Globally, fish are facing the pressure of increasing thermal variability, which challenges the physiological capacity and resilience of species at the level of the individual, population and community. As transition zones, estuarine ecosystems are becoming increasingly thermally dynamic, which affects their condition and function as nursery habitats. Assessing the thermal tolerance of estuary-associated species is critical to understanding their adaptive capacity to these increasingly variable environments. This study aimed to assess the thermal tolerance of dusky kob Argyrosomus japonicus to understand their capacity to adapt to thermal variability. Thermal tolerance was estimated for individual fish using a repeated measures (CTmin and CTmax) dynamic method, while physiological breakpoints were estimated by running a piecewise linear breakpoint analysis on estimates of the respiration rate (opercula beat counts) at every 1 °C degree change. Among 12 individuals, the thermal endpoint ranged from 8.1 °C to 32.6 °C, with an overall thermal breadth of 24.5 °C. Piecewise linear breakpoint analysis of respiration rate identified a lower breakpoint at 13 °C and an upper breakpoint of 28 °C, suggesting an optimal thermal range between these temperatures. There was considerable variability in the thermal endpoints and breakpoints of the individuals. Using a percentile ranking method, fish were categorised as broadly tolerant, intermediate or narrowly tolerant thermal performers. The broad thermal breadth and diversity in thermal physiological phenotypes suggests that this species may have considerable capacity to adapt to an increasingly variable environment. However, since environmental temperatures may exceed the upper breakpoint in upper reaches of an estuary and fall below the lower breakpoint near the mouth of the estuary in summer months, it is likely that further increases in variability may have implications for the way in which the species utilises the estuarine environment in the future.

Reports of learned behavioural adaptations of wild Cape stumpnose Rhabdosargus holubi (family Sparidae) were investigated in two estuarine systems in the Eastern Cape Province, South Africa. Underwater observational trials using video cameras demonstrated that fish rapidly responded to bait collection activities in the East Kleinemonde Estuary but not in the West Kleinemonde Estuary. By simulating the visual and auditory cues associated with the activity alone, evidence suggested that fish in the East Kleinemonde Estuary have learnt to associate these cues with the activity, likely allowing individuals to identify and respond to this novel foraging opportunity. It is suggested that historical rates of bait collection within these estuaries is likely the cause for differences in the fish behaviour between estuaries; furthermore, social learning in fish may play a role both in perpetuating learned behaviours in the systems where they occur, and learned behaviour may potentially be shared to new systems via fish migration. This study presents a rare example of natural behaviour conditioning of wild fish to an anthropogenic niche. Further investigations into the cognitive and behavioural processes that allow wild animals to respond to human disturbance should be expanded to better account for and manage the wide-ranging impacts of human disturbance.

The giant trevally Caranx ignobilis (family Carangidae) is an iconic apex predatory fish valued by recreational and commercial fisheries globally. A seasonal but episodic aggregation of up to ∼1 000 individuals takes place in the Mtentu Estuary in the Eastern Cape Province, South Africa, every year; however, the reason behind this unique aggregation is unknown. With the aim of understanding various aspects of their movement patterns, 10 individuals (sized 430–1 030 mm FL) were tagged with acoustic transmitters in the estuary and subsequently monitored by passive acoustic receivers in the estuary and adjacent coastal environment for up to 3.6 years. The results showed clear movement patterns, including repeated daily migrations between the upper estuary (∼4 km upstream) during the day and the mouth region or into the sea at night. Statistical modelling revealed that sea temperature was the main environmental variable associated with their pattern of daily estuarine presence/absence. During summer, the daytime temperature of the surface waters in the deep and thermally stratified Mtentu Estuary may be 3–6 °C warmer than the subsurface waters, providing a more suitable thermal environment for this tropical to subtropical species. Notwithstanding the possibility of other environmental drivers, we propose that adult C. ignobilis utilise the Mtentu Estuary during periods of cold coastal upwelling, primarily as a thermal refuge, specifically for daily re-warming after nocturnal feeding in colder water. The maintenance of this site as a no-take (no fishing) zone, as well as encouragement of tourism-linked incentives to increase local custodianship, are strongly recommended to protect this vulnerable aggregation.

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.

The marine shore-based recreational fishery (MSBRF) is the largest sector of the South African linefishery. The high rate of participation, widely dispersed effort, the diversity of motivations, attitudes and perceptions of its participants, and the large number of target species make this fishery a highly complex socio-ecological system with many governance challenges. Although improving the communication between fishers, scientists and managers is important to improve governance of the MSBRF, the views of multiple stakeholders in the MSBRF have not yet been synthesised. Here, horizon scanning was used to identify what stakeholders view as the 100 most-important research questions that need to be addressed to improve MSBRF management and sustainability. A total of 365 questions submitted by 175 stakeholders (63% anglers and spearfishers, 31% anglers, 3% spearfishers, 2% researchers, and 1% managers) were reviewed, thematised and refined by experts in the field to generate 100 priority research questions for the MSBRF. The 12 themes of these are: governance, management actions, resource monitoring, competing sectors, human dimensions, bioeconomics, compliance, education and engagement, catch and release, ecosystem impacts, marine protected areas, and threats to sustainability. This study aligns with the ecosystem approach to fisheries, can guide the development of research programmes, and will assist in co-management of the fishery by embedding the needs and opinions of stakeholders within the research process.

