Browsing by Author "Kerwath, Sven E"

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    Open Access
    A change of the seaward boundary of Goukamma Marine Protected Area could increase conservation and fishery benefits
    (2009) Götz, Albrecht; Kerwath, Sven E; Attwood, Colin G; Sauer, Warwick H H
    Goukamma Marine Protected Area (MPA) on the South African temperate South Coast has been shown to be effective in maintaining a spawning stock of roman, Chrysoblephus laticeps (Sparidae). The larval ecology and the oceanographic conditions in the area suggest a good potential for the enhancement of roman stocks outside the reserve through larval dispersal. A high rate of illegal fishing just inside the seaward boundary of the MPA could severely compromise its function. We suggest that a change of the seaward boundary of the reserve to coincide with a latitudinal line could increase its function as a harvest refuge for resident reef fishes such as roman, facilitate voluntary compliance and monitoring and prosecution of illegal fishing without a significant negative impact on the commercial linefishing fleet in the area. Simple adjustments such as the one proposed here could be attempted at a number of South African MPAs as they would be beneficial to achieve fishery and conservation goals alike.
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    A step-by-step framework to assess benefits of established temperate marine protected areas
    (2013) Götz, Albrecht; Kerwath, Sven E; Attwood, Colin G
    Marine protected areas (MPAs) have been advocated as a solution to the challenges of both conservation and modern fishery management, but their application remains controversial, partly because there are only general guidelines for evaluating their effectiveness. We propose a framework to specifically evaluate established MPAs in six steps. We tested the approach by reviewing published research and unpublished information on the Goukamma MPA in the centre of the South African temperate south coast. Information reviewed included effects on the structure of fish populations, catch and abundance indices of fish species, and ecosystem effects. We investigated factors that determine the usefulness of a MPA in fisheries management, including the movement behaviour of adult fishes, larval dispersal and fisher-displacement patterns. We found that differences in the rates of exploitation across the MPA border resulted in differences in abundance, size and condition of the main target species, roman (Chrysoblephus laticeps). The diversity and abundance of non-target fish species, and the composition of the benthic invertebrate community, were affected by the cessation of fishing. The potential for ‘spillover’ of adult roman might be limited to the vicinity of the MPA by their small home range, but there is potential for self-seeding and dispersal of roman eggs and larvae over wider areas. These theoretical considerations were confirmed by an analysis of catch data from before and after MPA implementation. The framework presented here may help to identify and fill gaps in the knowledge of established MPAs along South Africa’s temperate south coast.
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    Biology, movement behaviour and spatial dynamics of an exploited population of smooth hound shark mustelus mustelus around a coastal marine protected area in South Africa
    (2018) da Silva, Charlene; Attwood, Colin G; Kerwath, Sven E
    Aspects of the life-history, movement in relation to a Marine Protected Area (MPA) boundary, and short and long-term spatial behaviour in relation to environmental preferences of Mustelus mustelus were studied in the Saldanha Bay region on the west coast of South Africa. The overarching aim of this thesis was to examine the biological and ecological circumstances under which a MPA could provide effective protection to a commercially caught coastal shark from fishing activity. The pigmentation, reproductive biology, diet, growth and maturity of Mustelus mustelus was examined from 217 ranging from 381 to 1734 mm TL and 467 to 1267 mm TL for females and males, respectively. Sharks in the bay represented the largest females and males recorded worldwide. The seasonal changes in oocytes and testes development, embryo length and the occurrence of near-term and postpartum females indicated that female parturition and ovulation occurs between November and December after a gestation period of 10-11 months. The presence of juveniles, neonates and pregnant females inside the Langebaan Marine Protected Area indicates it to be a nursery ground for this species. The largest part of the diet of M. mustelus consisted of three species of crustaceans: Hymenosoma orbiculare, Upogebia africana, Callichirus kraussi. No ontogenetic