Mesopelagic fish often dominate the micronekton in the mesopelagic zone. Despite their ecological importance, information about their diversity, distribution and abundance in some areas of the Indian Ocean remains patchy and incomplete. Using data collected from three acoustic/trawl surveys spanning the western (WIO), central (CIO) and eastern (EIO) Indian Ocean regions in 2018, we investigated the composition and structure of the fish assemblages that were sampled using different pelagic trawl types, namely Macrozooplankton and Multpelt, which targeted the deep scattering layer (DSL) and shallow scattering layer (SSL). We assessed the structure of mesopelagic assemblages across the region at the level of genus and family, and analysed relationships between the samples with concurrently collected hydrographic data using multivariate statistics. Mesopelagic fish taxonomic richness varied across the region, as follows: WIO - 68 genera (13 unique), 34 families (4); CIO - 48 genera (6), 35 families (5); and EIO - 86 genera (26), 44 families (9). Although there was a pronounced difference in the structure of the assemblages by survey region, gear type and time of day (and many interactions between these), the assemblages were significantly impacted by water temperature, dissolved oxygen and aspects of the chlorophyll a environment- which varied across the sampled areas. Intriguingly, the spatial patterns (and diversity) observed when the mesopelagic fish assemblages were examined at the genus level were likewise reflected when the data were analysed at the family level. The implications of these results are discussed.
The reproductive success and long-term resilience of sea turtle populations crucially depends on the availability of adult males, yet studying these individuals remains challenging owing to their predominantly aquatic lifestyle. Sex determination in sea turtles is temperature dependent, with lower incubation temperatures producing males and higher temperatures producing females. Consequently, global warming could feminise hatchlings, thereby reducing male recruitment and exacerbating population vulnerability. This study provides the first systematic assessment of artisanal fishers' perceptions regarding changes in the abundance of male loggerhead turtles Caretta caretta in the eastern Atlantic. Drawing on the extensive experience of artisanal fishers (mean +/- SD = 38.6 +/- 7.9 years; n = 21) as a potential source of incidental loggerhead turtle bycatch, we conducted structured interviews with long-term fishers in six fishing communities on Maio Island, Cabo Verde, in regard to sightings of male sea turtles and mating events, as well as traditional uses of sea turtle parts. Their responses revealed a widespread perception of declining male abundance, mating activity and availability of organs used in cultural practices. These findings underscore the value of combining local ecological knowledge with biological monitoring to guide conservation strategies that reflect both ecological realities and cultural contexts.
Populations of blue whales Balaenoptera musculus and fin whales B. physalus were severely depleted by 20th-century commercial whaling. Historic whaling data suggest that the southeastern Atlantic Ocean was an important nursery and feeding area for both species. Here, we collate published and opportunistic sightings and strandings of two subspecies, the Antarctic blue whale B. musculus intermedia and southern fin whale B. physalus quoyi, in the southeastern Atlantic off the coasts of Namibia and South Africa, between May 1964 and March 2025. Blue whales were sparse, with 12 sightings, 1 stranding and 5 published records, whereas fin whales were seen more frequently, with 76 sightings and 6 strandings. Blue whale sightings in the Benguela ecosystem peaked between late spring and autumn, whereas fin whales appeared to be present throughout the year. The majority of offshore sightings were obtained from marine mammal observer data, which covered all months of the year but had the greatest effort between November and April. Although the total number of sightings of each subspecies was relatively low, sightings appear to be steadily increasing as these whale populations recover, with 95% of the observations recorded since 2012. Given the vulnerability of these populations and the anthropogenic risks the subspecies face across their ranges, their distribution and species-specific risks should be investigated, such as by using passive acoustic monitoring and increased presence of shipboard observers across commercial sectors, and as far as possible included in marine spatial development plans.
In December 2023, a pod of 15-20 killer whales Orcinus orca successfully predated on a likely juvenile (similar to 10-m long) humpback whale Megaptera novaeangliae that was part of a super-group (defined as feeding groups of 20+ individuals close to each other) on the west coast of South Africa. The event was recorded and photographed in detail by researchers from the University of Pretoria from a 6-m-long semi-rigid research vessel. The killer whales exhibited clear predation behaviour, with females and juveniles trying to drown the humpback whale by pressing their bodies on top of it to prevent it from breathing. Roughly one hour later, two additional male killer whales, which had not been active in the predation event up to this point, joined the pod and had a role in the final killing of the humpback whale. The kill occurred underwater and was inferred to have happened as pieces of blubber floated to the surface approximately a minute after the two males joined the pod. As humpback whales recover from whaling, and because super-groups of the species seem to use the west coast of South Africa over multiple months of the year, interactions between humpback whales and killer whales are likely to become more common.
