OpenUCT :: Browsing by Author "Jarre, Astrid"

Browsing by Author "Jarre, Astrid"

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Open Access
Above the surface, beneath the waves : contesting ecologies and generating knowledge conversations in Lamberts Bay
(2011) Rogerson, Jennifer J M; Green, Lesley; Jarre, Astrid
Based on fieldwork conducted over two months in 2010 in Lamberts Bay on the west coast of South Africa where the cold Benguela Currrent asserts its presence in water and wind, this dissertation aims to describe the ways that people come to know fish and the sea differently.
Open Access
Analysing modelled nearshore wave climate variability and change as relevant to the traditional handline fishery of the South African South Coast
(2018) Lyttle, Casey Tara; Jarre, Astrid; Backeberg, Björn; Rautenbach, Christo
The South Coast traditional handline fishing communities of South Africa are integrated into a complex ecosystem where human and natural components interact and overlap on many different spatial and temporal scales. The South Coast marine ecosystem, on which the fishers depend, already suffers from depleted fish stocks. The South Coast handline fishery is therefore vulnerable to added stresses such as those induced by climate change. While fishers have noted that deteriorating sea state and a declining number of sea days caused by shifts in wind patterns are affecting their livelihood, applicable scientific research and data on scales relevant to the fishers is insufficient. Insight into the complexities involved in climate change and local-scale responses of these highly integrated social-ecological system therefore remains sparse. While South Coast nearshore winds have been the subject of recent research, the wave climate aspect of the nearshore sea state has not. In a recent project conducted by the Department of Environmental Affairs and the Council for Scientific and Industrial Research, Simulating WAves Nearshore model outputs spanning 17 years (from 1997 to 2014) were produced for the South African coastline, including the South Coast. Wind (speed and direction) and swell (significant wave height, peak wave direction and period) outputs from the WaveWatch III model (provided by National Centre of Environmental Prediction, US) were used as boundary conditions. The present study uses these wave model outputs to conduct an investigation into the nearshore local-scale wave climate of four traditional handline fishing towns of the South Coast: Witsand, Still Bay, Gouritz and Mossel Bay. Results suggest that the shape and bathymetry of the coastal sites influence average significant wave height, peak wave directions, and seasonal variability of the approaching swell waves. This is due to the nearshore processes of refraction, bottom friction and sheltering by headlands from the approaching swell, driven by the offshore swell. Additionally, the presence of low peak period waves depended on the focussing of waves that were generated by easterly winds during summer (i.e., wind-waves, which are shorter period waves compared to swell) driven by the synoptic-scale winds. While summer afternoon waves remained higher than morning waves from 1997 to 2014, the significant wave height difference did not change over time; however, variability increased post-2006, particularly for sites more exposed to approaching swell. A regime shift in mean significant wave height was detected for 2006 across the South Coast, from lower to higher waves. The more exposed study sites showed a strong seasonality (higher waves during winter than summer), where the duration of summer conditions lengthened post-2006 during the period of higher significant wave heights. Significant wave height increased significantly from 1997 to 2014 across the South Coast. Since swell dominates across the South Coast, the observed regime shift (including interannual variability) and trend is likely to be attributed to offshore swell. The recent increase in wave height variability is in line with fishers observations where increase in climate variability has been observed. The increase in wave height is also in line with fishers' observations which state that the sea state has deteriorated, and sea days have decreased. Additionally, the lengthening duration of summer conditions in waves was also observed by fishers in terms of winds. This analysis of South Coast wave climate contributes to bridging the gap between the first hand observations of fishers and conclusions drawn from coarse resolution scientific data.
Open Access
Application of the sequential t-test algorithm for analysing regime shifts to the southern Benguela ecosystem
(2007) Howard, James A E; Moloney, Coleen; Jarre, Astrid; Clark, Allan
Long-term ecosystem changes, such as regime shifts, have occurred in several marine ecosystems worldwide. Multivariate statistical methods have been used to detect such changes, but they have to date not been applied to the southern Benguela ecosystem. A weakness of many of the methods is that they require long time series data and do not provide robust results at the end of time series. A new method known as the sequential t-test algorithm for analysing regime shifts (STARS) is applied to a set of biological state variables and environmental and anthropogenic forcing variables in the southern Benguela.
Open Access
Are distinct particle spectra an indication of the state of the phytoplankton community in St Helena Bay?
(2012) Crichton, Murray; Hutchings, Larry; Jarre, Astrid
The potential of phytoplankton particle spectra to be used as the basis for an indicator of the suitability of feeding habitat for fish recruits in the Southern Benguela was investigated. Phytoplankton samples collected on regular cruises on the St Helena Bay Monitoring Line (SHBML) off Elands Bay on the west coast of South Africa had been analysed with the Coulter Counter and formed the basis of this study. Chlorophyll a content of phytoplankton samples was also measured on monthly cruises and with total particle concentration (determined by the Coulter Counter) showed that overall phytoplankton cells represented a significant portion (76%) of samples collected. Surface particle spectra were constructed for the 12 stations on the SHBML for each of 15 monthly cruises between September 2000 and February 2007.
