Spawning Rainbow Trout Research Articles

Spawning interactions between hatchery‐reared and wild naturalized rainbow trout (Oncorhynchus mykiss, Walbaum, 1792) in high‐altitude tropical streams, Kenya

AbstractRainbow trout (Oncorhynchus mykiss) is among the most widely translocated fish species in the world. The current study evaluated the spawning interactions between naturalized and wild rainbow trout from two high‐altitude second‐order streams, in Kenya. Data on total length, weight, condition factor, fecundity, fertilization, egg diameter and fry survival were collected on spawning rainbow trout between March and December 2021. Length–weight relationship showed parabolic equations as W = 0.0144L2.900, W = 0.0069L3.0285 and W = 0.00027L3.175 for wild fish stock, hatchery‐reared and wild × hatchery‐reared rainbow trout, respectively. Total fecundity differed significantly among the hatchery‐reared, wild fish and the cross of the two (p < 0.05). The fertilization rate showed significant differences (p < 0.05), with no discernable difference observed between the hatchery‐reared and crossed (wild × hatchery‐reared). There was a positive correlation among the total fecundity to female egg weight, female body weight, fertilization rate and eyed egg survival in all the populations. Relative fecundity was significantly different among the three groups of fish (p < 0.05), but the differences between the hatchery and the crossed (wild × hatchery‐reared) fish showed no significant differences. We recommend the use of crossed (wild × hatchery‐reared) populations for fry production for use in aquaculture as they presented the highest fecundity and gave the best outcome of fry with high survival.

Simulating Spawning and Juvenile Rainbow Trout (Oncorhynchus mykiss) Habitat in Colorado River Based on High-Flow Effects

High flow generates significant alterations in downstream river reaches, resulting in physical condition changes in the downstream regions of the river such as water depth, flow velocity, water temperature and river bed. These alterations will lead to change in fish habitat configuration in the river. This paper proposes a model system to evaluate the high flow effects on river velocity, water depth, substrates changes, temperature distribution and consequently assess the change in spawning and juvenile rainbow trout (Oncorhynchus mykiss) habitats in the downstream region of the Glen Canyon Dam. Firstly, based on the 2 dimensional (2D) depth-averaged CFD (Computational Fluid Dynamics) model and heat transfer equation applied for simulation, three indices were simulated, namely depth, flow velocity and temperature distribution. Then, the spawning and juvenile fish preference curves were obtained based on these three indices and substrates distribution. After that, the habitat model was proposed and used to simulate the high flow effects on juvenile and spawning rainbow trout habitat structure. Finally, the weighted usable area (WUA) and overall suitability index (OSI) of the spawning and juvenile fish species were quantitatively simulated to estimate the habitat sensitivity. The results illustrate that the high flow effect (HFE) increased the juvenile rainbow trout habitat quality but decreased the spawning rainbow trout habitat quality. The juvenile trout were mainly affected by the water depth while the spawning rainbow trout were dominated by the bed elevation.

The fishing and natural mortality of large, piscivorous Bull Trout and Rainbow Trout in Kootenay Lake, British Columbia (2008-2013).

BackgroundEstimates of fishing and natural mortality are important for understanding, and ultimately managing, commercial and recreational fisheries. High reward tags with fixed station acoustic telemetry provides a promising approach to monitoring mortality rates in large lake recreational fisheries. Kootenay Lake is a large lake which supports an important recreational fishery for large Bull Trout and Rainbow Trout.MethodsBetween 2008 and 2013, 88 large (≥500 mm) Bull Trout and 149 large (≥500 mm) Rainbow Trout were marked with an acoustic transmitter and/or high reward ($100) anchor tags in Kootenay Lake. The subsequent detections and angler recaptures were analysed using a Bayesian individual state-space Cormack–Jolly–Seber (CJS) survival model with indicator variable selection.ResultsThe final CJS survival model estimated that the annual interval probability of being recaptured by an angler was 0.17 (95% CRI [0.11–0.23]) for Bull Trout and 0.14 (95% CRI [0.09–0.19]) for Rainbow Trout. The annual interval survival probability for Bull Trout was estimated to have declined from 0.91 (95% CRI [0.76–0.97]) in 2009 to just 0.46 (95% CRI [0.24–0.76]) in 2013. Rainbow Trout survival was most strongly affected by spawning. The annual interval survival probability was 0.77 (95% CRI [0.68–0.85]) for a non-spawning Rainbow Trout compared to 0.41 (95% CRI [0.30–0.53]) for a spawner. The probability of spawning increased with the fork length for both species and decreased over the course of the study for Rainbow Trout.DiscussionFishing mortality was relatively low and constant while natural mortality was relatively high and variable. The results indicate that angler effort is not the primary driver of short-term population fluctations in the Rainbow Trout abundance. Variation in the probability of Rainbow Trout spawning suggests that the spring escapement at the outflow of Trout Lake may be a less reliable index of abundance than previously assumed. Multi-species stock assessment models need to account for the fact that large Bull Trout are more abundant than large Rainbow Trout in Kootenay Lake.

