OpenUCT :: Browsing by Author "Field, John G"

Browsing by Author "Field, John G"

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Open Access
Acoustic investigation of the shoaling dynamics of sardine sardinops sagax populations : implications for acoustic surveys
(1997) Coetzee, Janet Claire; Field, John G; Champion, Harold
The total allowable catches (TA Cs) of both anchovy and sardine in South Africa are determined by assessment methods which rely on absolute estimates of recruitment and spawner biomass each year. The validity of the TAC set for each species each year is heavily dependent on the accuracy and precision of the biomass estimates. The method used for both anchovy and sardine has been acoustic surveys on a random stratified grid covering the known range of the species. These surveys have, however, been designed according to the life history patterns of anchovy, which result in a particular occupation of the pelagic environment. This study investigated the shoaling dynamics of sardine at various scales in both the horizontal and vertical dimension in an attempt to ascertain whether these surveys are adequate to characterise the occupation of space by sardine. Hydro-acoustic data collected during 5 meso-scale surveys in 1994 and 1995 was used to study the shoaling dynamics of sardine, Sardinops sagax on the western Agulhas Bank of South Africa. Results of this study indicate the presence of spatial structure in sardine aggregations at both the shoal and meso-scale level.
Open Access
The application of univariate and distributional analyses to assess the impacts of diamond mining on marine macrofauna off the Namibian Coast
(1999) Winckler, Heidi; Field, John G
This study is one of three based on grab samples of macrobenthos obtained before and at different times after mining for diamonds off the coast of Namibia. The first study dealt with multivariate clustering analysis of the first samples before and after mining. The second study focused on recovery times after mining and this study is aimed at estimating the amount of stress encountered by benthic communities, for comparision with the descriptive multivariate approach. Two research areas, classified as 'northern' and 'southern' were investigated. Data were aggregated and analysed at the genus level. Graphical and statistical analyses were conducted on the data which was classified in three ways. First, on all unmined sites from the two research areas together to test for natural site-to-site variability. Secondly and thirdly, each research area (north and south) was analysed separately to test for differences between unmined and mined sites at each area. Stress levels in the community were assessed by Caswell's neutral model (the Vstatistic) and by interpretation of the value of the W-statistic (a summary statistic of the ABC curves). Correlation techniques were applied to assess if there was any relationship between the diversity indices (as indicators of the influence of disturbance on community structure) on the one hand, and the environmental indicators of disturbance (percentage gravel, sand, mud) on the other.
Open Access
Assessing the effects of fishing on fish communities using South African case studies : empirical and theoretical approaches
(2007) Ghebrehiwet, Dawit Yemane; Field, John G; Leslie, Rob W; Shin, Yunne-jai
Currently heavy ﬁshing is recognized as one of the major threats to the structural and functional organization of marine ecosystems in many coastal nations. The threat is mainly the result of the inherent nature of the various ﬁshing activities: size selectivity, habitat destruction, biomass removal, and uncertainty in resource status and management of the resource. Thus this thesis investigates structural changes that result from ﬁshing. This thesis aims to answer whether there were changes in the structure of fish communities off the of South Africa using two case studies, to explore the response of fish communities to the proposed creation of Marine Protected Areas and to investigate the alternate application of spatially uniform and heterogeneous ﬁshing mortalities. The research questions of the thesis are answered through empirical analysis of landing data for the line ﬁshery and analysis of demersal trawl survey data from the south coast of South Africa, and analysis of output of the Individual Based Model OSMOSE applied to the southern Benguela. Structural changes in the landings from the line ﬁshery and south coast survey data are assessed using a variety of ecosystem indicators believed to capture such changes: size-based indicators {mean size, slope and height of the size spectra, mean Lmax7, proportion of size classes), species-based indicators (ordination by multidimensional scaling, and dendrograms, various diversity indices, dominance curves). Inferences are based on the reference directions of the indicators, according to the expected response of indicators to heavy fishing. Structural changes in the ﬁsh communities are observed, over the spatial and temporal bounds of the two case studies, to be the most likely cause of the observed changes is heavy fishing, although the influence of environmental factors cannot he ruled out. investigation of alternative implementation ﬁshing mortality using the simulation model OSMOSE showed that the system and species biomass do differ between the two implementations, but the variability in the system remains the same. The modelled response of ﬁsh communities to the introduction of Marine Protected Areas is an overall increase in relative biomass of large predatory fishes and a decline in the biomass of prey and competitor species.
