The movements of adult Atlantic salmon (<I>Salmo salar </I>L.) were investigated in relation to hydroelectric schemes in Scotland. Salmon were tagged with radio- and acoustic transmitters and released into the River Tummel, Perthshire, and River Conon, Ross-shire, and their subsequent movements recorded. Thirty-nine salmon were monitored in the vicinity of Pitlochry Dam on the R. Tummel. All fish that approached the dam successfully ascended a fish pass located here. Pass efficiency was estimated at 100%. Movements through the fish pass were related to water temperature and ambient light level. The three-dimensional movements of four salmon were recorded as they moved through a hydroelectric reservoir, Loch Faskally. Fish migrated to the head of the reservoir in under 48hours. Gross swim speeds were in the region 0.4 to 0.8mh^-1. Swimming depths were similar to mean loch depth. Two fish were recorded at shallower mean depths during the night than during the day. A total of 54 salmon were radio-tagged on the River Conon to determine the effects of a series of hydroelectric obstructions on upstream migration. Fish were either unable or unwilling to pass beyond each of the obstructions: only 20.7% of fish migrated beyond six obstacles, thus reaching their spawning grounds. Salmon were recorded moving through a 2.5km long pipeline into a separate tributary of the river. The muscle activity of three salmon were recorded, using electromyogram (EMG) telemetry, as they ascended Conon Falls fish ladder. The results show that fish are required to make use of anaerobic white muscle in order to ascend the structure. The highest mean levels of activity were recorded when fish were delayed in the ladder overnight. This finding is of great significance when designing new fish passes.

The mathematical modelling of fish behaviour in relation to towed fishing gear based on quantitative analysis of stimuli of the gear and fish response, was investigated in order to increase the selectivity and efficiency of the gear. The brightness contrast of the netting and other components of the fishing gear such as floats and bobbins was studied by comparing their luminance to that of their visual background of water, sand-cloud or sea bed. Luminance of each component was measured with a luminance meter in the tank and by the video image processing of nets and gears in the sea with light intensity and grey levels. The brightness contrasts of the gear are increased with diameter of twine, light level and viewing zenith angle. A number of contrast modules are developed, based on the measurements, for predicting the visual stimulus for reactions of fish to towed fishing gear. The underwater luminance against which a fish sees the net components is calculated for the prediction process. The visual stimuli of trawl were represented through the predicted light field and methods are developed for calculating the visibility of the trawl components and sand clouds as viewed from any fish position within the net. Fish behaviour in relation to towed fishing gear was observed and analysed using a net panel towed in the gantry tank and bottom trawls at sea observed by a SIT video camera system. When fish displayed stable swimming as in an optomotor response, the variations of relative swimming speed and angular velocity were relatively low and regular in their amplitude and period. However, when fish moved suddenly in panic reaction to escape or rise up, the variations of these measured components were high, irregular and nonperiodic.

This thesis was focused on the study of (i) individual feeding behaviour and growth performance of juvenile rainbow trout fed on normal and medicated diets and (ii) the responses of hepatic detoxification mechanisms in rainbow trout to dietary oxolinic acid and flumequine. During the first experiment, the development of dorsal and caudal fin damage over time was compared between 4 groups of rainbow trout fed on different ration levels (0.25, 0.5, 1.0 and 1.5% B.W. day^-1) and between individuals of different feeding rank within each group. The feeding and growth data indicated that the strength of the social hierarchy became more relaxed within increasing ration. The severity of both dorsal and caudal fin damage was significantly dependent on the ration size fed to the group, with lower ration groups sustaining more fin damage, whereas subordinate fish suffered the most dorsal fin damage within the two lower ration groups. Subsequently, the inter-individual variability in consumption and growth was studied in two groups of rainbow trout fed diets containing flumequine in different concentrations. Flumequine, at either concentration, had a deleterious effect on growth performance, even when palatability problems were not apparent, and resulted in dose-related increases in inter-individual variability in consumption and growth and changes in the pre-established feeding hierarchies. Cytochrome P450 biomarkers and ultrastructural alterations in hepatocytes were used to assess the toxicological responses to rainbow trout following oral administration of oxolinic acid or flumequine. Both antibiotics induced and/or activated CYP1A isoforms of P450 as measured using immunoblotting for P450 protein and a combination of selective substrates and inhibitors for P450 activity. The increase in the P450 activity was correlated with a significant increase, almost 2.5-fold, in the volume of rough endoplasmic reticulum. In addition, severe depletion of glycogen deposits, formation of lipid droplets, increased lysosomal activity as well as mitochondrial hyperplasia and high nuclear-cytoplasmic ratio in flumequine- and oxolinic acid-treated individuals respectively, were considered compensatory rather than degenerative phenomena, indicative of increased biotransformation activity.