Life history variation within and among naturalized rainbow trout populations of the Laurentian Great Lakes

Abstract We examined naturalized rainbow trout spawning stocks from across the Great Lakes to determine: i) how reproductive characteristics among adults within populations vary with sex, ontogeny and energetic status, and ii) if variations in growth and reproductive characteristics among populations reflect differences in the environments they inhabit. Females in spawning runs tended to be larger and older than males, but growth rates were similar between sexes. As predicted, females had larger and more lipid-rich gonads and livers, and lower somatic condition and muscle lipid concentrations than males. Within populations, gonad size, fecundity, and egg size were related primarily to body size, and secondarily to indices of adult condition. Gonadosomatic index (GSI) increased with body size in females and declined with body size in males. Egg size increased with female size and declined with liver size and/or lipid content, whereas relative fecundity showed opposite trends. In contrast, egg lipid content appeared to be more strongly related to liver size and lipid content (positive effects) than to female size. Among populations, growth rate and muscle lipid concentration declined from south to north, but contrary to predictions, none of the reproductive traits examined showed strong covariation with environmental gradients. Growth, energetic status, and reproductive investment of Great Lakes naturalized rainbow trout vary both within and among populations, but localized adaptation to environmental conditions appears to be subtle.

Effects of Increased Discharge on Spawning and Age‐0 Recruitment of Rainbow Trout in the Colorado River at Lees Ferry, Arizona

AbstractNegative interactions of Rainbow Trout Oncorhynchus mykiss with endangered Humpback Chub Gila cypha pose challenges to the operation of Glen Canyon Dam (GCD) to manage for both species in the Colorado River. Operations to enhance the Rainbow Trout tailwater fishery may lead to an increase in downstream movement of the trout to areas where they are likely to interact with Humpback Chub. We evaluated the effects of dam operations on age‐0 Rainbow Trout in the tailwater fishery to inform managers about how GCD operations could benefit a tailwater fishery for Rainbow Trout; although this could affect a Humpback Chub population farther downstream. A near year‐long increase in discharge at GCD in 2011 enabled us to evaluate whether high and stable flows led to increased spawning and production of age‐0 Rainbow Trout compared with other years. Rainbow Trout spawning was monitored by fitting a model to observed redd counts to estimate the number of redds created over a spawning season. Data collected during electrofishing trips in July–September and November were used to acquire age‐0 trout population and mortality rate estimates. We found that high and stable flows in 2011 resulted in 3,062 redds (1.7 times the mean of all survey years) and a population estimate of 686,000 age‐0 Rainbow Trout (second highest on record). Despite high initial abundance, mortality remained low through the year (0.0043%/d) resulting in significant recruitment with a record high November population estimate of 214,000 age‐0 Rainbow Trout. Recent monitoring indicates this recruitment event was followed by an increase in downstream migration, which may lead to increased interactions with downstream populations of Humpback Chub. Consequently, while our results indicate that manipulating flow at GCD can be used to manage Rainbow Trout spawning and recruitment, fisheries managers should use flow manipulation in moderation to minimize downstream migration in order to reduce negative interactions with other species in the Colorado River.Received September 8, 2014; accepted March 28, 2015

Three high flow experiment releases from Glen Canyon Dam on rainbow trout and flannelmouth sucker habitat in Colorado River

