Photoperiod regulation of plasma growth hormone levels during induced smoltification of underyearling Atlantic salmon

Seasonal Changes in Photoperiod: Effects on Growth and Redox Signaling Patterns in Atlantic Salmon Postsmolts

Farmed Atlantic salmon reared under natural seasonal changes in sea-cages had an elevated consumption of antioxidants during spring. It is, however, unclear if this response was caused by the increase in day length, temperature, or both. The present study examined redox processes in Atlantic salmon that were reared in indoor tanks at constant temperature (9 °C) under a simulated natural photoperiod. The experiment lasted for 6 months, from vernal to autumnal equinoxes, with the associated increase and subsequent decrease in day length. We found that intracellular antioxidants were depleted, and there was an increase in malondialdehyde (MDA) levels in the liver and muscle of Atlantic salmon with increasing day length. Antioxidant enzyme activity in liver and muscle and their related gene profiles was also affected, with a distinct upregulation of genes involved in maintaining redox homeostasis, such as peroxiredoxins in the brain in April. This study also revealed a nuclear factor-erythroid 2-related factor 2 (Nrf2)-mediated oxidative stress response in muscle and liver, suggesting that fish integrate environmental signals through redox signaling pathways. Furthermore, growth and expression profiles implicated in growth hormone (GH) signaling and cell cycle regulation coincided with stress patterns. The results demonstrate that a change in photoperiod without the concomitant increase in temperature is sufficient to stimulate growth and change the tissue oxidative state in Atlantic salmon during spring and early summer. These findings provide new insights into redox regulation mechanisms underlying the response to the changing photoperiod, and highlight a link between oxidative status and physiological function.

Elevated Water CO2 Can Prevent Dietary-Induced Osteomalacia in Post-Smolt Atlantic Salmon (Salmo salar, L.)

Expansion of land-based systems in fish farms elevate the content of metabolic carbon dioxide (CO₂) in the water. High CO₂ is suggested to increase the bone mineral content in Atlantic salmon (Salmo salar, L.). Conversely, low dietary phosphorus (P) halts bone mineralization. This study examines if high CO₂ can counteract reduced bone mineralization imposed by low dietary P intake. Atlantic salmon post-seawater transfer (initial weight 207.03 g) were fed diets containing 6.3 g/kg (0.5P), 9.0 g/kg (1P), or 26.8 g/kg (3P) total P for 13 weeks. Atlantic salmon from all dietary P groups were reared in seawater which was not injected with CO₂ and contained a regular CO₂ level (5 mg/L) or in seawater with injected CO₂ thus raising the level to 20 mg/L. Atlantic salmon were analyzed for blood chemistry, bone mineral content, vertebral centra deformities, mechanical properties, bone matrix alterations, expression of bone mineralization, and P metabolism-related genes. High CO₂ and high P reduced Atlantic salmon growth and feed intake. High CO₂ increased bone mineralization when dietary P was low. Atlantic salmon fed with a low P diet downregulated the fgf23 expression in bone cells indicating an increased renal phosphate reabsorption. The current results suggest that reduced dietary P could be sufficient to maintain bone mineralization under conditions of elevated CO₂. This opens up a possibility for lowering the dietary P content under certain farming conditions.

The effect of environmental stressors on growth in fish and its endocrine control

Fish body growth is a trait of major importance for individual survival and reproduction. It has implications in population, ecology, and evolution. Somatic growth is controlled by the GH/IGF endocrine axis and is influenced by nutrition, feeding, and reproductive-regulating hormones as well as abiotic factors such as temperature, oxygen levels, and salinity. Global climate change and anthropogenic pollutants will modify environmental conditions affecting directly or indirectly fish growth performance. In the present review, we offer an overview of somatic growth and its interplay with the feeding regulatory axis and summarize the effects of global warming and the main anthropogenic pollutants on these endocrine axes.

Anguillid Eels as a Model Species for Understanding Endocrinological Influences on the Onset of Spawning Migration of Fishes

Simple Summary

Endocrine regulation has been thought to play a major role in the onset of migration. Anguillid eels provide a good model for studying the onset mechanisms of migrations to breeding areas, because the process of the onset of migration occurs in inland waters. In this review, we summarize information about the silvering process in anguillid eels and the dynamics of mRNA expression of neurohormones and pituitary hormones, thyroid hormones, and sex steroids associated with the onset of the spawning migration. We also provide new results. Because 11-KT drastically increases during silvering, the role of 11-KT in the onset of spawning migration was discussed in detail.