Open Access
Assessing the effects of internal (trophic structure) and external (fishing and environment) forcing factors on fisheries off central Chile : basis for an ecosystem approach to management
(2008) Alarcón, Sergio Eduardo Neira; Moloney, Coleen; Cury, Philippe; Shannon, Lynne; Jarre, Astrid; Christensen, Villy
Human perception of sea fisheries has evolved from an inexhaustible resource paradigm towards a generalized concern on the degraded state of fish stocks and ecosystems. Accordingly, fisheries science and management are expanding from the traditional single-species approach towards an ecosystem approach to fisheries. Marine communities are organized as webs of interactions that are affected by external natural (climate) and anthropogenic (fishing) forcing, with their relative effects poorly known, but hypothesised to strongly depend on internal food web structure (i.e., who eats and controls whom). This thesis approaches relevant ecological considerations for an ecosystem approach to fisheries in the upwelling ecosystem off central Chile (33ºS-39ºS). The main objective is to assess the effects of internal (trophic structure) and external (fishing and environment) forcing factors at the fish stock and food web level in the study area. The methodology includes i) the construction of snapshot and dynamic food web models to test hypothesis of changes in the food web in the last century, and the relative contribution of fishing, trophic controls and bottom-up environmental variability to those changes, ii) the computation and analysis of a set of ecosystem indicators to test hypotheses of changes in different aspects of the exploited community (mean trophic level, age and length at maturity, network properties and system variability), iii) the analyses of the relationships between time series of abundance of species with known trophic interactions (Chilean hake-red squat lobster and Chilean hake-small pelagic fish) to test hypothesis of top-down and bottom-up control versus alternative hypotheses of fishing and/or environmental control in the same populations, and iv) simulation experiments to test hypotheses of ecosystem change and recovery under fishing and environmental forcing. Models and indicators are constructed using data series of abundance, catches, production, consumption and diets of the main functional groups in the study area. Snapshot and dynamic food web models are constructed and analyzed using the Ecopath with Ecosim software version 5.1 and routines therein. The observed trends in indicators and model results are in accordance with what is theoretically expected in stressed ecosystems (shift towards a food web dominated by short-lived, low trophic level and high turnover rate species), and suggest that the food web could be in a state that is more susceptible to external forcing. Fishing and the environment (bottom-up anomaly in PP) may have affected the upwelling ecosystem off central Chile both at the stock and at the food web level between 1970 and 2004. The effects of these forcing factors may have been mediated by trophic controls operating in the food web. There is also evidence to support the hypothesis that trophic controls beyond fishing, e.g., trophic (internal) and environment (external) may operate in the analysed populations and this information should be considered in their assessment and management. While target objectives are set and agreed, it is proposed that the main objective for the ecosystem approach to fisheries should be to avoid fishing-induced regime shifts, since results from simulation experiments suggest that fishing can induce ecosystem changes of lower recovery than bottom-up forcing.
Open Access
Assessing the implementation efficacy of an ecosystem approach to Fisheries management in the South African sardine fishery
(2015) McGregor, Emily Skye; Jarre, Astrid; Van der Lingen, Carl D; Wals, Arjen
An Ecosystem Approach to Fisheries management (EAF) offers a holistic approach for sustainable fisheries management by extending the traditional target resources-orientated management (TROM) to include wider social-ecological dimensions of fisheries. An EAF requires balancing of multiple, often conflicting objectives, effectively dealing with complexity and uncertainty, and engaging with diverse groups of stakeholders. Various tools within the field of Multi-criteria Decision Analysis provide a formal approach which takes explicit account of multiple criteria, while effectively dealing with risk and uncertainty. A knowledge-based tool was developed in this thesis to assess the efficacy of EAF implementation for the ecological well-being dimension in the South Africa sardine fishery. An iterative, participatory approach was adopted for its implementation. The modelling philosophy applied a rapid prototyping approach, and an applied research perspective was employed to direct the research. A broad group of stakeholders participated in indicator selection, tool design, and interpretation. The knowledge-based tool provided a hierarchical framework for seven specific management objectives to which eleven ecological indicators were linked. Time series (1987-2009) were collated for each indicator, and a utility approach was used to transform indicators to a common scale. Weights for indicators and objectives were agreed to by stakeholders and combined through the objectives' hierarchy using weighted means. The resulting outputs were discussed in detail during focus group meetings to ensure that the tool was clearly presented and as intended helped improve the stakeholder's understanding of the process. It was confirmed that the 20 knowledge-based tool presents a transparent, repeatable and scientifically defensible approach, suitable to meet management requirements. The tool development process was useful in bringing diverse stakeholder groups together, and through applying the tool as a boundary object, has helped to bridge the boundary between the TROM and EAF research communities. Encouraging stakeholder interaction offers opportunities for social learning, which if carefully facilitated through the tool development process is likely to enhance the outcomes of this process and support more generally in bridging boundaries to EAF implementation. The combined focus on tool development and social processes supports effective implementation of an EAF in the South African small pelagic fishery and provide a model for other fisheries.
