Bibliography: leaves 122-135. / The successful aquaculture of the abalone Haliotis midae requires a multi-disciplinary approach. Experiments were designed to provide insight into various aspects of abalone biology and seawater system design. A comparative evaluation of a closed and open seawater system for an H. midae hatchery was performed. Monthly seawater temperatures and nitrite levels were higher in the closed system. The salinity of the closed system seawater varied more than that of the open system. The pH of seawater in the closed system varied between 7.7 and 8.2. This was lower than the 7.9 to 8.35 pH range of the open system seawater. An open system is thus clearly preferable to a closed system. Haliotis midae reproductive condition can be assessed visually by examining the shape and colour of the broodstock gonads. Spawning experiments showed that abalone should be starved for at least 24 hours prior to spawning induction. Haliotis midae can be induced to spawn by treatment with hydrogen peroxide when exposed to seawater at a pH of between 9.0 and 9.9 (males and females) Spawning can reliably be induced by using final hydrogen peroxide concentrations in the range of 7 to 25 mM for male and female H. midae. Gravid broodstock should be exposed to hydrogen peroxide for 100 to 300 minutes during spawning induction. There was no significant impact observed for prior conditioning of the brood-stock to the spawning tanks, lunar phase and timing of spawning induction relative to sunset. Haliotis midae larval toxicity to chlorine, copper and ammonia was investigated. The LT50 to a chlorine concentration of 0.06 mg/l was 170 minutes. For copper the LT50 of larvae exposed to 0.12 mg/l was 53 minutes. The LT50 of ammonia at 5 mg/l was 600 minutes. A brief synopsis of the major findings is presented in the last chapter. Some discussion on the future prospects of the abalone industry is also provided.

Bibliography: leaves 182-198. / Three different techniques are used to established statistical relationships between annual pelagic fish species catches viz. pilchard, horse mackerel and chub mackerel, and monthly environmental indices. The three techniques are Spearman's Rank correlation, multiple regression and cross-correlations of the Box-Jenkins time series approach. The first method yielded interesting results but only in terms of the nature of the relationship not in terms of the response effect. In the same way the multiple regression analysis showed inherent problems in the interpretation of the results due to serial correlation in all fish catches and environmental data series. The Time-Series analysis yielded more coherent results, presumably due to the fact that the data series were pre- whitened to remove serial correlation. Monthly means of sea surface temperature, north-south wind component, west- east wind component and sea level were used as environmental indices. Over the period 1950 to 1985, annual pilchard catches are found to be negatively correlated to sea surface temperature in the Namaqualand and Agulhas Bank areas, but are positively related to sea surface temperature in the south western Cape area. With regard to wind components, northerly and westerly winds in the Namaqualand and south western Cape areas are found to be significantly correlated to annual pilchard catch, whereas in the Agulhas Bank southeasterly - and southwesterly wind are found to be predominant. Warmer waters improved horse mackerel catches in all areas, immediately in the Agulhas Bank area but affecting catch the following year in the other two areas. In the Namaqualand area predominantly northwesterly winds are found to favour horse mackerel catch immediately and this response is probably due to availability. The same wind orientation is found in the south western Cape area but with one-year-lag. In the Agulhas Bank area the favourable wind orientation is southerly with a zero lag and westerly with one-year-lag. Cooler waters influence chub mackerel catches favourably in all areas and in all seasons. In the Namaqualand area the effect of the sea surface temperature is immediate in all but the winter season. Northwesterly winds are most important in this area having an immediate effect from winter to summer. Cooler temperatures in the south western Cape area the previous year improve the annual chub mackerel catch. Northerly and easterly winds favour catch immediately during summer while northwesterly winds are most important during spring. In the south coast area, southwesterly winds improve catches immediately during spring and summer but south-westerly winds during autumn are related to improved catches the following year. These results are not easy to interpret because of problems in the quality of the catch data. These problems may be identified as constraints due to the effect of the different age-classes to the catch and the variability in the effort exerted by the fishery onto the resource. Furthermore, age-length distributions have changed recently for all three species, so that it is difficult to establish relationships which link up to specific effects of the environment on specific age- classes. It is assumed, however, that a zero-lag in the relationships between fish catches and environmental variables represents availability rather than environmental effects on spawning, recruitment or stock abundance.