Abstract

Anguillid eels are the iconic example of catadromous fishes, because of their long-distance offshore spawning migrations. They are also a good model for research on the onset mechanisms of migrations to breeding areas, because the migrations begin in inland waters. When eels transform from yellow eels to silver eels, it is called silvering. Silver eels show various synchronous external and internal changes during silvering, that include coloration changes, eye-size increases, and gonadal development, which appear to be pre-adaptations to the oceanic environment and for reproductive maturation. A strong gonadotropic axis activation occurs during silvering, whereas somatotropic and thyrotropic axes are not activated. Among various hormones, 11-ketotestosterone (11-KT) drastically increases during spawning migration onset. Gradual water temperature decreases simulating the autumn migratory season, inducing 11-KT increases. Administration of 11-KT appeared to cause changes related to silvering, such as early-stage oocyte growth and eye enlargement. Moreover, 11-KT may be an endogenous factor that elevates the migratory drive needed for the spawning migration onset. These findings suggested that water temperature decreases cause 11-KT to increase in autumn and this induces silvering and increases migratory drive. In addition, we newly report that 11-KT is associated with a corticotropin-releasing hormone that influences migratory behavior of salmonids. This evidence that 11-KT might be among the most important factors in the spawning migration onset of anguillid eels can help provide useful knowledge for understanding endocrinological mechanisms of the initiation of spawning migrations.

The Effect of Continuous Light on Growth and Muscle-Specific Gene Expression in Atlantic Salmon (Salmo salar L.) Yearlings

Photoperiod is associated to phenotypic plasticity of somatic growth in several teleost species, however, the molecular mechanisms underlying this phenomenon are currently unknown. The effect of a continuous lighting (LD 24:0), compared with the usual hatchery lighting (HL) regime, on the growth rate and gene expression of myogenic regulatory factors (MRFs: MyoD1 paralogs, Myf5, and MyoG) myosin heavy chain (MyHC), and MSTN paralogs in the white muscles of hatchery-reared Atlantic salmon yearlings was evaluated over a 6-month period (May–October). The levels of gene expression were determined using real-time PCR. Continuous lighting was shown to have a positive effect on weight gain. MyHC, MyoD1c, MyoD1b, and MSTN1a/b mRNA expression was influenced by the light regime applied. In all the studied groups, a significant positive correlation was observed between the expression levels of MRFs and MSTN paralogs throughout the experiment. The study demonstrated seasonal patterns regarding the simultaneous expression of several MRFs. MyoD1a, MyoG, and MyHC mRNA expression levels were elevated in the mid-October, but MyoD1b/c, and Myf5 mRNA levels decreased by the end of this month. In general, the findings showed that constant lighting affected the regulatory mechanisms of muscle growth processes in salmon.

Effects of a temperature rise on melatonin and thyroid hormones during smoltification of Atlantic salmon, Salmo salar

Smoltification prepares juvenile Atlantic salmon (Salmo salar) for downstream migration. Dramatic changes characterize this crucial event in the salmon's life cycle, including increased gill Na⁺/K⁺-ATPase activity (NKA) and plasma hormone levels. The triggering of smoltification relies on photoperiod and is modulated by temperature. Both provide reliable information, to which fish have adapted for thousands of years, that allows deciphering daily and calendar time. Here we studied the impact of different photoperiod (natural, sustained winter solstice) and temperature (natural, ~ + 4° C) combinations, on gill NKA, plasma free triiodothyronine (T3) and thyroxine (T4), and melatonin (MEL; the time-keeping hormone), throughout smoltification. We also studied the impact of temperature history on pineal gland MEL production in vitro. The spring increase in gill NKA was less pronounced in smolts kept under sustained winter photoperiod and/or elevated temperature. Plasma thyroid hormone levels displayed day-night variations, which were affected by elevated temperature, either independently from photoperiod (decrease in T3 levels) or under natural photoperiod exclusively (increase in T4 nocturnal levels). Nocturnal MEL secretion was potentiated by the elevated temperature, which also altered the MEL profile under sustained winter photoperiod. Temperature also affected pineal MEL production in vitro, a response that depended on previous environmental acclimation of the organ. The results support the view that the salmon pineal is a photoperiod and temperature sensor, highlight the complexity of the interaction of these environmental factors on the endocrine system of S. salar, and indicate that climate change might compromise salmon's time "deciphering" during smoltification, downstream migration and seawater residence.

