Male Atlantic Salmon Parr Research Articles

Smolting in post‐sexually mature male Atlantic salmon (Salmo salar L.) parr in the wild

AbstractConflicts can arise in developmental pathways that prevent an individual entering different developmental life stages that result in the expression of different phenotypes within a specific time period. In salmonids, theory suggests that sexual maturation may inhibit subsequent smolting within the same 12‐month period and that this is partly the result of the time and the apparently conflicting physiological changes for these processes to occur, and partly because of the energy requirements for these physiologically taxing processes. This study tested whether sexually mature male Atlantic salmon (Salmo salar L.) parr, caught in the autumn, would subsequently smolt the following spring. Through individual identification using PIT telemetry, minimum estimates of 3.0% (n = 6/203) and 5.9% (n = 1/17) of Atlantic salmon parr that were sexually mature in two river catchments during the autumn were subsequently identified as smolts in the following spring. We therefore suggest that, in line with previous studies on domesticated Atlantic salmon and laboratory‐based experiments, there is no developmental conflict but that life‐history expression is mediated by environmental and genetic processes.

Sexual maturation in Atlantic salmon male parr may be triggered both in late summer and early spring under standard farming conditions

Male Atlantic salmon (Salmo salar) display different sexual strategies, maturing either as parr during the freshwater phase (as sneaky spawners), or as post smolts following one or several years at sea. First sexual maturation (puberty) occurs at different times depending on environmental and genetic factors. To improve our knowledge on the timing (age and season) of first sexual maturation in Atlantic salmon male parr, we investigated pubertal activation in second generation farmed salmon from the Norwegian river Figgjo, reared under natural conditions of photoperiod and water temperature. Histological analysis, in combination with morphometric measurements, plasma androgen levels and pituitary gonadotropin gene expression analysis revealed that, as previously reported, some male parr initiated early sexual maturation in spring at one year of age. Interestingly, some male parr were observed to initiate sexual maturation already in autumn, six months after hatching (under-yearlings), much earlier than reported in previous studies. One-year old maturing males showed a low induction in gonadotropin levels, while under-yearling maturing males displayed a significant increase in fshb transcripts as compared to immature/early-maturing fish. Plasma testosterone, detectable also in immature/early-maturing males, increased constantly during testes development, while 11-ketotestosterone, undetectable in immature/early-maturing and early maturing males, increased during more advanced stages of maturation. A mild feminization of the testes (ovotestes) was detected in a subset of samples. This study brings new knowledge on the little investigated field of sexually maturing under-yearlings in Atlantic salmon. This is also the first study comparing the physiology of under-yearling vs one-year old maturing male parr, thus bringing new insights to the remarkable plasticity of Atlantic salmon puberty.

Environmentally relevant concentrations of the common anxiolytic pharmaceutical oxazepam do not have acute effect on spawning behavior in mature male Atlantic salmon (Salmo salar) parr

Low concentrations of psychiatric drugs are commonly found in surface waters around the world, and their ability to alter behaviors in aquatic fauna has raised concerns about ecological effects. One critical effect that contamination can have in aquatic systems is modifications of reproductive behaviors. In this study, we hypothesized that oxazepam, a common anxiolytic drug and aquatic contaminant previously shown to induce anti-anxiety behavior in salmonids, also affects the reproductive behavior of Atlantic salmon (Salmo salar). To test this hypothesis, small sexually mature Atlantic salmon males (mature parr) were exposed to concentrations of oxazepam comparable to that of effluent water (1.9 µg/L). After 5 days of exposure, their reproductive behavior was compared with that of unexposed control individuals. Behavior was monitored in a large-scale flume aquarium that also contained a large male and a female to mimic natural spawning conditions for Atlantic salmon. Three behaviors were quantified for the mature parr; (a) amount of courting behavior towards female, (b) proximity to female, and (c) amount of attacks received from the large male. In contrast to our hypothesis, no difference in either courting behavior, proximity to female, or received aggression could be found between exposed and control male parr. Hence, our results indicate that there is no acute effect of the highest levels of oxazepam concentration reported for urban environments on spawning behavior of mature male parr, although chronic effects could not be ruled out.

