Fifty-five-year longevity for the largest member of the family Sparidae: the
endemic red steenbras Petrus rupestris from South Africa
AH Andrews1* , MJ Smale2,3, PD Cowley4 and N Chang5
ABSTRACT
A previous study that explored the age and growth of red steenbras Petrus rupestris (Valenciennes, 1830), a large sparid (family Sparidae, seabreams or porgies) endemic to South Africa and reported to approach 2 m in length, provided estimates as a moderately slow-growing species with a maximum age near 30 years. The stock is considered collapsed and a fishing moratorium was imposed in 2012, resulting in this species being assessed as Endangered by the IUCN. One consideration in addressing population status is validation of life-history traits. In this study, estimates of age for red steenbras from thin-sectioned otoliths were reassessed visually in terms of both the original ages and revised estimates using a different age-reading pattern. The revised ages exceeded the original ages by up to 30 years, with a maximum estimated age of 55 years from a well-defined otolith section that provided a basis for the revised age-reading procedure. Bomb radiocarbon (14C) dating revealed there was an offset in the timing of the 14C rise on the Agulhas Bank that, when coupled with considerations for regional oceanography, provided support for the revised age-reading pattern and an estimated longevity that exceeds
50 years. These findings were further supported by the fortuitous recapture of a tagged red steenbras that was at liberty for 22 years.
Keywords: age validation, bomb radiocarbon dating, carbon-14, copper steenbras, endangered species, fishery closure, otoliths
Environmental influence on phytoplankton communities in the northern
Benguela ecosystem
R Barlow1,2* , T Lamont2,3, D Louw4, M-J Gibberd2, R Airs5 and A van der Plas4
ABSTRACT
An investigation of surface phytoplankton communities was undertaken on the shelf of the northern Benguela upwelling ecosystem during austral autumn (May) and spring (September), along latitudinal transects at 20º S and 23º S, from 2 to 70 nautical miles offshore, as well as on a zigzag grid located between these transects. Microscopic identification of the phytoplankton and CHEMTAX analysis of pigment biomarkers were used to characterise the community composition. During May 2014, warmer, more-saline water with a shallower upper mixed layer corresponding to periods of less-intense offshore Ekman transport was encountered on the shelf. Satellite imagery indicated high phytoplankton biomass extending for a considerable distance from the coast, and CHEMTAX indicated diatoms as dominant at most of the stations (52–92%), although dinoflagellates were dominant at some inshore localities (57–74%). Species of Chaetoceros, Bacteriastrum and Cylindrotheca were the most abundant, with abundance of the Pseudo-nitzschia ‘seriata-group’ being particularly high at a number of stations. In September 2014, more-intense wind-forcing resulted in a deeper upper mixed layer and stronger upwelling of colder, less-saline water. Elevated phytoplankton biomass was confined close to the coast, where diatoms accounted for most of the population (54–87%), whereas small flagellates, such as prasinophytes, haptophytes and cryptophytes, as well as the cyanobacterium Synechococcus, dominated the communities (58–90%) farther from the coast. It is hypothesised that stronger upwelling and deeper vertical mixing in September of that year were not conducive for widespread diatom growth, and that small flagellates populated the water column by being entrained from offshore onto the shelf in the upwelled water that moved in towards the coast.
Keywords: Benguela upwelling ecosystem, CHEMTAX, hydrography, pigment biomarkers, SE Atlantic, species composition, surface waters
The autumn occurrence and spatial distribution of cetaceans in the waters
of Mauritania during a geophysical survey in 2012
G Russell*, A Sánchez-Cabanes and M Nimak-Wood
ABSTRACT
This study aimed to identify the occurrence and spatial distribution of cetaceans in Mauritanian waters (eastern Atlantic). A geophysical survey was conducted over 92 days between September and December 2012. Over 967 km was surveyed, with 1 017 h of observer effort recorded. Visual watches for marine mammals resulted in 228 cetacean sightings of 16 different species, including 12 odontocetes and 4 mysticetes. The short-finned pilot whale Globicephala macrorhynchus was the most frequently encountered odontocete, followed by the
Atlantic spotted dolphin Stenella frontalis. Of the mysticetes, the sei whale Balaenoptera borealis and blue whale B. musculus were encountered most frequently. We used generalised additive models and generalised linear models to examine the relationships between environmental factors and cetacean distribution. The results showed depth had a significant influence on presence of the short-finned pilot whale, which preferred waters deeper than 1 800 m. The model for mysticetes identified bathymetry as a significant environmental predictor, whereas slope of the ocean floor had a positive effect but was not significant. This study contributes to our understanding of
cetaceans in Mauritanian waters, using data from a platform of opportunity.
