Behavioural and heart rate responses to food limitation and predation risk: an experimental study on rainbow trout

Food-restricted rainbow trout Oncorhynchus mykiss maintained a lower basal heart rate than satiated fish, probably as a result of reduced metabolic rate. Food-restricted fish were also more active during feeding and were more willing to take risks than satiated fish. Both satiated and food-restricted fish were positioned lower in the tank after the predator attack. Heart rate increased more during feeding in the food-restricted fish compared to the satiated, but energy status had no general effect on the relation between heart rate and behaviour. Hence, the increase in heart rate was mainly a response to the more active foraging behaviour in the food-restricted fish. Moreover, behavioural activity in the food-restricted fish was associated with a higher heart rate after the predator attack than when the fish was undisturbed, which may reflect physiological preparation for flight. These findings suggest that behavioural and cardiac responses are coadapted to meet variation in food availability and predation risk in the wild.     

Effect of AC electroshock on blood features in juvenile rainbow trout

Brief AC electroshock caused significant elevations in plasma cortisol, glucose, osmolality, and chloride levels in rainbow trout within 15min, but plasma protein was reduced at 3h. Circulating granulocytes had declined by 15min, but other haematological features remained unchanged. Thus, AC electrofishing would not be suitable for monitoring physiological conditions in trout if short delays are encountered before blood is sampled.     

Response of rainbow smelt, Osmerus mordax (Mitchill), eggs to low pH

The eggs of rainbow smelt were incubated at pH c . 4.5, c . 5.5 and c ontrol ( c . 6.8) for durations of 2–14 days to assess their vulnerability to acid precipitation. These eggs are likely to be exposed to marked pH depressions during their incubation in small streams owing to run-off from snow melt and spring rains. Survival after 48 h exposure was not related to pH, although longer exposure to low pH significantly decreased survival. The developmental stage at initial exposure was also important in determining survival. Overall egg survival varied from 0–87% at pH c . 4.5,0–89% at pH c . 5.5, and 44–100% at control pH levels, (c. 6.8).     

Competitive ability is related to metabolic asymmetry in juvenile rainbow trout

Of 55 size–matched pairs of juvenile rainbow trout Oncorhynchus mykiss , each consisting of one high and one low relative metabolic rate fish (relative size difference <5%), the high relative metabolic rate fish was dominant in 36 pairs (65·5%), significantly more often than expected by chance. The probability of a fish winning was related to its relative metabolic rate (measured prior to introduction into the test arena); the higher the relative metabolic rate of a fish compared to its opponent, the greater its probability of being dominant. In the 36 pairs of fish where the low relative metabolic rate fish was the subordinate, its competitive ability was significantly correlated with how closely the two fish were matched in terms of their relative metabolic rates. The smaller the difference between the relative metabolic rates of the dominant high relative metabolic rate fish and subordinate low metabolic rate fish, the greater the competitive ability and feeding success of the low relative metabolic rate subordinate. However, no such relation was found in the 19 pairs of fish where the high relative metabolic rate fish was the subordinate. In these pairs, the competitive ability of the subordinate high relative metabolic rate fish was correlated with its size relative to its dyad partner. The larger the dominant low relative metabolic rate fish, the lower the competitive ability of the subordinate high relative metabolic rate fish.     

The effect of humic substances on the toxicity of aluminium to adult rainbow trout, Oncorhynchus mykiss (Walbaum)

Some physiological parameters were measured in adult rainbow trout during a 10-day exposure to 180 μg Al_total l⁻¹ in acid water (pH 4.7) with or without humic substances (10 mg l⁻). The fish were acclimatized to pH 5.0 for 7 days prior to the experimental treatments.
Chemical analyses revealed that, in the presence of human substances, 74–80% of the A1 was organic bound, while in the absence of humic substances most of the Al(987percnt;) occurred in the inorganic form.
Al bound to humic substances (13–150 μg l⁻¹) did not alter the plasma NaCl-concentration, nor the haematocrit value, of rainbow trout during an exposure period of 10 days. This contrasts with the high death rate obtained within 2–3 days when most of the A1 (175 μg l⁻¹) was in the inorganic form. The lethality was accompanied by a 25% decrease in the plasmaconcentration of NaCl and a doubling of the haematocrit value. Bulk analysis revealed that when the metal was present in inorganic forms the total Al content of the gills (75 μg A1 g⁻¹ wet weight) was 15 times higher than when it was present as bound to the humic substances. These experiments showed that the accumulation of A1 at the gills was accompanied by physiological disturbances, both being a function of the chemical speciation of Al.     

