Browsing by Author "Hutchings, Laurence"

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    The Feeding ecology of, and carbon and nitrogen budgets for, sardine sardinops sagax in the Southern Benguela upwelling ecosystem
    (1999) van der Lingen, Carl David; Hutchings, Laurence; Field, John G
    Combined laboratory and field studies were employed to examine the feeding ecology of sardine Sardinops sagax in order to evaluate conflicting hypotheses regarding the trophic position of clupeoids in upwelling ecosystems, and to compare the trophodynamics of sardine with those of the co-occurring anchovy Engraulis capensis. Carbon and nitrogen budget models constructed using data from these studies were used to quantify the effect of particular food environments upon sardine growth. Sardinops sagax is primarily a filter-feeder, with food particles <1230μm total length eliciting a filtering response while larger particles elicit particulate-feeding at low concentrations and filter-feeding at high concentrations. This species is able to retain cells as small as 13μm, feeds at near-maximum efficiency when filterfeeding, and displays size-selectivity during particulate-feeding. Significant linear relationships between respiration rate and swimming speed obtained for sardine demonstrate that filter-feeding is the most energetically cheap feeding mode. Although omnivorous, sardine absorbs carbon and nitrogen more efficiently from zooplankton than from phytoplankton. Gastric evacuation follows an exponential pattern in sardine, and is influenced by food type; phytoplankton is evacuated faster than zooplankton. Feeding periodicity in sardine is size dependent; small fish show a feeding peak at, or around, sunset whereas larger fish appear to feed continuously. Estimates of daily ration range between 0.99 to 7.58% wet body mass.d-¹, depending on fish size and food type. Sardine stomach contents are numerically dominated by small particles, principally dinoflagellate phytoplankton, but the majority of the sardine's dietary carbon is derived from zooplankton, principally small calanoid and cyclopoid copepods. The budget models indicate that sardine is capable of positive growth under most of the trophic conditions it is likely to encounter in the southern Benguela upwelling system. The classical hypothesis that the high abundance of clupeoids in upwelling ecosystems results from their phytophagy is rejected; like anchovy, sardine are primarily zoophagous. However, these two species are trophodynamically distinct and show resource partitioning on the basis of prey size; sardine consume small zooplankton whilst anchovy consume large zooplankton. This difference is likely to contribute to regime shifts observed between these two species.
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    Patterns of distribution and spatial indicators of ecosystem change based on key species in the Southern Benguela
    (Public Library of Science, 2016) Watermeyer, Katherine E; Hutchings, Laurence; Jarre, Astrid; Shannon, Lynne J
    Several commercially and ecologically important species in the southern Benguela have undergone southward and eastward shifts in their distributions over previous decades, most notably the small pelagic fish sardine Sardinops sagax and anchovy Engraulis encrasicolus . Understanding these changes and their implications is essential in implementing an ecosystem approach to fisheries in the southern Benguela and attempting to appreciate the potential impacts of future environmental change. To investigate possible impacts of these shifts at an ecosystem level, distribution maps for before (1985-1991), during (1997-2000) and after (2003-2008) the shift in small pelagic fish were constructed for 14 key species from catch and survey data, and used to calculate spatial indicators including proportion east and west of Cape Agulhas, relative overlap in biomass and area, index of diversity, connectivity. Potential interactions on the south and west coasts were also compared. For several species (redeye; chub mackerel; kingklip; chokka squid; yellowtail), previously unidentified increases in the proportion of biomass east of Cape Agulhas were shown to have occurred over the same period as that of small pelagic fish, although none to the same degree. On average, overlap with small pelagic fish increased over time and overall system connectivity was lowest in the intermediate period, possibly indicating a system under transition. Connectivity declined over time on the west coast while increasing on the east coast. Distributions of other species have changed over time, with the region east of Cape Agulhas becoming increasingly important in terms of potential trophic interaction. Variations in distribution of biomass and structural complexity affect the trophic structure and hence functioning of the system, and implications should be considered when attempting to identify the possible ecosystem impacts of current and future system-level change.
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    The development and decline of phytoplankton blooms in the southern Benguela upwelling region
    (1986) Brown, Penelope Cranstoun; Field, John G; Hutchings, Laurence
    [page ii, 27 missing] These "Redfield productivity 14 estimates" were similar to C-uptake productivity but lower than estimates obtained from changes in particle volume. If the period of maximum nutrient decrease was used for the calculations, the "Redfield productivity 14 estimates" lie between the C-uptake and particle volume estimates. 14 Daily rates of C-uptake water column productivity ranged between 0.94 and 14.01 g C.m- 2 .d- 1 (mean 3.80 g C.m- 2 .d- 1) and were similar to or higher than productivity estimates reported for other upwelling areas. Phytoplankton biomass in the upper 50 metres ranged between 8 and 506 mg phll �- m- 2 (mean 208 mg chll �.m- 2); on average, about half the biomass occurred below the 1% light level indicating that self-shading is an important factor limiting primary production in the Benguela upwe!ling system. The temporal scale of phytoplankton bloom development was investigated in terms of changes in chlorophyll � concentrations in the euphotic zone. The build up and decline of the primary phytoplankton (diatom) bloom in newly upwelled water occurred within 6-8 days. The initiation of blooming appears to be controlled by the stability of the water body (vertical and horizontal), and the decline of the bloom was usually associated with reduced nutrient levels and is considered to result mainly from phytoplankton cells sinking out of the surface layers. Dispersive processes may also contribute to bloom decline. Zooplankton grazing made little impact on the phytoplankton community. The wide coastal band of chlorophyll-rich water seen in satellite images to well beyond the outer limit of the drogue tracks, suggests that both regenerated nutrients and new nutrients (entrained into surface waters after the primary bloom), maintain primary production in shelf waters at moderate levels than the 6-8 days suggested by this study.
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    Zooplankton of the Cape Peninsula upwelling region
    (1979) Hutchings, Laurence
    The vertical and horizontal distribution of meso-zooplankton in the vicinity of an extremely active upwelling site in the southern Benguela Current is described. Zooplankton samples collected in oceanic, newly-upwelled and mature upwelled water showed that temperature, chlorophyll and time of day had considerable effects on the vertical distribution of different species. Most species underwent limited daily vertical migrations within, the upper mixed layer, while concentrations of animals decreased markedly below the thermocline except where sinking had occurred, when an inverted distribution, particularly of juveniles, was observed. The Bray-Curtis dissimilarity coefficient was used to distinguish sample groups within the data. Differences in environmental parameters between groups were examined using the Mann-Whitney U-test. Species groups were separated using both the Bray-Curtisand McConnaughey coefficients and their relative merits are discussed. Samples from the upper mixed layer in upwelled water contained the greatest zooplankton standing crop in terms of dry weight, with Centropages brachiatus the most abundant organism. The frontal zone, with a strong thermal gradient, contained animals with distinct warm and cold water affinities as well as a group of species closely associated with the temperature gradient .The near-surface distribution of the bulk of the population, combined with the lack of cool-water continuity to the south, poses especial problems for the maintenance of the upwelled water community in the area during the upwelling season. It is suggested that sinking at the oceanic front combined with periodic shoreward flow of the upper layers when the southerly wind stress is relaxed, are sufficient to allow zooplankton to replenish rapidly from the north and offshore .