Open Access
At the interface : marine compliance inspectors at work in the Western Cape
(2014) Norton, Marieke; Green, Lesley; Jarre, Astrid
The Western Cape fisheries are heavily contested. Primary concerns in the contestations are over access to marine resources, which have been regulated through the Marine Living Resources Act of 1998. At the centre of these conflicts, is the figure of the marine compliance inspector, whose task is to enforce the state’s version of nature onto the collective of resource users. This thesis, based on 18 months of ethnographic fieldwork alongside inspectors of the Department of Agriculture, Forestry and Fisheries: Fisheries Branch in the Western Cape, explores the everyday human interactions on which the implementation of marine resource law depends. Exploring interactions between inspectors and resource users, the dissertation seeks to contribute to the task of reimagining fisheries governance. Drawing on ethnographic material deriving from participation in inspection duties; observations of fishing behaviour; conversations with inspectors, resource user and marine resource management officials; and analysis of texts such as relevant legislation and job descriptions, I argue that the issue of non-compliance in marine fisheries in the Western Cape can only be partially understood by the framework offered in extant South African compliance scholarship, which has focused largely on the motivations of resource extractors, or the formulation of law and policy. Given that compliance functions are part of the wider social spectrum of contestation and that the compliance inspectors are the interface between the government of South Africa and its fishing citizens, the study explores the real effects of state-citizen-nature contestations on environmental governance, and presents evidence in support of an argument that the design of the job of marine compliance inspector itself needs to be re-conceived. While compliance is a central feature of fisheries management, the performance of its personnel is taken for granted as the simple implementation of institutional policy, in a number of ways. Efforts to address conflicts will fall short of the goal of providing solutions if the assumptions about nature and humanity that current marine resource legislation embodies are not questioned, and this will exacerbate existing suffering in the ecology of relations between state, science, public and marine species.
Open Access
Climate change and small-scale fishing in South Africa: a community scale social vulnerability assessment for the southern Cape handline fishery
(2022) Andra, Kayla; Jarre, Astrid; Karenyi, Natasha
Climate change is majorly affecting the quality and quantity of marine organisms, as well as people's livelihoods. Coastal communities, small-scale fishers, and fishing-reliant individuals are especially vulnerable to climate change impacts (and other stressors) that alter the state and availability of ocean resources. Therefore, implementing integrated management approaches (such as the Ecosystem Approach to Fisheries (EAF)) is crucial to address these vulnerabilities. This study concerns the vulnerability to the impacts of climate change of fishers who act as crew members in the southern Cape commercial handline fishery. The southern Cape is a rural, peri-urban, and urban region characterised by agriculture, fishing, tourism, and retirement services as major economic activities. Aside from their documented social, governance, and economic stressors, small-scale fishers in the southern Cape also face biophysical stress (i.e., changes in wind, rainfall, and sea state). The Global learning for understanding local solutions (GULLS) survey instrument and a social vulnerability framework were created to assess vulnerability and its comprising concepts (sensitivity, exposure, and adaptive capacity) for coastal communities experiencing notable climate change. This study presents the first quantitative analysis of the data collected in the southern Cape in 2014-15. Univariate and multivariate analyses were performed to answer four key questions to investigate whether social vulnerability (as well as sensitivity, exposure, and adaptive capacity) differed among the communities, how variable vulnerability is within the communities, what drives the differences among the communities and to gain insight on the implications for issues of scales. The fishing communities differed significantly in their overall social vulnerability scores. Sensitivity and exposure were similar between the fishing communities. Sensitivity was the main driver of vulnerability for all fishers due to their attachment to fishing as an occupation, low self-sufficiency, and attachment to their communities. There was no significant difference in the dispersion (homogeneity) of the fishers' responses within the communities. The results also corroborate previous qualitative research, showing that variations between the communities are driven by adaptive capacity. The “component” scale (the second scale of the four-scale GULLS framework) yielded the most beneficial results and is recommended for future analyses. In addition, recommendations are made for future surveys to address uneven weighting, fundamental system changes (such as COVID-19), and questions irrelevant to the southern Cape small-scale fishers. Overall, with these recommendations, an improved survey offers a quicker methodology that can easily be communicated with various decision-makers and paves the way for consistent temporal comparisons that stimulate a long-term understanding of vulnerability. Most importantly, these recommendations and methods can contribute to the Ecosystem Approach to Fisheries (EAF) implementation in the southern Cape and the sustainability of this marine social-ecological system.