Constraints of body size and swimming velocity on the ability of juvenile rainbow trout to endure periods without food

The hypothesis that body size and swimming velocity affect proximate body composition, wet mass and size‐selective mortality of fasted fish was evaluated using small (107 mm mean total length, L _T) and medium (168 mm mean L _T) juvenile rainbow trout Oncorhynchus mykiss that were sedentary or swimming ( c . 1 or 2 body length s⁻¹) and fasted for 147 days. The initial amount of energy reserves in the bodies of fish varied with L _T. Initially having less lipid mass and relatively higher mass‐specific metabolic rates caused small rainbow trout that were sedentary to die of starvation sooner and more frequently than medium‐length fish that were sedentary. Swimming at 2 body length s⁻¹ slightly increased the rate of lipid catabolism relative to 1 body length s⁻¹, but did not increase the occurrence of mortality among medium fish. Death from starvation occurred when fish had <3·2% lipid remaining in their bodies. Juvenile rainbow trout endured long periods without food, but their ability to resist death from starvation was limited by their length and initial lipid reserves.     

Effects of food deprivation on 24 h-changes in brain and liver carbohydrate and ketone body metabolism of rainbow trout

Plasma glucose, lactate and acetoacetate, brain glycogen and acetoacetate, and liver acetoacetate, glycogen and lactate in fed rainbow trout exhibited daily changes. However, no daily changes were observed in the activities of the brain enzymes glycogen synthetase, 6-phosphofructo 1-kinase, and lactate dehydrogenase. Depending on the length of the previous fasting period most daily changes observed in the metabolic parameters of fed fish disappeared, except for liver acetoacetate levels, which displayed daily changes in both fed and fasted fish. These results suggest that feeding is an important factor regulating most daily changes in the brain and liver carbohydrate and ketone body metabolism of rainbow trout.     

Involvement of cortisol and sirtuin1 during the response to stress of hypothalamic circadian system and food intake-related peptides in rainbow trout,Oncorhynchus mykiss

Stress is conditioning animal welfare by negatively affecting a wide range of physiological and behavioral functions. This may be applied to circadian physiology and food intake. Cortisol, the stress-related hormone, may mediate such effect of stress, but other indirect mediators might be considered, such as sirtuin1. Then, either the independent modulatory effect or the existence of any interaction between mediators may be responsible. The circadian system is the main modulator of several integrative mechanisms at both central and peripheral levels that are rhythmically presented, thus influencing different processes such as food intake. In this way, food intake is controlled by the circadian system, as demonstrated by the persistence of such rhythms of food intake in the absence of environmental external cues. Our study aimed to evaluate the daily profile of hypothalamic mRNA abundance of circadian clock genes (clock1a, bmal1, per1 and rev-erbβ-like), and food intake regulators (crf, pomc-a1, cart, and npy) in rainbow trout (Oncorhynchus mykiss), the impact of stress on such rhythms, and the involvement of cortisol and sirtuin1 as mediators. Four cohorts of trout were subjected to 1) normal stocking density (control group), 2) high stocking density for 72 hours (stress group), 3) normal stocking density and implanted with mifepristone, a glucocorticoid receptors antagonist, and 4) mifepristone administered and stressed for 72 hours. Fish from each group were sampled every 4-h along the 24-h LD cycle, and cortisol, glucose and lactate plasma levels were evaluated. Hypothalamic mRNA abundance of clock genes, food intake regulators, glucocorticoid receptors and sirtuin1 were qPCR assayed. Our results reveal the impact of stress on most of the genes assayed, but different mechanisms appear to be involved. The rhythm of clock genes displayed decreased amplitude and averaged levels in stressed trout, with no changes of the acrophase being observed. This effect was not prevented by mifepristone. On the contrary, the effect of stress on the daily profile of crf, pomc-a1, and npy was totally prevented by mifepristone administration. Accordingly, cortisol appears to mainly mediate the effect of stress on food intake regulators through binding to specific glucocorticoid receptors within trout hypothalamus, whereas sirtuin1 is apparently mediating such effects on the circadian system in the same brain region. Further research must be performed to clarify those mechanisms through which stress influences food intake and the circadian oscillator within the same brain region, hypothalamus, in rainbow trout, and the interaction among them all.     