Genetic growth potential, rather than phenotypic size, predicts migration phenotype in Atlantic salmon

Knowledge of the relative importance of genetic versus environmental determinants of major developmental transitions is pertinent to understanding phenotypic evolution. In salmonid fishes, a major developmental transition enables a risky seaward migration that provides access to feed resources. In Atlantic salmon, initiation of the migrant phenotype, and thus age of migrants, is presumably controlled via thresholds of a quantitative liability, approximated by body size expressed long before the migration. However, how well size approximates liability, both genetically and environmentally, remains uncertain. We studied 32 Atlantic salmon families in two temperatures and feeding regimes (fully fed, temporarily restricted) to completion of migration status at age 1 year. We detected a lower migrant probability in the cold (0.42) than the warm environment (0.76), but no effects of male maturation status or feed restriction. By contrast, body length in late summer predicted migrant probability and its control reduced migrant probability heritability by 50-70%. Furthermore, migrant probability and length showed high heritabilities and between-environment genetic correlations, and were phenotypically highly correlated with stronger genetic than environmental contributions. Altogether, quantitative estimates for the genetic and environmental effects predicting the migrant phenotype indicate, for a given temperature, a larger importance of genetic than environmental size effects.

Muscle-specific gene expression and metabolic enzyme activities in Atlantic salmon Salmo salar L. fry reared under different photoperiod regimes

This study was conducted to characterise the muscle-specific gene expression, energy metabolism level and growth rates of Atlantic salmon Salmo salar L. reared under different photoperiod regimes. The effects of two photoperiod regimes - LD 16:8 (16 h light:8 h dark) and LD 24:0 (24 h light:0 h dark) over a period of 3 months (August to October) on growth, energy metabolism enzyme activities (cytochrome c oxidase, COX; lactate dehydrogenase, LDH; and aldolase) and the gene expression levels of myogenic regulatory factors (MRFs - MyoD1 paralogues (MyoD1a, MyoD1b, MyoD1c), Myf5, MyoG), myostatin paralogues (MSTN-1a, MSTN-1b, MSTN-2a) and the fast skeletal myosin heavy chain (MyHC) in the muscles of Atlantic salmon underyearling fry (0+) were investigated. The experiment was conducted in a fish hatchery with natural variations in water temperature. The results were compared with those obtained in salmon reared under the lighting conditions of a fish hatchery (HL, hatchery lighting). The results revealed that the fry reared under constant light (LD 24:0) grew faster and were bigger at the end of the experiment. Fishes reared within the photoperiod regime LD 16:8 had a lower growth rate. COX activity was lower in fish under the LD 16:8 regime compared with the LD 24:0 group. The LDH and aldolase enzyme activities were higher in the group with constant light in comparison to control in the beginning of September. The expression level for all of the genes studied variated during the duration of the experiment, and MyHC, MyoG, MyoD1a and Myf5 expression depended on the light regime as well. The more noticeable changes in gene expression occurred in October. The MyHC and MyoG mRNA levels increased, accompanied by MyD1c gene expression, in both groups that had additional lighting (LD 16:8 and LD24:0) at the beginning of October and were higher than the HL group. In the HL group, the elevation of MyHC and MyoG mRNA was gradual during October, but there was a sharp increase in Myf5 expression at the beginning of October. MyoD1 paralogues differently expressed during the experiment. The MyoD1a mRNA level was elevated at the end of October along with MyHC and MyoG expression, but MyoD1b and MyoD1c mRNA levels decreased along with Myf5 gene expression. The expression of MSTN paralogues were elevated with increases in MyHC and MRFs transcripts. These findings show that constant light has a positive effect on the growth rate of salmon, affecting the aerobic and anaerobic capacity in their muscles. The alterations in muscle-specific gene expression between the groups with different light indicated that the mechanisms for regulating muscle growth processes in fish depend on photoperiod duration.

Atlantic salmon (Salmo salar L.) post-smolts challenged two or nine weeks after seawater-transfer show differences in their susceptibility to salmonid alphavirus subtype 3 (SAV3)

Background

Pancreas disease (PD), caused by salmonid alphavirus (SAV), is an important disease affecting salmonid aquaculture. It has been speculated that Atlantic salmon post-smolts are more prone to infections in the first few weeks following seawater- transfer. After this period of seawater acclimatization, the post-smolts are more robust and better able to resist infection by pathogens. Here we describe how we established a bath immersion (BI) model for SAV subtype 3 (SAV3) in seawater. We also report how this challenge model was used to study the susceptibility of post-smolts to SAV3 infection in two groups of post-smolts two weeks or nine weeks after seawater - transfer.