shift in diet was found for M. mustelus from Langebaan Lagoon. M. mustelus grow relatively rapidly, matured early (between 3 and 6 years) and attained a maximum observed age of 13 years. The movements of individual Mustelus mustelus in and adjacent to a small closed area (Langebaan Lagoon MPA, 34 km2 ) situated on the West Coast of South Africa were investigated over two years using acoustic telemetry. Sharks spent the majority of the time (in hours, average 79%) inside the Langebaan Lagoon MPA, and some sharks (n = 2 of 15 recorded during a full year) did not leave the reserve during the observation period. Time spent inside the closed area and the number of crossings of its boundary was strongly influenced by season. Sharks concentrated inside the closed area during summer, while they were widely distributed throughout the study area during winter months. A combination of shallow and sheltered waters in close proximity to the Saldanha Bay port and other boataccess points would normally make this summer aggregation highly vulnerable to fishing activity. The residency of M. mustelus within the closed area suggests that spatial protection may be effective for this species. Acoustic telemetry and in situ environmental data were used to investigate movement of M. mustelus in relation to changing environmental conditions over long (seasonal) and short (20 min) time scales. Results of Generalised Additive Mixed Modeling (GAMMs) indicated no significant influence of tide or moon phase and only a weak influence of diel period on movement and direction of movement. The thermal preference for M. mustelus was between 18 and 22 C as determined by GAMMS. Absolute temperature and the relative change in temperature at the shark’s position were the best predictors for shark movement and its direction in summer, explained 4.4 and 42.7 % of the deviance, respectively. This study provided evidence that M. mustelus inside the embayment decide their position within their area of residency according to their thermal preference and that temperature change constitutes the trigger that determines movement direction. This study confirms that M. mustelus are resident within the Saldanha embayment and distinct by diet, life-history parameters and colouration from stocks elsewhere. M. mustelus from this group are more fecund and larger in body size than those from all other populations globally, possibly due to the favourable temperature conditions in the warm sheltered lagoon and the existence of a MPA closed to fishing which includes preferred habitat for all life- IX history stages of this species. In the absence of a comprehensive stock assessment and species-specific management, well-positioned closed areas that include preferred habitat can aid the sustainability of coastal shark fisheries.
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    Fishery, population dynamics and stock assessment of geelbek (Atractoscion aequidens), a commercially important migrant fish species off the coast of South Africa
    (2018) Boyd, Danielle Winona; Altwegg, Res; Winker, Henning; Kerwath, Sven E
    Geelbek (Atractoscion aequidens) is an important fish species in South Africa's linefishery, a fishing sector defined by its fishing gear of rod and reel or handline. Distributed from Cape Point (34°21'S, 18°29'E) on the south west coast to Kosi Bay (26°51'S, 32°53'E) on the east coast, they are targeted throughout their range by the commercial linefishery, recreational anglers and small-scale fishers. The majority of geelbek are caught on the Agulhas Bank during austral summer. Due to current minimum size limits of 600 mm (total length, TL), well below the 50% size-at-maturity (950 mm TL), the majority of the catches are comprised of immature fish, making the stock vulnerable to growth overfishing. Adults (>5 years) migrate seasonally to spawn off KwaZulu-Natal and congregate in offshore shoals at night. These spawning aggregations allow fishermen to catch large numbers of fish, making geelbek also vulnerable to recruitment overfishing. This study aims to improve understanding of the fishery and population dynamics of geelbek to help inform natural resource management of the geelbek linefishery. A stock assessment of South African geelbek was undertaken to fulfil this aim. For this purpose, spatially and seasonally explicit equilibrium per-recruit and dynamic age-structured operating models were developed for geelbek to account for the dynamic in stock structure as a result of the intra-annual coastal migration and differences in the vulnerability of life history stages to varying fishing pressure along South Africa's coastline. These models were developed using statistical programming environment R. The models were parameterised and calibrated using length and catch data