Rainbow Trout Salmo Gairdneri Research Articles

Immunoreactivity to gonadotropin-releasing hormone and gonadotropic hormone in the brain and pituitary of the rainbow trout Salmo gairdneri.

Immunoreactivity to gonadotropin-releasing hormone (GnRH) and gonadotropic hormone (GTH) was studied at the light-microscopical level in the brain and pituitary of rainbow trout at different stages of the first reproductive cycle using antisera against synthetic mammalian GnRH and salmon GTH. GnRH perikarya were localized exclusively in the preoptic nucleus, both in the pars parvicellularis and the pars magnocellularis. A few somata contacted the cerebrospinal fluid. Not all neurosecretory cells were GnRH-positive, indicating at least a bifunctionality of the preoptic nucleus. We recorded no differences between sexes or stages of gonadal development in the location of GnRH perikarya, whereas gradual changes were found in staining intensity during the reproductive cycle. GnRH fibres ran from the partes parvicellularis and magnocellularis through the hypothalamus and merged into a common tract at the transverse commissure before entering the pituitary. In the pituitary, GnRH was localized in the neural tissue of the neurointermediate lobe and, to a lesser extent, in the neural protrusions penetrating the proximal pars distalis. The bulk of GTH-positive cells was situated in the proximal pars distalis. Some cells were found more rostrally amidst prolactin cells or in the neurointermediate lobe. Only a limited number of GTH cells appeared to be in close contact with GnRH-positive material.

Release of erythrocytes from the spleen during exercise and splenic constriction by adrenaline infusion in the rainbow trout

Erythrocyte-supplying function of the spleen was examined in the rainbow troutSalmo gairdneri under exercise. The spleen showed remarkable reduction, about 70% in weight and about 85% in hemoglobin content, after forced exercise of 15 min. The amount of erythrocytes released from the spleen was 2.33 ml/kg body, and this amount corresponds to about 20% of the total volume of circulating erythrocytes in resting condition. No damage was observed at the spleen, splenic artery and splenic vein after the exercise. Examination of the vascular system by a corrosion casting method showed that no place other than the venous circulation exists for the erythrocytes released from the contracted spleen. The spleen was strongly constricted by infusion of adrenaline into the organ. These facts imply that the fish spleen supplies stored hemoglobin into the circulating blood in response to an increased demand of oxygen during exercise, under the control of the sympathetic nervous system.

Concentrations of Cd, Sr, and U in Fish and Water Samples Collected from a Small Stream Receiving Uranium Mine Drainage

ABSTRACT The concentrations of cadmium (Cd), strontium (Sr) and uranium (U) were determined in whole eviscerated rainbow trout (Salmo gairdneri), trout liver, and water samples collected from a small stream receiving uranium mine discharge in northeast Washington state during high and low flow periods. Average water concentrations of Cd, Sr, and U downstream of the mine discharge increased 15, 5, and 100-fold, respectively. Cadmium levels increased in fish and liver samples during both sample periods. The weighted average fish concentration was eleven times higher than that for the control stream. There was no increase in fish and liver Sr levels below the mine discharge during either sample period. Increases in uranium concentrations were detected only in the fish where the weighted average low flow value was 15 times higher than the control stream value.

Hypothalamic control of prolactin release in the rainbow trout,Salmo gairdneri: in vitro studies

Hypothalamic control of prolactin (PRL) release in immature rainbow troutSalmo gairdneri was investigated using anin vitro perifusion system of the rostral pars distalis. Hypothalamic extract of trout induced a dose-dependent stimulation of PRL release. A similar effect was observed when infusing the medium from a 24h static incubation of the hypothalamus. Extracts from different control tissues (muscle, liver, gut) did not changein vitro release, thus confirming the specificity of this stimulatory effect. Hypothalamic extract from adult male rat, known to contain PRL release inhibiting factors, stimulatedin vitro PRL secretion in rainbow trout. This suggests that PRL cells are predominantly influenced by PRL releasing factors. Measurement of TRH and serotonin content in trout hypothalamus indicated consistent physiological levels of these two factors. HPLC studies of hypothalamic extract showed that immunoreactive - TRH eluted at the same place as labelled TRH standard. Moreover, pizotifen, a serotonin antagonist, partially inhibited the stimulation observed with trout hypothalamic extract. These results suggest that, in immature rainbow trout, PRL release is under stimulatory hypothalamic control and that serotonin and probably TRH play a major role in this control.