Methods

Post-smolts, two weeks (Phase-A) or nine weeks (Phase-B) after seawater- transfer, were infected with SAV3 by BI or intramuscular injection (IM) to evaluate their susceptibility to infection. A RT-qPCR assay targeting the non-structural protein (nsP1) gene was performed to detect SAV3-RNA in blood, heart tissue and electropositive-filtered tank-water. Histopathological changes were examined by light microscope, and the presence of SAV3 antigen in pancreas tissue was confirmed using immuno-histochemistry.

Results

Virus shedding from the Phase-B fish injected with SAV3 (IM Phase-B) was markedly lower than that from IM Phase-A fish. A lower percentage of viraemia in Phase-B fish compared with Phase-A fish was also observed. Viral RNA in hearts from IM Phase-A fish was higher than in IM Phase-B fish at all sampling points (p < 0.05) and a similar trend was also seen in the BI groups. Necrosis of exocrine pancreatic cells was observed in all infected groups. Extensive histopathological changes were found in Phase-A fish whereas milder PD-related histopathological lesions were seen in Phase-B fish. The presence of SAV3 in pancreas tissue from all infected groups was also confirmed by immuno-histochemical staining.

Conclusion

Our results suggest that post-smolts are more susceptible to SAV3 infection two weeks after seawater-transfer than nine weeks after transfer. In addition, the BI challenge model described here offers an alternative SAV3 infection model when better control of the time-of-infection is essential for studying basic immunological mechanisms and disease progression.

Electronic supplementary material

The online version of this article (doi:10.1186/s12985-016-0520-8) contains supplementary material, which is available to authorized users.

An industry-scale mass marking technique for tracing farmed fish escapees

Farmed fish escape and enter the environment with subsequent effects on wild populations. Reducing escapes requires the ability to trace individuals back to the point of escape, so that escape causes can be identified and technical standards improved. Here, we tested if stable isotope otolith fingerprint marks delivered during routine vaccination could be an accurate, feasible and cost effective marking method, suitable for industrial-scale application. We tested seven stable isotopes, (134)Ba, (135)Ba, (136)Ba, (137)Ba, (86)Sr, (87)Sr and (26)Mg, on farmed Atlantic salmon reared in freshwater, in experimental conditions designed to reflect commercial practice. Marking was 100% successful with individual Ba isotopes at concentrations as low as 0.001 µg. g-1 fish and for Sr isotopes at 1 µg. g-1 fish. Our results suggest that 63 unique fingerprint marks can be made at low cost using Ba (0.0002 - 0.02 $US per mark) and Sr (0.46 - 0.82 $US per mark) isotopes. Stable isotope fingerprinting during vaccination is feasible for commercial application if applied at a company level within the world's largest salmon producing nations. Introducing a mass marking scheme would enable tracing of escapees back to point of origin, which could drive greater compliance, better farm design and improved management practices to reduce escapes.

Long-term effects of photoperiod, temperature and their interaction on growth, gill Na⁺, K⁺-ATPase activity, seawater tolerance and plasma growth-hormone levels in Atlantic salmon Salmo salar

This study was undertaken to examine the long-term effects of photoperiod, temperature and their interaction on growth, gill Na⁺,K⁺-ATPase (NKA) activity, seawater tolerance and plasma growth-hormone levels in Atlantic salmon Salmo salar pre-smolts and smolts. The fish (mean ± s.e. initial body mass = 15·9 ± 0·4 g) were reared on two photoperiods (continuous light, LL, and simulated natural photoperiod, LDN, 60° 25' N) and two temperatures (8·3 and 12·7° C) from June to May of the following year. Mean body mass was affected by photoperiod, temperature and their interactions. Both temperature groups on LL developed peak levels in gill NKA activity from October to November, 4-5 months prior to the natural season for the parr-smolt transformation. Fish at 12° C showed peak levels in NKA activity 4-6 weeks before the fish at 8° C. Fish in all four experimental groups showed maximum NKA activity within a similar size range (113-162 g). The present findings further indicate that smoltification in S. salar is to some extent driven by size, and that S. salar will develop smolt characteristics, e.g. a marked increase in NKA activity, within a similar size range. Faster-growing S. salar will, thus, reach this size threshold at a relatively younger age.