The morphological structure of fish otoliths is under dual regulation by genetic and environmental factors, and has been successfully used to aid stock delineation. This study examined whether differences in sagittal otolith shape outlines could be used to delineate potential stocks of the spotted grunter Pomadasys commersonnii (family Haemulidae) in South Africa. We hypothesised that P. commersonnii from two biogeographically distinct estuaries exhibit differences in their otolith outlines, and that there will be differences in the otolith shape of P. commersonnii captured in the Breede Estuary pre-2002 versus post-2019, considering recent research that has revealed changes in their movement patterns. Significant differences were found in the outlines of P. commersonnii otoliths both spatially and temporally. These findings demonstrate that while the overall otolith shape is genetically constrained, finer details may vary under environmental influences.
In Morocco, diverse saline ecosystems support both industrial salt production and the growth of Dunaliella salina, a halophilic unicellular green microalga in class Chlorophyceae, and well-known for its applications in aquaculture, cosmetics, food and pharmaceuticals owing to its significant carotenoid-producing potential. This study compares the growth and carotenoid production of six strains (DUN1-DUN6) isolated from salt ponds in southern and central Morocco to identify high-performing local strains. Among these, strains DUN3, DUN4 and DUN5 exhibited exceptional carotenoid accumulation (maximum of similar to 18 mg l-1). Notably, strain DUN5 shows promise for industrial applications, combining high carotenoid yield with a cell density of 2 & times; 105 cells ml-1. These findings highlight the potential of D. salina as a sustainable source of beta-carotene. This research advances the sustainable valorisation of Morocco's saline resources and promotes economic opportunities through the industrial exploitation of this valuable microalga.
This study examined the geographical and seasonal distribution of wave parameters (significant height, period, direction) and wave energy in the Gulf of Guinea using MFWAM (M & eacute;t & eacute;o-France WAve Model) reanalysis data from 1993 to 2019. The model was validated utilising satellite, buoy and coastal camera data. The results indicate that significant wave height varies from similar to 0.5 to 1.5 m, with peak values occurring in boreal summer. Wave orientations predominantly originate from the southwest, with extended periods observed during the summer months. Offshore wave power ranged from similar to 4 to 20 kW m-1, with the value increasing as waves approach the coast due to the shoaling effect. Seasonal fluctuations demonstrate increased wave energy from May to September. The model marginally overestimates wave heights; however, the bias was deemed acceptable. This study emphasises the wave energy potential along the Gulf of Guinea coast, particularly adjacent to Benin, where measurements have surpassed 18 kW m-1. The findings endorse forthcoming coastal energy strategies and importantly indicate that harnessing wave energy could advance the renewable energy sources in West Africa.
The genus Sebastes (rockfishes) of family Scorpaenidae (scorpionfishes) contains over 110 species, of which the false jacopever Sebastes capensis is the only one found in South African waters. Although it is a commonly caught linefish along the west coast of South Africa, information about this species is limited. In this study, biological data were extracted for 269 specimens (size 205-372 mm total length [TL]), caught from boats using hook-and-line, to describe the life-history traits of this poorly known rocky-reef fish. The length-weight models indicated hyperallometric growth for females (W = 5.08 & times; 10-6 TL3.23) and isometric growth for males (W = 1.39 & times; 10-5 TL3.04) for fish in the age range 3 to 23 years. The best-fit von Bertalanffy growth model for females was TL = 358.1(1 - e-0.105(t + 5.70)), and for males TL = 355.4(1 - e-0.138(t + 3.21)). Males outnumbered females in the catches at a ratio of 2.68:1. Females matured at age 6 years and males at age 4 years. The spawning period lasted from late autumn to early spring (May-September). Notably, females are capable of storing sperm in their ovaries over the reproductive period; the presence of oocytes of different developmental stages in the ovaries suggests multiple brooding is a feature of its reproductive strategy. Brood fecundity was estimated at similar to 40 000-100 000 embryos. Sebastes capensis is a benthos-associated opportunistic and generalist predator. As a slow-growing and late-maturing rockfish, the species is potentially vulnerable to overfishing, particularly given its prevalence in catches of recreational and commercial boat fishers along South Africa's west coast.