Open Access
Climate variability in social-ecological systems of the Southern Cape: integrating farming and fishing perspectives
(2018) Ward, Catherine Dale; Jarre, Astrid; Kemp, Georgina Cundill; Midgley, Guy
Large scale shifts playing out on global climate levels are manifesting locally in the South Africa’s terrestrial and marine ecosystems, where it is difficult to predict how different habitats may respond to these changes in natural systems, particularly at local levels. For example, the highly complex nature of climate variability in the southern Cape and on the Agulhas Bank, coupled with the lack of long-term environmental monitoring data, has resulted in knowledge gaps on how climate impacts these local social-ecological systems. This thesis focuses on bringing together knowledge systems from farmers, handline fishers and local scientific weather sources to examine climate variability in terrestrial and marine social-ecological systems of the southern Cape, in order to bring local perspectives into conversation with scientific data outputs. Through examining different knowledge systems in parallel and overlaying different perspectives and observations, this thesis contributes towards a better understanding of complex systems change, linked through the common thread of climate variability under a resilience lens, at the local scale of the southern Cape and Agulhas Bank. This thesis also contextualises responses to change under the theme of climate variability from farmers’ and fishers’ perspectives, and shows how different theoretical discourses can work in a complementary fashion to address complexity. The terrestrial component of this thesis examined local agricultural perspectives by surveying southern Cape farmers, and built in terrestrial scientific data through looking at local climate in relation to farming perspectives. Observations on terrestrial rainfall and temperatures were collected through interviews with 50 farmers, along with shared rainfall records from 13 farming families and ten official weather stations in the area. Fisher perspectives in relation to climate variability were then integrated with marine scientific data to examine the marine component of the Agulhas Bank. Fisher observations of climate variability were examined by drawing on existing research conducted through the South Coast Interdisciplinary Research Project. Marine wind data were obtained through model outputs from NCEP-DOE Reanalysis and a recent scatterometer-based product. Overlaying these different bodies of knowledge reduced the uncertainties associated with any single set of observations and confirmed two environmental regime shifts in the region, in the mid-1990s and end-2000s. Local climate knowledge of farmers and fishers also overlapped and corroborated these environmental regime shifts. Changes in prevailing wind direction, rather than wind speed, were more prominent over time. While no clear trends of change over time were found in rainfall and temperature time series, decadal variability was present and after the mid-2000s, the onset of seasonal autumn rainfall was found to have shifted to a month later. Knowledge disconnects were broadly related to scale mismatches between fisher observations and marine data tendencies; complexities around freshwater availability; and shifting baselines of natural resources concerning present versus past variability observed by farmers and fishers. Responses to climate variability were complex and other stressors associated with economic and political challenges were usually seen as a greater threat to local livelihoods. However, climate stressors can push social-ecological systems into vulnerable states if not well integrated into adaptation strategies, which can have serious implications for future food and job security in the southern Cape. Local-based case studies such as this one increase understanding of local social-ecological systems under global change in an effort to contribute to future adaptation strategies in the southern Cape region.
Open Access
Considering one's option when the fish leave: a case study of the traditional commercial handline fishery of the Southern Cape
(2015) Gammage, Louise Carin; Jarre, Astrid; Mather, Charles
Today, many pressures (socio-economic, resource scarcity, policy, and regulation) make fishers and their communities vulnerable on a variety of fronts. These pressures threaten fishing communities along the South African coast. Both natural and social changes in the traditional handline fishery affect the social-ecological system of a region as a whole. Fishers need to cope with these local global changes and require systems that support their strategies to achieve resilience. Furthermore, stressors that drive variability in the fishery system occur on multiple temporal and spatial scales thereby exposing fishers and communities to multiple stressors. The impact and interplay of these stressors at multiple scales need to be taken into account to develop a clear understanding of social-ecological linkages if sustainable livelihoods are to be promoted and guaranteed. There is however a shortage of appropriately scaled, context-specific data that is needed to inform various decision-making processes. To this end, participant-led research was conducted in six communities in the Southern Cape, where 50 participants were interviewed over a period of 6 months in 2013/2014. The interviews were an attempt to gather and record perceptions and knowledge regarding stressors that are responsible for the social- ecological system and ultimately affect the fishers' ability to fish successfully. Based on this knowledge, the research was aimed at gaining insight into what strategies are currently employed to ensure sustainable livelihoods. The data presented does not only offer valuable insights into the day-to-day experiences of the group of fishers, but also expose various knowledge gaps that exist in micro-scale interactions that influence the fishery system. This is achieved by first providing an analysis of various stressors, which include the impacts and responses to climate variability, challenges presented by policy and regulatory frameworks, social and economic considerations, challenges presented by infrastructure and political considerations. The adaptation, coping, and reaction strategies implemented are analysed using a place-based context and variability of strategies employed between each specific place is discussed. Apart from highlighting knowledge gaps, the development of a more complete understanding of current reacting, coping and adaptive strategies as well as the drivers behind the decisions contained in this thesis, provides valuable insight into a fishery system that is not well-described which underscores the need for context-specific research at smaller scales.