In vitro hepatic monodeiodination of L-thyroxine and the temporal effect of 17β-estradiol on deiodination in rainbow trout (Salmo gairdneri Richardson)

Thein vitro hepatic monodeiodination of L-thyroxine (T4) to triiodo-L-thyronine (T3) in rainbow trout (Salmo gairdneri) was found to be pH- and temperature-dependent, and was related to the amount of homogenate in the reaction vessel, suggestive of an enzyme-regulated event. Dithiothreitol (DTT) introduced into the reaction medium stimulated T3 production in a dose-related manner, whilst 6n-propyl-2-thiouracil (PTU) inhibited T3 production, also in a dose-related manner. The conversion was stimulated in the presence of light and depressed at buffer concentrations of less than 0.1 M.Prior treatment of fish with an intraperitoneal slow-release implant containing 17β-estradiol (E2), at doses which are known to induce chronic mild elevations in plasma E2 levels, elicited a biphasic response to E2 as regards hepatic T3 production from T4 with a depression of T4 to T3 conversion evident within 1-2 days after implantation, and a subsequent stimulation of T3 production evident 56 days after, implantation. This increased hepatic deiodinase activity after chronic exposure to E2 at physiological doses was accompanied by a 3.5 fold increase in Vmax without a significant change in Km, suggesting the presence of an increased amount of the enzyme.

Iron Resistance of Hepatic Lesions and Nephroblastoma in Rainbow Trout (Salmo gairdneri) Exposed to MNNG

Histochemical markers are important for the early detection of chemically initiated neoplasia in experimental animal studies. The marker, iron resistance, was evaluated in the Shasta strain of rainbow trout (Salmo gairdneri). Twenty-one-day-old trout embryos were exposed to 100 ppm aqueous N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) for 30 min in a static water bath. Fish were fed a semipurified diet, and sampled monthly from the 4th to the 9th month. Two days before sampling, fish were iron-loaded with a single ip dose of 0.30 mg iron dextran/100 g body weight. Livers and kidneys were conventionally processed to paraffin sections for iron, or hematoxylin and eosin (H&E) staining. Normal hepatocytes accumulated iron in pericanalicular locations, but in hepatocytes from carcinogen-altered foci and tumors, iron staining was clearly reduced or absent. Normal renal tubule cells exhibited slight to moderate iron staining, while those from nephroblastoma were iron resistant. These results establish iron resistance as a property of preneoplastic and neoplastic trout hepatocytes and nephroblastoma cells for the first time. Iron resistance may offer a practical histochemical marker in experimental fish models of hepatocellular carcinoma and nephroblastoma.

Isolation and characterization of the rainbow trout erythrocyte band-3 protein.

Rainbow trout (Salmo gairdneri) band-3 protein was isolated from trout erythrocyte plasma membranes by a combination of preparative SDS/PAGE and electroelution. High purity and recovery of the plasma membranes were achieved by a new method. This was demonstrated using 4,4'diiso-thiocyano[3H2]dihydro-stilbene 2,2'disulfonic acid (3H2DIDS) which specifically labels band-3 protein. On SDS/PAGE, band-3 protein yields a similarly diffuse pattern, as does mammalian band-3 protein, with an apparent Mr of 116,000. In situ chymotryptic cleavage/cross-linking experiments with 3H2DIDS reveal that the fragments cross-link as in human and mouse band-3 proteins but that there are minor differences. Treatment of trout erythrocytes with trypsin results in cleavage of the band-3 protein. Purified polyclonal antibodies raised against trout band-3 protein react with trout band-3 protein and do not crossreact with mouse or human band-3 protein. They react specifically with only one chymotryptic fragment of trout band-3 protein.

Rapid Branchial Excretion of Dietary Quinoline by Rainbow Trout (Salmo gairdneri)

Excretion of the nitrogen heterocycle 14C-quinoline was characterized in rainbow trout (Salmo gairdneri) following its ingestion with food. Quinoline-derived radioactivity was readily absorbed from the stomach as shown by peak serum levels that occurred 4–8 h after feeding. Pharmacokinetics were described using a two-compartment body model with first-order absorption and disposition; estimated half-lives for the α and β phase were 4.1 and 54.1 h, respectively. About 98% of the ingested dose was absorbed by gut epithelium before fecal excretion. Depending on dose, 71–83% of the ingested radioactivity was excreted during the first 24 h after feeding. Branchial elimination was the primary route of excretion since all other routes (fecal, biliary, urinary, and dermal) contributed < 5% of the eliminated dose within 48 h after ingestion. There was evidence for saturation of minor excretory pathways as the dose was increased from 1 to 100 mg quinoline∙kg−1 body weight. Apparent spillover occurred into the branchial route at the 100 mg∙kg−1 dose. Dietary quinoline was eliminated across the gills as parent compound, independent of the dose.