Genomic arrangement of salinity tolerance QTLs in salmonids: a comparative analysis of Atlantic salmon (Salmo salar) with Arctic charr (Salvelinus alpinus) and rainbow trout (Oncorhynchus mykiss)

Background

Quantitative trait locus (QTL) studies show that variation in salinity tolerance in Arctic charr and rainbow trout has a genetic basis, even though both these species have low to moderate salinity tolerance capacities. QTL were observed to localize to homologous linkage group segments within putative chromosomal regions possessing multiple candidate genes. We compared salinity tolerance QTL in rainbow trout and Arctic charr to those detected in a higher salinity tolerant species, Atlantic salmon. The highly derived karyotype of Atlantic salmon allows for the assessment of whether disparity in salinity tolerance in salmonids is associated with differences in genetic architecture. To facilitate these comparisons, we examined the genomic synteny patterns of key candidate genes in the other model teleost fishes that have experienced three whole-genome duplication (3R) events which preceded a fourth (4R) whole genome duplication event common to all salmonid species.

Results

Nine linkage groups contained chromosome-wide significant QTL (AS-2, -4p, -4q, -5, -9, -12p, -12q, -14q -17q, -22, and −23), while a single genome-wide significant QTL was located on AS-4q. Salmonid genomes shared the greatest marker homology with the genome of three-spined stickleback. All linkage group arms in Atlantic salmon were syntenic with at least one stickleback chromosome, while 18 arms had multiple affinities. Arm fusions in Atlantic salmon were often between multiple regions bearing salinity tolerance QTL. Nine linkage groups in Arctic charr and six linkage group arms in rainbow trout currently have no synteny alignments with stickleback chromosomes, while eight rainbow trout linkage group arms were syntenic with multiple stickleback chromosomes. Rearrangements in the stickleback lineage involving fusions of ancestral arm segments could account for the 21 chromosome pairs observed in the stickleback karyotype.

Conclusions

Salinity tolerance in salmonids from three genera is to some extent controlled by the same loci. Synteny between QTL in salmonids and candidate genes in stickleback suggests genetic variation at candidate gene loci could affect salinity tolerance in all three salmonids investigated. Candidate genes often occur in pairs on chromosomes, and synteny patterns indicate these pairs are generally conserved in 2R, 3R, and 4R genomes. Synteny maps also suggest that the Atlantic salmon genome contains three larger syntenic combinations of candidate genes that are not evident in any of the other 2R, 3R, or 4R genomes examined. These larger synteny tracts appear to have resulted from ancestral arm fusions that occurred in the Atlantic salmon ancestor. We hypothesize that the superior hypo-osmoregulatory efficiency that is characteristic of Atlantic salmon may be related to these clusters.

Diurnal expression of clock genes in pineal gland and brain and plasma levels of melatonin and cortisol in Atlantic salmon parr and smolts

In Atlantic salmon, the preadaptation to a marine life, i.e., parr-smolt transformation, and melatonin production in the pineal gland are regulated by the photoperiod. However, the clock genes have never been studied in the pineal gland of this species. The aim of the present study was to describe the diurnal expression of clock genes (Per1-like, Cry2, and Clock) in the pineal gland and brain of Atlantic salmon parr and smolts in freshwater, as well as plasma levels of melatonin and cortisol. By employing an out-of-season smolt production model, the parr-smolt transformation was induced by subjecting triplicate groups of parr to 6 wks (wks 0 to 6) under a 12 h:12 h light-dark (LD) regime followed by 6 wks (wks 6 to 12) of continuous light (LL). The measured clock genes in both pineal gland and brain and the plasma levels of melatonin and cortisol showed significant daily variations in parr under LD in wk 6, whereas these rhythms were abolished in smolts under LL in wk 12. In parr, the pineal Per1-like and Cry2 expression peaked in the dark phase, whereas the pineal Clock expression was elevated during the light phase. Although this study presents novel findings on the clock gene system in the teleost pineal gland, the role of this system in the regulation of smoltification needs to be studied in more detail.