Gnrh receptor gnrhr2bbα is expressed exclusively in lhb-expressing cells in Atlantic salmon male parr

Gonadotropin-releasing hormone (Gnrh) plays a major role in the regulation of physiological and behavioural processes related to reproduction. In the pituitary, it stimulates gonadotropin synthesis and release via activation of Gnrh receptors (Gnrhr), belonging to the G protein-coupled receptor superfamily. Evidence suggests that differential regulation of the two gonadotropins (Fsh and Lh) is achieved through activation of distinct intracellular pathways and, probably, through the action of distinct receptors. However, the roles of the different Gnrhr isoforms in teleosts are still not well understood. This study investigates the gene expression of Gnrhr in the pituitary gland of precociously maturing Atlantic salmon (Salmo salar) male parr. A total of six Gnrhr paralogs were identified in the Atlantic salmon genome and named according to phylogenetic relationship; gnrhr1caα, gnrhr1caβ, gnrhr1cbα, gnrhr1cbβ, gnrhr2bbα, gnrhr2bbβ. All paralogs, except gnrhr1caα, were expressed in male parr pituitary during gonadal maturation as evidenced by qPCR analysis. Only one gene, gnrhr2bbα, was differentially expressed depending on maturational stage (yearly cycle), with high expression levels in maturing fish, increasing in parallel with gonadotropin subunit gene expression. Additionally, a correlation in daily expression levels was detected between gnrhr2bbα and lhb (daily cycle) in immature fish in mid-April. Double fluorescence in situ hybridization showed that gnrhr2bbα was expressed exclusively in lhb gonadotropes in the pituitary, with no expression detected in fshb cells. These results suggest the involvement of receptor paralog gnrhr2bbα in the regulation of lhb cells, and not fshb cells, in sexually maturing Atlantic salmon male parr.

Effects of androgens on the leptin system in immature male Atlantic salmon parr

Leptin modulates all levels of the reproductive endocrine axis in mammals, and in turn, both leptin and the leptin receptor are regulated by sex steroids. The aim of this study was to investigate if sex steroids regulate the leptin system also in fish. Immature one-year old male Atlantic salmon parr were implanted with Silclear capsules that were either empty or filled with 11-ketoandrostenedione (11KA) or testosterone (T) and the effects of 35-days treatment were investigated on measures of maturation, gene expression of leptin (lepa1, lepa2), leptin receptor (lepra1) and circulating plasma leptin. Both 11-KA and T stimulated the reproductive axis by increasing testes weight and up-regulated pituitary lh-β mRNA levels and for T also fsh-β. T up-regulated transcription levels of lepa1 and lepra1 in the pituitary, while 11-KA had no effect. Leptin receptor expression in the testis was unaltered by either androgen. T up-regulated lepa1 mRNA levels significantly also in the liver, but had no effect on lepa2, and 11KA did not affect hepatic gene expression of either lepa1 or lepa2. Plasma leptin levels did not differ significantly between treatments. The results indicate that androgens regulate gene expression of leptin and the leptin receptor in different tissues in fish and that the effects of leptin might be tissue specific considering plasma levels remained unaltered. Overall, the results suggest a role for leptin in fish reproduction, where sex steroids are able to regulate components of the leptin system differentially in liver and important tissues of the reproductive axis.

Do Atlantic salmon parr trade growth against immunity?

Life history theory predicts trade-offs between reproduction and investments into other expensive physiological functions such as growth and immunity. The Atlantic salmon (Salmo salar L. 1758), a species with alternative reproductive strategies and indeterminate growth, offers an excellent opportunity to test for the existence of such trade-offs. We tested the hypothesis that reproduction suppresses immunity and physiological condition by comparing the measures of innate immunity, redox balance and body condition in reproductive (sneaker) male vs. non-reproductive female Atlantic salmon parr in suboptimal and good quality spawning site. Reproductive males did not appear immunocompromised or in poorer condition than non-reproductive females, suggesting that milt production per se did not exert sex-specific somatic costs under the study conditions. We did not find any evidence that the capability of phagocytes to produce an oxidative burst upon stimulation in vitro (whole blood chemiluminescence, WBCL response) covaried with measures of redox balance (spleen total glutathione concentration or superoxide dismutase activity). Quality of the spawning sites influenced the measured parameters. Fish collected from the river with a good quality spawning were heavier than those in suboptimal spawning site. In both study sites and irrespective of sex, shorter fishes had stronger WBCL responses than longer fishes. This finding is consistent with a concept that different individuals within populations can follow different life history or physiological strategies with some individuals prioritizing somatic growth over immune responsiveness and vice versa.

