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In recent years, freshwater fish farmers have come under increasing pressure from the Water Authorities to control the quality of their farm effluents. This project aimed to investigate methods of treating aquacultural effluent in an efficient and cost-effective manner, and to incorporate the knowledge gained into an Expert System which could then be used in an advice service to farmers. From the results of this research it was established that sedimentation and the use of low pollution diets are the only cost effective methods of controlling the quality of fish farm effluents. Settlement has been extensively investigated and it was found that the removal of suspended solids in a settlement pond is only likely to be effective if the inlet solids concentration is in excess of 8 mg/litre. The probability of good settlement can be enhanced by keeping the ratio of length/retention time (a form of mean fluid velocity) below 4.0 metres/minute. The removal of BOD requires inlet solids concentrations in excess of 20 mg/litre to be effective, and this is seldom attained on commercial fish farms. Settlement, generally, does not remove appreciable quantities of ammonia from effluents, but algae can absorb ammonia by nutrient uptake under certain conditions. The use of low pollution, high performance diets gives pollutant yields which are low when compared with published figures obtained by many previous workers. Two Expert Systems were constructed, both of which diagnose possible causes of poor effluent quality on fish farms and suggest solutions. The first system uses knowledge gained from a literature review and the second employs the knowledge obtained from this project's experimental work. Consent details for over 100 fish farms were obtained from the public registers kept by the Water Authorities. Large variations in policy from one Authority to the next were found. These data have been compiled in a computer file for ease of comparison.

This thesis provides a detailed study of methods for dissolving oxygen in water to reduce water requirements for fish farming. The principal sources of oxygen are air or pure oxygen gas. Aeration methods have the distinct advantage of the universal availability of air. However, the effectiveness of such methods is diminished by the presence of nitrogen in the air and, in general, the maintenance of dissolved oxygen levels above 70% saturation is likely to result in excessive power requirements. Pure oxygen has five times the solubility of oxygen in air and it is possible, therefore to achieve much higher transfer rates. However, oxygen is expensive and its economic use is essential: it is important, therefore, to dissolve a high proportion of the oxygen. Four distinct oxygenation systems were evaluated by the author. A detailed analysis of a column oxygenator is given first. The column was designed so that the oxygen bubbles generated are trapped within the column until dissolved. In seawater, much smaller bubbles are formed and this led to the development of a jet oxygenator which disperses gas rubbles within the rearing tank. Both the above systems were designed primarily for oxygenating recycled tank water. For oxygenating a primary water source, a U -tube device was evaluated. Lastly, the possibility of supporting fish stocks without any external power source, other than a pressured supply of oxygen from a liquid oxygen store, was considered. Experience of running commercial-scale oxygenation systems in high-intensity fish farms has made it possible to estimate operating costs of both aeration and oxygenation systems. The significance of these costs is discussed.

No description available.

Fish populations were sampled by electrofishing at a variety of sites on the Rivers N. Esk (1992-3) and Dee (1993-4) for comparison with sawbill duckling diets from the same rivers. Sampling methods are described and discussed in relation to other methods and in terms of possible biases. Within electrofished samples species distribution were similar on the two rivers, but more fish were caught per unit effort on the N. Esk. Catch results are described in terms of fish numbers and catch weights of each species from different sections of river and from different sampling times over the summers. Special attention is paid to the distribution and size frequency of salmon, because of their importance in the fish communities of the two rivers and as food for sawbill ducklings. The diets of goosander and merganser ducklings on the study rivers are quantified in terms of species of prey in the diet, size/age classes of fish and relative proportions of these species and classes. Estimates of diet are made from the stomach contents of samples of ducklings. Species composition of the diets are then related to the distribution of fish in those parts of the rivers in which the ducklings forage. Sizes of fish in the diets are then related to the sizes of fish in the duckling foraging areas, taking into account the changes in fish size with location and overtime, and the size (and approximate age) of ducklings. For comparison with diets, fish length data from electrofished samples was interpolated by regression between pairs of consecutive samples and adjacent river sections. This provided expected mean river fish lengths for the dates and locations at which broods were sampled.

The potential role in pathogenicity of extracellular products (ECP) of the fish pathogen Aeromonas salmonicida was investigated. Evidence is presented that determinants of host damage are produced extracellularly by A. salmonicida. In vitro, the bacterium produced a number of extracellular enzymic and ctyolytic activities, including proteases and an heterogeneous haemolysin which was active against rainbow trout erythrocytes. Solubilization of trout erythrocytes in vitro resulted from the cooperative effects of the haemolysin and a caseinolytic protease. The caseinolytic protease was implicated in host damage; the in vivo effects of the caseinase may have been potentiated by haemolysin. The production of extracellular proteolytic and haemolytic activities by a number of isolates of A. salmonicida was studied; some potentially important differences between isolates were found. Evidence is presented that rainbow trout possess humoral mechanisms of resisting the toxic products of A. salmonicida. Proteolytic and haemolytic activities of ECP were inhibited, and the in vivo toxicity of ECP was neutralized, by normal trout serum. In vitro, factors in normal trout serum appearaed to form soluble complexes, as well as insoluble precipitates, with component(s) of ECP. In vitro, ECP caused a reduction in circulating levels of an alpha-2 macroglobulin analogue in rainbow trout. As putative determinants of pathogenicity, extracellular factors of A. salmonicida are potentially important in conferring protective immunity against furunculosis. The humoral immune response of rainbow trout and rabbits to components of ECP was investigated. At least 15 components of ECP, including the caseinase and haemolysin, were immunogenic in the rabbit. In the trout, antibodies to only 4 components of ECP were detected and no antibody to the caseinase or haemolysin was found. It is suggested that the effectiveness of furunculosis vaccines might be improved if the immunogenicity in trout of certain ECP components could be enhanced.