Pituitary and plasma growth hormone dynamics during sexual maturation of female Atlantic salmon

Growth hormone in fish regulates many important physiological processes including growth, metabolism and potentially reproduction. In salmonid fish, GH secretion is episodic with irregularly spaced GH peaks. Plasma GH reflects secretion episodes as well as the clearance rate of the hormone, and plasma levels may thus not always reflect the level of activation of the GH axis. This study measured the production dynamics of GH over a 17-month period in sexually maturing female Atlantic salmon which included final maturation and spawning. For the first time, the level of pituitary GH mRNA, pituitary GH protein and plasma GH protein were analyzed concurrently in the same individuals. mRNA and protein were extracted in parallel from the same samples with subsequent real time quantitative PCR to measure mRNA transcripts and radioimmunoassay to measure pituitary and plasma GH protein. Further, the effects of photoperiod manipulation on these parameters were studied. The results show no correlation between mRNA and protein levels except at some time points, and indicate that it is inappropriate to correlate pooled temporal data and averages in time series unless the relationship among the variables is stable over time. The results indicate complex and shifting relationships between pituitary GH mRNA expression, pituitary GH content and plasma GH levels, which could result from changes in clearance rather than secretion rate at different times and its episodic secretion. The study also suggests that there is a functionally important activation of the GH system during spring leading up to maturation and spawning.

Understanding the multifactorial control of growth hormone release by somatotropes: lessons from comparative endocrinology

Control of postnatal growth is the main, but not the only, role for growth hormone (GH) as this hormone also contributes to regulating metabolism, reproduction, immunity, development, and osmoregulation in different species. Likely owing to this variety of group-specific functions, GH production is differentially regulated across vertebrates, with an apparent evolutionary trend to simplification, especially in the number of stimulatory factors governing substantially GH release. Thus, teleosts exhibit a multifactorial regulation of GH secretion, with a number of factors, from the newly discovered fish GH-releasing hormone (GHRH) to pituitary adenylate cyclase-activating peptide (PACAP) but also gonadotropin-releasing hormone, dopamine, corticotropin-releasing hormone, and somatostatin(s) directly controlling somatotropes. In amphibians and reptiles, GH secretion is primarily stimulated by the major hypothalamic peptides GHRH and PACAP and inhibited by somatostatin(s), while other factors (ghrelin, thyrotropin-releasing hormone) also influence GH release. Finally, in birds and mammals, primary control of GH secretion is exerted by a dual interplay between GHRH and somatostatin. In addition, somatotrope function is modulated by additional hypothalamic and peripheral factors (e.g., ghrelin, leptin, insulin-like growth factor-I), which together enable a balanced integration of feedback signals related to processes in which GH plays a relevant regulatory role, such as metabolic and energy status, reproductive, and immune function. Interestingly, in contrast to the high number of stimulatory factors impinging upon somatotropes, somatostatin(s) stand(s) as the main primary inhibitory regulator(s) for this cell type.

Green light stimulates somatic growth in the barfin flounder Verasper moseri

We examined the effects of different light wavelengths-blue, green, and red-on the somatic growth of the barfin flounder Verasper moseri, a flatfish. The light sources used were fluorescent lamps and a combination of daylight and fluorescent lamps that produced ambient light. These light sources were filtered using blue, green, or red filters. During the experiments, the fish were reared in indoor tanks with running seawater of natural temperature and fed with commercial pellets twice daily until satiety. The tanks were white in color. Fish were exposed to constant light emitted from the fluorescent lamps (9:15, light:dark; 08:00-17:00, light) for 14 weeks from October or September to January or to ambient light with a 14-week natural photoperiod from September to December. The wavelengths that were filtered from the fluorescent lamp light modified the growth of the fish, i.e., fish reared under green or blue light exhibited a greater total length (TL; P<0.01) and body weight (BW; P<0.01) than those reared under red light. In contrast, in the case of fish exposed to filtered ambient light, fish reared under green light exhibited a greater TL (P<0.01) and BW (P<0.01) than fish exposed to other wavelengths-blue-, red-, and nonfiltered ambient light. Our results indicate that flounder growth was modified by certain wavelengths, namely, green and red light, which had growth-stimulating and growth-inhibiting effects, respectively.