Open Access
Assessing the rate of recovery of benthic macrofauna after marine mining off the Namibian coast
(1997) Van Der Merwe, Karen; Field, John G
The primary aim of this study was to assess the rate of recovery of macrobenthic communities after offshore marine mining. Three techniques, namely univariate, distributional and multivariate, were used to make this assessment. Two distinct areas, the northern and southern research areas, were investigated, and statistical and numerical analyses were conducted for each area independently. Data were aggregated to, and analysed at, the genus level. Replicates were arranged in temporal categories according to recent mining history. The northern research area appears to be affected by mining activity in terms of species composition, but not species diversity. Statistical testing detected significant differences between unmined replicates and all other temporal categories for this area, and this was also discernible in the cluster analysis and ordination plots. The overall picture generated suggests that the northern research area is affected immediately and severely by mining activity, resulting in rapid changes in species composition. However, the period of 15-19 months subsequent to mining is insufficient to allow the community to recover to a stable state. The southern research area, on the other hand, shows a slightly different scenario, with mining activity having a severe and immediate impact on both species composition as well as species diversity. Recently mined sites were found to be significantly different from both unmined sites and sites mined 43-51 months ago. The latter two categories were not found to be significantly different from each other. The results suggest that the road to recovery in the southern research area is a slow, but steady one in terms of species composition. This was particularly apparent in the results of the "SIMPER" analysis where the level of similarity between temporal categories increased steadily with time after mining. The overall picture suggests that the area has recovered substantially after 43-51 months, and that the community approximates that of the unmined area with regard to species composition. Geological analyses were also conducted, with results indicating a prevalence of fine surficial sediment particles in unmined sites, and coarse surficial sediment particles in recently mined sites. Percentage gravel, in particular, was found to be a reliable indicator of the condition of a site with regard to the level of disturbance. Altered stratigraphy and changes in particle size distribution as a result of mining activity are considered to have a noticeable effect on the structure of benthic communities. A number of taxa were found to be particularly reliable as indicator species. In both the northern and southern research areas, polychaetes (specifically Prionospio pinnata and the Lumbrineris genus) were abundant in unmined sites as well as in sites mined 43-51 months ago in the southern research area. Individuals of the genus Nassarius, on the other hand, were scarce in unmined sites, but abundant in recently mined sites. These taxa appear to be reliable indicators of the level of recovery attained in previously mined areas.
Open Access
Assessment of patterns in the Namibian hake fishery based on commercial fisheries data, in relation to environmental factors
(2003) Voges, Elizabeth; Field, John G; Gordoa, A
This thesis explores the relationships between commercial data and those obtained from research surveys in an attempt to broaden the database available for management, and to cover seasonal and inter-annual changes in density estimates of Cape hake (Merluccius capensis). It also attempts to explain hake variability of in terms of environmental indices based on satellite remote sensing, carefully chosen to reflect underlying oceanographic processes. The influence of various factors on catch rates (CPUE) is investigated using a general linear model.
Open Access
Biology and ecological energetics of the supralittoral isopod Ligia dilatata
(1979) Koop, Klaus; Field, John G
It is believed that studies on individual species populations are important to gain a full understanding of ecosystem dynamics and that data gained from these studies can be incorporated into complex but meaningful models. This approach was adopted in a study of the complex kelp bed ecosystem off the west coast of South Africa initiated in 1975. Pilot studies indicated which plant and animal species should receive particular attention and several workers engaged in detailed studies of population dynamics and ecological energetics (Greenwood, 1974; Muir, 1977; Shafir, 1978; Dieckmann, 1978; Griffiths and King, 1978). A feature of the kelp bed system is that large masses of plant material are exported due to the heavy wave action experienced (Field et al., 1977; Velimirov et al., 1977). Attempts have been made to trace the fate of the kelp which is exported to adjacent beaches. A study by Muir (1977) showed the importance of kelp debris for the sandy beach amphipod Talorchestia capensis. The present study concerns the population dynamics and energetics of the supralittoral rocky shore kelp-eating isopod Ligia dilatata Brandt.