Keywords: generalised additive model, generalised linear model, marine mammals, mysticetes, odontocetes, seismic data, short-finned pilot
whale, West Africa
The occurrence of mycotoxigenic fungi in abalone feed in South Africa
MR Greeff-Laubscher1 , I Beukes2, GJ Marais3 and K Jacobs1*
ABSTRACT
Abalone feed in South Africa is mainly produced from locally sourced grains and has soy or fishmeal as a protein source. Contamination with fungal species associated with grain-based animal feed is, therefore, a predictable risk. Various Fusarium, Penicillium and Aspergillus species are known to produce mycotoxins when colonising grains. The aim of this study was to investigate the occurrence of toxigenic fungal species in the ingredients of compound abalone feeds as well as in final abalone-feed products. In this study, 248 fungal isolates were obtained from feed ingredients and compound abalone feeds collected from three different South African abalone farms and one feed supplier. Morphological and multigene phylogenetic analyses were used to identify the fungi isolated. Fusarium, Penicillium and Aspergillus species were consistently isolated. The presence of these species does not always result in mycotoxin contamination.
Keywords: aquafeeds, Aspergillus, fungal isolates, Fusarium, molecular identification, multigene phylogenetic analysis, mycotoxins, Penicillium
Kelp forests in False Bay: urchins vs macroalgae in South Africa’s
southwest coast biogeographical transition zone
K Morris1 and LK Blamey1,2*
ABSTRACT
Kelp forests along South Africa’s southwest coast occur in a region of biogeographical overlap. Commonly referred to as transition-zone kelp forests, these dynamic ecosystems are particularly susceptible to grazing from species such as urchins. This study explores relationships between urchins and macroalgae in transition-zone kelp forests along the western side of False Bay. Six kelp forests in this region were sampled. At each site, the urchin Parechinus angulosus and kelp Ecklonia maxima were counted, percentage cover of understorey algae was recorded, and drift algae were collected. Despite variability in urchin and algal cover across sites, kelps generally increased from north to south, while urchins did the opposite. Urchins were negatively associated with macroalgae, although this relationship was weak. A localised threshold of 50 urchins m–2 was identified, above which kelp density failed to increase above 10 m–2 and percentage cover of understorey algae usually remained below 20%. Surprisingly, no relationship was discovered between drift algae and attached algae. The findings highlight the complexity of these cool-water environments. To better understand the role of urchins in this system, particularly under changing climate trends, experimental research into the feeding behaviour and possible effects of urchins on
attached algae in the presence/absence of drift algae is advised.
Keywords: drift algae, Ecklonia maxima, grazer, Parechinus angulosus, seaweed, temperate reef
Extreme significant wave height climate in the Gulf of Guinea
AA Osinowo1* , EC Okogbue2 , EO Eresanya1 and OS Akande3
ABSTRACT
This article investigates spatio-temporal trends for different return periods of extreme significant wave height (SWH) in the Gulf of Guinea (GG), northeastern tropical Atlantic Ocean, based on a 37-year (1980–2016) wave hindcast. High-resolution reanalysis windfield datasets were used to force the spectral wave model WAVEWATCH III. The wave hindcast information was validated using data gathered from the US National Data Buoy Center. The model performance was adequate. In a spatial analysis, the trends were less than 0.3 m decade−1 in all parts of the GG, and were increasingly positive westwards, extending to the far western part of the GG; trends below 0.01 m decade−1 dominated in the eastern part and some areas of the northern part of the gulf. Temporal analysis showed that the trends were negative in all cases. Spatio-temporal trends in the return periods for the 99th-percentile wave height were generally weak. Also, trends in the yearly, seasonal and monthly means of extreme SWH all generally increased from east to west in the GG. Furthermore, temporal trend analysis showed that extreme SWH exhibited an increasing trend of 0.0041 m y–1 throughout the 37-year period; by season, it exhibited a declining trend of −0.0005 m y–1 in winter, and an increasing trend of 0.0048 m y−1 in summer. The observed
increasing positive trend of extreme SWH westward in the GG, however, suggests an increasing storminess towards the western part of the gulf, with potential implications for coastal flooding and erosion, and consequences for coastal structures.