Neuropeptide Y-like immunoreactivity in the cardiovascular nerve plexus of the elasmobranchs Raja erinacea and Raja radiata

The distribution of nerves showing neuropeptide Y (NPY)-like immunoreactivity in the cardiovascular system of elasmobranchs and teleosts has been investigated. Two species of teleosts, the rainbow trout (Salmo gairdneri) and the Atlantic cod (Gadus morhua), and three species of elasmobranchs, the spiny dogfish (Squalus acanthias), the little skate (Raja erinacea) and the starry ray (Raja radiata), were used in this study. An innervation of the cardiovascular system by an NPY-like substance was found only in the two species of Raja. A rich innervation was encountered in these skates, with the highest density of fibres in the wall of the ventricle, the conus arteriosus, the coeliac artery and smaller mesenterial vessels. In the vessels, the fibres formed a plexus at the adventitio-mediol border. Few fibres were found in the walls of the dorsal aorta, the sinus venosus and the atrium, and no fibres were observed in the walls of the ventral aorta. Falck-Hillarp fluorescence histochemistry showed the presence of a rich innervation of arteries and arterioles of the gut by catecholamine-containing nerve fibres.

Ultrastructural study of the response of eosinophil granule cells to Aeromonas salmonicida extracellular products and histamine liberators in rainbow trout Salmo gairdneri richardson

Intraperitoneal injections of Aeromonas salmonicida extracellular products (ECP), Compound 48 80 and Concanavalin A were found to degranulate the eosinophil granule cells (EGC) in the lower intestine and rectum of the rainbow trout Salmo gairdneri Richardson. Ultrastructurally, the EGC response resembled the anaphylactic granule extrusion of mammalian mast cells. Varying degrees of granule vacuolation and loss of electron density occurred. Labyrinthine channels were observed at the peak of degranulation. EGC response however, differed from mammalian mast cells in two respects. Firstly, degranulation involved the release of intact electron lucent granules and the subsequent disintegration of the granule matrix extracellularly. Mammalian mast cells on the other hand, release their granules by direct exocytosis. Secondly, the 48 80 and Con A-stimulated EGC degranulation was inhibited by antihistamines, promethazine and cimetidine. In mast cells, antihistamines do not prevent granule release but block histamine receptors in target cells. The degranulation of the EGCs was a non-cytotoxic event and the cells were capable of regeneration. As soon as the cells lost most of their granules, increased cytoplasmic activity was observed. This involved the expansion of the Golgi-endoplasmic reticulum complex.

Immunocytochemical Localization and Biochemical Characterization of α‐Melanocyte‐Stimulating Hormon in the Brain of the Rainbow Trout, Salmo gairdneri

Abstract The distribution of alpha-melanocyte-stimulating hormone (alpha-MSH)-like immunoreactivity in the central nervous system of the rainbow trout Salmo gairdneri was investigated by indirect immunofluorescence and peroxidase-antiperoxidase techniques, using a highly specific antiserum generated in rabbits against synthetic alpha-MSH. Immunoreactive perikarya were exclusively observed in the basal hypothalamus within the pars anterioris of the nucleus lateralis tuberis. In this region, a moderate number of small stained cell bodies were observed surrounding the dorsal wall of the anterior infundibular recess. These immunoreactive cells were organized in rostro-caudal rows extending over the whole portion of the nucleus. Positive fibres originating from these perikarya were visualized in the dorsal posterior lobe and the ventral hypothalamus. A dense tract of immunoreactive fibres projected ventrally through the pituitary stalk and terminated in the neurohypophysis. The concentrations of alpha-MSH in different regions of the brain were measured by means of a sensitive and specific radioimmunoassay. The dilution curves obtained with synthetic alpha-MSH and serial dilutions of diencephalon, mesencephalon, medulla oblongata, telencephalon or pituitary extracts were strictly parallel. The highest concentration of alpha-MSH in brain was found in the diencephalon (1.31 +/- 0.07 ng/mg protein). In contrast alpha-MSH was not detectable in cerebellar extracts. Reverse-phase high-performance liquid chromatography and radioimmunoassay were used to characterize alpha-MSH-like peptides in the trout brain and pituitary. Two major forms of immunoreactive alpha-MSH were resolved by high performance liquid chromatography in hypothalamic extracts; these peptides exhibited the same retention times as des-Na-acetyl alpha-MSH and its sulfoxide derivative, respectively. Additional peaks of alpha-MSH immunoreactive material were detected in pituitary extract. These latter peptides coeluted with authentic alpha-MSH, diacetyl alpha-MSH and their sulfoxide forms. These results provide the first evidence for the presence of alpha-MSH in the brain of a teleostean fish. Our data indicate that, in the brain, the immunoreactivity corresponds to the non-acetylated form of alpha-MSH, while three different types of alpha-MSH-like molecules (namely deacetylated, monoacetylated, and diacetylated forms) coexist in the pituitary. It thus appears that, in salmonoid fish, mono- or diacetylation of the N-terminal serine residue of aL-MSH only occurs at the pituitary level.