Open Access
A decision tree framework for assessing status of exploited marine ecosystems under changing environmental conditions
(2018) Lockerbie, Emma Margaret; Shannon, Lynne J; Jarre, Astrid
The removal of marine species through fishing has impacted marine ecosystems for thousands of years. The pressure of exploitation on marine ecosystems has now reached a point at which there is serious concern over ecosystem well-being on a global scale. There has, therefore, been a global move towards an ecosystem approach to fisheries management. The objective of this study was to develop a decision tree framework to assess the status of exploited marine ecosystems, which could be successfully applied to numerous ecosystems and guide decision support under changing conditions. This work was based on that of the IndiSeas project, which makes use of indicators designed to detect the impacts of fishing on marine ecosystem around the world. A suite of indicators, selected from those utilised in the IndiSeas project, was divided into ecological and fishing pressure indicators. Ecosystem specific suites of environmental indicators were also included, allowing the framework to ascertain the impacts of environmental variability on ecosystem components. This is an important addition as currently many assessments of the impacts of fisheries do not account for the influence of the environment. The framework was developed for the Southern Benguela ecosystem and then applied, with minor adjustments to account for ecosystem-specific characteristics, to the South Catalan Sea and North Sea. Indicator time series were analysed making use of linear regressions, resulting in the assignment of a score between one and five, depending on the direction and significance of trends. Data series were divided into distinct periods based on known environmental changes or shifts within ecosystems. Careful consideration was given as to whether fishing and environmental indicator trends could explain the observed trends in ecological indicators. A method of score adjustment was then developed to account for the impacts of both fishing and environmental variability on ecological indicators. Correlations were conducted to detect potential redundancies of ecological indicators and weightings were applied to decrease the contribution of correlated indicators to overall ecosystem trends. However, as correlations differed between indicators and amongst ecosystems, it was necessary to adjust the applied weightings for individual ecosystems. Results for the Southern Benguela classified the ecosystem as neither improving nor deteriorating during Period 1 (1978-1993) and Period 2 (1994-2003). During Period 3 (2004-2010) the ecosystem was classified as possibly improving. The South Catalan Sea was classified as possibly deteriorating during Period 1 (1978-1990) and neither improving nor deteriorating during Period 2 (1991-2010). The North Sea ecosystem was classified as neither improving nor deteriorating during Period 1 (1983-1992). During the second (1993-2003) and third (2004-2010) periods the ecosystem was categorised as possibly improving. When assessing fisheries impacts at an ecosystem scale there are typically high levels of uncertainty. However, this thesisoncluded that the development of a scoring and weighting system, alongside the addition of environmental drivers and the inclusion of expert knowledge throughout the applications of this framework, has allowed the developed decision tree framework to successfully categorise the three ecosystems. It is anticipated that the knowledge that this framework will add to current methods of generating advice for fisheries management will aid future decision support within these ecosystems.
Open Access
Development of a scenario-based approach for responding to change in fishery systems: a case study in the small-scale fisheries of South Africa's Southern Cape
(2019) Gammage, Louise Carin; Jarre, Astrid; Mather, Charles
Small-scale fishers and the communities they support face a range of challenges brought on by change in their marine social-ecological systems (SES). The resulting complexity and uncertainty hamper their ability to achieve sustainability while holding implications for decision-making at various scales: fishers need to respond proactively to change at smaller scales of operation while managers need to apply the principles of ecosystem-based management approaches such as an ecosystem approach to fisheries management (EAF) at larger scales. Using the small-scale fishing communities of South Africa’s southern Cape as a case study, this thesis explores how structured decision-making tools (specifically causal mapping, Bayesian belief networks and scenario planning) can be applied in an interactive and iterative scenario-based approach with disenfranchised fishers in support of decision-making at multiple scales. Specifically, this thesis aims to (1) determine and describe major stressors in the fishery system of the southern Cape using the perspectives from the crew component of its line fishery; (2) establish what interactions and feedback loops (drivers of change) exist and interact at various scales; (3) use Bayesian belief network modelling in an iterative participatory process to establish the prominent drivers of change within the fishery system (from the crew perspective); (4) develop, together with fishers, four stories of what the future may hold for one of the towns using an iterative participatory scenario planning exercise, based on some of the principles of transformative scenario planning approaches; (5) evaluate the contextual suitability of the application of the various tools used throughout the research process and recommend next steps in a larger scenario planning process; and (6) create an opportunity for fishers to engage in a process that could enhance their understanding of possible change response strategies through learning, thereby increasing adaptive capacity in the support of the implementation of an EAF in South Africa. As a start, drivers of change were established and documented, complementing earlier research. This was done to ensure that all user groups’ views were represented in an initial causal map showing the drivers of change in the fishery system. In the causal mapping process, stakeholders from towns across the research area mapped out drivers of change in an iterative process. The causal maps not only helped to frame the system but also revealed important hidden drivers of change as well as feedback loops. The Bayesian belief network and scenario story development took place in the town of Melkhoutfontein. Bayesian belief networks provided insights into system uncertainty while serving as a problem reframing tool. The outputs of both the causal maps and Bayesian networks were then used to construct four scenario stories depicting possible futures in 30 years, based on inputs obtained from research participants in a visioning workshop. These scenarios not only provided examples of plausible futures under certain conditions but also promoted new ways of thinking about the drivers of change and their likely effects, highlighting the interconnectedness in the system. Implementing the overarching approach has provided marginalised fishers with an opportunity to freely air their views while engaging with new tools. The process does not only benefit fishers and their communities (at the small scale) but also provide valuables insights into how fishers view and experience the marine SES of the southern Cape. Moreover, the approach has identified ways in which challenges presented by scale in SES can be better addressed to ensure more effective decision-making in the pursuit of sustainability. This understanding and insight are integral for moving closer to the implementation of the EAF in South Africa, where the integration of the social dimensions of marine social-ecological systems into coherent evaluation and planning continues to be problematic.
