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.     

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.     

Sibling-size variation in brown trout Salmo trutta in relation to egg size and stream size

Sibling-size variation (SSV), estimated as the coefficient of variation of egg size, was investigated for 13 populations of brown trout Salmo trutta . SSV was negatively correlated with mean egg size both at the population and individual levels. After correction for the effect of mean egg size, SSV was also negatively correlated with stream size. These results provide new information about how salmonid SSV can vary at different ecological scales (individual, population and region). The results are discussed in light of competing theories for explaining SSV: (1) the passive effect hypothesis, stating that egg size variation follows passively from selection on egg size and (2) the bet-hedging hypothesis, stating that high SSV is adaptive in unpredictable environments.     

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.     

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.     

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.     

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.     

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.     

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.     

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.     

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.     

Effects of predation, trout density and discharge on habitat use by brown trout, Salmo trutta, in artificial streams

1. The effects of predation risk, fish density and discharge on habitat use by juvenile brown trout, Salmo trutta, in four artificial streams were studied. Each stream contained three habitats, riffles, runs and pools, the latter two each being further divided into shallow margins and deeper mid-regions. 2. The presence of northern pike, Esox Indus, caused trout to decrease use of pool midregions, where pike also occurred, and to increase use of other habitats. Increasing the number of trout caused trout to increase use of pools and the shallow margins of runs. Decreasing discharge reduced the area of the run and pool margins covered by water, thereby reducing use of these areas by trout. 3. Habitat selection indices for the different treatments were calculated. The data indicated that riffles and the mid-regions of runs were preferred habitats, whereas run margins and pools were inferior habitats used when intraspecific fish densities were high. 4. Despite density- and discharge-dependent habitat use by trout, the number of trout consumed by pike was independent of trout density and discharge. 5. The results reveal the flexibility of habitat use by trout and illustrate the potential danger of applying data on habitat use in one stream to others where habitat availability and bioric interactions may differ.     

Observations on the structure of brown trout, Salmo trutta Linnaeus, redds

This paper describes the gravel type and structure of brown trout spawning gravels in Teesdale, the flow characteristics of the spawning sites and the size of redds. Factors influencing redd size are investigated and it is concluded that female size is a major factor. Relationships are established between various redd characteristics and the fork length of female fish. The findings are discussed in relation to the existing literature concerning salmonid spawning sites.     

Effects of hydropeaking on the spawning behaviour of Atlantic salmon Salmo salar and brown trout Salmo trutta

An in situ camera set‐up was used to study the spawning activity of Atlantic salmon Salmo salar and brown trout Salmo trutta throughout two consecutive seasons in a spawning area affected by hydropower‐related pulse flows due to hydropeaking. The purpose was to test whether the flow variation discouraged spawning in shallow areas or motivated spawning into areas with elevated risk of incubation mortality. There were more S. salar observed on the spawning ground during days with high discharge. The presence of S. salar in the spawning grounds was not affected by the hydropeaking cycles of the preceding night. Female S. salar were observed preparing nests within the first hour after water discharge had increased to levels suitable for spawning. In contrast, the number of S. trutta was not correlated with flow and nest preparation was also observed at a discharge corresponding to the lowest discharge levels during a hydropeaking cycle. Survival was generally high in nests excavated the following winter, with only 5·4% suffering mortality due to dewatering. The results suggest that S. salar may respond rapidly to variable‐flow conditions and utilize short windows with suitable flows for spawning. Smaller S. trutta may utilize low‐flow conditions to spawn in areas that are not habitable by larger S. salar during low flow.     

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.     

Rates of gastric evacuation in brown trout, Salmo trutta L.

Brown trout of similar length and weight were fed a standard meal which contained a known number of food organisms of the same size-group and taxon (seven taxa were used). The weight of digestible organic matter in a trout stomach decreased exponentially with time. i.e. at a constant relative rate. At a particular water temperature, the food organisms were either evacuated from the stomach at similar rates (Group 1: Gammarus pulex, Baetis rhodani, Chironomidae, Oligochaetes) or at progressively slower rates (Group 2: Protonemura meyeri, Hydropsyche spp., Tenebrio molitor). Rates of gastric evacuation were not significantly different for food organisms of different size groups of the same taxon, or for different sized meals, or for different sizes of trout (range 20–30 cm), or for mixed and multiple meals (three meals over 16 h). Times are given for the gastric evacuation of 50%, 75%, 90% and 99% of the digestible organic matter in a meal. Starvation periods of 1, 2, 3,4 and 5 days prior to feeding did not affect evacuation rates. The rates were slightly, but not significantly, slower for starvation periods of 6 and 7 days, and were often significantly slower for starvation periods of 10, 15 and 20 days. Evacuation rates increased exponentially with increasing water temperature. It was possible to estimate both the rate and time for the gastric evacuation of different meals at water temperatures between 3–8°C and 19·1°C.     