Regulation of the seasonal leptin and leptin receptor expression profile during early sexual maturation and feed restriction in male Atlantic salmon, Salmo salar L., parr

In mammals, leptin acts as an adiposity signal and is a crucial link between nutritional status and the reproductive axis. So far the link between leptin and energy balance during sexual maturation in teleosts has been poorly investigated. In this study, seasonal gene expression changes in two leptin genes (lepa1 and lepa2) and the leptin receptor were investigated during early sexual maturation in male Atlantic salmon parr under fully fed (control) and feed restricted conditions from April through September. Both Atlantic salmon lepa1 and lepa2 in the liver and lepr in the brain were significantly down-regulated in non-maturing control males in early spring, coinciding with the start of the growth and fat accumulation. In maturing control males, hepatic leptin expression increased during mid-spermatogenesis and lepa1 and lepa2 mRNA levels were up-regulated by 7.7 and 49 times respectively during final maturation. For the first time in a fish species, a significant up-regulation of lepr expression was observed in the testis throughout mid to late spermatogenesis. Feed restriction decreased the incidence of sexual maturation by 53% and highly up-regulated both leptin genes in the liver and the leptin receptor in the pituitary. This study shows that hepatic lepa1 and lepa2 expression and lepr expression in the testis is affected by early sexual maturation in male Atlantic salmon parr. Fast growth and high fat stores are associated with low leptin levels while feed restriction has a stimulatory effect on hepatic leptin and leptin receptor gene expression in the pituitary, suggesting a role for leptin other than that as an adiposity signal.

Leptin in fish: possible role in sexual maturation in male Atlantic salmon

Leptin plays an important role in energy homeostasis and reproductive function in mammals, while its function in fish is still poorly understood, especially its role in reproduction. In the present study, leptin gene expression and circulating leptin plasma levels were measured during sexual maturation in male Atlantic salmon parr. In maturing male salmon, higher hepatic leptin (lepa1) gene expression levels were observed during mid-spermatogenesis compared to immature fish. An upregulation of leptin during sexual maturation has also been observed in a few other teleost species. The physiological significance of elevated leptin levels during the maturational process in teleosts remains to be explored.

Reduced daylength stimulates size-dependent precocious maturity in 0+ male Atlantic salmon parr

To study relationships between fish length, photoperiod and the onset of precocious maturity in 0+ parr during intensive rearing of Atlantic salmon (Salmo salar L.), 0+ juveniles of 55–89mm length were individually tagged with Passive Induced Transponders (PITs) in May and exposed to reduced daylength (LD12:12) in May, June or July, or kept as control fish under continuous light (LD24:0). Relationships between the length of the fish and maturity were studied by the use of probabilistic maturation reaction norms. The incidence of mature males and the proportion of fish of lower modal group size in autumn were highest in the groups exposed to short days in May, and lowest under continuous light. In contrast with the expectation that high growth rates promote maturation, the future mature male parr were smaller than the immature males at the start of the experiment, and they also grew more slowly during the subsequent maturation process. Variability in condition factor was low until autumn, when the condition factor of maturing males rose well above that of immature males during autumn as the fish grew from ~80 to ~95mm length. Reduced daylength increased the probability of the onset of precocious maturity in the fish smaller than, but not above a threshold length of ~90–100mm (9 – 11g). Intensively reared parr of 60–90mm, and possibly also smaller fish, may be particularly sensitive to photoperiod manipulations that may influence the probabilities of fish adopting one of the three life-history alternatives, to enter lower or upper mode or to mature precociously.

