Browsing by Author "Kerwath, Sven"
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- ItemOpen AccessAge, growth and per-recruit assessment of the Saldanha and Langebaan stock of Chelon richardsonii(2018) Horton, Matt; Kerwath, Sven; Parker, DenhamChelon richardsonii are omnivorous, particle feeders found specifically within inshore and estuarine habitats on the west and south coast of South Africa. They are the primary target of the gillnet and beach-seine fishery in this region. Despite being managed through a multifaceted approach of gear restrictions and total allowable e↵ort, the fishery is thought to be oversubscribed and the stock is regarded as being overfished. The social and economic importance of this fishery necessitates an update of the life history parameters of C. richardsonii to enable an accurate assessment the current status of the stock. The fishery in Saldanha and Langebaan was described via investigating changes in sexratio, mean length (mm) and standardised catch-per-unit-e↵ort (CPUE). Firstly, exploration of sex ratio indicated a significant switch between the two periods (1998-2002 and 2017), resulting in a predominantly male biased population (1.7 males: 1 female). Secondly, through investigation of three length-frequency distributions of commercial catch of C. richardsonii (1998-2002, 2009-2011 and 2017) a reduction in mean total length (TL) of 36.5 mm was observed. Lastly, the standardisation of the Netfishery CPUE for the time series of 2008-2016 through the application of a Generalised Additive Mixed Model (GAMM) showed a reduction of approximately 30% in relative abundance of C. richardsonii. Chelon richardsonii exhibited a fast growth, a maximum age of six and matured relatively early at two years old. Growth was best described using a three parameter von-Bertalan↵y growth model; where L1 is the asymptotic length, K is the rate at which L1 is reached and t0 is the age when the average length is zero. The data collected in 2017 expressed two problems. Firstly, as a result of high gillnet selectivity, smaller individuals within younger age classes were missing. Secondly, due to growth overfishing and/or a small sample size (n = 353) older and larger adults were missing. Consequently, this increasesd K and decreased L1 to biologically implausible values (female original growth: L1 = 257.450 mm, K = 0.610 year-1 and t0 = -0.040 year). As a result L1 was fixed in accordance to a historic Lmax, in order to overcome these issues and produce biologically plausible growth parameters. Growth di↵ered significantly between males and females, hence female growth was subsequently used for the spawner biomass-per-recruit analysis in the proceeding chapter (L1 = 347.400 mm, K = 0.235 year-1 and t0 = -0.833 year). Total mortality (Z) and average natural mortality (M ) were estimated as 1.466 year-1 and 0.329 year-1, respectively. i 0 ii Growth and mortality was constant in Chelon richardsonii throughout Saldanha and Langebaan and despite potential emigration out of the bay, the sup-population of C. richardsonii was considered to be a discrete stock for the purpose of this study. A spawner biomass-pre-recruit model, based on the growth and mortality parameters calculated in Chapter 3, revealed that the stock is heavily depleted and recruitment is likely to be seriously impaired (spawner biomass-per-recruit = 5.5% of pristine levels). It must be acknowledged that the results of a per-recruit stock assessment heavily depend on the growth model parameters. In contrast, the model indicated an optimally exploited stock when the original growth parameters were applied (spawner biomass per-recruit = 76.2% of pristine levels). Considering results from Chapter 2 and the justifications for fixing L1 the plausibility of the second scenario being true is less likely. Acknowledging the temporal, spatial and sample size limitations of this study conclusions made will require definitive future examination. Regardless, a reduction in fishing e↵ort and further restrictions in mesh sizes are suggested to facilitate the replenishment and sustainable use of the stock.
