Browsing by Author "Gibbons, M J"

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    Egg and larval ecology of anchovy (Engraulis capensis) and sardine (Sardinops sagax) in the Southern Benguela ecosystem
    (1998) Fowler, Justine Lindsay; Gibbons, M J; Field, John G
    Temporal and spatial changes in the abundance and distribution of anchovy and sardine eggs and larvae in the southern Benguela ecosystem were investigated during 14 cruises over 2 consecutive summer upwelling seasons as part of the South African Sardine and Anchovy Recruitment programme (SA-SARP). Cruises were conducted each month between August 1993 and March 1994 (SARP I) and September 1994 and March 1995 (SARP II). Anchovy spawning was largely confined to the western Agulhas Bank, reaching a definite peak during spring (October and November). Sardine ·spawning extended up the west coast ·duiing·periods when anchovy spawning reached a peak on the western Agulhas Bank. However, sardine eggs were dense on the western Agulhas Bank during peak spawning activity which appears to be bimodal, reaching a peak in the early spring (August/September) and again in late summer (February). · Although the midshelf region on the W AB appeared to be the centre of both anchovy and ·sardine spawning in the southern Benguela ecosystem, these data suggest that anchovy and sardine spawning activity may be both spatially, and temporally separated to some extent. Mean anchovy egg densities on the W AB decreased by 53 % from 1993/94 to 1994/95 while a slight increase in mean sardine egg density (16 %) over the entire region was observed from one season to the next. Lloyd's Patchiness Index was used to investigate the distribution patterns of spawning products and the results showed that the distribution of both anchovy and sardine eggs and larvae was extremely patchy, but that the eggs of both species were more patchy than their larvae. Anchovy eggs were most abundant at sea surface temperatures ranging from 16-20°C. Changes in the extent of 16-l 9°C water (as a measure of spawning habitat) and the abundance of large copepods within this region reflect intra-annual changes in the intensity of anchovy spawning activity. Sardine eggs were found predominantly in water of between 14.5-21.5°C. The seasonal intensity of sardine spawning appears to vary independently of the area of 16-19°C water, but· a positive correlation between the spawning activity of sardine and the monthly chlorophyll a concentrations on the W AB suggests that spawning may be related to their feeding conditions. A comparison of egg and larval distributions with current features from selected months during SARP supports previous studies which indicate that the frontal jet plays an important role in the transport of the early life history stages of anchovy and sardine but that the position of such transport can vary between the 200 m and 500 m isobaths. Areas of possible egg loss included the W AB, the offshore currents which can develop west of the Cape Peninsula and the outer branch of the jet off Cape Columbine. However, there is evidence that onshore currents further north may transport the eggs back to the region of the jet. The monthly estimates of anchovy egg mortality were highly variable and imprecise and these results were probably due to a combination of small sample sizes and reduced egg abundances, usually encountered at the start and end of the anchovy spawning season. The annual estimates of anchovy egg mortality during November between 1984 and 1994 were positively correlated with the patchiness of anchovy eggs during the same cruises. The effect of cannibalism in areas where eggs are particularly dense was invoked as a possible explanation for this result. It was concluded that the spawning behaviour of anchovy and sardine may be influenced by sea surface temperatures, feeding conditions for adult fish and advective processes in the southern Benguela ecosystem and these factors were discussed in terms of their possible impact on recruitment.
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    Studies on the meiofauna of rocky shores
    (1988) Gibbons, M J; Griffiths, Charles L
    Annual macrofaunal and meiofaunal standing stocks were estimated on an exposed rocky shore along the west coast of False Bay, South Africa, using comparable area based sampling techniques. While meiofaunal densities exceeded those of macrofauna in all zones, by an overall ratio of approximately 400:1, macrofaunal biomass exceeded that of meiofauna by an overall ratio of 10:1. The numbers of meiofauna were not evenly distributed across the shore but varied with the algal standing stocks in each zone and their sediment load. By incorporating turnover ratios from the literature, mean annual productivity ratios were calculated which suggested that meiofauna were responsible for 25 of total (excluding bacterial) secondary production. To follow this up, the impact of wave exposure on the meiofauna of one species of alga (viz. Gelidium pristoides) was examined on five shores around False Bay. Meiofaunal densities (dominated by animals between 63um-280um) were significantly greater on sheltered than exposed shores. As the minimum width of Gelidium fronds exceeds that of these permanent meiofauna, and tufts offer little resistance to wave action, only those individuals living in the dense, holdfast region of plants could escape the impact of waves on exposed shores. Total meiofaunal biomass per plant remained constant irrespective of shore type, due to the greater numbers of juvenile bivalves and amphipods on exposed shores. Algal and herbivore biomass were not significantly different between shore types around False Bay and therefore, the proportional contribution by meiofauna to total secondary production on sheltered shores was predicted to be greater than on exposed shores, where the biomass of macrofaunal filter feeders was very high. It has previously been argued that differences in meiofaunal communities between plant species are a result of differential surface area, number of habitats and refugia from predation. The possible fate of meiofaunal productivity as food for higher trophic levels (fish) and the mediating role played by algal complexity was investigated in a series of carefully designed laboratory and field experiments.