Rainbow trout eggs Salmo gairdneri, Richardson, were incubated under a range of different environmental conditions. Recovery of bacteria from egg surfaces revealed that increased water temperature, slow water flow rates and high egg density all significantly increased egg surface bacterial populations. Live eggs were mainly colonized by Cytophaga sp., pseudomonas fluorescens and Aeromonas hydrophila. In contrast, dead eggs supported considerable numbers of fluorescent Pseudomonas sp. Analysis of potential nutrient sources for bacteria colonizing live egg surfaces revealed that small amounts of amino acids, phosphate and potassium may be lost by incubating eggs. Subsequently these nutrients were shown to be capable of supporting limited bacterial growth and reproduction. Dead eggs `leaked' increased amounts of the above nutrients which in turn supported higher bacterial numbers. In addition, biochemical analysis of eggs revealed amino acids and fatty acids that might be utilized by bacteria colonizing dead egg surfaces. Assessment of adhesion properties of bacteria frequently recovered from egg surfaces revealed high cell surface hydrophobicity as an important factor in successful egg colonization. Analysis of egg mortalities from groups of rainbow trout and brown trout (S.trutta L.) eggs maintained under two different incubation systems revealed that potentially a close correlation existed between egg surface bacterial numbers and mortalities in the egg during incubation. Innoculation of newly-fertilized eggs with bacteria demonstrated that groups of eggs supporting high numbers of P.fluorescens suffered significantly higher mortalities during the early part of their incubation. Exposure of incubating eggs to oxolinic acid, chlortetracycline and chloramphenicol demonstrated that numbers of bacteria on egg surfaces could be significantly reduced. However, as no corresponding increase in egg hatching success was revealed, the treatment of incubating eggs with antibiotics or antimicrobial compounds can not be recommended. In commercial hatcheries bacteria are only likely to be responsible for egg deaths during incubation when environmental conditions are unfavourable. High water temperatures, slow water flow rates and high egg density all lead to increased bacterial number of egg surfaces, reduced water circulation and low levels of dissolved oxygen. Under such circumstances sufficient amounts of dissolved oxygen may not be available to support developing embryos.

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.

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This thesis aims to contribute to a better knowledge of several aspects of the biology of L. piscatorius, namely biometry, growth and reproduction. The different subjects in this thesis were studied by examining fresh material obtained from commercial landings and research vessel surveys. L. piscatorius presents significant biometric differences between sexes (at 99% level of confidence). Small males are heavier than females of the same size. Conversely, large females are heavier than males of the same size. Attempts to estimate L. piscatorius' age were made using otoliths. Three other hard structures were used to ascertain consistency of the age readings: vertebrae, illicia and teeth. All structures showed seasonal growth increments. However, these growth increments were easier to interpret in vertebrae. Illicia sections were the most difficult structure to interpret. Although all structures chosen presented many sub-annual checks, illicia displayed them more closely than the others; making age readings more difficult. Teeth were considered inadequate for age readings because they may not be permanent structures. The accuracy and precision of age readings in otoliths were not determined; firstly, the real age of each fish was unknown. Secondly, there was no other independent reader to test precision between readers. The age readings in sagittae were validated and indirectly by back calculation, edge formation and marginal growth. The results obtained from age readings in otoliths were used to directly estimate growth parameters for L. piscatorius by sex. The study of the general morphology and histology of the gonads showed that L. piscatorius have unusual gonads. The testes are tubular, and bean shaped in transverse section while ovaries are confluent and form a single, flattened tube.

A total of 6512 specimens of Loligo forbesi Steenstrup (Cephalopoda : Loliginidae), obtained from Scottish fishing ports and research vessels during 1984/86, was used to investigate aspects of fishery biology of the species in Scottish waters. L. forbesi exhibits sexual dimorphism in size, males had a larger overall mean size (DML, weight) than females, and for each sex the mean size was larger in those from the north-eastern Atlantic than in those from the North Sea. In length-weight relationship, females had a higher regression slope than males and this relationship for squids from Rockall was significantly different to those from the North Sea. Multivariate analyses on measurements of body parts showed that Rockall squid, also, exhibited less variation in body form but distinct differences from those of the west coast and North Sea. Recruitment of L. forbesi occurred during April/May and November/December, the monthly growth rate after recruitment varied between 12.5 to 28.6mm DML/month for males and 12.5 to 17.5mm DML/month for females; for each sex the November/December cohort had lower growth rate. The logistic growth model best fitted the progression of modes of monthly length frequencies. Age determinations estimated from statolith markings varied from under 3 months for 30mm DML individual to 28 months for 571mm DML male individual. Sex ratio in L. forbesi varied significantly from 1:1 throughout the year with males exceeding females except during winter when the number of females may be equal to/or more than males. Mean size at maturity was 205mm DML for males and 192mm DML for females. Spawning occurred during December to March and the peak spawning period occurred in February throughout the study area. In this species, maturity coefficient and gonad-somatic index were the best indices for identification of spawning season. Fecundity is related to size and the average number of eggs per female was 6500.High proportions of specimens with 'empty' stomach were observed during the winter. The prey of L. forbesi was fish, crustaceans, molluscs and polychaetes in decreasing order of incidence. Identified fish prey were Argentina sphryaena, Trisopterus esmakii, Merlangius merlangus and Ammodytes sp.