Recruitment and turnover of populations of brown trout, Salmo trutta, in the upper River Wye, Wales

The age structure, abundance, net immigration and loss of trout populations were investigated in eleven tributary sites and two main river sites on the River Wye. Marking experiments indicated that recruitment to the site populations was mainly by immigration which began during the O-group phase and continued, with seasonal fluctuations, throughout life. Immigrant fish had a higher probability of being lost from the populations than residents providing a statistical distinction between mobile and sedentary components of the population. The turnover of the site populations is described using a simple exponential model, and half-times at different sites (stream length, 50 m) ranged from 140 to 430 days. Some factors affecting half-times are briefly discussed.     

Thermoregulatory behavior of brown trout,Salmo trutta

Brown trout, Salmo trutta, were allowed to thermoregulate individually in an electronic shuttlebox. Pooled data for 6 fish showed a diel pattern of preferred temperature, with a diurnal minimum of 10.3°C, an early nocturnal maximum of 13.7°C, a less pronounced mid-scotophase minimum of 11.7°C, and a secondary dawn maximum of 12.8°C, in a somewhat crepuscular pattern. The 24-hour mean preferendum was 12.2°C.     

Movements of brown trout, Salmo trutta L., in the River Gwyddon, South Wales

Movements of native brown trout, Salmo trutta , 1 + and older, were limited with up to 93% of recaptured marked fish occupying sites where previously caught. Movements > 50 m were rare with the majority < 15 m. Between sampling occasions, the population could be separated into a static component and a smaller mobile component but there was no evidence of a permanently mobile group. The proportion of mobile native fish increased after stocking with hatchery-reared trout. Displaced native trout (1 + and older) showed the ability to home from 75 m upstream to 111 m downstream of a release site.     

Haemolytic activity in the serum of brown trout, Salmo trutta

Brown trout serum contains a natural, spontaneous, antibody-independent lytic activity and a haemolysin antibody complement-mediated lytic activity against unsensitized and trout antibody-sensitized sheep erythrocytes, respectively. The use of various activators and inactivators of the mammalian complement system demonstrated that trout serum possesses complement or complement-like components similar in activity to those present in the classical and alternative pathways found in mammals. A single injection of trout with sheep erythrocytes stimulated the production of antibody-secreting cells in lymphoid organs and increased the levels of natural haemolysins. A second injection of sheep erythrocytes further raised the haemolysin values and antibody-secreting cell counts. Serum complement from homologous or closely related fish species was more effective for use in the haemolysin and antibody-secreting cell assays than that from heterologous sources, except guinea pig. Based on physico-chemical properties, gel filtration and immunoelectrophoretic studies, natural and induced anti-sheep erythrocyte haemolysins were found to be similar molecules and are possibly high molecular weight IgM antibodies.     

Conservation and management of brown trout, Salmo trutta, stocks in Wales by the Welsh Water Authority

SUMMARY. 1. The diversity, value and status of the trout resource within the Welsh Water Authority area is described.
2. Reported angling catches of sea trout have increased in recent years but there is an apparent decline in brown trout stocks. Factors affecting the distribution, status and diversity of the trout resource are identified and discussed.
3. Investigations are being carried out to evaluate the apparent problems and to provide information required to formulate management solutions. A management strategy is proposed which allows for the maintenance and development of the resource, whilst ensuring the preservation of strains of trout which have conservation value in their own right.     

The immune response of the brown trout Salmo trutta to lipopolysaccharide

Brown trout produced high molecular weight, thermostable, dithiothreitol sensitive, non-precipitating, complement-fixing antibodies and agglutinins to lipopolysaccharides after intramuscular injection with adjuvant. Antibodies were first detected on Day 14 and reached maximum titres after 56 to 63 days when a single injection was given. When either a second or a third injection was administered maximum titres occurred 34 to 40 days after the injection. After each injection the titres increased significantly, and the protein concentration of the sera was significantly decreased. In cellulose acetate electrophoresis experiments those bands which migrated in the β- to γ-globulin regions were increased.
Antibody-secreting and antigen-binding cells were detected on Days 8 and 4 respectively and maxima were reached between Day 16 and Day 18. The number of cells per 10⁶ lymphoid cells was higher in the spleen than in the kidney.     