Open Access
Carbon and nitrogen flux through the Cape rock lobster jasus lalandii (H. Milne Edwards), with particular reference to the nearshore Benguela system
(1989) Zoutendyk, Peter; Field, John G; Branch, George M
Observations and experiments on Jasus lalandii were undertaken over the period 1977 to 1986 to quantify the passage of carbon and nitrogen through populations at Oudekraal and Robben Island on the west coast of South Africa. The terms of the energy flow equation, C=P+R+G+UE+Ux+Uo+F (where C=consumption, P=production, R=respiration, G=reproduction, UE=Exuvial loss, Ux=excretion of exogenous nitrogen, Up =excretion of endogenous nitrogen and F=faeces), were quantified by combining field data and laboratory results. With this information the role played by J. lalandii in the kelp-bed ecosystem under the influence of the Benguela current was explored and quantified.
Open Access
Characterizing and comparing the spawning habitats of anchovy (Engraulis capensis) and sardine (Sardinops sagax) in the Southern Benguela upwelling ecosystem
(2002) Twatwa, Nandipha M; Field, John G; Van der Lingen, Carl D
The spawmng habitats of anchovy (Engraulis capensis) and sardine (Sardinops sagax) in the southern Benguela upwelling ecosystem were characterized by comparing the data on abundance and distribution of eggs of the two species with environmental variables. Data were collected from two different survey programs: (a) 14 SARP (Sardine and Anchovy Recruitment Programme) cruises, conducted monthly for 2 spawning seasons between August 1993 and March 1994, and September 1994 and March 1995, off the west coast of southern Africa from Cape Columbine to Cape Agulhas; (b) annual November/December spawner biomass surveys conducted from 1984-1999 along the South African coast and covering the continental shelf between Hondeklip Bay on the West Coast and Port Alfred on the South Coast. A CalVET net was used to collect fish eggs at stations on a survey grid, and physical and biological data were collected concurrently with egg samples. Physical variables measured included sea surface temperature, nitrate concentration, water depth, salinity, current speed, wind speed and mixed layer depth, whereas biological variables included phytoplankton biomass (as 50m integrated chlorophyll a) and zooplankton biomass and production. The spawning habitats selected by anchovy and sardine were identified by constructing quotient curves derived from egg abundance data and individual environmental variables. Anchovy and sardine spawning probability with reference to the distribution of environmental variables was examined using overlay operation analysis. Relationships between eggs of the two species and the environmental variables were verified using multivariate co inertia analysis. Relationships among the environmental variables were examined through cluster analysis. During SARP surveys, single parameter quotient analysis indicated that the spawning habitats of these two species were most dissimilar in terms of water depth, sea surface temperature, current speed and zooplankton biomass, and most similar in terms of salinity and phytoplankton biomass. Coinertia analysis using all environmental variables as inputs showed a positive association between anchovy eggs and salinity and sea surface temperature, and a negative association between anchovy eggs and secondary production and phytoplankton biomass. Sardine eggs were strongly positively associated with phytoplankton biomass. During spawner biomass surveys, anchovy and sardine spawning habitats appeared to differ with respect to sea surface temperature, wind speed and current speed, and overlapped in terms of water depth, phytoplankton biomass, zooplankton biomass and production. Anchovy eggs were strongly positively associated with SST, salinity, mixed layer depth and zooplanktop production. Sardine eggs were strongly positively associated with current speed and zooplankton biomass.
