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No description available.

No description available.

No description available.

Fish behaviour, mediated primarily through the visual system, is known to be fundamental to the functioning of trawled fishing gears. Experiments were designed to examine the behavioural reactions of fish to visual stimuli associated with panels of meshes and their constituent components in order to increase our understanding of the principles underlying mesh penetration. Behavioural reactions of mackerel and haddock were observed in a large arena to a constricting funnel of two frames of meshes. Fish were seen to avoid the meshes where possible rather than pass through them. This reluctance to approach and penetrate meshes could be reduced by inducing the fish to race to a conditioned feeding stimulus, thereby reducing reaction distance and time. A greater avoidance reaction was seen to meshes which presented a stronger visual stimulus due to greater contrast with the water background. The orientation of twine also affected reaction behaviour. Fish avoided horizontally orientated parallel twines more than vertically oriented twines, while twine sloping diagonally upwards (with respect to the direction of travel of the fish) elicited a greater avoidance reaction than those sloping down. There was little difference between behavioural reactions to netting panels of square or diamond meshes. The behavioural reactions of mackerel to a tunnel of netting constricting from above and below as well as the sides, were examined. The presence of top and bottom panels increased the number of fish passing through the meshes on the sides. However, the orientation of twines on the top and bottom panels had little effect on the behaviour of the fish. The avoidance behaviour shown to meshes could also be reduced by the introduction of a strong visual stimulus or a physical block at the narrow end of a constricting funnel of two netting frames.

Sea lice Lepeophtheirus salmonis are the major pathogen affecting the salmon farming industry. Current methods of control, especially in the UK, are largely ineffective as they do not affect the chalimus stages of L. salmonis and are toxic to fish. Greater control of L. salmonis through improved management, more effective compounds, disease control and stock selection were examined. The epidemiology of L. salmonis on Atlantic salmon Salmo salar production sites was examined at a number of farms within Loch Sunart. Intensity of infection was found to be influenced by both management practices and seasonal variations. By instigating a series of co-ordinated, synchronous, strategic treatments throughout the loch system during the winter, initial chalimus levels during the spring were significantly reduced by 90% (p<0.001). Lice numbers for the rest of the production cycle were significantly lower (p<0.01). In addition there was a reduction in the number of treatments needed, an increase in the interval between treatments, reduced fish mortalities and improvements in fish harvest quality. There was no significant reduction in mean chalimus numbers observed at the control site where treatments were not co-ordinated. The strategic treatments were repeated in the Loch Linnhe system during 1997 resulting in an 87% (p<0.01) reduction in initial chalimus numbers. The efficacy of the synthetic pyrethroid, cypermethrin (Excis RTM) was assessed against L. salmonis chalimus. Exposure of the chalimus to 5mug I-1permethrin for 1 h resulted in a significant reduction (83%, p<0.001) in the mean number of chalimus 12 d post-treatment. The developmental stages observed in the untreated group were predominantly pre-adult II males. The majority of the remaining lice observed in the treated group were chalimus III indicating a profound effect of cypermethrin on the rate of development of L. salmonis. At 1 pg I-1permethrin for 1 h there was no significant reduction in the mean number of lice compared to controls (p=0.161) although there was a significant delay in the rate of development. A number of immunosuppressants were administered to Atlantic salmon and their effect examined upon the settlement and survival of Lepeophtheirus salmonis. Corticosteroid was found to increase the mean numbers of L. salmonis observed post-challenge by 59% (p<0.05) compared to controls. Administration of immunosuppressive factors isolated from Aeromonas salmonicida induced a 70% (p<0.01) increase in mean numbers of L. salmonis observed. There was no effect on the rate of development, nor tissue distribution of L. salmonis observed on the test fish compared to controls. The control of L. salmonis on production sites is examined in relation to disease status of the fish as well as the potential for effective immune modulation. Different stocks and families of Atlantic salmon Salmo salar were examined for variable susceptibility to settlement and survival of L. salmonis. Differences in susceptibility of up to 55% (p<0.01) in mean lice numbers were observed between stocks. Variability in susceptibility to settlement and survival, as well as damage caused by L. salmonis was also observed between families of Atlantic salmon.

