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Tuna and tuna-like species are highly mobile and migratory, and therefore assumed to have single highly connected populations over large ocean regions. However, empirical data on the genetic population structure of such species in the western Indian Ocean (WIO) remain limited. Understanding these structures is crucial for managing stocks across territorial and transnational waters. This study tested the hypothesis of single genetic stocks of skipjack tuna Katsuwonus pelamis and kawakawa Euthynnus affinis within the WIO region, specifically off Kenya, Tanzania, Mozambique, South Africa and the Seychelles. Samples were collected from commercial fishers, between September 2019 and July 2020. Genome-wide single-nucleotide polymorphism (SNP) data for skipjack (7 005 loci, average H O = 0.206) revealed low overall genetic differentiation (global F ST = 0.003; between-sample F ST = 0–0.013, where F ST is the genetic-distance value), but significant genetic differences between skipjack taken from a northern sampling region (i.e. central Tanzania, Kenya and Sri Lanka) and from a southern region (i.e. southern Tanzania, Mozambique and South Africa), with samples from the Seychelles falling closer to the southern grouping. SNP data for kawakawa (14 806 loci, average H O = 0.2585) indicate a patchy distribution of low but significant genetic differentiation among these WIO sites (global F ST = 0.018; between-sample F ST = 0.003–0.036), yet with no obvious geographically-based pattern. In comparison with data from previous studies, we conclude that there is evidence of subpopulations of skipjack and kawakawa within the WIO, and that a precautionary approach should be adopted for future fishery management.

This study determined the biometry, hatchability and growth performance of a strain of the brine shrimp Artemia franciscana, a species now widely introduced globally, obtained from salt farms in the Tanga Region, Tanzania. The cysts were collected from salt pans using a 100–150-μm mesh net, and then processed and packed in airtight containers. In the laboratory, cysts were incubated in beakers at 28 °C, 35 ppt and pH 8. After 24 hours, newly hatched nauplii were transferred to rearing tanks, in triplicate, and cultured for 15 days while fed the microalga Nannochloropsis oculata. Cysts of a strain of A. franciscana from Vĩnh Châu, Vietnam, were cultured as the control. The cysts from Tanga showed greater mean capsulated cyst diameter (213.4 μm) and chorion thickness (9.7 μm) than cysts from Vĩnh Châu (208.3 μm, 8.3 μm). A higher hatching percentage and hatching efficiency were recorded for the Vĩnh Châu Artemia (p < 0.05). Nevertheless, nauplii of the Tanga Artemia exhibited a significantly higher growth rate and survival percentage (74.56%) than the Vĩnh Châu Artemia (53.85%). These findings highlight the potential of the Tanga strain for pond culture, suggesting their suitability as a live feed to enhance larviculture outputs.

Intertidal communities are predictably affected by both physical and biological factors, with herbivory as a particularly important factor. Instead of dense beds of macroalgae, which typically characterise the mid-eulittoral zone along the south coast of South Africa, at Kalk Bay this zone is dominated by a generalist grazer, the goat’s eye limpet Cymbula oculus. Despite its voracious grazing behaviour, this species of true limpet (family Patellidae) appears to positively influence macroalgal richness through epibiotic interactions. This study aimed to determine the role of C. oculus as an alternative settlement substrate for several macroalgal species that would otherwise be consumed by this grazer. Additionally, we wanted to determine whether the limpet’s density was on the increase, and, if so, what such an increase might mean for a future scenario at the site. Data collection and sampling took place within quadrats along transect lines. The quadrats were photographed to determine macroalgal cover on the limpet shells and on the surrounding primary rocky substrate, and analysed using ImageJ software. Additionally, C. oculus density was estimated, as well as macroalgal biomass on both the limpet shells and the primary rocky substrate. It was found that C. oculus shells are an important secondary substrate for macroalgae, and that species of foliose and turfy macroalgae benefit more than encrusting macroalgal species. The mean density of C. oculus, at 20 (SE 2.22) ind. m−2, was significantly greater than that reported two decades ago. The potential exists for increased cover abundance of chemically and morphologically defended macroalgae on the primary rocky substrate, a future scenario that seems increasingly possible given the increased C. oculus densities.

Skipjack tuna Katsuwonus pelamis and kawakawa Euthynnus affinis are economically important tuna species targeted by artisanal fishers in the western Indian Ocean. Both species are currently considered sustainably fished despite substantial catches. Even so, ongoing management is crucial to maintain healthy populations. A comprehensive understanding of the size structure and reproductive biology of these tuna species across a broad spatial scale is essential for accurate stock assessments. This study investigated spatial and gear-based seasonal variations in the size structure of landed catches and describes the reproductive traits of the two species, including spawning season, size at maturity and sex ratio. Data were collected from the artisanal fishery in Kenya, Tanzania and Mozambique, and the recreational fishery in South Africa. The results revealed seasonal variations in size structure for both species, potentially influenced by fishing methods and local environmental conditions. The landings were biased towards male fish for both species, with sex ratios of 58% and 53% males for skipjack and kawakawa, respectively. Size at maturity (L 50) was estimated at 42.0 cm fork length (FL) for skipjack females and 47.0 cm FL for males, and kawakawa at 44.0 cm FL for females and 45.3 cm FL for males. At all study sites, spawning occurred year-round, with peak spawning activity from October to March, which coincides with the northeast monsoon. The findings provide updated information on the reproductive demographics of skipjack and kawakawa, which is crucial for assessing stock status and implementing sustainable fishery management practices in the region.