Environmental stress and the survival of brown trout, Salmo trutta

SUMMARY. 1. The hypothalamic-pituitary-interrenal axis of the brown trout, Salmo trutta , is activated in response to most forms of environmental stress. This results in an elevation of blood cortisol levels.
2. Experimental elevation of blood cortisol levels in otherwise unstressed brown trout caused a dose-dependent increase in mortality rate due to disease. In our studies., Saprolegnia-infection , furunculosis and bacterial fin-rot were the principal diseases.
3. Chronic cortisol elevation also suppressed several of the endocrine processes controlling sexual maturation, resulting in a significant reduction in the size of the gonad in both male and female fish.
4. It is argued that many of the deleterious effects of sublethal pollution (including acidification) on natural trout populations can be attributed to chronically-elevated blood cortisol levels and that a knowledge of such physiological changes would allow an assessment of the impact of pollution events and act as an early warning of potential disease and recruitment problems.     

Downstream movement of juvenile brown trout,Salmo trutta,L. in the tributaries of Loch Leven,Kinross, Scotland

Anomalous individuals in five species of marine polychaetes are described. The anomalies fall into three structural types, viz, duplication, bifurcation and common variation. Included in the first are double ventral cirrus in Perinereis nuntia vallata and duplicate operculum in Hydroides homoceros. Bifid antenna in Lysidice collaris and forked anal papilla in Armandia leptocirris constitute the second type. The third includes supernumerary eyes in Arabella iricolor iricolor and variations in opercular crown in, again, Hydroides homoceros.     

Rates of gastric evacuation in piscivorous brown trout, Salmo trutta

SUMMARY. 1. The dry weight of food remaining in the stomachs of piscivorous trout decreased exponentially with time. Gastric evacuation rates increased exponentially with increasing temperature but were unaffected by predator size, meal size or type of fish prey.
2. Mathematical models were developed to estimate both the rate and time for the gastric evacuation of different meal sizes (expressed as dry weight), and were applicable to piscivorous trout of different sizes (length range 10–32 cm) feeding on trout fry or sticklebacks at different temperatures (range 5–18°C).
3. The wet weight of food in the stomachs also decreased exponentially with time, but evacuation rates both increased with temperature and decreased with increasing meal size; the latter relationship occurred because relative rates of water loss from a meal also decreased with increasing meal size. Use of wet or dry weights can therefore lead to different conclusions about the effect of meal size on evacuation rates.
4. When piscivorous trout were fed three consecutive meals of varying size, the models predicted the total dry weight of food left in the stomach, but not the weight remaining for each individual meal. Interactions between meals led to an increase in evacuation rates for meals consumed early in the series and a decrease in evacuation rates for later meals.
5. Evacuation rates for piscivorous trout were compared with those for trout feeding on invertebrates in an earlier study, and were close to those for caddis larvae as prey, higher than those for mealworms and lower than those for a variety of invertebrate prey. Although a great deal is now known about the daily food intake and growth rates of trout feeding on invertebrates, there is little comparable information for piscivorous trout.     

Early-age changes in oxidative stress in brown trout,Salmo trutta

Fish are often used as models for studies investigating the ability of xenobiotics to induce oxidative stress, though age or developmental stage of the individuals studied has been given little attention. Oxidative stress in other organisms is associated with aging as well as with periods of rapid growth, which occurs in young brown trout. We measured protein carbonyls, 20S proteosome activity and glutathione (GSH) levels in farmed Salmo trutta in four different age groups from 5 months to 3 years. We found an increase in protein carbonyls and a decrease in 20S proteosome activity in both brain and liver tissues of the fish with increasing size and age. Total GSH levels in liver tissue declined as fish aged and the GSSG:GSH ratio increased. Five month and 1 year old trout were treated with paraquat (PQ) to induce oxidative stress. Five month old fish showed no changes in the measured parameters while 1 year old fish had both an increase in protein carbonylation in liver tissue and a decrease in 20S proteosome activity in brain tissue. These results indicate that oxidative stress biomarkers are affected by age or rapid growth in brown trout, and that individuals of different ages respond differently to oxidative stress induced by PQ.     