A critical review of adaptive genetic variation in Atlantic salmon: implications for conservation

Here we critically review the scale and extent of adaptive genetic variation in Atlantic salmon (Salmo salar L.), an important model system in evolutionary and conservation biology that provides fundamental insights into population persistence, adaptive response and the effects of anthropogenic change. We consider the process of adaptation as the end product of natural selection, one that can best be viewed as the degree of matching between phenotype and environment. We recognise three potential sources of adaptive variation: heritable variation in phenotypic traits related to fitness, variation at the molecular level in genes influenced by selection, and variation in the way genes interact with the environment to produce phenotypes of varying plasticity. Of all phenotypic traits examined, variation in body size (or in correlated characters such as growth rates, age of seaward migration or age at sexual maturity) generally shows the highest heritability, as well as a strong effect on fitness. Thus, body size in Atlantic salmon tends to be positively correlated with freshwater and marine survival, as well as with fecundity, egg size, reproductive success, and offspring survival. By contrast, the fitness implications of variation in behavioural traits such as aggression, sheltering behaviour, or timing of migration are largely unknown. The adaptive significance of molecular variation in salmonids is also scant and largely circumstantial, despite extensive molecular screening on these species. Adaptive variation can result in local adaptations (LA) when, among other necessary conditions, populations live in patchy environments, exchange few or no migrants, and are subjected to differential selective pressures. Evidence for LA in Atlantic salmon is indirect and comes mostly from ecological correlates in fitness-related traits, the failure of many translocations, the poor performance of domesticated stocks, results of a few common-garden experiments (where different populations were raised in a common environment in an attempt to dissociate heritable from environmentally induced phenotypic variation), and the pattern of inherited resistance to some parasites and diseases. Genotype x environment interactions occurr for many fitness traits, suggesting that LA might be important. However, the scale and extent of adaptive variation remains poorly understood and probably varies, depending on habitat heterogeneity, environmental stability and the relative roles of selection and drift. As maladaptation often results from phenotype-environment mismatch, we argue that acting as if populations are not locally adapted carries a much greater risk of mismanagement than acting under the assumption for local adaptations when there are none. As such, an evolutionary approach to salmon conservation is required, aimed at maintaining the conditions necessary for natural selection to operate most efficiently and unhindered. This may require minimising alterations to native genotypes and habitats to which populations have likely become adapted, but also allowing for population size to reach or extend beyond carrying capacity to encourage competition and other sources of natural mortality.

Growth hormone and insulin-like growth factor-I act together and independently when regulating growth in vertebral and muscle tissue of atlantic salmon postsmolts

Aiming to elucidate the role of GH and IGF-I with regard to vertebral and white muscle growth, gene expression of the GH and IGF-I receptors (ghr and igf-Ir, respectively) and local IGF-I (igf-I) were analyzed during spring growth (January-June) in Atlantic salmon postsmolts. One group of fish was reared under natural light (NL), while one group was reared under continuous light (LL). Growth rate of fork length was higher in the LL group for a short period after onset of continuous light (LL: 0.50+/-0.02 mm day(-1), NL: 0.43+/-0.01 mm day(-1)) and for a longer period at the end of the experiment in June (LL: 1.18+/-0.06 mm day(-1), NL: 0.75+/-0.02 mm day(-1)). Likewise, growth rate in length of vertebra No. 40 in the LL group was higher than in the NL group the first period after onset of light (LL: 0.015+/-0.002 mm day(-1), NL: 0.008+/-0.001 mm day(-1)). Plasma GH levels peaked in late February and were higher in the LL group than in the NL group (LL: 7.27+/-0.61 ng ml(-1), NL: 2.60+/-0.50 ng ml(-1)), whereas plasma IGF-I levels peaked in early February and were unaffected by photoperiod. ghr expression was upregulated in late February in liver (12-fold), white muscle (6-fold) and vertebral tissue (3-fold) and higher in the LL group than in the NL group (2-fold) in vertebral tissue in late March. White muscle expression of igf-I and igf-Ir decreased from initial levels throughout the experiment. Hepatic gene expression of igf-I doubled in both groups in late February, followed by a 4-fold upregulation in June in the LL group only. Vertebral tissue expression of igf-I (4-fold) and igf-Ir (6-fold) increased in May and were unaffected by photoperiod. One exception was a smaller upregulation of igf-I (2-fold) in the LL group in early February. In conclusion, GH appears to have an initial role in stimulating vertebral growth, while IGF-I seems to stimulate growth during late spring. It is suggested that local IGF-I acts as a paracrine agent, evaluated from the concurrent upregulation of igf-I and igf-Ir. The upregulation of ghr in white muscle tissue, concurrent with a downregulation of muscle igf-I and igf-Ir, indicate that GH stimulated growth or metabolism independent of IGF-I.