Open Access
The diet, reproductive biology age and growth of yellowtail, Seriola lalandi, in South Africa
(2014) Dunn, Kieron; Attwood, Colin; Jarre, Astrid; Kerwath, Sven
Yellowtail, 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.
Open Access
Ecology and ecophysiology of Zostera capensis: responses and acclimation to temperature
(2020) Lawrence, Cloverley Mercia; Pillay, Deena; Jarre, Astrid; Bolton, John
This study aimed to understand the ecology of the threatened, habitat-forming seagrass, Zostera capensis in Langebaan Lagoon, a marine protected area that forms part of the West Coast national park, South Africa; and the physiological strategies that allow this habitat to persist in sub- and supra-optimal temperatures. First, the environmental drivers responsible for spatial and temporal variability in Zostera and its associated macro-epifauna were determined. Secondly, the effects of temperature and the role of algal grazing in maintaining seagrass performance under temperature stress were investigated. Seasonal field collections of seagrasses and their associated macro-epifauna were undertaken, along with regular measurements of key environmental variables. Thereafter, mesocosm experiments were performed to measure the responses of Zostera to thermal stress and grazing using morphometry, fluorometry, chromatography and biochemistry. Significant seasonal variability in seagrass distributions with severe diebacks in summer were found, which influenced associated macro-epifauna communities. Populations responded to environmental stress by changing their growth form producing small-leaves in high densities in the high intertidal, while those in low intertidal and subtidal areas produced sparse, large-leaved populations. These distinct populations supported unique faunal diversities, which were dominated by grazing invertebrates. Temperature was a consistent driver of seagrass density and leaf size, while turbidity and exposure were key environmental factors that influenced macro-epifaunal patterns. Macro-epifaunal abundances were highly positively correlated with seagrass leaf width and biomass. Different growth forms displayed different responses to thermal stress, including a higher photosynthetic rate, and accumulations of carbon and nitrogen as phenolic compounds, in small- compared to large-leaved plants. This implies that large-leaved populations are more vulnerable to stress from fouling, which was evident in their larger epiphyte loads, compared to small-leaved populations. In addition, grazers were ineffectual at regulating epiphyte growth which increased under warming conditions. These findings suggest differences in resilience between sub-populations of Zostera, and attest to their capacity to recover from environmental stress. They further emphasise the significance of identifying characteristics and acclimation strategies that allow habitats to persist under climate change, and thus sustain biodiversity and productivity, as well as continue to provide important ecosystem services.
Open Access
Ecosystem effects of bottom trawling in the Benguela current system : experimental and retrospective data analyses
(2011) Mafwila, Samuel Kakambi; Field, John G; Jarre, Astrid; Shannon, Lynne
This thesis investigates ecosystem effects of bottom-trawling on demersal fish assemblages (1990-2006) off Namibia and their relationship to environmental variables.
Open Access
Effects of demersal trawling on marine infaunal, epifaunal and fish assemblages: studies in the southern Benguela and Oslofjord
(2009) Atkinson, Lara Jane; Field, John G; Jarre, Astrid; Shannon, Lynne; Hutchings, Larry
This thesis investigates the impacts of the demersal trawl fishery on infaunal, epifaunal and fish assemblages in the southern Benguela upwelling system for the first time. In the absence of representative areas of similar habitat protected from trawling in the southern Benguela region, infaunal and epifaunal assemblages were compared between heavily and lightly trawled areas to assess the impacts of the otter-trawl fishery. Infauna were sampled at four sites, from southern Namibia to near Cape Town by means of five replicate grab samples at each paired heavily and lightly trawled area. Invertebrate epifauna were sampled at two sites in heavily and lightly trawled areas using a finemeshed otter trawl. Sites ranged in depth from 350-450 m in unconsolidated sediment habitat. Epifaunal assemblages showed greater differences at heavily trawled areas with significantly reduced species diversity, average number of species and individuals. Several epifaunal species were absent from heavily trawled areas highlighting their vulnerability to impacts of trawling. Multivariate analyses show significant differences in composition of both infaunal and epifaunal assemblages among the sites and between trawling treatments at all sites. The results of this study suggest that intense trawling activities are at least partially responsible for significantly altering benthic community composition, affecting epifauna to a greater measurable extent than infauna. Biological Traits Analysis (BTA) was used to explore potential changes in ecological functioning of benthic assemblages, comparing areas exposed to heavy and light trawl intensities in the Benguela system. BTA incorporates biological traits (life-history, morphology and behaviour) of infaunal and epifaunal species with biomass, capturing a broad range of information of marine benthic assemblages. Seventeen percent of the infaunal traits analysed showed a significant difference between heavily and lightly trawled areas. Twenty-four percent of epifaunal biological traits investigated were significantly different between areas of heavy and light trawling. This study suggests that more intense trawling modifies some trait constituents of the benthic