Putative immune response of rainbow trout, Salmo gairdneri, to Diplostomum spathaceum infections

The immune response of rainbow trout to infection with the cercariae of the digenean parasite Diplostomum spathaceum was investigated. Rainbow trout infected with cercariae at weekly intervals, or injected with a suspension of dead cercariae, did not produce a specific humoral response against D. spathaceum cercariae, as tested using immunoperoxidase, agglutination and cytotoxicity assays. However, a significant difference occurred in the rate of infection of rainbow trout given weekly exposure to cercariae of D. spathaceum in winter and summer. Rainbow trout injected with a suspension of dead cercariae acquired significantly fewer metacercariae than controls when exposed to a challenge infection. This suggested a specific immune response by the host and is the first example of a reduction in the infection rate of rainbow trout immunized against a digenean parasite, when exposed to a challenge infection.     

Exercise training and the stress response in rainbow trout, Salmo gairdneri Richardson

Plasma levels of catecholamines, cortisol, and glucose were monitored in rainbow trout during a 6-week forced swimming exercise programme. Compared to resting non-exercised controls, resting trained fish had lower levels of epinephrine, norephinephrine, cortisol, and glucose during the last 3 weeks of training. Initially, trained fish that were swimming had higher levels of epinephrine than resting trained fish. After 2 weeks of exercise, swimming did not significantly elevate epinephrine levels in trained fish. Glucose levels were consistently greater in swimming fish than in resting fish. At the end of the training period, exercised trout had lower (15–20%) oxygen consumption rates while resting or swimming than unexercised fish.
After a 5-month forced swimming exercise programme plasma levels of catecholamines and glucose were monitored in trained and untrained cannulated rainbow trout after 2 min of mild agitation. Trained fish showed an immediate (within 1 min) increase in the levels of epinephrine, but not norepinephrine and a delayed (within 15 min) increase in the levels of plasma glucose. Epinephrine levels returned to pre-stress levels within 15 min. Untrained fish had no significant increase in the plasma levels of norepinephrine, epinephrine, or glucose.     

Acclimation-induced changes in toxicity of arsenic and cyanide to rainbow trout, Salmo gairdneri Richardson

Pre-exposure of rainbow trout, Salmo gairdneri to sublethal levels of arsenic or cyanide, resulted in significant changes in the incipient lethal levels of the respective toxicants. Fish gradually increased their tolerance to arsenic (as arsenite) by 47% during a three week preexposure to 0.22 of the incipient lethal level. Mean whole-body aresenic concentrations increased from 2 to 27 μg g⁻¹ dry weight during the first week, then decreased to a stable level of 15 after two and three weeks. Fish decreased their tolerance to cyanide (as HCN) by 32 and 15% after one and two weeks of pre-exposure to 0–35 of the incipient lethal level. This initial sensitization disappeared after three weeks. Renal tubular parenchymatous oedema was observed after two and three weeks of cyanide pre-exposure. Neither arsenic nor cyanide affected growth at these levels of pre-exposure.     

Cytochemical demonstration of acid phosphatase activity in the pineal organ of the rainbow trout,Salmo gairdneri

Summary Activity of acid phosphatase (ACP) was investigated cytochemically in the pineal organ of the rainbow trout, Salmo gairdneri. Intense reaction product for ACP activity was observed (1) in lysosomes varying in size and shape and (2) in endoplasmic reticulum associated with the Golgi complex of (i) the pineal photoreceptor and supporting cells, (ii) vascular endothelial cells, and (iii) macrophages inhabiting pineal lumen, parenchymal epithelium and perivascular spaces. This localization of ACP is discussed with particular reference to the capacity for lysosomal digestion in a pineal organ combining photoreceptive and secretory functions, and lacking a blood-brain barrier, as holds true for the pineal of the rainbow trout. Taking advantage of its capacity for endocytotic uptake and lysosomal digestion, the pineal organ of the rainbow trout may serve as a barrier between the blood circulation and the cerebrospinal-fluid compartment. Furthermore, the macrophages may be considered as an essential component in pineal function of fish.Fellow of the Alexander von Humboldt Foundation.Fellow of the Alexander von Humboldt Foundation.     