The ecology of brown trout Salmo trutta in English chalk streams

SUMMARY. .1. Chalkstream trout are fast-growing, short-lived with a stable age structure that is a consequence of the lack of extremes in the abiotic environment, especially of water temperature and flow.
2. Levels of secondary (invertebrate) production arc high and there is no evidence that interspecific or intraspecific competition for food resources limits growth.
3. Dietary studies show a broad similarity between trout and other species, but indicate differences that reflect variations in the micro-habitat distributions between fish species.
4. Trout numbers appear to be limited initially by the availability of gravel spawning areas, and then by areas suitable for newly-emerged fry. Reduction in stream discharge in the spring, either naturally or by man, can lower the number of 0+ trout that survive.
5. Eels are not important predators on trout eggs or fry. but a reduction in pike numbers can lead to a decrease in the mean weight of pike. Small pike do not pose a serious threat to stocked yearling trout.     

Morphology of the pronephros of the juvenile brown trout, Salmo trutta

The pronephros in juvenile brown trout (Salmo trutta) consists of a large ovoid renal corpuscle and a pair of tubules. The corpuscle is retained for 11 months, after which the glomerulus regresses. The glomerular arteries come directly from the dorsal aorta. The interstitium is permeated with venous blood vessels that arise from the anterior cardinal veins and are closely apposed to the tubules. Two distinct segments of the pronephric tubular system are distinguished by the histological and ultrastructural features of their component cells: 1) a short, transitional neck in which cells change from capsular epithelium to columnar epithelium, typical of tubules; 2) the convoluted segment composed of cells similar to first proximal tubular cells of the opisthonephros with well-formed brush borders, apical vesicles that vary in size and number along this segment, and lysosomes. Pinocytosis and exocytosis are also evident in this segment. The tubular system increases in length and in its convolutions until about week 9, when the opisthonephros develops. Distally each tubule connects with a Wolffian duct, with cells marked by the absence of apical inclusions and the presence of a uniform brush border, numerous mitochondria, and elaborate infolding of the basalar membrane. Nephrostomes, which are often characteristic of pronephroi, are not present. Cells with long cilia are found throughout the tubular system but are most characteristic of the neck and Wolffian-duct segments.     

Sexual size dimorphism in anadromous brown trout Salmo trutta

Anadromous trout Salmo trutta exhibits sexual size dimorphism (S_SD); females were larger than males in populations where male mean total length (L_T) at maturity was below 49 cm and females were smaller than males when mean male L_T was above 49 cm, the slope of the regression of female on male L_T was 0·59. In streams with mean annual discharge below 41 m³ s^?1, flow added significantly to a model with S_SD as the dependent variable and male mean L_T at maturity as the first predictor variable. There was a slight increase in S_SD with increasing latitude, which may result from an increase in male size with increasing latitude.     

Allozyme variability in brown trout Salmo trutta in Chile

1. Brown trout ova were imported during the last century from different locations in Europe to establish populations in Chilean rivers (South America). The rivers are currently occupied by naturalized populations that have adapted to very different environmental conditions, such as areas of semi-desert in the north, or rainy and cold areas in the south.
2. In this first study in this geographical area, electrophoretic variability of proteins encoded by twenty-five loci was screened in seven populations from northern to southern Chile.
3. The results show significant heterogeneity of allelic frequencies between populations in seven of eleven polymorphic loci detected. The estimated value of genetic diversity 0.1274 ( H _T) is higher than that observed in populations from areas of natural distribution of this species. However, only 12.64% of this genetic diversity was found between samples ( G _ST), indicating a low genetic divergence among Chilean populations. The observed associations among the Chilean and 'modern' group of European populations suggests the probable origin of the new populations.     

