The chronic degradation of coral reefs globally and its negative impact on coastal communities such as those in southern Sri Lanka dependent on the marine ornamental trade for their livelihood forms the focus of this study. Attempts to improve the conservation status of Sri Lanka's coral reefs and their associated fauna have failed because they omit to address the social circumstances of local people. Such social-ecological systems require an integrated approach, which provides holistic reasons for the degradation of natural ecosystems and livelihoods of coastal people. The aim of this study was therefore to ascertain the current sustainability of the marine ornamental trade in southern Sri Lanka through an interdisciplinary study employing a participatory bottom-up approach, and derive from findings alternative pathways to restore and maintain the health of the reef and thus provide better livelihoods for the fishing communities. Findings confirm both the fragile state of nearshore coral reefs, their fish populations and the precarious nature of local communities’ livelihoods. Historical and recent environmental and anthropogenic impacts reduced resilience in all trade sectors and current fishing practices and the unjust supply chain compound these effects. Therefore, a holistic co-management framework is recommended that recognises local ecological knowledge and involves fishing communities as citizen scientists to improve monitoring and also provides communication channels to facilitate interaction within and across all groups of the ornamental trade. In this way, all actors are involved in making decisions and taking responsibility for the management of the supply chain at their particular level. This single, coherent framework would thus employ diverse groups and ways of doing as a resilience strategy to halt the degradation and reinvigorate the reef for more sustainable utilisation whilst simultaneously developing highly acceptable alternative income generating livelihoods, such as the community-based aquaculture experiment undertaken during this study.

Three trials were conducted to investigate the effects of dietary immuno-stimulant supplementation on various physiological, haematological and immunological parameters of ornamental fish following stress and vaccination. The first series of experiments investigated the mucosal stress response of mirror carp, (Cyprinus carpio), using urinalysis strips and established stress indicators within peripheral blood. A netting stressor, which combined aerial exposure and confinement, of 6 minutes duration was applied and the response of fish was measured 1.5, 3, 5, 24 and 168 hours post-stress. Mucosal haemoglobin and protein were established as consistent indicators of stress and strong relationships could be drawn between them and plasma glucose concentration and peripheral monocytes and lymphocytes counts, firmly establishing them as secondary stress indicators. Visual assessment of the strips was found to overestimate mucosal parameters compared to results obtained using instrument based analysis, but the underlying changes were similar. Further experiments conducted on goldfish, (Carassius auratus), and tilapia, (Oreochromis niloticus x. 0. mossambicus), showed that the mucosal response to stress was conserved between fish species. The second trial investigated the potential of dietary immuno-stimulants to modify the immune status of fish and their response to stress. Fish were fed either a control, beta-glucan (0.2 % w/w), nucleotide (0.2 % w/w) or combined supplement (0.2 % beta-glucan + 0.2% nucleotide) diet for 3 weeks at 1 % BW.dˉ¹ and then to the same stressors as used in Trial 1. Significant stress-induced changes in mucosal protein (p = 0.006), plasma glucose (p = <0.004) and protein concentration (p = <0.003), peripheral lymphocytes (p = 0.001) and monocytes (p = <0.001) and blood NBT reduction (p = <0.001) were observed. Fish fed diets containing immuno-stimulants shower a quicker recovery from the stress-induced hyperglycaemic response than those fed the control diet. Fish that received the beta-glucan diets showed lower blood NBT reduction immediately post-stress than all other diets. The final trial investigated the effects of the control, beta-glucan and combined-supplement diet on the response of fish to vaccination. Fish were fed the diets for 2 weeks prior to and 1 week following IP vaccination against Aeromonas salmonicida at 1.0 % BW.dˉ¹. The immune response was measured every 2 weeks for 12 weeks following vaccination. Vaccination induced a significant stress response as seen by a large hyperglycaemic response (p = <0.05). Significant reductions in peripheral monocyte count were seen following vaccination (p = <0.04). All groups showed elevated NBT reduction at 4 weeks post-vaccination. Serum lysozyme activity showed evidence of long term immuno-suppression following vaccination. No clear effect of the immuno-stimulating diets could be found on any of the physiological or immune parameters tested. The trials conducted found that ornamental fish have a well developed mucosal stress response and this can be used to make inferences about the stress state of fish. The effects of beta-glucans and nucleotides on either the response to stress and immune status were subtle. The nucleotide supplement appeared most effective at reducing the effects of stress, whilst beta-glucans may exert more influence on the immune parameters tested. However, no evidence of synergy between the two types of immuno-stimulant was found in fish fed both immuno-stimulants together.

Research has been actively looking for alternative feed ingredients to reduce the reliance of the aquafeed industry on marine ingredients, namely fish meal (FM) and fish oil (FO). In this context, insects, in particular housefly (Musca domestica) and black soldier fly (BSF, Hermetia illucens) larvae, have been identified as promising candidates. Although a global insect farming industry is emerging, it is for now constrained by regulatory and technical bottlenecks that raise the question ‘where and how insect-based products could be integrated into aquaculture’. The literature indicated a high interspecies variability of the results when replacing FM with insect meals in fish diets and previous work failed to consider the existing challenges related to the insect production to demonstrate commercial relevance and applicability. In this thesis, maggot meals (MM) and frass (insect digestate) were assessed as strategic feed ingredients for two commercially important farmed species: Atlantic salmon, (Salmo salar) and Nile tilapia (Oreochromis niloticus), in their relevant contexts. Case studies showed that both housefly and BSF MM are high quality feed ingredients and suitable alternative to FM. Specifically, dietary inclusions of up to 200 g/kg of crude or defatted housefly larvae meal did not compromised the feed digestibility and utilisation and the growth performance and body composition of salmon parr (freshwater stage), compared to a FM-based control diet. Hormone (17α-methyltestosterone) treated diets containing between 250 and 1000 g/kg BSF or housefly meal were found as effective as a commonly used pure hormone-treated FM in sex-reversal process leading to 99.8 to 100% males, high survival and evenness of the fish produced. In a commercial diet for advanced nursing of Nile tilapia fingerlings, up to 80 g/kg BSF meal was included without impairing the fish performance and body composition; dietary inclusion was limited by the lipid content of the crude MM. Finally, BSF frass derived from brewery spent grains or processed food wastes were found more effective when used as soil bio-fertilisers with minimum application rate of 10.0 tonnes/ha or 5.0 tonnes/ha, respectively (for a spring onion culture), rather than supplemental feeds for tilapia farmed in semi-intensive conditions (fertilised pond). The study also indicated that site-specific conditions should be accounted to support appropriate and sustainable use of insect-based products but in any case, juvenile fish should be strategically targeted given their requirements. It is expected that this approach, could support the sustainable intensification of aquaculture and contribute more broadly to food security whilst contributing to the development of a circular economy.