Gene expression profiling during spermatogenesis in early maturing male Atlantic salmon parr testes

The initiation of sexual maturation and spermatogenesis are complex processes that require the highly coordinated regulation of a number of key genes. The endocrine system plays crucial roles in these processes, but the precise mechanisms involved in sexual maturation of fish are poorly understood. We investigated the expression of genes encoding proteins involved in sex steroid biosynthesis (Ff1b (FTZ-F1 homolog), steroidogenic acute regulatory protein (StAR), cytochrome P450 cholesterol side-chain cleavage enzyme (P450scc), 3β-hydroxysteroid dehydrogenase/Δ 5–Δ 4–isomerase (3β-HSD), cytochrome P450 17α-hydroxylase/17,20-lyase (P450c17), cytochrome P450 11β-hydroxylase (P45011β) and 11β-hydroxysteroid dehydrogenase (11β-HSD)) and the anti-Müllerian hormone (AMH) homolog during early sexual maturation of one-summer-old male Atlantic salmon parr by RT-PCR. Genes encoding Ff1b, StAR, 3β-HSD, P450c17 and 11β-HSD were upregulated during spermatogonial proliferation. During the course of spermatogenesis expression profiles of Ff1b, StAR, 3β-HSD, P450scc, P450c17, P45011β, and 11β-HSD were similar; transcript levels being low during early stages, then strongly increasing during spermiogenesis. These results indicate that coordinated de novo transcription of genes encoding StAR as well as 3β-HSD, P450c17 and 11β-HSD might be required for sex steroids production during the initiation of spermatogenesis in salmon. In contrast, transcription levels of AMH were comparatively high in immature testes, decreased when spermatogenesis was initiated, and were lowest during spermiogenesis, suggesting that AMH suppression plays a crucial role in the process of spermatogenesis in salmonids. Correlation analyses show that FSH and LH might be differentially involved in the regulation of several of these genes studied.

Expression of gonadotropin and gonadotropin receptor genes during early sexual maturation in male Atlantic salmon parr

Atlantic salmon males may mature already as small parr in freshwater. Sexual maturation in teleosts as in vertebrates is characterized by the activation of the brain-pituitary-gonad axis. The endocrine regulation of early puberty is still not well understood. In the present study, one-summer-old male Atlantic salmon parr were sampled regularly from December several months prior to the beginning of spermatogenesis until spawning in October. Pituitary expression levels of follicle-stimulating hormone (FSH) and luteinizing hormone (LH) beta subunit genes were analyzed in parallel with testis expression of FSH receptor (FSHR) and LH receptor (LHR) genes by RT-PCR and plasma 11-ketostestosterone (11-KT) was measured. Expression levels of FSHbeta, low during winter and spring started to increase prior to the onset of gonadal growth at the end of May while LHbeta mRNA levels were hardly detectable. Both gonadotropin receptor genes were expressed in immature testis with FSHR transcripts being more abundant (8-fold). FSHR transcript levels increased in parallel to FSHbeta levels from early spermatogenesis onwards, while LHR mRNA started to increase prior to any large changes in LHbeta expression. Both transcript levels of LHbeta and LHR were highest during spermiation. Plasma 11-KT increased at the beginning of spermatogenesis reaching highest levels at spermiogenesis suggesting a possible role of FSH in inducing 11-KT production during early spermatogenesis while LH stimulates via its specific receptor 11-KT production at spermiogenesis. The commitment into sexual maturation appears to be dependant on both the presence of FSHR in immature testis and the increase of FSH expression.

Spawning behaviour and success of mature male Atlantic salmon (Salmo salar) parr of farmed and wild origin

We examined genetic differences in the reproduction of an alternative maturation phenotype in Atlantic salmon (Salmo salar) by comparing the spawning behaviour and success among farmed, first-generation hybrid, and wild mature male parr raised in similar environments. Parr competed for spawning opportunities in the presence of either wild or farmed large males. There were no consistent differences among groups in aggression; however, there were differences in spawning participation with respect to large male origin and among parr types. There was a strong negative temporal effect on mature male parr spawning participation that differed by parr type; wild and farmed parr were most likely to participate in early spawnings, with hybrids being the most likely to participate in late spawnings. Overall, parr were also less likely to participate in spawnings involving large farmed males. Variance in reproductive success was higher among parr than among large males. Our results are consistent with the hypothesis that there are genetically based behavioural differences among mature male parr of farmed and wild origin that may potentially lead to differences in reproductive success.

Mature atlantic salmon (Salmo salar L.) male parr are attracted to ovulated female urine but not to ovarian fluid.