Keywords: buoy data, extreme wave height, mid-Atlantic Ocean, regional model, return periods, spatio-temporal trends, wave climate, wave hindcasts
Life-history traits of seabream Boops boops (Linnaeus, 1758) in the
central eastern Atlantic Ocean (Canary Islands)
A Massaro1,2* and JG Pajuelo2
ABSTRACT
Life-history traits of the sparid Boops boops are studied for the first time in an isolated Atlantic population. Boops boops is a hermaphrodite species characterised by late gonochorism or non-functional hermaphroditism. The 1:0.852 ratio of females to males was significantly different from unity. Both sexes had similar size distributions; however, females outnumbered males at sizes of between 16 and 18 cm TL, whereas males predominated among fish larger than 28 cm TL. The average lengths at first maturity for females and males were 16.02 (SD 0.28) and 16.49 (SD 0.25) cm TL, respectively, with no significant difference between sexes. Spawning-capable females were first observed in January, at a low percentage, and were present until May. The gonadosomatic index of both females and males peaked in February. Vitellogenesis did not begin synchronously, and the simultaneous presence of oocytes in all stages of development indicated an asynchronous mode of ovarian development. The presence of postovulatory follicles, together with tertiary-yolk-vesicle oocytes, indicates that the species is a multiple spawner. An age–length key showed an age-group composition for females of between 0 and 4 years, whereas males were between 0 and 5 years. The von Bertalanffy growth parameters for females were
L∞ = 32.3 cm TL, k = 0.44 y–1 and t0 = −0.81 y, and for males they were L∞ = 35.3 cm TL, k = 0.38 y–1 t0 = −0.80 y.
Keywords: age, gonad histology, growth, maturity, otoliths, reproductive cycle, sex ratio, spawning period
Distribution and habitat use of Indo-Pacific bottlenose dolphins Tursiops
aduncus along the south coast of South Africa
OA Vargas-Fonseca1,2*, SP Kirkman3 , D Conry1,2, GM Rishworth2,4 , V Cockcroft1 and PA Pistorius1,2
ABSTRACT
The distribution and habitat use of the Indo-Pacific bottlenose dolphin Tursiops aduncus along the south coast of South Africa was investigated using bi-monthly boat-based surveys, from 2013 to 2016, over a distance of 145 km between the Goukamma and Tsitsikamma marine protected areas (MPAs). Survey effort totalled 6 239 km and 750.6 hours. Encounters occurred throughout the area (n = 200) and more frequently during winter. Pod sizes ranged from 1 to 350 animals (mean = 45), with calves mostly associated with larger groups. During 70% of the encounters, at least a single calf was present (up to 80 calves; mean = 5) and calves were more prevalent during summer. Encounters occurred mostly in shallower (mean = 11.3 m) and warmer (mean = 17.4 °C) waters. Groups aggregated in larger sizes in colder waters. Most encounters were in Plettenberg Bay and the Goukamma MPA, both of which are characterised by sandy bottoms and sheltered areas. Encounter rates were lowest in rocky and exposed areas, but dolphin groups in such locations were larger and usually travelling. There was a relatively low association of T. aduncus groups with MPAs, except for a sandy area in the Goukamma MPA in particular, suggesting some mismatch between favourable T. aduncus habitat and habitat protection.
