Consumer preference for commercially reared fish products that resemble their wild counterparts has resulted in the supplementation of pigments called carotenoids into aquafeeds to promote a pink-red colour in the flesh of salmonid fish. To date synthetic forms of these pigments have been commonly utilised to achieve this desired colouration, with the carotenoid astaxanthin being the regular choice for the feed manufacturer. However, increase in consumer demand for farmed fish products reared on natural feed additives has evoked an interest in natural sources of astaxanthin that could be successfully used to pigment salmonid fish efficiently. In the current study, the microalga Haemalococcus pluvialis has been assessed as a potential feed supplement to pigment the flesh of rainbow trout (Oncorhynchus mykiss). More specifically, those natural characteristics that may well limit the absorption and utilisation of astaxanthin from this source have been assessed individually and discussed from a physiological standpoint. The cell wall of Haemalococcus pluvialis when cracked efficiently presents no limitation to the absorption and utilisation of astaxanthin from this source. Indeed, the cell wall remnants help to prevent oxidation of astaxanthin in the feed compared to cell wall free extracts of carotenoid from the same source. However, esterified astaxanthin (which this algae predominantly contains) is not absorbed as efficiently as unesterified synthetic astaxanthin. Furthermore, the extent of esterification is negatively related to the absorption of astaxanthin. Regional variation in ester hydrolysis along the gastrointestinal tract combined with gut transit time of the ingested feed may explain these limitations. However, despite limitations in absorption, the muscle deposition of astaxanthin supplied as esters does not significantly differ from the unesterified form. The optical purity of astaxanthin esters from this source does not prejudice the final deposition of astaxanthin in fish tissues. An in vitro model has been developed to assess the absorption of astaxanthin at the intestinal level in salmonid fish in order to define absorption characteristics of carotenoids under different abiotic and biotic conditions. The absorption of astaxanthin seems to occur in a linear passive manner into the intestinal tissue. Although size of the fish does not affect the absorption of astaxanthin, temperature does have a significant effect. Although there were no significant differences in absorption between Atlantic salmon (Salmo salar) and rainbow trout, absorption tended to be greater in the latter species and merits further study.

A rainbow trout (Slamo gairdneri Richardson) population of IPN virus carriers was studied over a one-year period using both homogenisatian and co-cultivation for virus isolation. The percentage of virus-yielding fish was high between March and June but then declined. This was diametrically opposite to the trend in the serum antibody levels indicating that the marked humoral immune response resulted in a very significant reduction in the virus titres. The highest isolation rate was obtained from the kidneys after co-cultivation (from seventeen of the twenty-three virus-positive fish) underlining the very high sensitivity of this recently developed method for virus detection. Twelve of the twenty-three virus-positive fish yielded virus from the pyloric caeca after homogenisation. Virus was occasionally isolated from the faeces indicating that this may well be a possible avenue for horizontal transmission of the virus. No virus was ever detected in gonadal tissue. The virulence of the rainbow trout virus was enhanced in various ways and used to infect tilapia Oreochromis spilurus Gunther of different ages through a variety of routes. Fry infected by direct immersion, orally and by force feeding showed little or no signs of infection. Intraperitoneally and intramuscularly injected fingerling and adult fish developed marked haemorrhaging, severe loss of skin mucus and up to 50% mortalities were recorded. Gross pathology included enlarged and liquifying liver, gastroenteritis and mild brain haemorrhaging. Histopathologically there was extensive cellular vacuolation and degeneration as well as marked leucocytic infiltration in the liver, intestine and swimbladder. Eosinophilic granule cell infiltration of the intestinal wall was also very prominent. Virus was recovered from several organs and determination of virus titres revealed that active viral replication had occurred in the tilapia tissues, a finding further supported by electron microscopical evidence. The fish showed a clear humoral antibody response.

