A livelihood fishers catch for the day frequently includes multiple species including undersized white steenbras
Protecting South Africa’s Endangered White Steenbras: Lessons from Two Estuaries
The white steenbras (Lithognathus lithognathus), an endemic South African fish, is an important species targeted and caught by recreational and livelihood fishers in coastal and estuarine environments. Due to a combination of overfishing and environmental pressures, white steenbras populations have plummeted, resulting in the species being rated as Endangered on the IUCN’s Red List for Threatened Species. Focusing on case studies from the Swartkops and Knysna estuaries, our recent publication explores the challenges facing white steenbras conservation. Through these case studies, unique aspects of the problem are highlighted, offering valuable lessons for conservation efforts.
White steenbras are categorised as estuarine-dependent marine species with larval and early juvenile fish recruiting into estuaries where they remain for the first three to four years of their lives before the subadults migrate back out to sea. Growth rates of white steenbras are initially very rapid, with a monthly increment of 13 mm estimated for individuals under one year old. This rapid growth rate declines as the fish ages, with the maximum age of white steenbras at least 25‒30 years, corresponding to a maximum recorded length of 1376 mm. Similar to many other fish species, white steenbras rely on the shallow, sheltered and food-rich habitats found within estuaries for both feeding and protection from predators.
Although white steenbras occur in all types of estuaries, the Swartkops and Knysna estuaries are two key nursery areas within the ‘core’ distribution range of white steenbras. Both estuaries are relatively large and have permanently open mouths enabling unrestricted recruitment into and migration out of the estuaries. A diversity of shallow nursery habitats are also available for the juveniles of this species in both systems. However, these ecosystems face intense pressure threatening the long-term survival of this already vulnerable fish.
Case Study 1: Swartkops Estuary
The Swartkops Estuary, located within the Eastern Cape Province, is an important, permanently open system. However, white steenbras catches in the Swartkops Estuary have declined significantly over time, reflecting increased fishing pressure and environmental changes. The estuary has a long history of fishing, with evidence of intensive angling and netting going back more than a century.
Although concerns were already being raised around white steenbras populations and overfishing during the early 1900s, historical records from this time indicate white steenbras catches were common. For example, a single seine net haul in 1915 captured 120 individuals, including large adults up to 15.4 kg. However, by the 1970s, studies reported a stark decline, with fewer than ten white steenbras caught in over 50 gillnetting events between 1975 and 1979. Furthermore, angler surveys from 1972 to 1978 recorded just 339 white steenbras being caught compared to almost 9000 of the similarly sized spotted grunter.
Continuous pollution from nearby urban and industrial areas has compounded the fishing pressure and further degraded the estuary. Overall, the Swartkops Estuary exemplifies a shifting baseline, where historical data highlight the profound long-term decline in white steenbras abundance due to cumulative human impacts. Today, Swartkops is a stark example of a collapsed fishery, where adult white steenbras are practically absent, and juveniles are rare. As a result, it seems spotted grunter has taken over as the dominant species caught by recreational anglers and livelihood fishers in the estuary, filling the ecological niche once occupied by white steenbras.
Case Study 2: Knysna Estuary
Knysna Estuary, located along South Africa’s Garden Route, is one of only two systems within the white steenbras distribution range classified as an estuarine bay, the other being Langebaan Lagoon. The large and deep mouth enables a huge volume of water to move in and out on each tide and consequently the lower sections of the system are very similar to the adjacent ocean in terms of water temperature and salinity.
During 1978-1980, seine net surveys recorded white steenbras as the third most abundant marine fish species, with catches dominated by early juveniles that recruited seasonally and grew rapidly. However, subsequent surveys have revealed declines in abundance. In 1994, 29 white steenbras were caught from 26 hauls, representing 1.1% of the total catch, and by 2023, this had decreased to 56 individuals from 60 hauls, representing 0.2% of the total catch. In all surveys, juveniles dominated catches, with no significant representation of larger adults. The species now constitutes a small proportion of total biomass and abundance compared to historical levels.