Effects of temperature and a piscivorous fish on diel winter behaviour of juvenile brown trout (Salmo trutta)

1. Low winter temperatures constrain predator‐detection and escape capabilities, making poikilotherms vulnerable to predation. Investigations of temperature effects on predator–prey interactions can therefore be of special importance in light of ongoing climate change, where winter temperatures are predicted to increase substantially at northern latitudes.
2. Behavioral responses of stream fishes to terrestrial predators in winter are well recognised, whereas responses to predatory fish have received little attention. Using stream flumes, we examined the anti‐predator behaviour of one‐summer‐old brown trout (Salmo trutta) at 3 and 8°C in the presence and absence of burbot (Lota lota) under night, dawn, and daylight conditions. Burbot was placed upstream of the trout, separated by net screens.
3. Lower temperature and the presence of burbot reduced trout activity. Light increased trout shelter use, and trout sheltered more in the presence of burbot. An interaction between the presence of burbot and light conditions affected trout position in the flumes: at night and dawn, trout positioned themselves further downstream when burbot were present than when absent, whereas during the day, trout maintained the same position in the presence or absence of the predator.
4. Our results suggest that piscivorous fish, in addition to terrestrial predators, shape the behaviour of prey fishes in streams during winter. We show how predator avoidance results in altered diel patterns of juvenile brown trout under winter conditions, and that temperature has additional effects on trout behaviour.

     

Daily energy intake and growth of piscivorous brown trout,Salmo trutta

• 1 The chief objectives were to determine the daily energy intake and growth of piscivorous brown trout (Salmo trutta), and to compare the observed values with those expected from models developed previously for brown trout feeding on freshwater invertebrates. Energy budgets for individual fish were obtained from experiments with 40 trout (initial live weight 250–318 g) bred from wild parents, and kept at five constant temperatures (5, 10, 13, 15, 18 °C) and 100% oxygen saturation. Each trout was fed to satiation on freshly killed sticklebacks (Gasterosteus aculeatus) over a period of 42 days.
• 2 Energy intake (C_IN cal day^‐1) and growth (C_G cal day^‐1) were measured directly and energy losses (C_Q cal day^‐1) were estimated by difference (C_Q = C_IN ‐ C_G). All three variables increased with temperature. A model previously used to predict the daily energy intake (C_IN(INV)) of trout feeding to satiation on invertebrates was adapted, by changing only one parameter, to provide an excellent model (R² = 0.998) for predicting the mean daily energy intake (C_IN(ST)) for the piscivorous trout. Values of C_IN(ST) were 58% (range 48–67%) higher than those for C_IN(INV). A simple model was also developed to estimate mean daily energy losses for piscivorous trout (R² = 0.999). Both models were combined to provide excellent estimates of the daily energy gain (growth) of the piscivorous trout, and this was about three times that for trout feeding on invertebrates. The optimum temperature for maximum growth in energy terms increased from 13.9 °C for trout feeding on invertebrates to 17.0 °C (range 16.6–17.4 °C) for piscivorous trout.
• 3 The models are basically an extension of those developed for trout feeding on invertebrates. They show clearly how energy intake, growth, and the optimum temperature for growth increase markedly when trout change their diet from invertebrates to fish. The implications of this are discussed and it is shown that, in theory, these increases should continue if a more energy‐rich diet was utilised by the trout.

     

Effects of gravel size and peat material concentrations on embryo survival and alevin emergence of brown trout,Salmo trutta L.

Embryo survival and alevin emergence pattern of brown trout were studied in simulated redds with different homogeneous gravel sizes and different concentrations of peat material. Optimal survival (95%) occurred in 18 mm gravel and survival decreased with decreasing gravel size. High concentrations (40%) of peat material resulted in low survival (65%). The proportion of premature emerging alevins increased in finer gravels and at high peat concentrations. Premature alevins had a large yolk sac and are probably very vulnerable to predators.