Interest in the subject of fish welfare is continuing to grow, with increasing public awareness and new legislation in the UK. Water quality has long been recognised as being of prime importance for welfare: water provides the fish with oxygen and removes and dilutes potentially toxic waste metabolites. This thesis investigates the interactions between water quality and the welfare of farmed rainbow trout (Oncorhynchus mykiss Walbaum). A literature review was undertaken to identify current recommended water quality limits for the health and welfare of farmed rainbow trout. Contradictions in the literature regarding suggested ‘safe’ water quality limits were also identified, as were deficiencies in some of the methods used to arrive at conclusions for recommended limits. The literature relating to the effects of poor water quality on welfare were also reviewed. The review ends with a discussion about water quality monitoring in the context of on-farm welfare assessment and how the information might be used in such a scheme. A telephone survey of UK rainbow trout farmers was undertaken to ascertain the level of water quality monitoring currently conducted. Participants in this study accounted for over 80% of 2005 UK rainbow trout production. It was established that 54% of farmers monitored dissolved oxygen to some extent and 69% monitored temperature, the most commonly measured water quality parameters and among the most important for health, welfare and growth. Subsequent visits were made to a sample of the participants in the telephone survey to obtain more detailed information of the farming operations, such as frequency of water quality monitoring, retention of production data and slaughter methods. Monitoring water quality will be an integral part of any on-farm welfare assessment scheme, and while measuring some water quality parameters requires specialist equipment, farmers should be able to monitor the essential parameters, dissolved oxygen and temperature. Any on-farm welfare assessment scheme for rainbow trout should incorparate fish-based measures in addition to resource-based parameters in order to provide as complete an overview of trout welfare as possible. An epidemiological study was undertaken to investigate the current status of welfare on UK rainbow trout farms and to identify risk factors for welfare. Forty-four trout farms from throughout the British Isles were visited between July 2005 and April 2007, sampling a total of 3700 fish from 189 different systems. Farms were visited twice, once in winter and once in summer, to account for any seasonal differences in fish physiology and environmental conditions. Data were collected on a range of fish parameters, together with background information on the batch from which the fish originated. Particular emphasis was placed on water quality due to the potential effects this can have on welfare. The water in each system sampled was monitored for 24 hours, with measurements of dissolved oxygen, temperature, pH, specific conductivity and ammonia taken every 15 minutes. A welfare score was developed for each fish using a multifactorial method, combining data on the condition of the fins, the condition of the gills, the stress hormone cortisol, the splenosomatic index and the mortality levels for the population of fish in the system. Using this welfare score and the individual components of the score as response variables, multi-level models were developed using the water quality, system and husbandry data collected. The primary risk factor that was associated with deteriorating welfare was disease. The purpose for which the fish was being farmed was also important, as fish farmed for the table market had on average worse welfare than those farmed for restocking fisheries. Seasonal effects, linked to higher water temperatures in summer, were associated with poorer welfare scores. Aside from seasonal effects, there is not much evidence that poor water quality is a major problem for the welfare of farmed rainbow trout in the UK. While deteriorating water quality certainly has the potential to affect the welfare of farmed rainbow trout, water quality measurements were within recommended ranges for the majority of farms visited. The results of this epidemiological study suggest that factors other than water quality may have a greater impact on trout welfare, such as exposure to diseases and production differences between farming for the table and restocking markets.

An optimal method for stunning animals before slaughter should result in instantaneous and irreversible insensibility. Today, there are various stunning and slaughter practices used around the world for farmed fish. With aquaculture being a growing food sector, the welfare of the animals used has become increasingly important in the consciousness of consumers, researchers and regulatory bodies. With growing research into the subject matter, an overview to summarize and examine how these practices impact on the welfare of the fish, and how well they conform to animal welfare legislation, is warranted to minimize the suffering of farmed fish. Stunning practices used in aquaculture include methods such as electrical and percussive stunning, carbon dioxide and asphyxiation. Each with its own level of effectiveness in terms of how fast the method results in loss of consciousness, whether or not the effect is reversible and how the welfare of the fish is affected as determined by behavioural and physiological stress responses. It was concluded that there is no unambiguous answer as to which stunning method is optimal in regard to animal welfare in modern day aquaculture. The optimal method for a given facility is influenced by factors like practicalities relating to each individual method as well as legislation on EU and national level. Suggestions were made for future research.