Open Access
Combined fishing and climate forcing in the southern Benguela upwelling ecosystem: an end-to-end modelling approach reveals dampened effects
(Public Library of Science, 2014) Travers-Trolet, Morgane; Shin, Yunne-Jai; Shannon, Lynne J; Moloney, Coleen L; Field, John G
The effects of climate and fishing on marine ecosystems have usually been studied separately, but their interactions make ecosystem dynamics difficult to understand and predict. Of particular interest to management, the potential synergism or antagonism between fishing pressure and climate forcing is analysed in this paper, using an end-to-end ecosystem model of the southern Benguela ecosystem, built from coupling hydrodynamic, biogeochemical and multispecies fish models (ROMS-N 2 P 2 Z 2 D 2 -OSMOSE). Scenarios of different intensities of upwelling-favourable wind stress combined with scenarios of fishing top-predator fish were tested. Analyses of isolated drivers show that the bottom-up effect of the climate forcing propagates up the food chain whereas the top-down effect of fishing cascades down to zooplankton in unfavourable environmental conditions but dampens before it reaches phytoplankton. When considering both climate and fishing drivers together, it appears that top-down control dominates the link between top-predator fish and forage fish, whereas interactions between the lower trophic levels are dominated by bottom-up control. The forage fish functional group appears to be a central component of this ecosystem, being the meeting point of two opposite trophic controls. The set of combined scenarios shows that fishing pressure and upwelling-favourable wind stress have mostly dampened effects on fish populations, compared to predictions from the separate effects of the stressors. Dampened effects result in biomass accumulation at the top predator fish level but a depletion of biomass at the forage fish level. This should draw our attention to the evolution of this functional group, which appears as both structurally important in the trophic functioning of the ecosystem, and very sensitive to climate and fishing pressures. In particular, diagnoses considering fishing pressure only might be more optimistic than those that consider combined effects of fishing and environmental variability.
Open Access
Comparative ecology of the copepods calanoides carinatus and calanus agulhensis in the Southern Benguela and Agulhas Bank ecosystems
(2003) Huggett, Jenny Ann; Field, John G; Hutchings, Larry
The aim of this study was to investigate the niche separation between these two species, using zooplankton net samples collected during biannual hydroacoustic surveys of pelagic fish between 1988 and 2000, as well as results from ship- and laboratory-based egg production and feeding experiments. C. carinatus is most abundant on the productive central West Coast, but also occurs at relatively low abundance on the Agulhas Bank, mainly on the outer shelf.
Open Access
Comparative laboratory study of photoacclimation in selected dinoflagellate and diatom species of the Benguela ecosystem
(2010) Balarin, Marianne G; Field, John G; Barlow, RG; Pitcher, Grant C
In their natural environment the diatoms (mixers) are exposed to fluctuations in incident irradiance due to vertical displacements in the water column induced by turbulence whereas the dinoflagellates (migrators and layer-formers) tend to control their vertical positions. In these two groups of phytoplankton physiological acclimation to these fluctuations results in the variation of a number of measurable photosynthetic parameters and variables which can be evaluated in controlled laboratory experiments. The processes can be expressed in time scales from seconds to hours e.g. carbon to chlorophyll ratio changes over several hours. Photoacclimative responses in five species of dinoflagellates (Alexandrium catenella, Protoceratium reticulatum, Prorocentrum micans, Prorocentrum triestinum, Gymnodinium zeta) and three diatom species (Chaetoceros sp., C. capense, C. cf. pendulus) were investigated with respect to parameters of P versus E curves (P*m, Î±* and Ek) and variability in chemical composition (C and N), photosynthetic capacity, pigment ratios, maximum quantum yield and chlorophyll to carbon ratio. These species were grown at irradiances of 33 (LL), 178 (ML) and 647 Î¼mol quanta . m-2 . s-1 (HL) at a 12:12h day: night length at 17 ÂºC. The photosynthetic parameters, pigment concentration and Chl a-specific absorption were mostly affected by photoacclimational status. Species-specific differences were observed at the three different light levels in P versus E curves, pigment concentrations, absorption, carbon to chlorophyll a ratios and quantum yield. Photoprotective carotenoids (diadinoxanthin and diatoxanthin) were significantly different relative to Chl a in eight species of dinoflagellates and diatoms.