- ItemOpen AccessThe diet, reproductive biology age and growth of yellowtail, Seriola lalandi, in South Africa(2014) Dunn, Kieron; Attwood, Colin; Jarre, Astrid; Kerwath, SvenYellowtail, Seriola lalandi, is an important line-caught fish in South African waters, yet little information is available on their life-history. This study aims to add information on the diet and feeding habits, reproductive biology and the age and growth of yellowtail in South Africa. The diet of 62 yellowtail caught in the Western Cape of South Africa between 2011 and 2012 was investigated. Fish sampled by line and speargun ranged from 488 to 916 mm fork length (FL). Prey items were removed from stomachs, sorted, counted and weighed in order to calculate the percentage number (% N), percentage weight (% W), frequency of occurrence (% F) and index of relative importance (% IRI). Of the 62 stomachs examined 11 (17.7 %) were empty, 16 (26.0 %) contained only unidentifiable remains and 47 (82.5 %) contained identifiable remains. Prey items covered 18 species belonging to five classes: bony fishes, crustaceans, cephalopods, polychaetes and bivalves. Small pelagic fish were the dominant prey type, followed by crustaceans. The remaining taxa were of negligible importance. Some dietary differences were observed between sample areas. Most notable was the increased importance of crustaceans at Dassen Island on the West Coast compared to the sites at Robben Island, False Bay and Struisbaai. The reproductive characteristics of yellowtail were documented from fish collected from 1974 to 2012. Samples were collected from Cape Infanta on the South Coast to Lamberts Bay on the West Coast of South Africa. Histological validation of macroscopic staging criteria revealed that active and developing ovaries are commonly staged incorrectly. A protracted spawning season from November to February with peak spawning in December and January was deduced from GSI values. No hydrated eggs were observed. Females matured at 550 mm FL (95 % CI = 532 - 570 mm) and males matured at 585 mm FL (95 % CI = 555 - 619 mm). The age and growth characteristics of yellowtail in South African waters were determined from readings of whole sagittal otoliths collected from 1974 to 2012. Whole otoliths were considerably easier to read than sectioned otoliths. A total of 524 whole otoliths were taken from fish ranging from 430 to 1080 mm FL, of which 141 (27 %) were discarded and 384 (73%) were readable. Agreement between all three readers was 13 % (n = 50) and between any two was 71 % (n = 274). Maximum ages for male and female yellowtail were 7 and 8 years respectively. Age at 50 % maturity (A50) for males it was 2.3 years while females matured (A50) at 3.6 years. Von Bertalanffy growth parameters did not differ between males and females (P > 0.05). A statistical penalty was used to keep the estimated growth parameters within biological limits and produced a von Bertalanffy growth equation with an L∞ and K of1064 mm and 0.17 y-1 respectively. The growth performance index (φ) of yellowtail in South African waters was found to be 3.51. This is high for the family Carangidae but on par with other species in the genus Seriola. The life history characteristics for yellowtail in South African waters closely resemble those of other yellowtail populations. The diet of yellowtail in South African waters represents that of a robust generalist feeder that is not reliant on specific prey for its survival. The age, growth and reproductive characteristics of yellowtail in South African waters indicate that they are a fast growing and relatively early maturing species. These life-history characteristics indicate that the stock is resilient in relation to other line-fish species, but the large proportion (41%) of fish caught below the 50 % size at maturity suggests that a revision of the minimum size limit should be considered.
- ItemOpen AccessLife history study of red stumpnose (Chrysoblephus gibbiceps), a South African endemic seabream(2013) Van Zyl, Megan Elizabeth; Attwood, Colin; Kerwath, SvenSeabreams are an important family of fishes in the southern African recreational and commercial linefisheries. This family is known for its extreme longevity, slow growth and wide variety of reproductive styles. Red stumpnose are one of the most iconic species within this family, yet biological information is lacking on this species. Six hundred and seventy-eight specimens were collected from 1986 to 2012, ranging in size from 165 to 600 mm fork length. All specimens were dissected and morphological measurements taken. Gonads, stomachs and otoliths were removed from 237 individuals and presented. Otoliths were sectioned and independently aged by three readers, age agreement was reached on 183 fish.
- ItemOpen AccessLife history traits that predispose South African linefishes to overexploitation(2018) Haupt, Meghan; Kerwath, Sven; Parker, Denham; Winker, HenningGlobally, the status of many fish stocks remains unknown, of which the majority fall under data-limited small-scale fisheries. Management decisions in most of these fisheries are difficult due conflicting objectives and views from fisheries managers and scientists. In South Africa, the traditional boat-based ‘linefishery’ provides such an example of a small-scale, multi-species fishery with a long history. The historical de facto open access nature of this fishery resulted in continuous declines in catches of many linefish species, and in 2000 the fishery was declared to be in a state of emergency. This led to a reduction of up to 70% within the fishery, among other measures, such as introductions of size and bag limits. Assessing the status of linefish species is difficult due to a lack of reliable long-term data for the majority of species. The aims of this study were therefore: (1) to quantify the stock status for all linefish species with available life history and size composition information, (2) compare current and historical stock levels to ascertain if the reduction in effort facilitated any recovery in linefish species and (3) correlate the current stock status estimates to life history traits to identify simple indicators of resilience to exploitation. For this purpose, length frequency data from 1988-1990 and 2008-2010 and biological parameters sourced from literature were used to conduct per-recruit analysis to estimate spawner biomass depletion (SBD) for both time periods. The majority of the 26 species analyzed, (68%) showed improvements in spawner biomass between the two time periods, with 12 species undergoing a change in stock status (i.e. improving from collapsed or overexploited). Specifically, increases in length-at-capture (Lc) as well decreases in fishing mortality (F) facilitated recovery for many species. Asymptotic length (L∞), as well as the ratio between Lc / L∞ and Lc / Lopt (where Lopt is the optimum length) were found to be significantly correlated to spawner biomass depletion. Kruskal Wallis analyses revealed that only movement pattern had a significant relationship to SBD, more specifically, migratory species were significantly more depleted than resident ones. This study identifies simple indicators that, in the absence of conventional stock assessments, provide fisheries managers with a fundamental understanding of a species’ susceptibility to overexploitation – offering another decision making tool for use in data poor fisheries such as the South African linefishery.