Responses of pineal photoreceptors in the brook and rainbow trout

Summary Electron microscopy of the pineal receptor cells in light- and dark-adapted brook trout, Salvelinus fontinalis and the rainbow trout, Salmo gairdneri, revealed no significant differences in the tubular and filamentous elements of the inner segment, neck and supranuclear regions. However, changes in synaptic relations between the photoreceptor and nerve cell were induced by light and darkness. In the light-adapted state, the synaptic relationship between axon terminals and photoreceptor basal processes predominates, while in darkness the synapses between photoreceptor basal processes and ganglion cell dendrites are more prominent. Further, in darkness, the photoreceptor basal processes show a number of synaptic vesicles and synaptic ribbons. These findings suggest that the sensory function of the fish pineal is enhanced during darkness but inhibited by light, and that the synaptic relationships are involved in the control of sensory activity in the pineal photoreceptor and ganglion cells. These results corroborate those of electrophysiological studies in that the maximal spontaneous discharge frequency of the ganglion cells occurs in the dark, and it also shows a burst when light is removed. The typical chemical synapse between the axon terminal and the photoreceptor basal process in light seems to function as an inhibitor.The authors thank Dr. Mary Ann Klyne for her assistance in several aspects of this work. Financial assistance was provided by the NSERC of Canada and the Ministry of Education of Québec     

Influence of growth rate on white muscle dynamics in rainbow trout and brook trout

In trout, fast growth stimulated white muscle fibre hypertrophy ( P 0·001) and hyperplasia ( P <0·01) in outer fibres but not in deep fibres. Glycogen was most prevalent in outer fibres ( P <0·01) and in brook trout ( P <0·01) that on average had three to four times larger fibres than rainbow trout.     

Influence of light and food on the circadian clock in liver of rainbow trout,Oncorhynchus mykiss

Several reports support the existence of multiple peripheral oscillators in fish, which may be able to modulate the rhythmic functions developed by those tissues hosting them. Thus, a circadian oscillator has been proposed to be located within fish liver. In this vertebrate group, the role played by the circadian system in regulating metabolic processes in liver is mostly unknown. We, therefore investigated the liver of rainbow trout (Oncorhynchus mykiss) as a potential element participating in the regulation of circadian rhythms in fish by hosting a functional circadian oscillator. The presence and expression pattern of main components of the circadian molecular machinery (clock1a, bmal1, per1 and rev-erbβ-like) were assessed. Furthermore, the role of environmental cues such as light and food, and their interaction in order to modulate the circadian oscillator was also assessed by exposing animals to constant conditions (absence of light for 48 h, and/or a 4 days fasting period). Our results demonstrate the existence of a functional circadian oscillator within trout liver, as demonstrated by significant rhythms of all clock genes assessed, independently of the environmental conditions studied. In addition, the daily profile of mRNA abundance of clock genes is influenced by both light (mainly clock1a and per1) and food (rev-erbβ-like), which is indicative of an interaction between both synchronizers. Our results point to rev-erbβ-like as possible mediator between the influence of light and food on the circadian oscillator within trout liver, since its daily profile is influenced by both light and food, thus affecting that of bmal1.     

Cortisol and immune characteristics in rainbow trout (Oncorhynchus mykiss) selected for high or low tolerance to stress

The influence of exposure to stressors on cortisol and the non-specific immune traits lysozyme and serum haemolytic activity were examined in second generation rainbow trout ( Oncorhynchus mykiss ) selected for either high or low serum cortisol level following a confinement stress. Lysozyme and serum haemolytic activity were also assessed, together with levels of specific antibodies against Aeromonas salmonicida A-layer, Vibrio salmonicida O-antigen and Vibrio anguillarum O-antigen, following injection of vaccines against these pathogens.
Significant differences in mean cortisol levels between the two selection lines were observed, but in only one of two stress experiments was the 'high-stress' line found to have the higher cortisol level; in the other experiment the 'high-stress' line had significantly lower cortisol levels than the 'low-stress' line. Lysozyme levels were in four of four assessments higher in the high-stress line than in the low-stress line, whereas components of serum haemolytic activity tended to be lower in the high-stress line than in the low-stress line. Levels of specific antibodies against all three bacterial pathogens were elevated following the injection of the vaccines. Only antibody production against A. salmonicida A-layer was significantly different between the two lines, the higher production of antibody being in the high-stress line.     