Open Access
Competition for anchovy (Engraulis capensis) and sardine (Sardinops sagax) between the Cape Gannet (Morus capensis), Cape fur seal (Arctocephalus pusillus pusillus) and the purse-seine fishery on the west coast of South Africa
(2002) Pillay, Pavitray; Field, John G; Crawford, RJM; Oosthuizen, WH
Competition for pelagic ﬁsh resources was investigated by assessing the overlap in the food base of three land-based predators: Cape gannet (Morus capensis), Cape fur seal (Arctocephalus pusillus pusillus) and humans operating via a purse-seine ﬁshery. Multivariate analysis the diet composition of gannets and seals and the catch composition of the pelagic ﬁshery indicated that there were three ""feeding regimes"" during the 21-year study period: (1978-1998), an anchovy-dominated regime (1978-1983), an intermediate regime (1984-1990) and a sardine-abundant regime (1991-1998). It further showed that anchovy (Engraulis capensis) and sardine (Sardinops sagax) contributed substantially to the catches of all three top predators. Analysis of size-distributions of catches showed that the three predators generally caught the same sizes of anchovy and sardine, reﬂecting very little resource partitioning. The analysis demonstrated stronger competition between gannets and the purse-seiners than between gannets and seats, especially during seasons of poor recruitment. Furthermore, seasonal differences in the catches of the three predators are related to the recruitment, growth and migration of the prey species. The study conﬁrmed previous observations that gannets prefer sardine to anchovy by showing gannet exploitation of sardine in the early 19905, when anchovy was still abundant. The gannets, which are species-speciﬁc feeders, may be impacted by substantial removals of pelagic ﬁsh resources by a large seal population and a large ﬁshery.
Open Access
A computer simulation of the population dynamics of the Cape hakes (Merluccius capensis and M. paradoxus)
(1978) Mertens, Robert George Stephen; Field, John G; Newman , G G
Catch and effort statistics for the Cape hakes (Merluccius capensis and M. paradoxus) have indicated a substantial decrease in the stock size over the period 1955-74; consequently, a catch projection model was designed to investigate the hake population. Where appropriate, available data have been included in the model; however, some further research was required: 1. The monthly pattern of availability for 1974-76 was examined using South African data; it was found to be similar to that reported for earlier years, and the later figures were used. 2. Assuming that selection curves for different mesh sizes are identical except for their position on the X-axis, a method to calculate selection values for combinations of mesh size and fish length has been outlined, and used with data presented by Bohl et al. (1971). 3. An investigation was made of two methods of estimating natural mortality (M), utilizing the results of Virtual Population Analysis (V.P.A.). In both cases the criteria used to judge M were found to be insensitive, and therefore neither can be used. 4. The stock estimates obtained using V.P.A. were applied to two stock/recruit curves. The goodness of fit in both cases was poor, and nearly identical.
Open Access
Distribution and abundance of Cape hakes (Merluccius capensis and Merluccius paradoxus) in relation to environmental variation in the Southern Benguela system
(2000) Millar, Dinah Lynn; Field, John G
Standard Geographical Information Systems (GIS) techniques and Generalized Additive Models (GAM), nonparametric regressions without the assumptions of normality or linearity of traditional regression methods, were used to test the hypothesis that trends in hake (M. paradoxus and M. capensis) distribution and abundance are related to ocean environmental conditions (including bottom temperature, bottom oxygen concentration, sea surface temperature and sea surface minus bottom temperature, as a measure of stratification or mixing) and location (including longitude, latitude and bottom depth). Hake and physical data, from seventeen biomass surveys undertaken by Marine and Coastal Managment between 1984 and 1997 in the southern Benguela ecostystem, were used to test these relationships.