The possible role of phytoplankton in the nutrition of the larvae of the prawn M. rosenbergii was investigated in this study which is conducted in two parts. In the first section of Part I, the utilization of the yolk and its lipids in the egg and first two larval stages was examined. It was found that the yolk lipids in the egg undergoes very little change during egg development and that these lipids provide for 94% of the metabolic requirements of the first stage larvae. In section two of part one, the changes in the lipids and fatty acids in larvae reared in phytoplankton rich cultures and in clear water were examined to determine whether the phytoplankton had any effect on the composition of these lipids. As no overall significant differences were found in the lipid and fatty acid compositions of larvae reared in phytoplankton rich or clear water systems, it was concluded that phytoplankton had no significant effect on the composition of lipids in M. rosenbergii larvae. In part two of the study the uptake and fate of dissolve ¹⁴C-glycine was examined. The uptake of ¹⁴C-glucose and ¹⁴C-labelled algal exudate was also examined. It was found that a mechanism for the uptake of these dissolved organic substances does exist in the larvae. It was also shown, however, that the dissolved organic compounds are not of major importance as a source of energy in the nutrition of the larvae. The possible role of dissolved organic compounds is discussed. It was concluded that phytoplankton does not play an important role as an energy source for the larvae of M. rosenbergii.

Bibliography: leaves 120-128. / Community structure of intertidal rocky shore mussel beds has been previously shown to be strongly influenced by trophic interactions, particularly interactions between predators of mussels and their prey. This investigation involved, firstly, construction of models to simulate trophic interactions in the mussel beds of the west coast of South Africa under different conditions of wave exposure and shore height and, secondly, field-caging experiments to investigate predation on mussels around the entire coast of South Africa. These two studies allowed for a comparison between predator interactions in the model and in the field.

Lampanyctodes hectoris, commonly called lanternfish, is a myctophid and was first recorded in the South African pelagic catches in 1969. The average annual landings since then have been approximately 4% of the pelagic catch, the largest being 42 560 metric tons in 1973 (10,45% of the total landing). Maurolicus muelleri, commonly called lightfish, is a sternoptychid, and is not taken commercially. An attempt was made to document the basic biology of these two species before possible commercial fishing pressure brings about a change in these parameters. Samples of L. hectoris taken from commercial catches and research cruises, and samples of M. muelleri obtained from research cruises were analysed to obtain the biological parameters. L. hectoris spawning in the southern Benguela Current System occurs throughout the year, but mainly during the late winter to early spring with a peak in spring. M. muelleri spawning occurs throughout the year with a peak in spring/early summer and a smaller peak in winter. Maturity analysis show the same pattern, and also indicate that M. muelleri spawning has both a vertical and a horizontal dimension and its eggs were found in the temperature range 14 to 11°c.

The primary aim of this study was to investigate changes in benthic community structure and to assess how they relate to environmental factors using multivariate and distributional techniques.

Includes bibliographical references. / The effects of the exploitation of prawns (Callianassa & Upogebia spp.) in Langebaan Lagoon, primarily for bait collecting, were assessed using field experiments and observations. Langebaan Lagoon has been a proclaimed National Park for 20 years and a proclaimed Marine Protected Area since 2000. The area is zoned for different human uses, about one third being reserved as a sanctuary, one third for non-extractive visitation and one third for extractive purposes including bait-collecting and fishing. Four species of prawns were identified in Langebaan Lagoon - C. kraussi, C. rotundicaudata, U. africana and U. capensis.