Assessing the status of British wild brown trout, Salmo trutta, stocks: a pilot study utilizing data from game fisheries

SUMMARY. 1. This paper describes an analysis of results from a questionnaire (Giles, 1987) which requested information on the status of game fish stocks (especially wild brown trout Salmo trutta L.) from those Game Conservancy members who own or manage fisheries. Also included are data arising from fishery log book records which are used to indicate trends in brown trout catch per unit effort (CPUE) over past decades.2. The key finding from questionnaire returns was the widespread reporting of apparent declines of wild brown trout stocks in a total of twenty‐seven sites throughout Britain. The affected waters are geographically widespread from the south‐west of England through southern and south‐east England, south Wales, northern England and Scotland including the Western Isles. Suspected stock declines of migratory (sea) trout, salmon Salmo salar L. and grayling Thymallus thymallus L. were also reported. Where brown trout occur alone, a significantly higher proportion of questionnaire returns recorded a stock decline compared with fisheries containing both brown and rainbow trout Salmo gairdneri Richardson.3. Trout catch records from a small selection of differing fishery types are presented and the value of such data in fish stock assessment is discussed.

メダカおよびニジマス卵膜の陽イオン交換体としての性質

メダカおよびニジマス卵膜の陽イオン交換体としての性質

Decrease in the bactericidal activity of normal serum during the spawning period of rainbow trout.

The effects of seasonal changes and sexual maturation on the serum bactericidal activity in rainbow trout Salmo gairdneri were investigated.Two fish populations separated in spawning season were studied. One population reared under natural environmental conditions had spawning period between September and November, while the other reared under controlled day length had spawning period between April and June.The sera collected from trout were pooled together and mixed with an Escherichia coli suspension. This mixture was incubated at 25°C for 6h. After incubation the number of viable bacteria were counted by the plate counting method and the survival rate was calculated.Among the heated (45°C for 30 min) sera, the bacteria multiplied about 10 times during the 6h incubation. In contrast, the bacterial count in the normal sera decreased in number to about one tenth of the initial count after 6h. However, the normal sera sampled during spawning period showed an increase of bacteria. This change of the serum bactericidal activity level was observed in both fish populations. In addition, the bactericidal activity level of the sera from fish before being stripped was lower than that from fish after being stripped. From these results, it is probable that the decrease of the serum bactericidal activity during the spawning period increase the susceptibility to disease in fish.

Comparison of aggregation behaviors in human and fis blood.

Aggregation behaviors of human platelets and carp Cyprinus carpio, rainbow trout Salmo gairdneri, and red seabream Pagrus major thrombocytes were studied by an electronic aggregometer and microscopic observation.Measurement was done by using electronic aggregometer. The irreversible aggregation responses were recognized in all rainbow trout and red seabream blood similar to human blood with the addition of sodium arachidonate (AA). However, the response of thrombocytes in carp blood was not induced by the addition of AA, but the reversible aggregation response was recognized in four out of twelve samples. The aggregation was not induced by the addition of sodium eicosapentaenoate (EPA) or docosahexaenoate (DHA) in fish blood like human blood. It is assumed that the differences in prostaglandin (PG) syntheses among them cause the different behaviors in the blood aggregation.Moreover, in the microscopic observation, human platelets and all fish thrombocytes aggregated in the presence of ionophore A23187, thrombin, and collagen, but did not aggregate in the presence of EPA, DHA, PGF2α, PGD2, or ADP. On the other hand, in the presence of AA, PGE2 and platelet activating factor (PAF), the aggregating responses varied among species tested.