Open Access
Distribution and mixture of Cape and Cunene horse mackerel, Tachurus capensis and Tracherus trecae in the Angola-Benguela front in relation to environmental and other factors
(2001) Duarte, Agostinho Domingos Caholo; Field, John G; Cury, P; Bianchi, G
This thesis makses an analysis of survey data of horse mackerel catch per unit effort and acoustics data from R/V Dr. Fridtjof Nansen over twelve years in the region of the Angola-Benguela front. The main objectives are: to characterize the pattern of distribution and mixture of Cunene and Cape horse mackerel in the area around the Angola-Benguela front and to study the relationships between the distribution of T. capensis and T. trecae and the movements of the Angola-Benguela front. The role of sea surface temperature (SST) was also examined, assuming that this environmental parameter would be related to the seasonal variation in distribution of both species of horse mackerel, at least in the overlap area.
Open Access
Distribution and population dynamics of Euphausia lucens (Euphausiacea) in the southern Benguela current
(1987) Pillar, Stanley C; Field, John G
The thesis first reviews the research on zooplankton ecology in the Benguela system and then evaluates the contribution of euphausiids to the zooplankton biomass of the southern Benguela region. The study further investigates the population dynamics and maintenance of the dominant euphausiid, Euphausia lucens, principally in the St Helena Bay region because of its importance as a recruitment area for the pelagic fishery.
Open Access
Distributions of physical habitats and benthic macroinvertebrates in Western Cape headwater streams at multiple spatial and temporal scales
(2005) Schael, Denise Marie; King, Jackie; Day, Jenny; Field, John G
The research in this thesis examines the links between benthic macroinverebrate (invertebrate) distributions and aspects of the physical habitat at different spatial and temporal scales in mountain and foothill zones of rivers in the Western Cape, South Africa. A geomorphological hierarchy was used as the spatial template for studying invertebrate distributions. The different levels of the hierarchy are: catchment, segment, zone, reach, morphological unit and hydraulic biotype. Hydraulic biotypes are visually defined flow and substratum combinations. The hierarchical levels studied in this thesis were catchment, zone, reach and hydraulic biotype.
Open Access
Disturbance and temporal variability in invertebrate assemblages in two South African rivers
(2009) Ractliffe, Sylvia Georgiana; King, J M; Field, John G
This thesis presents an examination of the relationship between floods as disturbances, the disturbance regime and the temporal dynamics of invertebrate assemblages, over the short term and at intra- and inter-annual time scales in the Molenaars and Berg Rivers in the Western Cape of South Africa. Invertebrate responses to individual floods were investigated by a field study that links the displacement of river-bed stones by a flood to change in invertebrate densities and community and population structure from before to after flood events. The magnitude of the hydraulic force acting on each marked stone during the peak of each flood was also calculated, providing a second measure of physical disturbance. Multivariate analyses of similarity, hierarchical clustering and multidimensional scaling were used for analysis of invertebrate patterns before and after floods. Size frequency data for 28 species or genera were analysed to explore changes in population structure over the flood season. Flood records were developed from the daily discharge hydrological record of both study rivers collected by the South African Department of Water Affairs and Forestry. Flood frequency analysis and descriptive statistics were combined with graphical methods to describe the flood regime and to test flood predictability in these rivers. Quantitative monthly samples of invertebrates from the Molenaars River collected over 17 months were used together with a further 2 ½ years of semi-quantitative monthly data, to identify intra- and inter-annual patterns in communities. Multivariate analysis of community patterns was combined with a range of indices that reflect community persistence and stability over periods longer than one generation. Population dynamics of the common species were also studied. Life history attributes, specifically seasonality of life cycle stages and generation time, were explored using size frequency data from the samples.
Open Access
Ecophysiology of the black mussel Choromytilus meridionalis (Krauss)
(1980) Griffiths, Roberta Joyce; Field, John G
The thesis decribes the reproduction, population dynamics and production, filtration, respiration and assimilation of the black mussel Choromytilus meridionalis (Kr.) at Bailey's Cottage, False Bay, South Africa. The reproductive season and gonad development is described from monthly measurements made over a period of 4 years. Spawning usually extends from July to February and is characterised by peaks of gamete release interspersed with regeneration of the gonad. Individual fecundity varies from year to year and increases with increasing body size, representing 61-97% of production. Although large volumes of gamete material are emmitted annually, spat settlement at the study site is only successful at 4 to 6 year intervals. The population dynamics at different shore levels is described in detail.
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.