In recent years, dam management guidelines need to estimate the effects of downstream flow on environment. However, high-flow alters physical conditions such as water depth, flow velocity and water temperature in the downstream side of the river. These changes will lead to alteration in fish habitat of the river. In this paper, an eco-hydraulic model was used to determine the levels of fish habitat in the Colorado River downstream from the Glen Canyon Dam. This model has been proposed by combining hydrodynamic and heat transfer model with a habitat suitability index model based on fish preference curves. Rainbow trout (Oncorhynchus mykiss) and flannelmouth sucker (Catostomus latipinnis) were chosen as target species because they represent both native and non-native fish in the study area. Flow velocity, water depth and water temperature were selected as the suitability indicators. The hydrological data from three high-flow experiments were analyzed to determine the effects of high-flow on the Colorado River ecosystem. Numerical model simulations were undertaken as follows: firstly, based on the hydrodynamic and heat transfer equation, three hydraulic factors including water depth, velocity and temperature distribution were simulated and the associated suitability for each was obtained based on the fish preference curves. Later, the habitat suitability equation was developed to simulate the target species’ habitat situation in high flow effects. Finally, the WUA (weighted usable area) and OSI (overall suitability index) of the spawning fish species were quantitatively calculated to evaluate the sensitivity of the high flow. The results show that the effects of three HFE are more severe in spawning flannelmouth sucker than in spawning rainbow trout. It is worth noting that the one day HFE was beneficial to spawning rainbow trout and harmful to spawning flannelmouth sucker but in the HFE of more than 8 days, both the spawning habitats were completely destroyed.

Molecular Markers for Variation in Spawning Date in a Hatchery Population of Rainbow Trout (Oncorhynchus mykiss)

We examined the distribution of alleles at 63 microsatellite loci distributed across 29 linkage groups in broodstock females from a commercial population of rainbow trout spawning on different dates throughout the season (August to January). A total of 368 females, 184 and 117 females from each of the tail-ends of the spawning distribution and a subsample of 67 females spawning in the middle, were used to detect marker-trait associations. Twenty-one loci in a subset of genomic regions (RT-5, 7, 8, 10, 12, 14, 15, 22, 23, 24, 25, 29, 30, and 31) were significantly associated with variation in spawning date. Many of these markers localize to regions with known spawning date quantitative trait loci based on previous studies. An individual assignment analysis was used to test how well the molecular data could be used to assign individuals to their correct spawning group, and markers were given a ranking reflecting their contribution to the accuracy of assignment. The top 15 ranked markers were successful at assigning the majority of females to the correct spawning group based on genotype with an average accuracy of 76%. The most likely genes that could contribute to these differences in spawning date are discussed. Together, these data indicate that the loci could be incorporated into a selection index with phenotype data to increase the accuracy of selection for spawning date.

IMPORTANCE OF TRIBUTARY STREAMS FOR RAINBOW TROUT REPRODUCTION: INSIGHTS FROM A SMALL STREAM IN GEORGIA AND A BI‐GENOMIC APPROACH

ABSTRACTTributaries of tailwater fisheries in the southeastern USA have been used for spawning by stocked rainbow trout (Oncorhynchus mykiss), but their importance may have been underestimated using traditional fish survey methods such as electrofishing and redd counts. We used a bi‐genomic approach, mitochondrial DNA sequences and nuclear microsatellite loci, to estimate the number of spawning adults in one small tributary (Cabin Creek) of the Chattahoochee River, Georgia, where rainbow trout are known to spawn and have successful recruitment. We extracted and analysed DNA from seven mature male rainbow trout and four juveniles that were captured in February 2006 in Cabin Creek and from 24 young‐of‐year (YOY) trout that were captured in April 2006. From these samples, we estimated that 24 individuals were spawning to produce the amount of genetic variation observed in the juveniles and YOY, although none of the mature males we sampled were indicated as sires. Analysis of the mitochondrial D‐loop region identified four distinct haplotypes, suggesting that individuals representing four maternal lineages contributed to the offspring. Our analyses indicated that many more adults were spawning in this system than previously estimated with direct count methods and provided insight into rainbow trout spawning behavior. Copyright © 2011 John Wiley & Sons, Ltd.

Correlation of Fluvial Rainbow Trout Spawning Life History with Severity of Infection by Myxobolus cerebralis in the Blackfoot River Basin, Montana