Keywords: conservation, generalised linear mixed-effects models, Goukamma, habitat preference, line-transect survey, marine protected area,
Plettenberg Bay, Tsitsikamma, western Indian Ocean
Age, growth and natural mortality of blue jack mackerel Trachurus
picturatus (Carangidae) from the Canary Islands, Spain (NW Africa)
A Jurado-Ruzafa* and MTG Santamaría
ABSTRACT
Status assessment of exploited fish populations is a general requirement to ensure sustainability of fisheries in both global and local contexts. The blue jack mackerel Trachurus picturatus (Bowdich, 1825) remains unassessed in a data-limited fishery in the eastern central Atlantic, within the waters of the Canary Islands (Spain) off the coast of northwest Africa. This is the most southwestern targeted fishery in the species’ distributional range. To obtain basic life-history information, a total of 2 472 specimens were analysed, ranging from 10.2 to 31.9 cm total length, from samples collected monthly between March 2005 and March 2006. Length–weight relationships (a = 0.0023; b = 3.418; R2 = 0.97) showed no significant difference between sexes. Deposition periodicity and coherence of the ageing criteria were verified by analysis of the monthly evolution of the otolith edge. Ages were estimated by counting annual growth rings on whole otoliths, with these fish ranging from age 0 to 6 years. The von Bertalanffy growth model, which was used to estimate growth parameters from both direct readings and back-calculated length-at-age data, gave good statistical fits. The parameters from the direct readings were: L∞ = 34.9 cm; k = 0.214 y–1; t0 = −2.545 y (R2 = 0.84), and the natural mortality rates estimated from empirical methods varied between 0.20 and 0.55 y–1.
Keywords: ageing criteria, eastern central Atlantic, data-limited fishery, otolith shape, small pelagic fish
Population fluctuations of Cerithidea decollata (Gastropoda: Potamididae)
in mangrove habitats of the St Lucia Estuary, South Africa
JL Raw1,2* , GM Rishworth1 , R Perissinotto1 and JB Adams2
ABSTRACT
Mangrove macrobenthos species are used as ecological indicators as they are sensitive to changes in sediment properties. In this study, the population density of the common mangrove whelk Cerithidea decollata was assessed during different environmental conditions in the St Lucia Estuary, South Africa. Previously, this species was found to persist through both hypersaline and freshwater-dominated conditions. The natural variability in C. Decollata populations could provide information on the potential for this widespread species to be used as an ecological indicator in mangroves. We found that snail population density as well as sediment conductivity, moisture content and organic content differed between three mangrove sites that were monitored between 2010 and 2015. The relationship between snail population density and physicochemical characteristics of the sediment was therefore investigated using a mixed-effects model, and sediment conductivity was found to be the best predictor of C. decollata abundance. The resistance of this species to environmental variability could inform on resilience to ecological shifts, which is important when measuring responses associated with climate change.
Keywords: abundance, eurytolerant invertebrate, indicator species, mixed-effects model, sediment conductivity, sediment moisture content,
sediment organic matter
Unique co-occurrence of two genera of cymothoid ectoparasitic isopods
on the same individual fish host
RL Welicky* and NJ Smit
ABSTRACT
Parasitism of a host fish by a single species of cymothoid is regularly reported. For the first time on a temperatefish host and in South African waters, we observed two species of cymothoids, the flesh-infesting Anilocra capensis and the tongue-infesting Ceratothoa africanae, simultaneously infesting the same individual hottentoseabream Pachymetopon blochii. Each P. blochii harboured an ovigerous and non-ovigerous A. capensis and an ovigerous and non-ovigerous C. africanae. Cymothoid co-occurrence is rarely reported, and this is the first report of two cymothoid species infesting a single fish host from Africa. Globally, it is the first record of flesh- and tongueinfesting cymothoids parasitising the same individual fish.
Keywords: Anilocra, Ceratothoa, Cymothoidae, marine fish, parasite, South Africa, southeastern Atlantic Ocean