The saccus dorsalis of the rainbow trout,Salmo gairdneri Richardson

The saccus dorsalis of the brain of the rainbow trout, Salmo gairdneri Richardson, has been investigated by means of histological, cytochemical, enzyme-cytochemical, electron microscopical autoradiographical techniques. The saccus dorsalis is a rostro-dorsal evagination of the diencephalic roof, and consists of a partly folded epithelial wall separating the cerebrospinal fluid from the meningeal matrix fluid. The well-developed vascular system around the epithelial wall, consisting of capillaries with different diameters, seems to be part of the pineal vascular system. No structures were found that may be involved in a possible mechanical or nervous blood flow control. The single-layered epithelium consists of highly specialized cells of one specific type. These cells are mainly characterized by infolded basal membranes, long microvilli of a peculiar shape, non-folded lateral membranes bordering intercellular spaces, apical concentrations of elongate and cup-shaped macromitochondria, a basally located rough endoplasmic reticulum, an apically situated smooth endoplasmic reticulum and apical concentrations of micropinocytotic vesicles. Morphological evidence is presented of a multiple function of these cells: (1) fluid secretion, (2) extrusion of low molecular weight organic substances into the ventricular system, (3) uptake of high molecular weight substances, and (4) uptake of low molecular weight organic substances (aminergic neurotransmitters [GABA]) from the cerebrospinal fluid. The significance of light and dark cells is discussed. Indications of a possible innervation of the saccus dorsalis epithelial cells were not observed. The functional significance of the saccus dorsalis (possible analogue of the choroid plexus?) is discussed.     

The response of the haemoglobin system of fed and starved rainbow trout, Salmo gairdneri Richardson, to bleeding

Seven days after a 15% reduction in blood volume by bleeding, the haemoglobin system of rainbow trout showed a modification in relative abundance of two of the four haemoglobins present in the haemolyate. The least anodic non-Root effect haemoglobin increased proportionally whereas the major component, with its Root effect properties, showed a decline. A total of 30 days of starvation did not influence the relative proportions of the fractions. However, bleeding of these starved S. gairdneri provoked a shift similar to that of fed trout. In this case, however, both non-Root haemoglobins, and the minor Root fraction, increased proportionally. The major Root fraction declined. The response of the starved group was more acute since all four components were significantly affected in contrast to two fractions in fed animals.     

Seasonal changes in reproductive function of the rainbow trout (Salmo gairdneri)

The sequential changes in serum oestradiol 17β (measured by specific radioimmunoassay), vitellogenin (measured by phosphoprotein phosphorus content) and total calcium (measured by fluorimetry) in male and female rainbow trout under a simulated natural photoperiod cycle were investigated. Resting levels of 130 pg/ml serum oestradiol in both male and female fish were found in April and May. In the female, levels reached a peak of 4800 pg/ml in October, and almost returned to resting levels just prior to spawning in mid-January. No significant change from resting levels was seen in the male fish.
In both male and female fish levels of 25 μg/ml serum phosphoprotein phosphorus and 10–14 mg % total serum calcium were found from April to July. Coincident with the rise in oestradiol 17β, in the females serum levels of phospho-protein phosphorus and total calcium increased to 400 uβg/ml and 58 mg% respectively just prior to spawning. In the males no significant change in either of these values was observed throughout the cycle. These results strongly support the hypothesis that photoperiod is the major factor in the environmental control of reproductive activity in the rainbow trout.     

Compensatory growth in the rainbow trout, Salmo gairdneri Richardson

The effect of a period of starvation and subsequent refeeding on the weight and length of rainbow trout at different times of the year has been investigated. Fish that have been starved for 3 weeks and then fed for 3 weeks show a weight gain equivalent to or greater than that of fish fed normally for the 6 week period, in four out of the five periods studied. The study provided evidence of the adaptation of the fish to starvation followed by what may be termed compensatory growth once feeding was resumed. The length changes of the fish indicate that the weight gains were due to growth rather than increases in gut fat deposits or increased water uptake.     

Avoidance response of two-year-old rainbow trout, Salmo gairdneri Richardson, to air-supersaturated water: hydrostatic compensation

Vertical swimming responses of 2-year-old rainbow trout were tested with air saturated and air supersaturated water in tank experiments. The level of supersaturation varied between 115 and 125% total gas pressure. Control groups offish were kept in tanks with saturated water. No consistent vertical avoidance response was observed in the fish tested, but the mortality of fish restricted to the upper 30 cm of the tanks was significantly greater than fish free to sound to the whole depth of the tanks. It is concluded that the Norwegian stock of rainbow trout do not avoid air-supersaturation at levels from 115 to 125% TGP by active hydrostatic pressure compensation. An incidental type of hydrostatic pressure compensation seemed to be taking place during these experiments, as indicated by different levels of mortality in tanks with different depths. This incidental type of hydrostatic compensation may explain the observed difference in tolerance to supersaturation between wild fish and fish kept in experimental tanks.     

Kidney function in response to salt feeding in rainbow trout (Salmo gairdneri Richardson)

1. The effect of high levels of dietary salt up to 12% NaCl on kidney function in freshwater rainbow trout was investigated. 2. Renal response to dietary NaCl load includes increases in urinary flow rate and glomerular filtration rate, together with a slight reduction in ionic reabsorption capacity. 3. The renal salt excretion rate, which was doubled to about 100 microM/kg/hr, in fish fed the high salt diet, is not entirely a consequence of a reduction in tubular ionic reabsorption but also of increased glomerular filtration. 4. The role of the endocrine system in control of renal salt excretion is discussed.