Mitochondrial DNA variation in River Usk brown trout, Salmo trutta

Mitochondrial DNA (mtDNA) polymorphism in the essentially non-anadromous River Usk brown trout Salmo trutta population was investigated by restriction analysis. Following mtDNA extraction and purification on caesium chloride density gradients, monomorphic restriction profiles were obtained with Hae III , Hind III , Sau 3AI and Xbal . However, the restriction endonucleases Ava II and HinfIl proved informative. The distribution of four composite genotypes found within the Usk system was heterogeneous, and a fifth genotype appeared exclusively in an outgroup sample from the adjacent River Wye drainage. The source of the observed genetic variation is discussed in relation to estimated divergence times for Usk mtDNA genotypes and the stocking history of the catchment.     

A new energetics model for brown trout, Salmo trutta

1. The chief objective of the present study was to develop a functional model for the daily change in the total energy content of a brown trout, Salmo trutta , (equivalent to growth when positive) in relation to the difference between energy intake (energy content of food) and energy losses (metabolism + losses in faeces and excretory products). Energy budgets for individual fish were obtained in earlier experiments with 210 hatchery trout (live weight = 11–270 g) kept at fairly constant temperatures (mean values ranging from 3.6 to 20.4 °C), but without strict control of temperature or oxygen, and in later experiments, with 252 trout (1–300 g) bred from wild parents and kept at five constant temperatures (5, 10, 13, 15 and 18 °C) and 100% oxygen saturation. Each trout was fed a fixed ration of shrimps, Gammarus pulex, the ration level varying between zero and maximum. 2. Energy intake (C_IN, cal day^??1) was measured directly and expressed as a proportion (p) of the maximum energy intake (C, cal day^??1), the latter being estimated from a model developed earlier. In a new model, energy losses (C_Q, cal day^??1) were expressed as a function of temperature, fish weight and ration level. This model was continuous over the 3.6–20.4 °C range, had twelve fitted parameters and was an excellent fit to the data for the 462 trout (P < 0.001, R² = 0.9970). In an extended model, the weight exponent for energy losses was not assumed equal to that for energy intake, the difference between the two exponents being very small, but significant, with a slight improvement in the fit of the model (R² increased to 0.9972). 3. The limits of model use were discussed. An example of its utility was to elucidate the complex relationships between both positive (growth) and negative daily changes in the total energy content of the trout, and temperature, fish size and variable energy intake. The model has raised several questions for future work, including the effect of increasing energy intake by a change of diet from invertebrates to fish or fish pellets, and a comparison of growth models based on weight or energy changes.     

Conservation and management of brown trout, Salmo trutta, in Ireland

SUMMARY. .1. Ireland's fauna and flora are, compared with those of Britain and Continental Europe, relatively impoverished as a consequence of our glacial history. Salmonids are, however, well represented here and, because cyprinids are not. they make up a large proportion of the freshwater fish biomass. 2. It may be more accurate to regard our trout fauna as a mosaic of subspecies and races rather than a single species. Two principal immigrations are thought to have contributed to the extant strains one of which includes the valuable long-lived sea trout and freshwater ferox. Their fidelity to specific spawning areas isolates and maintains the purity of trout strains but certain aspects of spawning behaviour may leave some of them susceptible to cross fertilization with others. 3. Strains of trout may take distinguishable external forms or they may not be easily recognized by eye. In spite of their great interest in the conservation and management of trout, anglers are largely unaware of their genetic status. 4. Artificial restocking of salmonids has been carried out in Ireland since the nineteenth century; most effort has been expended on Atlantic salmon but, in the last 25 years, the restoration of trout populations depleted by pollution and arterial drainage has been the object of these exercises. There is concern for the consequences of wild trout strains being diluted by these efforts although, to date, evidence to support that happening is sparse. 5. Problems associated with deterioration in water quality have multiplied over the past 20 years and trout is the species most affected by fish kills. These tend to be seasonal, exacerbated by low water and dry weather. The smallest streams, which may be as narrow as 30 cm, which are the stronghold of trout in many catchments, are at greatest risk. 6. Ireland's cyprinid fauna results from introductions but it is rapidly developing - as is coarse angling. Roach is the species spreading fastest. Studies show that roach will share the diet of trout and. possibly more significant, will compete with them for the angler's fly.     