This study focuses on fish discards generated by demersal vessels fishing in Scottish waters (North Sea and West of Scotland). The analysis is based on data collected by scientific observers onboard commercial vessels during the past two decades (1975 - 1993). Chapter 1 introduces the problem of discarding, describes its implications for fisheries and the marine environment, and gives a brief overview of the sampling scheme. Chapter 2 uses generalised additive models (GAMs) and regression trees to investigate the main reasons that lead Scottish fishers to discard commercially important species (haddock - <I>Melanogrammus aeglefinus, </I>whiting - <I>Merlangius merlangus </I>and cod - <I>Gadus morhua</I>). Different discarding patterns are observed in inshore and offshore areas, with vessels fishing offshore discarding larger fish, irrespective of gear type. In inshore areas, discarding decisions are mainly driven by legal landing size, whereas offshore, high grading decisions due to market pressures and, possibly, quota limitations prevail. Chapter 3 explores the performance of several estimators of total discards and their variances in a simulated population of fishing trips. The simulated population is generated from the residuals of temporal models fitted to the catch and discards observations from the trips sampled under the Scottish discards monitoring scheme. The estimator currently applied to obtain annual estimates of total discards in Scotland is shown to be biased and imprecise, whereas several other estimators perform better under limited sampling coverage. The best estimators from chapter 3 are used in chapter 4 to estimate the total discards and their variances for species that were regularly discarded by Scottish vessels in the period 1988 - 1993. Overall, it is estimated that Scottish vessels generate annually around 100,000 tonnes of demersal fish discards in the North Sea and 30,000 tonnes in the West of Scotland. This suggests that close to 40% of the total demersal catch of Scottish vessels is annually wasted through discarding.

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Dietary factors are one of the most significant considerations in the regulation of appetite in fish since dietary nutrient and energy concentration modulate feed intake in the short and longer term. These interactions may also be important from a commercial aspect, since the objective of aquaculture is to obtain maximum growth, feed efficiency and consumer acceptance of the product. This thesis addresses the major dietary components which are likely to influence appetite in rainbow trout. These include dietary lipid level, carbohydrate content and degree of complexity as well as energy density and protein/energy ratio. Experimental data is presented which examines the influence of such factors on reed intake, growth performance, nutrient utilisation, gastric evacuation rate, return of appetite and changes with respect to the postprandial level of circulating plasma metabolites. It is proposed that rainbow trout have the capacity to regulate feed intake within specific constraints. On the other hand, trout become obese when offered high oil diets and fail to control feed intake in the short term, possibly due to the palatability of lipids. Regulation may also appear at a metabolic level following accumulation of lipids in adipose tissue. Gastric evacuation rate was probably the main factor in the short term influencing feed intake. This was irrespective of carbohydrate complexity or level in the diet. However, simple sugars might suppress the appetite of trout in the longer term. The biochemical status of liver via plasma glucose concentration may play a more important role compared to gastric fullness in the long- term regulation of appetite. It was postulated that X-radiography was a paramount technique for the quantification of sequential meals and return of appetite measurements in these investigations. The above parameters and their interactions were studied in relation to the physiological control of feed intake in order to develop a more defined model for such processes and to improve the optimum feeding regimes for rainbow trout under intensive production conditions. This is discussed within the wider concept of fish nutrition, and the implications for future research in this area are stated.