Includes bibliographical references (p. 101-115). / Spawning habitats of sardine and anchovy in eastern boundary systems are characterized in an attempt to understand the fluctuations observed in their abundance. These species have developed mechanisms that enable them to survive in these turbulent conditions. Spawning habitats were characterized by investigating the relationships between egg abundance and environmental data collected through two different survey programmes: SW APEL surveys (1978 -1985) and Nansen surveys (1999 - 2004). Single factor quotient techniques were used to define the selection of environmental conditions by these species for spawning.

Bibliography: leaves 64-69. / In the late nineteenth century, six North American mesquite taxa (Prosopis spp., Mimosaceae) were introduced into South Africa. They were to provide shade for livestock, and their pods were valued as a source of fodder. All Prosopis species in south Africa were recognized, as weeds in 1983. In 1987 a biological control programme using seed weevils (Coleoptera: Bruchidae) was launched against the two most invasive taxa, namely, P. glandulosa var. torreyana (L. Benson) M.C. Johnston and P. velutina Wooton. Algarobius prosopis (Leconte), the first biocontrol agent released, destroys large numbers of mesquite seeds annually, where livestock do not graze pods that are lying on the ground. Where pods are grazed, however, there is insufficient time for A. prosopis to inflict an adequate degree of seed damag'e before pods are eaten. For this reason another bruchid, Neltumius arizonensis (Schaeffer), which was reported to be capable of ovipositing on immature and mature tree-borne pods, was released in 1993. The establishment of N. arizonensis on mesquite at three sites in Western Cape Province was confirmed by monitoring N. arizonensis oviposition and emergence in the field. High levels of oviposition by N. arizonensis on tree-borne pods in June 1994 (39 and 29 'egg-seeds' at Onderplaas and Clanwilliam, respectively) were accompanied by high rates of trichogrammatid egg parasitism. Most pods had fallen to the ground by this time. The degree of egg parasitism was independent of N. arizonensis egg density. From December 1994 until June 1995, N. arizonensis egg densities were lower than those recorded in June 1994. The rate of field oviposition by N. arizonensis returned to high levels in August and September 1995, when 'tree pods' were again few in number. Neltumius arizonensis eggs were more abundant on 'tree pods' than on 'ground pods' in August and September 1995.

Includes bibliographical references. / A dispute arose in the Luderitz rock-lobster, Jasus lalandii, fishery as to whether declines in CPUE were due to changes in rock-lobster migration patterns or a reduction in fishable biomass. Rock-lobster migrations were studied at two sites by estimating in situ density in the 10-12m and 15-20m depth zones. Water temperature and dissolved oxygen concentration profiles were obtained at several sites at approximately monthly intervals. Observations over a period of six years indicated that rock-lobster underwent seasonal in- offshore migrations and that marked interannual variation in those migration patterns did, indeed, occur. The movements and their interannual variation were directly related to the inshore presence of water with very low dissolved oxygen concentrations. CPUE was closely related to the density of legal sized rock-lobster on the fishing grounds (observed by SCUBA diving) and hence it is feasible that at least a part of the decline in CPUE at Luderitz was due to an alteration in the "normal" pattern of in-offshore migration. The hypothesis that environmental change caused the major decline in rock-lobster catches at Luderitz was examined by time series analysis of 21 years of CPUE, wind, SST and sea-level data. Luderitz rocklobster CPUE was found to be negatively correlated with the southerly component of summer wind stress at Diaz Point six years previously. On this basis it was hypothesised that variable settlement of puerulus larvae is an important factor driving changes in recruitment to the fishable population. The mechanism may be increased mortality of larvae as they attempt to cross the continental shelf (due to greater northward and offshore drift during periods of strong southerly wind). The dramatic interannual changes in rock-lobster density provided an opportunity to carry out a "natural" experiment on the interaction between rock-lobsters and the associated benthic species assemblage. It was found that, although there were changes in the benthos (notably an increase and subsequent decline in whelk density), these were unrelated to variation in rock-lobster density. In addition, no relationship between per capita food availability and rock-lobster growth on four grounds, was apparent. As regards temporal changes in food supply and subsequent effects on rock-lobster growth rates; there was no temporal reduction in food supply at the main study site which could be related to the reduction in CPUE since 1988.