This thesis explores the physical and the socio-economic conditions for aquaculture in Tabasco, Mexico, seeking to find how aquaculture can best be supported to reduce poverty. On-farm physical, technical and social surveys were carried out in four municipalities, covering different physiographic regions. The surveys establish that the environmental conditions are frequently good for aquaculture. Despite efforts to bring farmers into more intensive commercial systems, aquaculture has developed mainly to improve subsistence, as one component of semi-subsistence farms. In order to evaluate these semi- subsistence systems, the thesis explores current debates on 'sustainability' and on 'subsistence'. As no single approach to sustainability per se seems appropriate to apply to these systems, new indicators and methods need to be developed which are appropriate to evaluate systems poor in cash generation but rich in social assets. Subsistence aquaculture is arguably more sustainable than commercial, monocultural aquaculture in environmental and social terms. It has become part of the economic diversity of the communities, increases food security, reduces the use of fossil fuels, promotes the careful management and recycling of wastes and the careful stewardship of natural resources, can help in the protection and enhancement of biological diversity and yields a feeling of self- empowerment. From the surveys, socio-economic and cultural realities are more important than physical conditions in determining the present state of aquaculture in Tabasco. The existence of some successful farmers' micro-businesses shows that once farmers receive continuous technical assistance, subsidies and access to markets, bringing them into commercial aquaculture is possible. Such help however, is rarely provided at present by the local extension institutions, which face internal problems resulting from inadequate budgets. This thesis argues that, in these circumstances, semi-subsistence aquaculture is a good option for the poor as many semi-subsistence systems are highly valued and have the potential to become more efficient and productive if locally based research is conducted with an understanding of farmers' cultures and motivations.

Studies were performed on two types of infection of teleost fish where viruses have been observed by electron microscopy: erythrocytic infections in the Atlantic Cod (Gadus morhua) and the Common Blenny (Blennius hpo lis) and lymphcystis disease., Searches were made for new, isolations of these infections Ja British coastal waters and on shores chiefly in the vicinity of Plymouth and Aberystwyth. In the absence of disease symptoms, the blood of fish was, screened for the presence of viral inclusion bodies by standard haematological methods. PEN in cod was found in the North Sea-. and in the Celtic Sea off southern Eire, thus extending the previous distribution data from the Atlantic-coastal waters. of North America. The blenny infection was also found in new sites on shores in the vicinity of Plymouth. Moreover, the cytology of these infectionswas as had been previously described. Collection data for the PEN infections showed an inverses; - relationship of infection incidence-with age for cod sample populations but no correlation was found for blenny sample populations. In addition, no external disease symptoms were observed in either type of infection. Concerning the recognition of the blenny infection, observations from maintaining blennies suggested the length of the natural infection might be inversely related to temperature; non-experimental longevities are quoted in this connection. The degree of infection in individual fish was estimated by light microscopy and the estimates for both erythrocytic infections cover the range 1-60% infection. Attempts were made to propagate the viruses in vitro using fish cell and organ cultures. Primary cell cultures were originated from tissues of the Blenny, Flounder, Plaice and Dab using the protocol in the literature for marine fish cell culture. Vigorous cell outgrowth was observed in the flounder cultures and in these the time to confluence was only 3-5 days. However, established secondary cultures could not be derived from tissues of either species. Plaice and dab cultures were used for virus inoculation but the virus from the blenny infection and lymphocystis virus could not be propagated. , Organ cultures were set up using skin blocks from the Flounder. With tris-buffered maintenance medium such cultures maintained histological integrity for 15 days. However, one - trial inoculation with lymphocystis virus showed no-integration or multiplication of the virus in the tissue. In connection with attempts to induce the blenny infection, the. effect of high temperature-in the Blenny was investigated. The infection was not induced over a9 day holding period but lytic effects on the erythrocyte nuclei were observed. The effect of the drug acetylphenylhydrazine (APH) in the Blenny was also investigated with the aim of reproducing its reported action of anaemia induction and ensuing erythropoiesis. Marked anaemia was produced but not erythropoiesis. However, this result could not nesessarily be interpreted as the effect of APH alone. The viruses were identified and characterized with emphasis on their mophology, using ultrathin sectioning, negative staining and shadowing methods. It was concluded that the virus from the Blenny and lymphocystis virus conform to the structural measurements in the literature but negative staining indicated that both viruses display unique core structures. These are discussed in the light of the knowledge of other DNA virus cores. The position of these viruses is further considered with respect to their classification in the virus family Iridoviridae.

No description available.