- ItemOpen AccessMicroplastic ingestion by two estuarine-associated mullet species, Chelon richardsonii and Chelon dumerili in the Breede Estuary, South Africa(2023) Elliott, Daniel; Kerwath, SvenMicroplastic (MP) ingestion in fish has the potential to cause a multitude of damaging effects to both organisms and the environment. Microplastics (MPs) are defined as solid plastic particles ≤ 5mm in size. The small size of MPs makes them extremely hazardous, as they imitate a range of food items on which organisms feed. Rivers and estuaries are links between terrestrial and marine ecosystems; thus, these systems hold key evidence of the magnitude and nature of MP pollution. Research particularly in the southern hemisphere is poorly documented for MP ingestion. This study shows MP ingestion by two mullet species (Chelon richardsonii & Chelon dumerili) in the Breede Estuary, South Africa. It was found that 98.4 % of the captured fish had ingested MPs and ingestion took place throughout the estuary reaches by both species. Multiple brown/orange fragments and the concentration of MPs found in the large mullet (≥200mm), were consistent with results found in sediment samples, indicating large (≥200mm) mullet intake MPs via sediment ingestion. The previous trend was not identified in small mullet (<200mm), thus indicating small mullet selectively fed on particles and MPs in the water column. Small juvenile fish (<200mm) ingested concentrations of MPs one order of magnitude greater than large (≥200mm) adult fish throughout the estuary, possibly reflecting the feeding transition from planktivorous juveniles to benthic adult detritivores. The MP conc. (MP.g-1) in small mullet (<200mm) was significantly (p<1.639e-12) higher compared to large mullet (≥200mm). A significant difference (p<0.008) was found between MP conc. (MP.g−1) and species, this interspecific differentiation was driven by small (<200 mm) fish, which selectively and visually feed. This study provides insights and new knowledge regarding MP pathways within food webs. It is imperative that a better understanding of MP pathways through marine food webs is established on a local, regional, and global scale in order to mitigate the resulting risks associated with fish, humans, and any other animal-related consumption. Keywords: Breede River . Estuary . Microplastic concentrations . Mugilidae . Sediment pollution . South Africa
- ItemOpen AccessRemotely operated vehicle exploring ichthyofauna association with habitat from shore-shelf, in an endemism hotspot in South Africa(2018) Button, Rio E; Kerwath, Sven; Parker, DenhamThe priority of understanding marine systems increases as expanding pressures are exerted on them. In South Africa, efforts are underway to utilise the ocean to stimulate economic growth which will result in further pressures on marine systems. The aim is to mitigate pressures, largely through proposed offshore marine protected areas (MPAs). One of these proposed MPAs is situated off the Kei river mouth in the Eastern Cape of South Africa. The area offshore of the Kei river mouth is an endemism hotspot, and a transitional zone between the Subtropical East Coast and the Warm Temperate South Coast. The region waslargely unexplored because of itstreacherous sea conditions and because it was no longer an area of commercial fishing importance. A remotely operated vehicle (ROV) explored the regions’ fish and benthic habitats and investigated their associations. This study’s results provide quantitative information on fish and habitat diversity in the intermediate depths off the Kei river mouth. Observations of rare, commercially important, yet critically endangered charismatic reef fish species were made. These included red steenbras (Petrus Rupestris), seventy-four (Polysteganus undulosus), red stumpnose (Chrysoblephus gibbiceps) and dageraad (Chrysoblephus cristiceps). Habitat types, including rhodolith beds, sponges, and deep-water corals were documented. Maximum predicted fish diversity corresponded with mid-continental shelf, which is incorporated into the regions proposed MPA. The highest fish diversity was 10km within the shelf edge (which had a depth of approximately 100m) and was associated with the most structurally complex habitat biota: Fan Coral. Depth was a fundamental predictor associated with the presence and abundance of species distributions. The results support the location of the proposed MPA and are a step forward in identifying critical habitat to protect diversity and endangered species, and thus contribute to the regions spatial management and governance.