assemblage in the southern Benguela region, confirming the sensitivity of functional traits analysis in detecting changes induced by trawling disturbance. Biological traits analysis of benthic invertebrates shows promise as a practical technique for incorporation into monitoring programmes and for developing indicators of benthic ecosystem health, needed for implementation of an ecosystem approach to fisheries management in South Africa. A lack of representative untrawled areas in the trawl grounds of southern Africa precluded investigations comparing trawl impacts with unfished reference sites. A bilateral agreement between South Africa and Norway (NORSA) provided the opportunity to conduct experiments in an untrawled area of Oslofjord, Norway, where a shrimp trawl fishery for Pandalus borealis operates nearby. Infauna Abstract 2 were sampled at four untrawled sites in Oslofjord with five replicate grabs after which an Agassiz beam sled was dragged across two of the sites (impact sites) four times, simulating a trawl disturbance. Infauna were re-sampled at all four sites immediately after trawling (post-impact), 14 days and 64 days after the impact to monitor recovery of infaunal populations. Multivariate analyses comparing the impact sites with paired control sites for each sampling occasion showed no significant differences in infaunal assemblages at any stage of the experiment. Whilst it is considered possible that the trawl simulation was not a sufficient impact to represent that of a commercial trawl effect, it is considered more likely that trawl activities in Oslofjord do not inflict measurable impacts on infaunal assemblages. However, the impact of trawling on epifaunal assemblages in Oslofjord was not investigated in this study. Annual research survey data collected over the past 24 years (1986-2009) provide an opportunity to explore long-term demersal fish assemblage composition changes on the west coast of South Africa. Differences in spatial (latitude and depth) and temporal (seasonal and annual) factors were examined using multivariate analyses. Possible long-term changes were investigated using the Sequential T-test Algorithm to detect Regime Shifts (STARS). Results indicate geographic differences in fish assemblage composition from the northern to the southern region on the west coast of South Africa. The fish community composition is also clearly influenced by depth with a distinct change in fish assemblages in the shelf break region between 300 m and 400 m. Multivariate analyses also show two clear temporal changes in assemblage composition, firstly, in the early 1990s and secondly, in the mid- 2000s. STARS analyses detect long-term shifts in 27% of demersal species with the majority of speciesâ shifts detected either in the early- to mid-1990s or in the past decade (2002 to 2009). Multivariate analyses among year groups reveal an increase in three fast-growing, early maturing species and decreases in two slow-growing, long-lived species. STARS analysis detected increases in two of the same fast-growing species, decreases in an additional four slow-growing, long-lived species, but four other slow-growing, long-lived species showed the opposite trend (i.e. increases). The hypothesis of an increase in fast-growing, early maturing species and a decline in slow-growing, longlived species in fished systems is therefore only partially supported by these findings. Shifts in demersal fish assemblages coincide temporally with spatial shifts observed in small pelagic species and west coast rock lobster. The shifts in the demersal fish assemblage composition detected in this study are probably a reflection of long-term indirect effects of fishing in combination with environmental changes. Abstract 3 The response of benthic invertebrate assemblages to two levels of fishing intensity in the southern Benguela region justifies regular monitoring of epifauna during existing annual demersal research surveys and infaunal monitoring through dedicated, periodic sampling initiatives. Demersal fish assemblage data should be regularly assessed for changes in community composition. Representative protected areas can serve as reference areas against which fishing impacts could be assessed and improve our understanding of ecosystem effects of demersal fishing.
Open Access
Estimating the spatial and temporal variability of primary production from a combination of in situ and remote sensing data a southern Benguela case study
(2013) Williamson, Robert I; Field, John G; Shillington, Frank; Jarre, Astrid; Potgieter, Anet
The aim of this thesis is to produce fine resolution estimates of primary production in three-dimensional space at the temporal scale that these events develop. It is hypothesized that complex relationships among time sequences of physical and biological processes that influence primary production can be automatically discovered from archives of data. This study uses an archive of in situ ship-board data containing subsurface temperature and phytoplankton distribution profiles. Each profile is associated in time and space with satellite remotely-sensed wind, sea surface temperature and surface chlorophyll a data. The bottom depth, season and location of each profile are also recorded. The archive of depth profiles is simplified by mapping each profile onto one of twelve representative profile clusters obtained using the k-means clustering algorithm so that each cluster contains a set of similar profiles and their corresponding data. Relationships between remotely sensed surface features and chlorophyll a profiles are first obtained from a static Bayesian network using same day data. This is then taken further by analysing time-series of satellite data to predict likely temperature and chlorophyll a profiles for each pixel of a 4 km resolution satellite image.