The behavioral responses to urine and ovarian fluids from conspecific and heterospecific ovulated females were studied in mature Atlantic salmon (Salmo salar) male parr in a two-choice fluviarium. The males reacted differently to the stimulants. They spent more time in water scented by urine from salmon or brown trout (Salmo trutta L.) females compared to the time in water with ovarian fluids from salmon females. Furthermore, the males were attracted to salmon female urine (different from an indifferent reaction). Males exposed to urine of either species had higher plasma concentrations of testosterone (T) compared to unexposed controls. Measurement of the concentrations of prostaglandin F2alpha (PGF2alpha) and its major metabolite 15-ketodihydroprostaglandin F2alpha (15-ketodihydro-PGF2alpha) showed that the concentrations of the substances were higher in ovarian fluids of both species compared to those in urine. PGF2alpha showed a greater difference between ovarian fluid and urine than its major metabolite. The results suggest that urine of both species, in contrast to ovarian fluid, contain substances that attract mature Atlantic salmon male parr and that the active substances are neither PGF2alpha nor 15-keto-PGF2alpha.

The impact of two pesticides on olfactory-mediated endocrine function in mature male Atlantic salmon (Salmo salar L.) parr

Short-term exposure of the olfactory epithelium of mature male Atlantic salmon parr to either the pesticide simazine (concentrations 1.0 and 2.0 μg l−1) or the pesticide atrazine (concentration 1.0 μg l−1) significantly reduced the olfactory response to the female priming pheromone, prostaglandin F2α. In addition, the reproductive priming effect of the pheromone on the levels of expressible milt was also reduced after exposure to the individual pesticides (simazine 0.1, 0.5, 1.0 and 2.0 μg l−1 and atrazine 0.5 and 2.0 μg l−1). When the olfactory epithelium was exposed to a mixture of simazine and atrazine, (concentrations of 0.5:0.5 and 1.0:1.0 μg l−1), there was no significant reduction in the olfactory response when compared to the single pesticides at equivalent concentrations. In addition, exposure to a mixture of simazine and atrazine had no synergistic effect on the priming response, and plasma levels of testosterone, 11-ketotestosterone and 17,20β-dihydroxy-4-pregnen-3-one were similar in the groups of male parr exposed to the individual pesticides. Although the levels of expressible milt were reduced in all groups, there were no significant differences between the different pesticide treatments. The results of the study suggest that the two s-triazine pesticides have an additive and not a synergistic impact on olfactory-mediated endocrine function in mature male salmon parr.

The effects of a synthetic pyrethroid pesticide on some aspects of reproduction in Atlantic salmon ( Salmo salar L.).

The synthetic pyrethroid pesticide cypermethrin, a known contaminant of tributaries supporting spawning salmonid fish, had a significant sublethal impact upon the pheromonal mediated endocrine system in mature male Atlantic salmon ( Salmo salar L.) parr. Previous studies have demonstrated that ovulated female salmon release a priming pheromone in their urine (considered to be the F-type prostaglandin, PGF 2α) which is subsequently detected by the olfactory system of mature male salmon parr and results in increased levels of plasma sex steroids and expressible milt. Exposure of mature male parr for a 5 day period to a water concentration of <0.004 μg l −1 cypermethrin significantly reduced or inhibited the olfactory response to PGF 2α. In addition, exposure of male parr to cypermethrin significantly reduced their ability to respond to the priming effect of the pheromone. The priming effect on milt and plasma 17,20β-dihydroxy-4-pregnen-3-one levels were abolished at water concentrations of <0.004 and 0.028 μg l −1cypermethrin, respectively. The effect of cypermethrin on the priming response did not appear to be due to a direct effect on the testes, since the ability of testes to respond to pituitary extract stimulation in vitro was not impaired in males exposed to cypermethrin. In addition, exposure of salmon milt and eggs to a concentration of 0.1 μg l −1 cypermethrin during fertilisation subsequently reduced the number of fertilised eggs. The results of the study suggest that low levels of cypermethrin in the aquatic environment may have a significant effect on Atlantic salmon populations through disruption of reproductive functions.