The pulmonate limpets Siphonaria concinna and S. serrata co-exist intertidally on many South African rocky shores. The two species are similar morphologically and in terms of their intertidal habitat and distribution; both lay gelatinous egg masses on the shore, but have different reproductive strategies. Siphonaria concinna has planktotrophic development involving high fecundity and small eggs. Field observations over 2 years showed that the majority of spawning occurs during summer. A discrete recruitment event follows an estimated planktonic period of 2 months. Siphonaria serrata has direct development with large eggs and lower fecundity. Spawning is seasonal, although egg masses can be found throughout the year. Newly hatched juveniles can only be detected after ∼2 months of growth. A single juvenile cohort follows ∼4 months after spawning. A comparison of the life-history traits of the two species revealed that S. concinna reproduces at a smaller size and juveniles grow faster but have a higher mortality rate than S. serrata juveniles. Adult growth rates for S. concinna are slower and likewise show higher mortality; maximum adult size is smaller and the lifespan is estimated to be shorter than that of S. serrata. Accordingly, the life history of S. serrata is characterised by reproduction at a larger size, and juveniles that grow slower and have a lower mortality rate than S. concinna juveniles. Adults grow to a larger size, suffer lower mortality and probably live longer. These life-history attributes correspond to the classic r- and K-selected traits.

Despite being important to artisanal and industrial fisheries, little is known about the role of skipjack tuna Katsuwonus pelamis and the eastern little tuna (or kawakawa) Euthynnus affinis in recreational fisheries. This study used a banner-recruited, online questionnaire of recreational tuna anglers in South Africa to characterise the role of these two species in this fishery and those that participate. Respondents (n = 108) were generally well-equipped with high-capacity skiboats and were highly specialised (high levels of fishing avidity and centrality-to-life). Many indicated that they actively targeted skipjack and eastern little tuna, suggesting that these species are not only bycatch. Both species had little food value to the respondents, who were more motivated to catch/target them for sport or for use as bait for more desirable species. Half the respondents perceived poor compliance in the tuna recreational fishery, and most felt that efforts at monitoring, control and surveillance were poorly implemented. However, many were willing to engage in mandatory reporting of tuna catches, and the majority supported a harvest moratorium for tuna if required. Perceptions of catch seasonality and trends generally aligned with the available data, suggesting that fishers’ ecological knowledge may be valuable for management of heterogenous subsectors of the recreational fishery.

This study investigates the value chain of Kenya’s artisanal tuna fishery targeting skipjack tuna Katsuwonus pelamis and kawakawa Euthynnus affinis. The study used an integrated approach combining a questionnaire survey and catch assessment data at four landing sites along the coast of Kenya to examine the socio-demographic profiles of key actors (i.e. fishers, traders, processors, and agents/investors), the structure and function of the value-chain nodes, and the associated economic benefits. The findings reveal that fishers, the primary actors, sell most of their catch (53%) to agents, with the remainder distributed to traders (20%) and processors (18%). Processors, predominantly women, play a key role in the value chain and had the highest net profit margin (49.5%). Limited post-harvest infrastructure, inadequate transportation and poor marketing conditions were identified as key challenges that impact the quality of the fish sold, and hence the income generated. These challenges disproportionately affect fishers who have limited access to market information and financial resources. The findings demonstrate the need for multi-level interventions to optimise benefits from the artisanal tuna fishery along the entire value chain, taking into consideration the economic, environmental and social dimensions.

Protea Banks, a reef off the east coast of South Africa and a renowned dive destination, attained marine protected area (MPA) status in 2019. This study investigates the economic significance of the shark diving industry at this MPA and examines diver experiences. Data were collected by means of a semi-structured questionnaire survey of 107 divers, between December 2018 and June 2019. Diver counts (883 launched divers; 95% CI = 760–1 025) estimated from the boat launch site monitoring system were used to estimate economic value through mean visitor expenditures. Of the respondents, 80.3% (n = 82) identified tiger sharks Galeocerdo cuvier and scalloped hammerhead sharks Sphyrna lewini as primary attractions within the MPA. Annually, the assessed economic value of the shark-diving sector at Protea Banks totals ZAR 39 875 891 (USD 2 574 298, at ZAR 15.49 = USD 1.00) (95% CI = ZAR 34 123 302–46 384 726), conferring substantial benefits to the local and national economies. Divers highly rated both the dive quality and operator standards, and despite sharks approaching to within an average distance of 2.2 m (95% CI = 1.4–3.0), none of the interviewed divers reported shark aggression, maintaining a sense of safety during encounters. A large majority (70.7%) expressed support for chum usage to enhance shark sightings. This research underscores the substantial economic contributions of this diving industry and illuminates perspectives crucial for shaping future conservation strategies, specifically regarding MPAs in South Africa.