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.

In most fisheries, the traditional centralised, top-down management approach is still prevalent despite its limited success. While a bottom-up ecosystem-based approach is increasingly preferred, a shift in focus from purely ecological considerations to a socio-ecological system approach is necessary. The latter approach requires understanding interrelationships between fishers, the environment and legislative bodies. We conducted an onsite survey of shore-based anglers in Dorob National Park, Namibia, in 2023, to assess angler motivations, compliance, behaviour and perceptions regarding the regulations and management of the fishery. Among the 234 anglers interviewed, the main subgroup comprised recreational anglers who fished primarily for recreation and food (but not subsistence). Only a small portion (n = 6 anglers, ∼3% of the sample) said they fished solely for livelihood support, and thus constituted small-scale or subsistence fishers. Twenty percent of the anglers reported deliberate non-compliance behaviour, attributing this to declining catches and a need to support their livelihood. We thematised the anglers’ perceived reasons for declines in primary target species (namely, silver kob Argyrosomus inodorus, west coast steenbras Lithognathus aureti and galjoen Dichistius capensis). The four most-cited themes were: (i) inadequate regulation of commercial boat fishing or recreational ski-boat fishing; (ii) non-compliance, overfishing or illegal fishing; (iii) climate/human-induced environmental changes; and (iv) poor enforcement of the fishing regulations. The anglers’ top suggestions for improving the shore-based fishery in Namibia included stricter or revised harvest control rules and improved enforcement. From these data we conclude that it is imperative that the suggestions and concerns of shore-based anglers be addressed by all stakeholders in the fishery, and that regulations aimed at promoting sustainable fishing should incorporate the ecological and social/human dimensions if the stock status and management of Namibia’s shore-based fishery is to improve.

The dusky kob Argyrosomus japonicus (family Sciaenidae) is a large-sized estuarine-dependent fish of high fisheries importance, with only ∼1.3% of the pristine stock remaining due to overexploitation, habitat loss and mismanagement. Its conservation is complicated by the species’ cryptic morphology and range overlap with congeners. Understanding movement patterns is critical for effective management. Historical catch-per-unit-effort data suggest annual adult spawning migrations from waters off the Western Cape and Eastern Cape to off KwaZulu-Natal, South Africa; however, contemporary acoustic telemetry data indicate higher adult residency than generally assumed. Therefore, this study analysed 39 years (1984–2022) of tag-recapture data from the Oceanographic Research Institute’s Cooperative Fish Tagging Project (24 520 tag-releases, and 1 600 tag-recaptures). After taxonomic screening, 18 488 tag-releases were retained for the dataset, of which 77% were juveniles, 18% were sub-adults, and 5% were adults. Of 1 420 recaptures, 80% were juveniles, 17% sub-adults, and 4% adults. Mean movement distances were 16 km for juveniles and 43 km for sub-adults, with only 18% and 34% of those groups, respectively, displaying movements of >100 km. Among 53 adults (mean movement of 114 km), 55% moved >100 km, but only 2 of these adults displayed eastward movement consistent with a spawning migration to the coast of KwaZulu-Natal. These results, combined with prior studies, support emerging paradigms of partial migration and the existence of highly resident subpopulations of A. japonicus within a metapopulation structure. High residency and limited connectivity among coastal zones across size classes (life stages) suggest that management should prioritise spatially explicit measures, such as estuary-specific protection, to conserve local populations and prevent further declines of this imperilled species.

Water temperature is a key abiotic factor affecting fish physiology, behaviour and distribution, with fluctuations impacting species assemblages and habitat use. The flathead mullet Mugil cephalus is a eurythermal species with a cosmopolitan distribution across tropical, subtropical and temperate coastal environments, and is prevalent in South African estuaries. However, little is known about its thermal limits in temperate regions, where future climate change is expected to increase the frequency and intensity of temperature extremes. This study quantified the lower and upper critical thermal limits (CTmin and CTmax) of M. cephalus from the warm-temperate Kowie Estuary, Eastern Cape Province, South Africa. Using the critical thermal methodology, 14 juveniles and 1 sub-adult M. cephalus were exposed to simulated heating and cooling events in a controlled laboratory setting. The results showed that M. cephalus that were acclimated in seawater (salinity 35) at 18 °C had a broad thermal breadth of 34.4 °C, with a CTmin of 3.2 °C and CTmax of 37.7 °C. Despite this wide thermal tolerance, physiological stress was observed at 13 °C and 29 °C—temperatures that align with seasonal extremes (the minimum winter and maximum summer water temperatures recorded) at the estuarine–riverine interface of the in situ environment. These findings suggest that while M. cephalus is unlikely to face its thermal limits in this estuary, some individuals may experience thermal stress, potentially influencing their seasonal habitat use. Global rising temperatures are projected to negatively impact estuarine species through increased intensity and duration of thermal stress. Therefore, understanding species’ physiological thresholds is crucial for predicting how individual species and entire ecosystems will respond to climate change, and for developing appropriate management strategies under shifting environmental conditions.