarctic Front and the Antarctic Polar Front, the Prince Edward Islands (PEIs) provide an essential breeding ground for top predators and are an ideal location to investigate disturbances linked to climate change. This study provides the first seasonal characterisation of surface hydrography at the PEIs, using satellite and reanalysis products from 1993 to 2016. Sea surface temperature (SST) showed consistently higher values to the north, while wind, currents and eddy kinetic energy all showed higher values to the south of the islands. The highest SST (8 °C) occurred in summer and the lowest (2 °C) in winter, with a one- to two-month delay between the maximum incoming solar radiation (in December–January) and the highest SST (in February). The highest wind<br>speed occurred in July (10.8 m s−1) and the minimum in February (7 m s−1). Geostrophic currents were four-times larger than Ekman currents, showing lower speeds between April and June (0.25–0.30 m s−1) and a peak in August (0.45 m s−1). There were no significant differences for SST and Ekman currents between the regions upstream and downstream of the PEIs. In contrast, surface total and geostrophic current velocities were weaker downstream because the islands act as a barrier to the flow. A zone of lower wind speed, coinciding with enhanced positive wind stress curl (WSC), favouring downwelling, occurred directly upstream throughout the year. Although WSC over the PEIs was negative (upwelling-favourable), no corresponding cooling was evident in SST. This seasonal&nbsp; characterisation provides a baseline against which interannual/decadal variability and future changes in these parameters can be assessed.</p> <p><strong>Keywords</strong>: Antarctic Polar Front, eddy kinetic energy, monthly climatology, satellite data, Southern Ocean, sub-Antarctic Front, surface currents, wind speed, wind stress curl </p> 2021-05-28T00:00:00+00:00 Copyright (c) Potential impact of marine heatwaves on selected phytoplankton adapted to the Gulf of Guinea during stable hydrographic periods 2021-05-28T11:49:05+00:00 E. Acheampong P. Mantey A. Weremfo <p>Reports suggest that the Gulf of Guinea (northeastern tropical Atlantic) frequently experiences marine heatwaves (MHW)—prolonged periods of anomalously warm seawater—of ≥1.5 °C above baseline. We assessed the likely impact of this anomaly on two microalgae, <em>Thalassiosira weissflogii</em> (diatom) and<em> Gymnodinium sp.</em> (dinoflagellate), adapted to the surface temperature (28 ± 1.5 °C) of the gulf during stable hydrographic periods. The algae were adapted over ~400 generations. They were assessed for specific growth rate (μ), dry weight, and protein content after exposure to 5 or 6 days of warming (+2 °C or +4 °C above the temperature of the stock cultures), in line with the minimum duration of MHW. Under each of the investigated warming scenarios, the effect of warming on the diatom was immediate, occurring during the first day of exposure, and μ had decreased by ~75% by the end of the warming period. In contrast, the warming effect on the dinoflagellate became significant during the second day, with μ reduced by ~78–86%. Also, the protein&nbsp; content of the dinoflagellate had been reduced by ~32% by the end of the warming period. The dry weight of <em>T. weissflogii</em> increased three-fold under +2 °C of warming. In contrast, the dry weight of Gymnodinium sp. decreased by ~78% and did not recover when warming was removed. These results highlight vulnerability of these algae to MHW and their unique responses to the anomaly.</p> <p><strong>Keywords</strong>: dry weight, <em>Gymnodinium sp</em>., laboratory experiment, marine microalgae, protein content, specific growth rate, temperature effect,<em> Thalassiosira weissflogii</em></p> 2021-05-28T00:00:00+00:00 Copyright (c) ross-shelf movement of <i>Chrysaora fulgida</i> (Scyphozoa; Discomedusae) off Namibia inferred from stable isotopes (δ15N and δ13C) 2021-05-28T11:55:22+00:00 H. Skrypzeck C.D. van der Lingen M.J. Gibbons <p>Large and small specimens of two species of metagenic Scyphozoa (true jellyfishes) can be found in nearshore waters off central Namibia throughout the year. Whereas populations of Chrysaora africana are largely restricted to inshore waters, <em>C. fulgida</em> occurs across the shelf, with small individuals found inshore and large individuals primarily found offshore. We examined stable isotopes δ15N and δ13C of both species in Walvis Bay and found that large-sized <em>C. fulgida</em> have lower δ15N values than small individuals and <em>C. africana</em> throughout the year. These differences are interpreted to reflect cross-shelf changes in δ15N baseline levels, with greater nitrogen recycling (and hence lower δ15N values) occurring offshore. The occasional/seasonal nearshore appearance of large <em>C. fulgida</em> with low δ15N values therefore implies routine, onshore advection. The values of δ13C did not show cross-shelf differences, which suggests that jellyfish populations across the shelf are supported by phytoplankton-based food chains. This study emphasises the value of using stable isotopes to examine the mesoscale structuring of jellyfish populations.