Determining the thermal tolerance limits of fishes is crucial for understanding their persistence in the face of anthropogenic climate change. This study used dynamic critical thermal assays to determine the upper (CTmax) and lower (CTmin) critical thermal limits of an estuary-dependent mugilid, the grooved mullet Chelon dumerili, through laboratory experiments with individuals captured from the Kowie Estuary on the south coast of South Africa. Fish were exposed to simulated heating and cooling events, and their ventilation rate (opercular beats per minute) was evaluated at every 2 degrees C of incremental change to assess the thermal stress response of this species prior to reaching the equilibrium endpoint. The CTmax range of the individuals was 35.1-37.9 degrees C (mean +/- standard deviation: 36.45 +/- 0.56 degrees C), and the CTmin range was 5.4-7.6 degrees C (6.63 +/- 0.64 degrees C), resulting in a broad thermal range (difference between CTmax and CTmin) of 28.7-30.4 degrees C (29.4 +/- 0.5 degrees C). Despite a broad thermal range, break-point analysis of the linear relationship between ventilation rate and water temperature indicated that the species has a narrower thermal range because the experimental fish experienced thermal stress prior to reaching the critical thermal limits (mean upper and lower break points of 30.8 degrees C and 10.7 degrees C, respectively). Our findings suggest that water temperature changes driven by climate change may reduce fish performance near a species' thermal break points, potentially impacting its population dynamics.
The St Lucia estuarine lake and Mfolozi River mouth on the Indian Ocean coast of South Africa were once a single, connected system but were artificially separated in 1952 to prevent siltation from the Mfolozi River catchment which risked shallowing the globally significant estuarine lake. This separation deprived the St Lucia estuarine lake of its primary freshwater source and threatened its integrity. A rehabilitation project financed by the Global Environment Facility was implemented in 2017 to re-link the system to restore natural mouth functioning and marine connectivity. This study investigated benthic macrofaunal changes associated with the Mfolozi River reconnection, over the period February 2015 to November 2018. A decrease in salinity, increased water level and silt buildup coincided with significant differences in the benthic macrofaunal assemblage. This shifted to the presence of taxa indicating the disturbance from siltier sediment and freshwater predominance, such as the invasive freshwater gastropod Tarebia granifera, the brackish-water tanaidacean crustacean Halmyrapseudes cooperi and chironomid larvae. The mouth reconnection has increased freshwater input into the St Lucia estuarine lake system, reviving it from a prolonged dry or hypersaline state; however, further intervention is required to prevent further silt input via the freshwater inflow as this risks changes to the benthic macrofaunal assemblage, increased sediment buildup and regular mouth closure.
Macroinvertebrates such as snails and crabs influence aspects of salt marsh structure and function through herbivory and bioturbation. However, the effects of physico-chemical variables and habitat composition on their abundance and distribution remain underexplored. This study examined the influence of environmental factors on snail and crab populations across salt marsh habitats in the Berg River Estuary on the west coast of South Africa, hypothesising greater abundance in the intertidal than in the supratidal zone, driven by salinity and elevation. Snail and crab abundance were assessed using manual counts in triplicate quadrats (10 & times; 10 cm for snails; 25 & times; 25 cm for crab burrows) across six transects. Physicochemical variables of sediment and groundwater were also analysed. Six snail species (Davisassiminea sp. 1, Davisassiminea sp. 2, D. capensis, D. globulus, Afrolittorina africana and Melanoides tuberculata) and one crab species (Hymenosoma orbiculare) were recorded. Crab burrow density differed significantly between habitats, with the highest abundance at creek edges (85.37 burrows m-2) and lowest in supratidal habitat (8.57 burrows m-2). Snail richness and abundance declined with elevation; Davisassiminea sp. 2 and D. capensis occurred only at creek edges. Habitat and environmental variables explained 32% of the variation in macroinvertebrate abundance, with sediment conductivity and organic content being key predictors. Generalised linear models indicated that species richness declined with increasing sediment redox potential (t = -2.25, p = 0.025), organic matter (t = -2.07, p = 0.040) and clay content (t = -3.07, p = 0.003). These findings highlight the influence of local environmental conditions in shaping and predicting species distributions under a changing climate.
Rising global concern regarding the conservation and management of shark species makes it essential to identify key ecological species to guide conservation efforts, especially species with more restricted distributions such as endemic species. The southern African endemic spotted gully shark Triakis megalopterus is currently listed as Least Concern on the IUCN Red List of Threatened Species. Though not commercially exploited, increased catches of this species by recreational fishers and high misidentification with other species of Triakidae in commercial fisheries are bringing uncertainty regarding the future of the population. Although T. megalopterus is known for localised behaviour, the extent of its site fidelity and fine-scale movements remain largely unknown. Using passive acoustic telemetry, this study assessed the species' residency, site fidelity and movement patterns along the Western Cape coast of South Africa. Overall, the data revealed low detections, suggesting that individuals are using coastal areas not currently covered by receivers. Tagged T. megalopterus exhibited seasonal detection patterns within their zone of tagging, with increased detections during winter months. A high number of the detections occurred after the estimated parturition period; thus, the existing receiver array might not cover potential nursery areas for the species. The results also show that some individuals do undertake long-distance movements to other zones of the coastline, possibly driven by the need to locate reliable food sources. Finally, the species did not significantly use marine protected areas (MPAs), highlighting the importance of better receiver coverage and the need to expand existing MPAs.