Morphological differences in the skin of marble trout Salmo marmoratus and of brown trout Salmo trutta

Despite being genetically very closely related, the marble trout Salmo marmoratus and the brown trout Salmo trutta exhibit marked phenotypic differences, particularly with regard to skin pigmentation. Histological analysis of skin from the head and gill cover of differently aged individuals of the two species was carried out in order to characterize differences in skin structure. The basic structure of skin of the individuals studied corresponded with that described for other salmonids, though the head epidermis was somewhat thicker in S. marmoratus than in S. trutta, thickening with age in both species. Numerous secretory goblet cells and sporadic secretory sacciform cells were observed in the upper and middle part of the epidermis in both species. Melanophores were present in both species only in the dermis, and were bigger in S. marmoratus and present at lower average density than in S. trutta, and more or less constant across all age classes. In adult S. marmoratus with fully established marble pigmentation, light areas at low density with small (i.e. aggregated) melanophores were present, while in S. trutta melanophores were more uniformly distributed.     

The growth of brown trout (Salmo trutta) in mild winters and summer droughts in upland Wales

SUMMARY. 1. We compared the observed annual growth of 0- and I-group trout in nine Welsh upland streams, with growth predicted from temperature assuming that this was the only limiting factor.
2. Autumn weights of second year fish were 51–67% of predicted ( G _max) values in 1988, but only 30–40% in 1989 and 1990 when drought occurred. Though initial weights of fry were unknown, simulations suggested that first year growth was also less than G _max, but with no obvious effect of drought.
3. To evaluate the possible effects of future climate change, we simulated stream temperature regimes 1.5–4.5°C above those of a recent year with temperatures similar to the long-term average. Growth was set at 60% G _max for both 0- and I-group, or at 40% for I-group to represent the effect of drought. As winter temperature increased, time to hatching and emergence decreased, for example by 56 and 49 days respectively for a rise of 3°C. 0-group growth was slightly enhanced at up to + 3°C but retarded at + 4.5°C. Simulations of I-group growth suggested that warmer winters could enhance trout growth while warmer summers would only increase growth if there were no adverse effects of drought.
4. We discuss many uncertainties in these simulations, which nevertheless suggest the magnitude of possible effects of climate change.     

The production of juvenile Atlantic salmon, Salmo salar in the upper Wye, Wales

The production of juvenile Atlantic salmon ( Sulmo salur L.) was investigated in 16 study sites in the upper Wye catchment during the period February 1975 to November 1976. The population structure was characterised by large numbers of 0+ fish whose abundance decreased with respect to time so that the numbers of each of the older year-classes was a function of the year-class strength of the original fry. The range of parr densities was similar to that recorded for other rivers: the estimates of 2+ smolts did not exceed about 0.04 m⁻². Production ranged from 0.3 to 11.0 gm⁻² a^−l; Of and 1+ fish contributed over 72% and 3+ fish less than 1 % of the total annug production. The relationship between production P (gm⁻² a⁻¹) and mean biomass B (gm⁻²) for all yeatclasses considered separately and in combination can be expressed by the relationship P=a^b . The values for a vary with age-composition and season but the values for b are not significantly different. Differences in growth rate of salmon between sites are small and differences in production result principally from differences in standing crop (biomass density). In most cases low values for salmon production can be attributed to poor spawning.