</p> <p><strong>Keywords:</strong> <em>Chrysaora africana,</em> coastal upwelling, eastern boundary currents, energy flow, jellyfish, metagenesis, molecular markers</p> 2021-05-28T00:00:00+00:00 Copyright (c) Three-dimensional modelling of the circulation in False Bay, South Africa 2021-05-28T12:01:26+00:00 F. Coleman G.P.J. Diedericks A.K. Theron J Lencarte Silva <p>False Bay is the largest coastal embayment along South Africa’s coastline. Despite notable contributions of various historical observational studies on the circulation within False Bay and the associated physical processes driving these flows, there remains significant uncertainty largely because the studies to date have been undertaken over short durations and on limited spatial scales. This article aims to expand upon the findings of these historical observational studies by providing a thorough evaluation of intra-annual hydrodynamics within False Bay, with particular emphasis on providing greater definition on current circulation. To evaluate the effects of different processes on coastal hydrodynamics, a 3D hydrodynamic model was developed which included the influence of spatially varying wind forcing and daily averaged temperature-depth profiles on the model boundaries and atmospheric heat exchange. The model results relating to current circulation are consistent with historical studies that described a cyclonic (clockwise)<br>circulation within the bay under southeasterly wind conditions. The development of this cyclonic pattern is primarily caused by the spatially varying wind field. During warmer months, upwelling events were noted at Cape Hangklip. Interestingly, under northwesterly wind conditions, the model results deviated from historical findings by showing a spatially uniform current field across the bay. Under these conditions, strong bottom return currents are generated, which contribute significantly to cold-water intrusion events within the bay.</p> <p><strong>Keywords:</strong> coastal engineering, coastal hydrodynamics, HYCOM data, hydrodynamic modelling, numerical modelling, spatially varying wind field, wind forcing</p> 2021-05-28T00:00:00+00:00 Copyright (c) Mobilisation and dynamics of energy reserves in different tissues of <i>Donax trunculus</i> (Bivalvia: Donacidae) in the Gulf of Tunis (eastern Mediterranean Sea, Tunisia) 2021-05-28T12:07:39+00:00 D. Boussoufa H. Chalouati N. Ghazali J.C. Navarro M. El Cafsi <p>Seasonal changes in condition index and the biochemical components (proteins, lipids and glycogen) of the gonad/digestive gland, foot, labial palp, mantle, gills and adductor muscles of <em>Donax trunculus Linnaeus</em>, 1758 from the Gulf of Tunis were monitored seasonally, from November 2006 to October 2007, in relation to environmental conditions and reproductive events. The condition index increased during late gametogenesis and the ripe stage, coinciding with enrichment of the water by phytoplankton, and decreased during late summer and autumn (i.e. the spawning and rest periods). Glycogen increased during early gametogenesis and peaked during winter, pointing to its mobilisation in the formation of active ripe gametes. The foot, gonad/digestive gland, and adductor muscle were the three major glycogen-reserve tissues. Protein content was high during the end of summer in the whole individual and during autumn in the gonad/digestive gland. Lipid content started to increase as gametogenesis began, reached its peak at gonad ripeness and during the early spawning stage (summer) and sharply declined due to the shedding of gametes (autumn). A transformation of glycogen for de novo synthesis of lipids, in the gonad/ digestive gland, was suggested during the later stages of the gonadic cycle, in support of gametogenesis. Temporal fluctuations in environmental factors, particularly temperature and food supply, drive the cycle of storage and utilisation of metabolic energy reserves which in turn govern gametogenesis in <em>Donax trunculus.</em></p> <p><strong>Keywords:</strong> bivalve tissue, environmental parameters, glycogen, lipids, nutritional quality, protein, reproductive cycle </p> 2021-05-28T00:00:00+00:00 Copyright (c) Large-scale movements and site fidelity of two bull sharks <i>Carcharhinus leucas</i> estimated from a double-tagging experiment at Réunion Island (southwest Indian Ocean) 2021-05-28T12:13:35+00:00 M. Soria Y. Tremblay A. Blaison F. Forget E. Crochelet L. Dagorn <p>Since 2011, the mean number of bites per year by bull sharks <em>Carcharhinus leucas</em> has increased markedly at Réunion Island. To predict areas and periods of increased risk, we need to better understand the space-use dynamics of individual sharks. In coastal waters off Réunion Island, two bull sharks, one of each sex, were double-tagged and tracked for 174 days (male) and 139 days (female) using pop-up satellite archival tags (PSATs) and acoustic transmitters. Both sharks spent most of their time inshore (58.1% for the male and 89.9% for the female). The female performed short excursions but typically remained inshore. The male alternated between spending residence time along the coast and undertaking wide-ranging movements, including one extensive open-ocean excursion to the vicinity of a seamount situated about 210 km from the island. Differences in the residency and home range between the two sharks probably<br>reflect different patterns of foraging and mating behaviours. Our results highlight the advantages of double-tagging in telemetry studies that attempt to estimate the degree of habitat fidelity of a species and illustrate the need to consider the movement patterns of sharks at different scales when developing efficient risk-mitigation management.</p> <p><strong>Keywords:</strong> philopatry, pop-up satellite archival tags, residence time, shark-bite management, telemetry, western Indian Ocean </p> 2021-05-28T00:00:00+00:00 Copyright (c) Strengthening the DNA barcode reference library for South African estuarine macrofauna 2021-05-28T12:20:36+00:00 C. Fagg N.L. Phair L. Claassens R.S.K. Barnes S. von der Heyden <p>Seagrass ecosystems face many anthropogenic pressures, yet globally there is a lack of knowledge of their associated biodiversity. Molecular barcoding can aid in biomonitoring efforts, but few South African invertebrates are accounted for. We provide 48 new sequences for 15 invertebrate species across a range of taxonomic groups sampled from Z<em>ostera capensis</em> meadows, eight of which were not previously barcoded. Our work presents an important effort to strengthen the mitochondrial DNA cytochrome c oxidase subunit I (mtDNA-COI) reference database for estuarine macrofauna in South Africa, supporting regional and global conservation efforts.</p> <p><strong>Keywords</strong>: biomonitoring, DNA barcoding, estuarine biodiversity, invertebrates, Knysna Lagoon, mtDNA-COI, seagrass meadows, <em>Zostera capensis&nbsp;</em> </p> 2021-05-28T00:00:00+00:00 Copyright (c)