Lack of species‐specific primer effects of odours from female Atlantic salmon, Salmo salar, and brown trout, Salmo trutta

We exposed, in two successive spawning seasons, individually placed precocious male Atlantic salmon (Salmo salar) and brown trout (Salmo trutta) parr to odour stimuli (ovarian fluid and urine mix) from ovulated conspecific or heterospecific anadromous females. Atlantic salmon parr had significantly higher plasma concentrations of the hormones 17α,20β‐dihydroxy‐4‐pregnen‐3‐one (17,20β‐P), 11‐ketotestosterone (11‐KT) and testosterone (T) after exposure to odours from conspecific females or from brown trout females compared to parr exposed to a control solution (0.9% NaCl). We did not observe any significant differences between the hormone levels in salmon parr exposed to the two female odours. The salmon parr exposed to conspecific odours had significantly higher volumes of strippable milt compared to the controls, but we did not find any significant differences when comparing the effect of the two female odours. Brown trout parr had significantly higher plasma 17,20β‐P levels following exposure to heterospecific female odours compared to control males, but there was no significant difference between males exposed to the different female odours. We did not observe any significant differences in plasma levels of T and 11‐KT and in milt volumes between exposed and control trout. Taken together, the results from both tested species indicate that the potency of heterospecific stimuli in stimulating increased plasma sex steroid hormone levels in male parr was as strong as stimuli from conspecific females. The results are discussed in connection to observed hybridisation between the two sympatric species.

Evidence of Successful Natural Reproduction between Brown Trout and Mature Male Atlantic Salmon Parr

Abstract Hybridization between Atlantic salmon Salmo salar and brown trout S. trutta is frequently reported, and the role of mature male Atlantic salmon parr has been suspected but never proven. Salmon fry were stocked into a headwater tributary stream of the Connecticut River, where no adult salmon are present. The stream has a self-sustaining population of brown trout. Enzyme electrophoresis revealed the presence of one hybrid (0.81% of the sampled population). The maternal species was identified as brown trout; therefore, the only possibility for the male parent was that it was a mature male Atlantic salmon parr. This is the first direct evidence of parr producing hybrid offspring in a totally natural setting and in the absence of any sea-return salmon.

Reproductive priming in mature male Atlantic salmon parr exposed to the sound of redd cutting

The sounds of a wild female Atlantic salmon cutting a redd were associated with significant increases in the levels of plasma 17,20β-dihydroxy-4-pregnen-3-one (17,20βP) and of expressible milt in mature male parr, comparable with levels of the steroid and milt produced in parr exposed to the priming pheromone, prostaglandin F2a. Hence auditory cues may have a significant role in synchronising reproductive physiology in Atlantic salmon.

Reproductive priming in mature male Atlantic salmon parr exposed to the sound of redd cutting

The sounds of a wild female Atlantic salmon cutting a redd were associated with significant increases in the levels of plasma 17,20β‐dihydroxy‐4‐pregnen‐3‐one (17,20βP) and of expressible milt in mature male parr, comparable with levels of the steroid and milt produced in parr exposed to the priming pheromone, prostaglandin F2a. Hence auditory cues may have a significant role in synchronising reproductive physiology in Atlantic salmon.

Feedback Control of Gonadotropins in Atlantic Salmon,Salmo salar,Male Parr: I. Castration Effects in Rematuring and Nonrematuring Fish

Sexual maturation of Atlantic salmon (Salmo salar) male parr is a seasonally recurrent “all or none” response; either a fish matures fully or it does not mature. To study whether gonadal feedback on gonadotropic hormones, GTH I and GTH II, is involved in the control of maturation, previously mature Atlantic salmon male parr were either sham-operated or castrated in spring. They were then sampled during the onset of gonadal growth (late June–early July) or shortly before the breeding season (late September). In autumn, sham-operated males separated into two groups: nonrematuring males with low pituitary and plasma levels of both GTH I and GTH II, and those rematuring with high levels of gonadotropins. Castrated males had low GTH I and GTH II plasma and pituitary levels, similar to those of the nonrematuring fish, suggesting positive feedback mechanisms, separating the sham-operated fish into low and high GTH level groups. In the summer, plasma GTH II was nondetectable in all fish. Pituitary GTH II content was lower in nonrematuring, than in rematuring males and was even lower in castrated fish. In contrast, castration increased pituitary and plasma levels of GTH I in the summer, suggesting a negative feedback at this reproductive stage. There were no significant differences in immunoassayable levels of GTH I in plasma in rematuring and nonrematuring sham-operated males at this time. The control of rematuration is complex and may involve factors other than circulating GTH I levels, possibly with differences in gonadal sensitivity to GTH I.