AbstractTo assess the exposure of Blackfoot River rainbow trout Oncorhynchus mykiss to the exotic parasite Myxobolus cerebralis, the causal agent of whirling disease, we investigated the spawning life histories of adult rainbow trout with respect to the distribution and severity of infection in spawning and early rearing areas in two distinct reaches of the Blackfoot River. Radiotelemetry confirmed that Blackfoot River rainbow trout express a fluvial life history and migrate from wintering sites within the Blackfoot River to spawning sites in the lower reaches of tributaries. Spawning peaked in late April, and fry emergence was estimated to occur within a narrow window of time in early July during the known period of high parasite exposure. However, the severity of infection varied between study reaches. Spawning of lower Blackfoot River rainbow trout was dispersed among the lowermost reaches of smaller, colder, higher‐gradient tributaries, most of which fell below our ability to detect infection. By contrast, a majority of the telemetered rainbow trout in the middle Blackfoot River spawned higher in the drainage and within a single, low‐gradient stream where fry emerged under infectious conditions. For fluvial rainbow trout, the risk of infection varies from the tributary to the subbasin scale and relates to the geographical arrangement and properties of the tributaries, the longitudinal relationship of the pathogen to spawning and early rearing areas, and the rate and timing of dispersing age‐0 fry to downstream parasite‐positive waters. Before the introduction of M. cerebralis, the middle Blackfoot River was identified as having recruitment limitations caused by winter mortality and anthropogenic activities. According to our results, riparian restoration and habitat enhancement with emphasis on migratory native fish within and upstream of the pathogen may buffer fish from the effects of the disease.

Molecular characterization of muscle atrophy and proteolysis associated with spawning in rainbow trout

Severe muscle deterioration is a physiological response to the energetic demands of fish spawning. This response represents a suitable model to study mechanisms of muscle degradation in fish where typical tetrapod methods, such as muscle unloading, are not applicable. Enzyme activities and mRNA accumulations of genes in major proteolytic pathways, including cathepsins, calpains and the multi-catalytic proteasome, were measured in white muscles of rainbow trout during spawning and post-spawning seasons of gravid fish for comparisons to sterile fish. Fertile fish at spawning had less muscle tissue and less muscle protein compared to sterile fish and post-spawning fertile fish. Muscle deterioration of the fertile fish during spawning was associated with greater mRNA accumulation and elevated activity of cathepsin-L. Concurrently, muscle of spawning fish showed increased mRNA accumulations of cathepsin-D, the calpain regulatory subunit and the proteasome catalytic subunit alpha without corresponding increases in enzyme activities. In addition, elevated activity and increased mRNA accumulation of caspase-9, but not caspase-3, were observed in fertile fish during spawning. This study indicates that cathepsins mediate protein catabolism during spawning in rainbow trout and the catabolic process may involve activation of the apoptosis mediator, caspase-9, but not the apoptosis executioner, caspase-3.

Migratory behaviour of spawning rainbow trout (Oncorhynchus mykiss) in the Tongariro River, New Zealand, after habitat alteration

The movements of 92 adult rainbow trout (Oncorhynchus mykiss) from Lake Taupo into the Tongariro River, New Zealand, were monitored using radio‐telemetry every 3 days during the main spawning period between May and November 2003. This study repeated one previously conducted in 1995. Differences in spawning site preference and resting locations between 1995 and 2003 probably reflect differences in the nature of the river as a result of major natural floods that occurred in 1998. Average migration times were slower than in 1995 and the correlation between flow and mean daily movement was less distinct owing to the dry and settled weather during July and August, although fish did respond to freshets when they occurred. Peak movement occurred between 1600 and 2000 h NZ Standard Time with no movement occurring between midnight and 0400. Fish adjusted their movement in response to changing photoperiod. Nineteen fish were observed above the mouths of natal tributary streams for several weeks in the main river stem before entering these tributaries.

Spawning Habitat Selection by Rainbow Trout in the Pere Marquette River, Michigan

We evaluated habitat features (i.e., substrate particle size, water depth, water velocity) at spawning redds and randomly selected reference locations (where spawning activity was not apparent) to determine importance of these features to rainbow trout (Oncorhynchus mykiss) spawning habitat use in the Pere Marquette River, Michigan, 1997 to 1999. Rainbow trout selected areas with small gravel, large gravel, and small cobble substrates for redd construction over areas with clay, silt, sand, and large cobble. Rainbow trout redds were located in areas where the stream velocity was significantly higher (F = 97.77, df = 630, p < 0.0001) than velocities that were recorded at reference sites, and redds were located in water that was significantly shallower than what was typically found in the study reaches (F = 113.84, df = 629, p < 0.0001). Water temperature did not affect selection of redd locations. Because rainbow trout spawn during spring in Michigan when water temperatures are cool, within-reach variation in water temperature may have minimal influence on choice of locations for redd construction. These microhabitat data provide an indication of rainbow trout spawning habitat utilization, and can be used to identify critical habitats that may serve as rainbow trout spawning sites, and provide guidance on targets for habitat improvement projects in the Pere Marquette River and elsewhere.

