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The work described in this thesis was conducted In rural Gambia and builds a body of evidence Incriminating the fly Musca sorbens as a vector of the blinding disease, trachoma, which is caused by ocular infection with Chlamydia trachomatis. Literature on hygiene promotion, environmental change and flies and trachoma is reviewed in the context of the SAFE strategy for trachoma control advocated by the World Health Organization M. sorbens was present throughout the year in trachoma endemic communities; was responsible for the majority of fly-eye contacts; C. trachomatis DNA was found on it; and trachoma transmission dropped when they were removed from the environment. In a large cluster-randomised trial communities receiving fly control with insecticide for six months had a mean reduction in trachoma prevalence of 56% (95% Cl 19-93%; P=0.01) compared to controls and 37% (4-70%; P=0.068) fewer new prevalent cases of trachoma. Breeding media choice experiments showed that isolated human faeces were the preferred larval medium for M. sorbens and were capable of supporting the production of large numbers of adults. However, other animal faeces were also able to support M. sorbens development. This suggested that a community-based strategy to reduce the quantity of human faeces on the soil surface by providing latrines would have the effect of reducing the population of M. sorbens, and hence reduce fly-eye contact and trachoma transmission. The provision of latrines gave encouraging results, which were not statistically significant; 30% less active trachoma than controls after six months (-22-81%; P=0.210) and 28% (-5-60%; P=0.146) fewer new prevalent cases. Provision of latrines warrants further investigation as a method to control trachoma, particularly when used in conjunction with other control methods. The potential role of fly control in the SAFE strategy for trachoma control is discussed.

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This study examined three aspects of the processes of host location and initial attachment of the infective copepodid stage of the sea louse, <i>Lepeophtheirus salmonis</i> Krøyer 1838, to its host Atlantic salmon, <i>Salmo salar </i>(L.). The role of host and non-host odours and derived chemicals in modifying copepodid behaviour was identified in laboratory bioassays. Seasonal and stage-specific changes in the energetics of the eggs and the free-living larval stages were investigated through respiration experiments and chemical analyses. The influence of realistic physical factors, light intensity, salinity and host swimming speed, on determining the sites of initial copepodid attachment were identified in flowing water conditions, by the use of flume studies. Exposure to original odours and extracts of host odours resulted in significant changes in non-oriented behaviour and movement patterns which would increase the probability of host encounter, whilst exposure to original and extracts of host odour gave near-significant levels of oriented movement to the odour source. The host-derived chemicals isophorone and 1-octen-3-ol appeared to induce activated and limited arrested behaviours in the copepodid, respectively. During all stages of embryonic and larval development a gradual reduction in weight and energy content with developmental stage of larvae was noted, as lipid and protein stores were preferentially catabolised to provide energy for morphological changes and increased locomotion of the later larval stages, as revealed by an observed increase in respiration rates with developmental stage. Seasonal changes in embryonic dry weight, energy and biochemical composition were masked by the presence of seasonally variable proteinaceous egg cases. The free living stages showed a mixed trend in reproductive strategy, similar to previous studies of <i>L. salmonis </i>eggs in response to seasonal changes in temperature and photoperiod, but also similar to trends observed in food-unlimited holopelagic copepod species. Light intensity, salinity and host swimming speed independently and interactively altered the distribution and total initial attachment of <i>L. salmonis</i> copepodids on host Atlantic salmon smolts.

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1. Although growth and moulting rates of larval and post-larval lobsters have often been monitored, very little is known about the effects of such regimes on the physiology of the animals, on their muscle properties, or on their performance and behaviour when released into the wild in re-stocking or ranching programmes. The extensive research on effects of environmental variables on fish development (Johnston and Temple, 2002), is unlikely to provide an accurate model for a crustacean such as the lobster, which have distinct larval and adult stages in different environments, and a development pattern with intermittent growth during the moult and metamorphosis. This project addresses these issues in the context of the practicalities of rearing the European lobster Homarus gammarus in a laboratory environment. It poses the question: do lobster rearing programmes produce animals that are suitably equipped to compete against their wild counterparts for food and space, once released? To address this question, groups of post-larval European lobsters, Homarus gammarus, were reared at 11°C, 15°C and 19°C from post-larval Stage 8 to 18 months old, and also from the egg through to 6 months of age at 15°C and 19°C at the larval rearing facilities of the Centre for Environmental, Fisheries and Aquaculture Science (CEFAS), Conwy, North Wales. 2. Rates of mortality were low at all rearing temperatures. Moulting frequencies were temperature-dependent, as shown by the complete moult records and also by the data based only on the median group. Long Inter-moult periods for the postlarval stages at 11°C indicate that this temperature was close to the null point for moulting. The standard measure of size, carapace length (CL) increased with age in a temperature-dependent manner from an initial value of 6 mm in the Stage 8 lobsters. Daily increases in CL were 18 mm, 28 mm for the 11°C, 15°C and 19°C groups respectively.