Open Access
Evaluating the implementation efficacy of an ecosystem approach to fisheries in the South African anchovy fishery
(2014) Astor, Nicole; Jarre, Astrid; Van der Lingen, Carl D
A knowledge-based tool was developed to assess the efficacy of the ecosystem approach to fisheries in the South African anchovy fishery. South Africa has agreed to implement an ecosystem approach to fisheries (EAF) management. The EAF management comprises of three dimensions; ecological well-being, human well-being and ability to achieve. The focus for this study was on the ecological well-being dimension. Meetings were held with experts to revise objectives stemming from issues in the anchovy fishery documented earlier and indicators linking to these objectives. A hierarchical tree was constructed using these indicators and objectives. The objectives were divided into pressure and state. The indicators were transformed using piecewise linear transformation on to a common scale from -1 to 1. Thresholds were also decided to assess when the EAF implementation was considered to be 'good' (1), 'ok' (0) and 'bad' (-1). Weighted means were applied through the hierarchy. The overall resulting output, the truth value, was then used to assess how well the ecological dimension of the anchovy fishery had been doing over time. This showed the implementation efficacy of an EAF within the ecological dimension had been doing badly in 1987-1996, and since 1997 the ecological dimension of the EAF has been doing better. Sensitivity tests on the weights also showed that the knowledge based tool was robust to changes in weights. This allowed for experts to have slight differences in opinion over the way the weights should be distributed, but without these differences in opinion affecting the overall truth value. Sensitivity tests on the thresholds of one of the indicators showed that the outcome of the indicator is sensitive to changes in the thresholds. However, when looking at the overall truth values or the objectives, the changes to the thresholds did not affect the resulting objective values or the overall truth value that much. Although the indicator values change with changes in the thresholds, looking at the whole knowledge based tool it is fairly robust to moderate changes in thresholds.
Open Access
Examining the feeding ecology of two mesopelagic fishes (Lampanyctodes hectoris & Maurolicus walvisensis) off the west coast of South Africa using stable isotope and stomach content analyses
(2016) Tyler, Tamsyn; Jarre, Astrid
Although mesopelagic fishes are an important component of marine food webs, the adaptive features used to facilitate niche partitioning among co-existing and presumably competing mesopelagic species is unclear. This study examined the trophic ecology of the two principal mesopelagic fishes off the west coast of South Africa (lanternfish Lampanyctodes hectoris and lightfish Maurolicus walvisensis) sampled during the spring 2014 and autumn 2015 cruises, using stable isotope and stomach content analyses. Stable isotope values were extracted from the white muscle tissue of fishes, but due to the high lipid content of both species, samples were processed in duplicate: δ13C was measured from lipid-extracted samples and δ15N from non-extracted samples. To validate the stable isotope results, stomach contents were examined and the relative importance of prey items was assessed using three measures: frequency occurrence (%F), numerical abundance (%N), and dietary carbon (%C). Both mesopelagic species occupied different isotopic niches that were separated by their δ15N values across a similar δ13C range. Furthermore, the relationship found between trophic position and standard length emphasizes the structuring effect of size within the assemblage, with the larger species (L. hectoris) occupying a higher trophic position than the smaller species (M. walvisensis). Although copepods dominated the diet of L. hectoris in terms of numerical abundance (42%), macro-zooplankton was by far the most important dietary component, with euphausiids contributing 53% of dietary carbon. Conversely, copepods - particularly Calanus sp. - were the most important component of the diet for M. walvisensis in terms of their occurrence (84%), numerical abundance (64%), and dietary carbon (67%). Though some dietary overlap exists between L. hectoris and M. walvisensis, the results of this study suggest resource partitioning within the mesopelagic assemblage, likely facilitated by differences in alimentary morphology (i.e. trophodynamically mediated), and possibly by differences in their respective foraging strategies. Similarly, ontogenetic shifts in trophic position were detected, which suggests that these adaptive features may also be used to mitigate intra-specific competition within populations. Furthermore, the trophic positions of both L. hectoris and M. walvisensis inferred from dietary and isotopic data signify that mesopelagic fishes (in the context of this study) are secondary and tertiary consumers in the marine ecosystem of the southern Benguela. Nevertheless, samples covering a larger area of the southern Benguela and multiple years would be needed for a more complete understanding of the trophic ecology of these two species.
Open Access
Growth and otolith zone formation of Namibian hake, merluccius capensis
(2012) Wilhelm, Margit Renate; Jarre, Astrid; Moloney, Coleen L; Roux, Jean-Paul; Durholtz, M Deon
Life history traits and tactics of commercially important Namibian shallow-water hake, Merluccius capensis, were investigated in relation to their environment. A time series of length-frequency distributions (LFDs) from otoliths collected from fur seal scat samples was used to identify cohorts and calculate the approximate hatchdates and growth rates of young M. capensis from 1994 to 2009. Monthly otolith samples of five of these cohorts (1996, 1998, 2002, 2005 and 2006) were used to evaluate the translucent zone periodicity over the first 21 months of their life. Additionally, LFDs from research surveys and commercial samples were used to calculate growth rates for M. capensis up to 65 cm total length (TL), and to further validate the translucent zone formation of three of the five cohorts (1996, 1998 and 2002) on fish up to 3.5 years old.