Browsing by Author "Hutchings, Larry"

Now showing 1 - 5 of 5
  • Thumbnail Image
    Item
    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.
  • Thumbnail Image
    Item
    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.
  • Thumbnail Image
    Item
    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.
  • Thumbnail Image
    Item
    Open Access
    A model of copepod population dynamics in the southern Benguela upwelling region
    (1995) Plagányi, Éva Elizabeth; Field, John G; Hutchings, Larry
    A simple population dynamics model is constructed to simulate temporal variability in the biomass of a dominant copepod Calanoides carinatus (Copepoda: Calanoida) along the West Coast region of South Africa. C. carinatus is extensively preyed upon by the commercially important anchovy Engraulis capensis and variability in zooplankton production may serve as an useful predictor of variability in anchovy recruitment levels. The model developed here circumvents the need to include a large number of parameters because it uses satellite-derived estimates of chlorophyll a concentration and sea surface temperature as primary inputs. Abundance estimates necessary to initialise the model are readily obtainable from biannual research cruises. The model successfully simulates observed features of a copepod population's response to pulses of upwelling and results obtained are consistent with data from field studies. The model is robust with respect to most of its parameters because minor changes in their values result in predictable changes in model output. The effect on model predictions of errors in field estimates is quantified. The model showed greatest sensitivity to parameters which are difficult to determine empirically, such as predator-induced mortality rates. Gaps in our present understanding of the nature and scale of processes affecting copepod egg abundance, survival and viability in the Southern Benguela system, were identified as the dominant impediment to attempts to simulate copepod population dynamics in the region. The Southern Benguela system is patchy on a range of different space and time scales. The effect of fine-scale distributional heterogeneity on mesoscale patterns of copepod productivity was investigated by assuming that spatial patchiness affected the degree of overlap between zooplankton and phytoplankton populations. The effect of spatial patchiness is particularly prevalent under poor feeding conditions, and may result in predictions based on average feeding conditions underestimating zooplankton production by as much as 30% in some circumstances. Estimates of zooplankton production are sensitive to both the spatial arrangement and intensity of food patches in a heterogeneous environment. There is a need to isolate the essential mechanisms causing distributional heterogeneity and to quantify the effect of spatial patchiness on model predictions to permit the correct averaging of model results over broad horizontal areas. Because of the model's sensitivity to the predator-induced mortality rate, a temporally and spatially integrated system is used to quantify this parameter as a function of varying patterns of predator and prey abundance. Shoals of anchovy recruits are explicitly modelled feeding on patches of C. carinatus prey, and the fish's performance is quantified through temporal and spatial integration of periods and patches of prey abundance and shortage. Constant high fish densities dampen the spatial variability in copepod abundance, whereas a pulsed predation pressure permits locally depleted copepod populations a short respite in which to recover some growth, thereby allowing the persistence of a few good prey patches which offer favourable energy returns for foraging fish. The model suggested that at high densities of anchovy recruits, predicted growth rates are strongly density-dependent and predation rates may exceed copepod production rates. Absolute measures of prey availability are sometimes unable to predict anchovy feeding success as mechanisms permitting temporal and spatial segregation play a vital role in synchronizing the relationship between fish predation pressure and prey turnover rates. The model emulates observed variability in anchovy growth rates and analysis of the output indicates that the availability of high sustained abundances of food along the West Coast may be a critical "bottleneck" contributing to the strength of recruitment to the pelagic puseseine fishery in South African waters. Observed chlorophyll a concentration and sea surface temperature data in 1971 and 1972 were used as inputs into an annual version of the basic model, and model-predicted patterns of copepod biomass were compared with observed patterns of zooplankton biomass in the two years. The ability of the model to simulate major differences in the general features observed in the two years supports its use as a tool to describe net patterns of zooplankton productivity over large horizontal areas. The model · identified the need to quantify the role of major size-class groups, such as the microzooplankton and macrozooplankton, in mediating the flow of energy from phytoplankton to fish.
  • Thumbnail Image
    Item
    Open Access
    The pelagic copepods of Lambert's Bay : an ecological study of a west coast fishing ground
    (1983) Hopson, Susan Kate Dickerson; Field, John G; Hutchings, Larry
    Monthly zooplankton samples were collected as part of the Cape Egg and Larva Survey from August 1977 - 1978. A line of stations from the coast to approximately 90 Kilometres offshore at Lambert's Bay was selected for intensive study because of its location in the middle of the recruitment area of South African anchovy and pilchard. Examination of physical features showed perennial moderate upwelling, with slight seasonal variation in frequency and intensity. Chlorophyll "a" levels were generally high. Zooplankton standing stock was surprisingly low considering the abundant phytoplankton, and this was attributed to intense predation resulting in high production/biomass ratios. Among the zooplankton, copepods were singled out for special attention because of their importance as food for larval fish. Only six species comprised 83% of the copepod community. These species displayed different spatial and temporal distribution, which appeared to be partly the result of size-determined life histories with indications of time/space niche partitioning. In comparison to the more intense seasonal upwelling area off the Cape Peninsula, there was a trend toward fewer species, larger animals, and more carnivores. This trend was related to changes in the physical and biotic environment.