The aims of this thesis were to investigate the effects of the main abiotic and biotic factors on growth and protein turnover in fish. The first experiments were concerned with the effects of seasonal acclimatisation and temperature acclimatisation on tissue-somatic indices, nucleic acid concentrations/ratios and protein synthesis rates in cod. It was concluded that low water temperature resulted in an increase in tissue RNA concentration and reduced RNA (translational) activities. At the same time, protein synthesis rates were similar between cold and warm acclimated fish. Increased tissue RNA concentration at low water temperatures therefore appeared to be a thermal compensatory mechanism on protein synthesis rates in cod. The next experiment investigated the effects of ovine growth hormone administration on protein turnover and RNA concentration/activity in rainbow trout. Growth hormone administration generally stimulated protein synthesis rates resulting in larger fish; little effects on rates of protein degradation were observed. Stimulated protein synthesis rates were the result of both increased RNA/protein ratio and/or RNA activity. The final experiment investigated the effects of long-term nutritional regime and short-term alterations in nutrition on indices of growth rate in cod, with the aim of identifying the most sensitive indicator(s) of growth rate (nutritional status). Of all the indices examined, liver-somatic index, stomach and intestine nucleic acid concentrations/ratios and ventricle stomach and intestine cytochrome c oxidase activities were generally the best correlates of long-term growth rate as well as being sensitive to alterations in nutritional level. Whilst these data gave credence to the use of various correlates of growth rate under constant laboratory conditions, the use of these measurements as correlates of growth rate of wild fish may be limited under seasonally fluctuating environmental conditions.

Sixty one year old (S1) and sixty two year old (S2) Atlantic salmon smolts derived from comparable egg stocks, were dye-marked and their growth and development individually monitored in a seawater cage for a period of 26 months. During that time, blood samples were withdrawn at regular intervals and their gonadal hormone content determined by radioimmunoassays. The growth of S1 fish, in both weight and length, was distinguished from that of S2s not only by its higher magnitude but also by its mode, S1s growing relatively more in length than in weight. These observations are discussed in relation to previous studies of growth bi-modality in freshwater. Growth was found to be highly seasonal. Although the period of maximum growth corresponded with maximum water temperature, sharp changes occurred in growth when the temperature was relatively steady. In spring, non-maturing fish grew relatively more in length than in weight and in the autumn more in weight than in length; the time of inflection was close to that of maximum day-length. Fish maturing after one sea-winter (grilse) were not recruited from the larger or faster growing members of the population. Grilse were larger than non-maturing fish of the same sea-age because they had experienced a surge of growth, postulated to be hormone induced, in the early stages of gonadal development. In the summer preceding spawning, maturing fish suffered a sharp decrease in specific growth rate. Changes in the digestive tract were rapidly reversed after spawning and are considered to be the result of inanition rather than its cause. Recovering fish (kelts) experienced a growth "rebound" in spring, showing higher specific growth rates than salmon of the same sea-age. A highly specific and sensitive radioimmunoassay technique was developed for the simultaneous determination of the three principal gonadal steroids oestradiol-17beta, testosterone and 11-oxotestosterone, together with rapid, but less specific individual assays for the first two of these, steroids. Use of these assays showed that in maturing female fish, blood serum levels of oestradiol-17beta reached maximum values of 1-3 microg/100 ml approximately one month before spawning, and testosterone titres peaked, at spawning time, at 4-8 microg/100 ml. In male grilse, serum titres of testosterone and 11-oxotestosterone reached their maximum values of 1-3 microg/100 ml and microg/100 ml in October and November respectively. Levels of the latter steroid remained high until March, long after the normal period of spawning. In salmon, early spring peaks in the serum concentrations of oestradiol-17beta and testosterone (in females) and of testosterone and 11-oxotestosterone (in males) were invariably associated with sexual maturation in the same year. Early peaces in serum oestradiol-17beta and 11-oxotestosterone but not in testosterone were noted in female and male grilse respectively. These observations suggest that the physiological decision to mature is taken during the late winter or spring preceding spawning. The incidence of grilse maturity was markedly higher in the experimental fish than in the control farm populations from which they were drawn. This is interpreted as the effect of stress on the decision to mature. Groups of fish from a population which had not matured as grilse were subjected to periods of food deprivation during winter and spring. Starvation and stress associated with handling in January or in mid February-mid March resulted in a significant increase in the proportion of fish maturing in the same year. Handling stress alone during mid March-mid April caused a significant increase in grilse maturation in a one sea winter stock. Stress supplied after that time did not significantly affect the percentage maturing as grilse. Administration of oral oestradiol for any monthly period from November to February induced a significant decrease in the proportions of sea-water smolts maturing as grilse, but not, apparently by a predominant affect on one sex.