Gastrointestinal responses of rainbow trout to dry pellet and low-fat herring diets

Long-term voluntary-feeding experiments were carried out on farmed, 2-year-old rainbow trout offered a commercial dry feed, or chopped low-fat Baltic herring. Despite large differences in dietary water, protein and lipid content, the trout adjusted their intake to consume similar amounts of dry matter. After an 18-week trial, the stomach volumes of the herring-fed trout were significantly larger (30–35%) than those fed on the dry diet. Greatest differences were observed when fish were fed one meal per day; increasing the number of daily feeding opportunities reduced these expected stomach volumes on each diet by 15–20%. The relative increase in stomach volume was shown to be due to growth of the cardiac stomach region (corpus) rather than the pyloric region, and not to muscle relaxation; the change was completed within 10 weeks. Data were collected in a separate study to investigate stomach size in fish (age 0+, 1+, 2+) of similar genetic backgrounds which had been grown using dry pelleted diets. Despite considerable variation between populations, stomach volume to body weight relationship was allometric ( S = a W ^b) with the exponent in the range of 0.3–0.4.     

Nocturnal emergence of juvenile rainbow trout from winter concealment relative to light intensity

This study examined the relationship between light intensity and the number of juvenile rainbow trout (Oncorhynchus mykiss) visible to a snorkeler during February in the Henrys Fork of the Snake River, Idaho, USA. Fish were concealed in the substratum during daylight. Emergence from concealment was observed from 30 to 80 min after real sunset time and began when stars were first visible (pyranometric irradiance, 4.5 × 10^–3 W^–2). Densities of visible fish were negatively correlated with light intensity (r ²=0.81,P<0.001). Later at night, densities decreased in the presence of moonlight and artificial light. Fish were observed to feed at night.     

The food of juvenile trout,Salmo trutta L., in Loch Leven,Kinross, Scotland

The food of juvenile brown trout in Loch Leven was studied by the examination of 662 stomachs of fish caught between October 1974 and December 1976. The food consumption was estimated only on the basis of the analysis of the stomach contents.The main food items of juvenile trout in the loch are chironomids, crustaceans and nematodes: these are abundant in the offshore mud zone of the loch.     

A comparison of the effect of long-term starvation on responses to low-temperature stress by juvenile rainbow (Oncorhynchus mykiss) and brown (Salmo trutta) trout reveal different responses in the two species

Biometric parameters and oxidative stress indicators were measured in liver and muscle samples from rainbow trout and brown trout juveniles exposed to a 45-day starvation period at low water temperature. As a general tendency, hepatic antioxidant enzyme activities in both species increased with fasting to eliminate the harmful effects of reactive oxygen species (ROS). However, the metabolic response to food deprivation in the muscle of each species was different. Lipid peroxidation levels in both species increased with starvation. We concluded that (1) low water temperature enhances ROS production in salmonids because of increased polyunsaturated fatty acid content in the cell membrane; (2) starvation significantly impaired the growth parameters of brown trout, yet the reverse was found for rainbow trout; and (3) despite this negative interaction, brown trout juveniles can physiologically tolerate oxidative stress caused by starvation and can therefore be cultivated under stressful conditions even in their early life stages.     

Trychophrya intermedia on the gills of rainbow trout acclimating to low ambient pH

Rainbow trout under low pH conditions acquire heavy infections with Trychophrya intermedia , not secondary to gill damage, which suggests that the parasite may have a primary effect on gill function in fish Under acid Conditions.     

Effects of diffusion limitation on immobilized nitrifying microorganisms at low temperatures

Activation energies of suspended and immobilized nitrifying bacteria were determined and compared to determine if diffusion limitation results in decreased sensitivity for temperature. The activation energy for the respiration activity of suspended Nitrosomonas europaea and Nitrobacter agilis was found to be 86.4 and 58.4 kJ mol(-1), respectively. The activation energy for oxygen diffusion in the support material, kappa-carrageenan, determined from the effect of temperature on the effective diffusion coefficient (D), was 17.2 kJ mol(-1). Consequently, the apparent actvation energy of diffusion limited cells should be lower. It was indeed shown that due to the effect of diffusion limitation and to temperature effects on the Monod constant K(s), the immobilized-cell activity was less sensitive to temperature. The apparent activation energy for immobilized Ns. europaea was between 28.6 and 94.2 kJ mol(-1) and for immobilized Nb. agilis between 1.4 and 72.9 kJ mol(-1), depending on the oxygen concentration and temperature. (c) 1995 John Wiley & Sons, Inc.