Effects of immunosuppressive agents on common carp infected with the haemoflagellate Trypanoplasma borreli

Trypanoplasma borreli, a haemoflagellate, occurs in the blood of European cyprinids (Lom 1979). It has been reportet to cause death in naturally infected carp (Hofer 1904), and in experimentally infected goldfish (Lom 1979) and carp (Lom et al. 1986). The importance of environmental stressors in disease outbreaks in fish has been pointed out by several authors (e.g. Snieszko 1974). Environmental stressors are accompanied by rapid changes in the concentration of several plasma chemicals especially corticosteroids (Mazeaud et al. 1977, Pickering & Duston 1983). A recent study on rainbow trout Salmo gairdneri with cortisol implants revealed an increased susceptibility of these fish to experimental infection with the haemoflagellate Cryptobia (syn. Trypanoplasma) salmositica (Woo et al. 1987). An increased susceptibility of goldfish Carassius auratus to infections with bacteria and parasites was also reported after X-ray irradiation (Shechmeister et al. 1962, Purdom & Woodhead 1973). In this communication, data on Trypanoplasma borreli infections of carp treated with hydrocortisole and Xrays are reported. Materials and methods. One-year-old 25 g carp were obtained from laboratory raised stock, kept in 80 1 tanks at 20 C and fed daily on fish food pellets (F-45, Rheinkrone, Wesel, FRG). Trypanoplasma borreLi was isolated from naturally-infected carp and maintained in the laboratory by fish-to-fish passage. A clone, established from this isolate, was also cultivated (by syringepassage) in laboratory-raised carp using the method

The function of the cardiac branch of the vagus and the location of its preganglionic neurons in the rainbow trout Salmo gairdneri.

The function of cholinergic vagal innervation and the locality of its preganglionic neurons were investigated using in situ preparations of the rainbow trout heart.The cardiac-visceral branches of the vagus were identified by vagal stimulation. The stimulation induced intense bradycardia with frequencies between 10 and 100Hz, while tachycardia was never produced even after atropinization, indicating that only the cholinergic vagus nerves were stimulated in the study. Subsequent beta-adrenoceptor blockade by propranolol intensified the stimulation-induced cardiac inhibition. It is suggested that the serum catecholamine levels in the preparations were kept high by secretion from chromaffin tissues and they produced a chronotropic acceleratory effect to the heart during the experiment.A group of preganglionic neurons of the cardiac-visceral trunk of the vagus were identified in the vagal motor nucleus of the medulla by retrograde labeling with HRP. They formed a longitudinal column within the nucleus at the level around the obex.

The presence of 17α,20β-dihydroxy-4-pregnen-3-one receptor activity in the ovary of the brook trout,Salvelinus fontinalis, during terminal stages of oocyte maturation

The presence of 17α,20β-dihydroxy-4-pregnen-3-one (17α,20β-DHP) oocyte receptor activity has been demonstrated in brook troutSalvelinus fontinalis. Scatchard analyses of the cytosol fraction during various terminal stages of oocyte maturation gave a high equilibrium association constant (Ka) value of 1.394±0.669 10(8)M(-1) (n=7) and low maximum binding capacities (Nmax). The association kinetics of the receptor was second order k+1=2.292×10(6)M(-1) sec(-1). The dissociation rate constant ka was 1.502×10(-2) sec(-1) for the first order dissociation reaction. The Ka=1.526×10(8)M(-1), when it was determined from k+1/k-1 a value close to that found from the Scatchard analysis. Competition studies showed the following binding affinities testosterone > 17α-HP > 17α,20β-DHP > Promegestone > progesterone > estradiol > pregnenolone; cortisol showed no competitive inhibition. Cytosolic extracts when pre-equilibrated with various labelled steroids and eluted from a Sephacryl S-300 column gave multiple specific binding peaks. On sucrose density gradient centrifugation specific binding was observed at 3.05 S in cytosol containing 0.15M sodium chloride buffer. The receptor lost binding activity when incubated with various proteases, but DNase and RNase had no effect. Blood plasma without heparin at (1∶10) dilution also bound [(3)H]17α,20β-DHP, Ka was 8.04×10(7) M(-1).The nuclear pellet extract (750×g) gave very little specific binding activity even at high radiolabelled steroid concentrations and a linear Scatchard plot was not obtained. Nevertheless the nuclear extract, after dextran-charcoal treatment, pre-equilibrated with [(3)H]17α,20β-DHP, bound specifically to DNA cellulose, and cytosol from the same oocytes also bound to DNA cellulose under similar conditions. Although specific binding to DNA cellulose was obtained the salt concentrations at which the steroid-receptor complex elution took place was not reproducible in both nuclear extracts and cytosol samples. Also binding activity was extremely small compared to the total cytosolic binding. The nuclear extract when pre-equilibrated with high concentrations (20 nM) of the labelled steroid and then chromatographed on Sephacryl S-300 column gave a specific binding peak which was similar to that of the cytosolic preparation.The receptor levels in cytosol decreased progressively during final maturation (Stages 1-7). There is preliminary evidence for the presence of 17α,20β-DHP receptor activity in cytosol of landlocked Atlantic salmonSalmo salar ouananiche, and rainbow troutSalmo gairdneri. The zona radiata fraction from late stages oocyes 5, 6, and 7 in brook and rainbow trout oocytes were isolated by ultracentrifugation; from this fraction a protein was characterized which covalently bound [(3)H]R5020 after photoaffinity labelling. The same protein also bound [(3)H]17α,20β-DHP after solubilization in Brig 35 buffer. The SDS gel electrophoresis subunit composition of the above protein was similar to the cytosol counterpart binding [(3)H]17α,20β-DHP, although the molecular weights were different. The blood sample [(3)H]R5020 binding component subunit composition was different from that of the membrane extracted protein. These results demonstrate the presence of 17α,20β-DHP receptor activity in the cytosol and zona radiata membranes of the oocytes during final maturation.