Within Knysna Estuary white steenbras face significant fishing pressures from recreational and livelihood fishers. Economic factors exacerbate pressures and despite regulations, including a minimum size limit of 60 cm TL and a daily bag limit of one, high retention rates of undersized fish persist. By monitoring angler catches, we have shown that white steenbras become susceptible to capture within their second and third years of life, reducing individuals’ chances of reaching maturity and replenishing adult stocks. Importantly, and in contrast to Swartkops, recruitment of juveniles into the system seems relatively stable. However, the high fishing mortality of undersized fish prevents population recovery, and sub-adult and adult fish are now rarely caught. The decline in white steenbras populations highlights the ongoing challenges of managing heavily exploited fisheries under socioeconomic and ecological pressures.
A rare catch of a legal sized white steenbras caught on Knysna Estuary.
An angler being interviewed by Scientific Services staff as part of an ongoing fishery monitoring programme on Knysna Estuary
Key Differences Between the Two Estuaries
The contrasting states of white steenbras populations in the Swartkops and Knysna estuaries reflect differences in local environmental conditions and levels of fishery regulation enforcement. The Swartkops Estuary has experienced a total collapse of its white steenbras population, with no adult fish recorded and few juveniles recorded in recent decades. Meanwhile, Knysna still supports a viable juvenile population, although it also faces declining numbers.
Swartkops suffers from significant industrial and wastewater pollution due to its location near a densely populated and industrial area. This pollution, combined with nutrient overload from agricultural runoff, degrades the water quality, further endangering fish populations. In contrast, Knysna has a smaller human population and less industrial activity, resulting in lower pollution levels. However, both systems face chronic nutrient input from failing wastewater treatment works.
A key factor in the implementation of remedial measures for the recovery of white steenbras stocks is the fact that SANParks is the environmental and management custodian for the Knysna Estuary. Thus the implementation of remedial measures to assist the recovery of the white steenbras has a greater chance of success in the Knysna rather than the Swartkops system.
Recommendations and conclusions for Conservation
To protect and restore white steenbras populations, we recommend a multi-faceted approach, including the establishment of Estuarine Protected Areas (EPAs), enhanced enforcement of fishing regulations, and greater public awareness. Declaring no-take EPAs within the Swartkops and Knysna estuaries could provide safe havens for juvenile white steenbras, thus allowing populations to recover. These no-take zones should be strategically placed, based on ecological data and habitat use patterns and would also provide protection to other overexploited fish species.
Regulations are only as effective as their enforcement. Authorities must prioritize enforcement in these estuaries, ensuring that size and bag limits are respected. Increased monitoring and strict penalties for non-compliance could help curb illegal fishing practices, giving young white steenbras a chance to mature and reproduce. At the same time local communities, recreational anglers, and small-scale fishers need to be engaged and included in conservation initiatives. Education campaigns can raise awareness of the white steenbras’ plight, while collaboration with fishers could encourage sustainable practices. As part of this engagement, authorities could highlight the potential for improved fish stocks over time if recovery measures are implemented.
Chronic pollution issues need to be resolved and habitat degradation prevented. Continuous monitoring of white steenbras populations and estuarine conditions is critical for assessing the effectiveness of conservation strategies. Scientific studies on habitat use, movement patterns, and population dynamics can guide adaptive management, thus ensuring that policies remain responsive to changing conditions.
The story of the white steenbras in South Africa’s estuaries is a cautionary tale about the consequences of overfishing and environmental degradation. While the Swartkops and Knysna estuaries differ in their current fish population dynamics and environmental pressures, both require urgent conservation actions to ensure the survival of this iconic species. By establishing no-take EPAs, strengthening fishing regulation enforcement, and fostering local support for conservation, South Africa has a chance to protect and revive white steenbras populations in both estuaries and along our coastline.