Proliferative kidney disease (PKD) is one of the most economically important diseases in commercial rainbow trout culture in Europe. It is recognised by the British Trout Association as the most significant problem affecting the industry in Britain, with annual losses in revenue in excess of £1.5 million, which represents a total loss to the industry of approaching 14%. The specific aims of this work were:- 1. To investigate aspects of the transmission and passage of infection to rainbow trout, with particular emphasis on the unknown alternate host and infective stage. 2. To determine environmental parameters which have an effect on the occurrence and severity of annual epizootics, and to see if early season conditions can be used to predict the arrival of the parasite and the time of each epizootic. 3. To test the efficacy of the antibiotic Fumagillin DCH in controlling the outbreak and severity of the disease on a commercial scale. 4. To examine the potential of biological methods of control, centering around a vaccination regime, in ameliorating the effects of the disease. Chapter 1 gave a general introduction to the problems posed by parasitic diseases in salmonid aquaculture and, more specifically, with important pathogens of the class Myxosporea. Chapter 2 dealt with the life cycle and transmission of PK'X'. A polyclonal antiserum to PK'X' was raised and used with fluorochrome techniques to label PK'X' cells within infected trout kidney. Screening oligochaetes with this diagnostic tool failed to show any evidence of infective stages to salmonids, as did random examination by light microscopy. Filtration experiments showed that the waterborne infective stage was capable of passage through 25<I>μ</I>m mesh screens, but was at least partially held back on 10<I>μ</I>m mesh. This suggested that, if an actinosporean, the infective stage is in the size range of an aurantiactinomyxon. Chapter 3 examined environmental factors which control the strong seasonal epidemiology characteristic of the disease. It was shown that the <I>intensity </I>of water temperature rises in spring promotes the early development of the disease.

Juvenile trout were found predominantly in the tributaries while older trout and juvenile salmon were found predominantly in the main river. Localised reductions in juvenile salmonid abundance in relation to the degradation of the stream habitat were demonstrated. HABSCORE habitat evaluation models appeared to be reasonably accurate in predicting juvenile salmonid numbers. The recruitment of juvenile trout from a tributary into the main river during spring was assessed by trapping. Predictive models based on environmental parameters accounted for 64% and 78% of the variation in the timing of migration of brown trout and trout smolts respectively. The fecundity of adult sea and brown trout was significantly and positively correlated with the fork length and the somatic weight of the female. Fecundity at a standardised body length varied between the brown trout and sea trout. Survival to hatching, of planted trout eggs was high at sites in the upper part of the catchment but was relatively low at sites in the mid and lower catchment. Low survival rates appeared to be related to a high degree of infiltration of fine organic sediments. Displaced trout failed to return to the home site. Home range estimates were less than 20 metres of stream. Large numbers of small trout were taken by anglers in the past. Recorded catches have decreased since 1980 while the mean weight has increased. Fishing effort was shown to be the most important determinant of catch. A small percentage of the anglers accounted for a disproportionately large percentage of the total catch and fishing effort. The age structure diversity of the catch was reduced in areas where the fishing effort was high.