from the National Marine Linefish System (NMLS) and life history parameters sourced from peer-reviewed literature. Per-recruit analyses were performed to estimate current stock-specific fisheries mortality rates and the spawner biomass depletion. These estimates were used as input into the stochastic age-structured simulation model and calibrated using available commercial catch data (1987 - 2011). The stochastic operating model was used to predict the probability of stock recovery and long-term sustainability under eleven alternative fisheries management strategies. The current stock status was estimated at 9.9% (approximately 10%) of the pristine spawner biomass (SB₀) using per-recruit analysis. This was compared to the stock depletion estimates of ~5 and 7% SB₀ from prior assessments conducted in the late 1990s and 1980s. This study indicated that there was a ~50 to 100% increase in spawner biomass over the past twenty years. However, this level of stock depletion is still considered critically low with respect to the previous limit management goal of increasing spawner biomass depletion rates above 25% SB₀, the collapsed limit reference level, advised by Griffiths in 1997. Eleven management strategies were simulated, examining the effects of decreases in harvest rates, closed seasons and areas and changes in minimum size limits, initiated in 2020, and tested over the medium (ten years) to long (twenty years) term. The least efficient management strategy was continuing at the status quo, with a minimum size limit of 600 mm (TL), which predicted only 1% and 2% increase in SB by 2030 and 2040, respectively. The most efficient in terms of a rapid recovery was a full fishery closure 'control scenario' (moratorium), which predicted a recovery to the threshold reference level for sustainable fishing at 40% SB₀ by 2025, and approaching pristine levels by 2040. Increasing the minimum size limit to the size-at-50%-maturity, 950 mm TL, had the second highest recovery rate, reaching 25% SB₀ by 2027, and nearing 40% SB₀ by 2035, at which point its trajectory is asymptotic to 40% SB₀. Decreasing the harvest rate by 50% across all regions and seasons had the third highest recovery rate, reaching 25% SB₀ by 2035, but levelling off thereafter. All the other management strategies resulted in slight stock recoveries, but with all stock trajectories remaining below 14% SB₀ in the long term. Additionally, the impact of various strategies, such as increasing the minimum size limit to the size-at-50%-maturity, 950 mm TL, were unequal, with the east coast experiencing increasingly higher catches over time, whereas the catches for the south south west coast declined drastically throughout the year, and did not improve with time. Such unequal distribution of the impact of management intervention is a consequence of the migratory life history of the geelbek stock. These results provide comprehensive insights into the population dynamics and current impacts on the geelbek stock, suggesting that this species remain severely depleted at ~10% SB₀. Rebuilding the stock to sustainable levels would require serious management intervention.
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    A survey of the metazoan parasite assemblage of snoek, Thyrsites atun (Euphrasen, 1791), off South Africa with an assessment of host-parasite relationships and potential biological tags
    (2015) Nunkoo, Muhammad Azher Irfan; Reed, Cecile C; Kerwath, Sven E
    Parasites are a ubiquitous but often unseen, unacknowledged and understudied component of biological communities. However, their roles in structuring ecosystems, their influence on the evolutionary history of host species and their potential applications are slowly being uncovered. The snoek, Thyrsites atun, is a nomadic predator native to the cold coastal waters of the southern Hemisphere. Being a major target of the South African inshore line-fishery, the snoek is a socioeconomically important species whose ecological significance in the southern Benguela should not be underestimated. This study aimed to survey the metazoan parasite community of snoek off South Africa, assess host-parasite relationships and evaluate the potential of parasites as biological tags for stock structure studies. Examination of 210 snoek (FL 411 - 1040 mm) revealed them to be host to 16 parasite taxa. These included 9 new host records (Tentacularia coryphaenae, Caligus coryphaenae, Caligus dakari, Corynosoma australe, Nothobomolochus fradei, Hatschekia conifera, Bolbosoma vasculosum, Rhadinorhynchus cadenati, Digenea sp.) and 4 new locality records (Molicola uncinatus, Pseudoterranova sp., C. dakari, B. vasculosum). A further three cosmopolitan taxa (Anisakis sp., Kudoa thyrsites, Hepatoxylon trichiuri) as well as Caligus zei were also recorded.