Effect of Carbon Monoxide Exposure on Gas Bubble Disease in Rainbow Trout (Salmo gairdneri)

Cumulative mortalities of 100, 50, 20, and 0% due to gas bubble disease (GBD) occurred on exposure of rainbow trout (Salmo gairdneri) for 180 min to supersaturated water at 136, 130, 124, and 116% total dissolved gas saturation (TDGS), respectively. At 130% TDGS, a prior exposure to carbon monoxide (CO), which converted 80% of the hemoglobin to carboxyhemoglobin (COHb), significantly prolonged survival time, but cumulative mortality was insignificantly reduced in the same group. Histologically, all supersaturation mortalities had branchial lesions for GBD (characterized by gas displacement of blood from the afferent arterioles of the gill filaments), while 70% had gas emboli in the retinal choroid gland. At a TDGS of 100%, these histological lesions were not observed and the CO 80% COHb conversion did not induce mortality within 120 min. Breathing movements of both control and CO-exposed fish slowed greatly when a TDGS of 130% was imposed; however, the COHb conversion initiated tachypnea and the CO-treated fish maintained a relatively higher respiratory rate when exposed to supersaturation conditions for a period which was proportional to their prolonged survival time. We concluded that the initial formation of gas emboli preceding fatal GBD was delayed as a result of a CO-inhibited function of hemoglobin.

Development of habitat suitability criteria for trout in small streams

AbstractWe observed 2863 trout in the wild to determine habitat utilization in small streams of the Kings River basin in California's Sierra Nevada mountains. The habitat utilization data were used to develop habitat suitability functions that provide input variables to the instream flow incremental methodology (IFIM) habitat assessment model of the U.S. Fish and Wildlife Service.Observations of habitat utilization of rainbow trout (Salmo gairdneri), brown trout (Salmo trutta), and brook trout (Salvelinus fontinalis) were obtained during the summer months of 1983 and 1984. The observations were made in small streams with discharges ranging from 0.7 m3 s−1 to 0.03 m3 s−1. The streams are at elevations of 1250 to 2530 m. Equal effort was applied to observing undisturbed trout in all habitat types. Snorkeling proved to be the most effective method of observation.Individual trout of all species and life stages were most often observed in the lower half of the water column, utilizing low‐velocity currents of less than 3.0 cm s−1.From the depth and velocity utilization data, several forms of habitat suitability functions were developed and evaluated: Univariate depth and velocity functions derived from frequency histogram analysis. Univariate depth and univariate velocity exponential polynomial models. Bivariate depth and velocity exponential polynomial models. Univariate exponential polynomial models provided the best fit to the data for each species, based on visual comparisons of response surfaces and contour plots, and comparisons of computed sums of squared errors. Bivariate models fitted to the data resulted in greater sums of squared errors than multiplicative aggragation of univariate models, and frequently predicted utilization at zero depth.The habitat suitability functions derived from the univariate exponential polynomial models provided the best input to the IFIM habitat assessment models.