Relation of Spawning and Rearing Life History of Rainbow Trout and Susceptibility toMyxobolus cerebralisInfection in the Madison River, Montana

Abstract The Madison River, Montana, is the site of a whirling disease epizootic among rainbow trout Oncorhynchus mykiss caused by the parasite Myxobolus cerebralis. We investigated how the timing and location of the spawning and rearing of rainbow trout influences the risk of exposure to the parasite. Sentinel fish exposures were used to assess the spatial and temporal variation in disease severity in 1998 and 1999. Redd counts, radiotelemetry of spawning rainbow trout, and electrofishing estimates of fry abundance indicated that during the critical 2–3-month posthatch period, when rainbow trout are most susceptible to whirling disease, the majority of fry in the Madison River population are concentrated within a relatively small portion of the upper drainage. Disease severity ranged from low to very high in this area of the river. Large temporal variation in disease severity was also evident: severity was significantly higher in the spring than in the fall and in 1999 than in 1998. These data indicate t...

Population structure of sympatric anadromous and nonanadromousOncorhynchus mykiss: evidence from spawning surveys and otolith microchemistry

Reproductive isolation between steelhead and resident rainbow trout (Oncorhynchus mykiss) was examined in the Deschutes River, Oregon, through surveys of spawning timing and location. Otolith microchemistry was used to determine the occurrence of steelhead and resident rainbow trout progeny in the adult populations of steelhead and resident rainbow trout in the Deschutes River and in the Babine River, British Columbia. In the 3 years studied, steelhead spawning occurred from mid March through May and resident rainbow trout spawning occurred from mid March through August. The timing of 50% spawning was 9-10 weeks earlier for steelhead than for resident rainbow trout. Spawning sites selected by steelhead were in deeper water and had larger substrate than those selected by resident rainbow trout. Maternal origin was identified by comparing Sr/Ca ratios in the primordia and freshwater growth regions of the otolith with a wavelength-dispersive electron microprobe. In the Deschutes River, only steelhead of steelhead maternal origin and resident rainbow trout of resident rainbow trout origin were observed. In the Babine River, steelhead of resident rainbow trout origin and resident rainbow trout of steelhead maternal origin were also observed. Based on these findings, we suggest that steelhead and resident rainbow trout in the Deschutes River may constitute reproductively isolated populations.

Competitive exclusion of brown trout Salmo trutta L., by rainbow trout Oncorhynchus mykiss Walbaum, in lake tributaries, New Zealand

Spawning runs of brown trout Salmo trutta L., and rainbow trout, Oncorhynchus mykiss Walbaum, were compared in three tributaries of two lakes for timing, numbers and redd characteristics. Redd characteristics for the two species in four other streams were examined. No significant differences in redd characteristics between the two species were observed, but in all lake tributaries the possibilities of superimposition of spring spawning rainbow trout on redds of brown trout were examined. A common feature was the high ratio of numbers of spawners to the area of preferred spawning gravel, and it was concluded that in the lake tributaries, superimposition resulted in increased mortality of brown trout eggs, and in one case elimination of the brown trout population. Management possibilities for control of the species proportions in comparable situations are discussed.

Growth and vitality in yellow forms of rainbow trout

Summary The colour genes controlling the palomino and albino phenotype in the spring spawning rainbow trout [Oncorhynchus mykiss (Walbaum 1792)] strain reared at the Department of Salmonid Research Rutki, Poland were found to have strong detrimental pleiotropic effects on growth and vitality. After 5 and 9 months of rearing, the differences in mean body weights in albino, palomino and wild-coloured trout were statistically highly significant, with albino showing the poorest, palomino as the intermediate and wild-coloured trout having the highest growth. An identical ranking of colour phenotypes was observed for survival. The influence of colour phenotypes on both traits showed a diminishing tendency with increasing age of fish. It is concluded that alleles controlling palomino and albino colouration cannot be recommended as genetic markers. If reference groups are needed, instead of albino trout, palomino trout should be used since their pleiotropic effects are less pronounced.