Ulva reticulata is a fast-growing green macroalga with high bioremediation and economic potential. Cultivation of this seaweed at Bambous Virieux in southeast Mauritius was undertaken to evaluate its growth potential and biological feasibility in a tropical lagoon. Over a 30-day trial, three 6 & times; 6 m rafts constructed from common aquaculture system materials supported substantial biomass gains, with initial mean wet weights of 106.29 g per thallus, and final biomass per cultivation unit as high as 1 886.83 g (Raft 2), representing an similar to 18-fold increase and 60.55 g mean daily growth. Growth differences across rafts were significant (chi & sup2; = 144, p < 0.001), with Raft 2 showing the highest consistency (standard deviation = 8.74 g). Water quality remained within optimal ranges, with no significant temporal or site-based fluctuations. Routine maintenance was successfully conducted by trained community members, demonstrating the viability of participatory management. Preliminary ecological observations indicated that the seaweed rafts attracted herbivorous reef fishes, suggesting additional biodiversity benefits. Although the seaweed cultivation system proved to be highly productive and ecologically sustainable, in potential community-led scale-up operations, the lagoon location might present logistical challenges for some coastal communities to access. Overall, the results suggest U. reticulata is a climate-resilient species for sustainable open-water seaweed farming to support 'blue growth' in small island states while mitigating aquatic ecosystem issues (like absorbing excess nutrients and carbon), thus aligning economic development with ecosystem health. Future research on U. reticulata cultivation should focus on understanding its seasonal variations, nutrient dynamics, and optimal densities to maximise farming productivity and ensure resilience against environmental changes.
Oceanographic features such as upwelling cells and currents contribute to shaping the evolutionary history of marine fishes, including species that may be important socio-economic resources. However, the impacts of these barriers are often cryptic and may change, particularly in a rapidly changing climate. Hence, sustainable fisheries management strategies need to take into consideration the influence of oceanographic barriers to the genetic structure of populations. Here, we evaluated the influence of the Benguela Upwelling System, a known barrier for warm-temperate fishes, on the evolutionary history of the blacktail seabream Diplodus capensis (family Sparidae), using eight microsatellite loci in conjunction with DNA barcoding of the cytochrome c oxidase 1 gene. Diplodus capensis is an important recreational and small-scale fishery species, distributed across southern Africa. Overall, there was significant genetic differentiation across the region, but the level of divergence varied with the genetic marker. Our results identified two isolated populations-in the northern and southern subsystems of the Benguela Upwelling System-with limited contemporary gene flow, and no sub-structuring detected within the subsystems. Because the species is vulnerable to exploitation, with declines in South Africa, our results suggest that management policies should aim at regional levels for the northern population (off Angola and Namibia), whereas the southern population (off South Africa) was identified as an isolated and highly connected population.
Along the south coast of South Africa, extreme upwelling events lead to rapid temperature declines and prolonged marine cold-spells, which have, on occasion, led to large-scale mortalities of coastal fishes. In a somewhat antagonistic process, fishing exploitation has been shown to target specific phenotypes, potentially selecting for higher physiological diversity within exploited populations, reducing resilience to adverse environmental conditions. This study investigated the effects of thermal stress and exploitation in two populations (exploited and unexploited) of the commercially and recreationally targeted red roman Chysoblephus laticeps (family Sparidae) during an intense upwelling and a marine cold-spell event, using acoustic telemetry data. The results show that the acceleration of tagged fish (a proxy for fish activity) during the upwelling event differed significantly between the two populations (p < 0.05), with fish in the unexploited population maintaining their activity, and fish in the exploited population exhibiting reduced activity. The activity of the fish was also significantly different between the populations during the marine cold-spell, with those in the exploited population exhibiting reduced activity over time compared with the unexploited population. These findings highlight the impact of thermal extremes on fish activity and suggest that exploitation may selectively remove individuals that are tolerant to these events, which might consequently reduce resilience of the fish population. Based on these findings, it is concluded that networks of marine protected areas can promote fish populations that are resilient to future climatic conditions.