Population structure of sympatric anadromous and nonanadromous &lt;i&gt;Oncorhynchus mykiss&lt;/i&gt;: evidence from spawning surveys and otolith microchemistry

Reproductive isolation between steelhead and resident rainbow trout (Oncorhynchus mykiss) was examined in the Deschutes River, Oregon, through surveys of spawning timing and location. Otolith microchemistry was used to de- termine the occurrence of steelhead and resident rainbow trout progeny in the adult populations of steelhead and resi - dent rainbow trout in the Deschutes River and in the Babine River, British Columbia. In the 3 years studied, steelhead spawning occurred from mid March through May and resident rainbow trout spawning occurred from mid March through August. The timing of 50% spawning was 9-10 weeks earlier for steelhead than for resident rainbow trout. Spawning sites selected by steelhead were in deeper water and had larger substrate than those selected by resident rain - bow trout. Maternal origin was identified by comparing Sr/Ca ratios in the primordia and freshwater growth regions of the otolith with a wavelength-dispersive electron microprobe. In the Deschutes River, only steelhead of steelhead mater - nal origin and resident rainbow trout of resident rainbow trout origin were observed. In the Babine River, steelhead of resident rainbow trout origin and resident rainbow trout of steelhead maternal origin were also observed. Based on these findings, we suggest that steelhead and resident rainbow trout in the Deschutes River may constitute reproduc- tively isolated populations.

Spawning of anadromous rainbow trout, Oncorhynchus mykiss (Walbaum): a threat to sea trout, Salmo trutta L., populations?

Interference competition for territorial space and food resources between progeny of anadromous brown trout, i.e., sea trout, Salmo trutta L., and non-indigenous anadromous rainbow trout, Oncorhynchus mykiss, was studied in stream channels under laboratory conditions. In addition, the two anadromous species were compared by measurements of fecundity, egg size, alevin size, growth rate, and timing of spawning migration. Competition was mediated through migration of sea trout parr out of the stream channels and the results suggest a negative influence on the sea trout in terms of displacement further downstream in the channels and lowered specific growth rate when fish of equal size were tested together. Given the size advantages normally found, caused by differences in spawning period, sea trout parr held their positions in the stream channels while the smaller rainbow trout parr moved out of the system or stayed in areas where sea trout were absent. This study was initiated because an increasing number of ascending rainbow trout was reported in many streams on Gotland. Spawning rainbow trout have been observed to have destroyed sea trout spawning redds. Despite rainbow trout spawning in many streams, only one descendant from these anadromous rainbow trout has been found to date.

Heterogeneity in the Distribution of Mitochondrial DNA Haplotypes in Female Rainbow Trout Spawning in Different Seasons

Analysis of females spawning throughout the year (October to late February) at a commercial fish farm in Ontario, revealed significant heterogeneity in the distribution of their mitochondrial DNA (mtDNA) haplotypes. These findings are consistent with previous data from this stock and indicate that female– daughter spawning times are temporally stable. This suggests a high maternal genetic contribution to this trait. Conversely, no temporal heterogeneity in the spawning times of females with different mtDNA haplotypes was observed in an unselected government (Ontario Ministry of Natural Resources) hatchery stock recently derived from a wild naturalized population. The government hatchery stock is normally spring spawning and has much higher levels of nucleon diversity compared with the cultured stock. Therefore, it appears that only a limited number of family lines within unselected stocks may show the propensity towards greatly accelerated spawning times. Furthermore, associations between spawning time and mtDNA haplotype may only become apparent following several generations of artificial selection.

The chinook salmon gonadotropin II beta subunit gene contains a strong minimal promoter with a proximal negative element.

The salmon pituitary expresses two distinct gonadotropins, gonadotropin I (GTHI) and gonadotropin II (GTHII). These two hormones are synthesized in distinct pituitary cells and secreted at different stages during the reproductive cycle. To study the transcriptional regulation of the hormone-specific beta-subunit of GTHII (sGTHII beta) gene, approximately 3.5 kilobases of the 5'-flanking region was characterized and sequenced. The pituitary specificity of sGTHII beta was examined by analyzing sGTHII beta promoter activity in homologous primary pituitary cells derived from spawning rainbow trout and in a collection of heterologous cell lines. Various lengths of the 5'-flanking region of the sGTHII beta gene were ligated into a vector encoding the bacterial chloramphenicol acetyltransferase (CAT) gene, and the resulting sGTHII beta/CAT chimeric constructs were analyzed using transient expression systems. Several constructs (-3500CAT, -1260CAT, -563CAT, and -39CAT) displayed readily detectable CAT activity in the pituitary cells derived from spawning male and female rainbow trout. In contrast, three of the constructs (-3500CAT, -1260CAT, and -563CAT) were expressed only at background levels in a variety of heterologous cell lines, suggesting that the 5'-flanking sequence of sGTHII beta contains information dictating its pituitary specificity. A silencer sequence (-95 to -35, pSil) was identified, which might function to repress sGTHII beta gene expression in the nongonadotropes or in the gonadotropes at developmental stages that precede final maturation and spawning.