Expression of endogenous and exogenous growth hormone (GH) messenger (m) RNA in a GH-transgenic tilapia (Oreochromis niloticus)

We have previously produced transgenic fish from crosses between a wild-type female tilapia (Oreochromis niloticus) and a G transgenic male. This line of growth-enhanced tilapia carries a single copy of a chinook salmon (s) growth hormone (GH) gene spliced to an ocean pout antifreeze promoter (OPA-FPcsGH) co-ligated to a carp beta-actin/lacZ reporter gene construct, integrated into the tilapia genome. Because little is known about the expression sites of transgenes, we have characterised the gene expression patterns of sGH and tilapia (t)GH in transgenic tilapia using a newly established real-time PCR to measure the absolute mRNA amounts of both hormones. The sGH gene, which was expected to be expressed mainly in liver, was also found to be expressed in other organs, such as gills, heart, brain, skeletal muscle, kidney, spleen, intestine and testes. However, in pituitary no sGH mRNA but only tGH mRNA was found. Tilapia GH mRNA in wild-type pituitary amounted to 226 +/- 30 pg/microg total RNA but in transgenics only to 187 +/- 43 pg/microg total RNA. Liver exhibited the highest level of sGH mRNA (8.3 +/- 2.5 pg/microg total RNA) but the extrahepatic sites expressed considerable amounts of sGH mRNA ranging from 4.1 +/- 2.0 pg/microg total RNA in gills to 0.2 +/- 0.08 pg/microg total RNA in kidney. The widespread expression of the sGH gene is assumed to be due to the tissue specificity of the type III AFP gene promoter. It is assumed that our transgenic experiments, which in contrast to some other approaches caused no obvious organ abnormalities, mimick the GH expression during ontogeny. Because sGH mRNA is expressed both in liver and in extrahepatic sites it may not only promote secretion and release of liver-derived (endocrine) IGF-I leading to an overall growth enhancement but also stimulate IGF-I expression within the different organs in a paracrine/autocrine manner and, thus, further promote organ growth.

Effect of seawater temperature on leucocyte populations in Atlantic salmon post-smolts

Using monoclonal antibodies (mAb) and flow cytometry, the distribution of immunoglobulin positive (Ig+) cells and neutrophils in isolated peripheral blood leucocytes (PBL) and head kidney leucocytes (HKL) from Atlantic salmon under-yearling out-of-season smolts was studied in the post-smolt period from 0 to 14 weeks after seawater transfer. A temperature acclimation was performed in freshwater, resulting in four groups maintained at a water temperature of 18, 14, 10 and 6 degrees C, respectively. The temperature for each group was kept constant for the remaining period and all groups were reared under a simulated natural photoperiod regime (60 degrees 25'N). Sampling of eight fish from each temperature group was performed at regular intervals from 0 to 14 weeks after seawater transfer, starting the day after transfer (week 0). The seawater temperature influenced the distribution of the leucocyte populations, and the effect was most prominent in PBL. The lower rearing temperature (6 degrees C) resulted in higher percentages of Ig+ cells in PBL compared to fish reared at the other temperatures. The high temperature (18 degrees C) resulted in higher proportions of neutrophils and lower proportions of Ig+ cells in PBL compared to fish from the other temperature groups. The observed differences were consistent throughout the 14-week experimental period. While the present study indicate that rearing water temperature influence the distribution of leucocytes in blood of Atlantic salmon post-smolts, the proportions of HKL populations do not seem to be dependent on temperature to the same extent. Comparing the temperature groups, no clear differences in the percentages of Ig+ cells and neutrophils in HKL were observed. Likewise, no evident time-related changes in the leucocyte profiles within each temperature group could be observed during the studied post-smolt period. Significantly, the results could indicate that the post-smolts reared at a temperature of 18 degrees C experienced thermal stress or a non-optimal environment.

Overwinter fasting and re-feeding in rainbow trout: plasma growth hormone and cortisol levels in relation to energy mobilisation

This study investigated the roles of cortisol and growth hormone (GH) during a period of fasting in overwintering salmonid fish. Indices of carbohydrate (plasma glucose, liver glycogen), lipid (plasma free fatty acids (FFAs)) and protein metabolism (plasma protein, total plasma amino acids) were determined, together with plasma GH, cortisol and somatolactin (SL) levels at intervals in three groups of rainbow trout (continuously fed; fasted for 9 weeks then fed; fasted for 17 weeks). In fasted fish, a decline in body weight and condition factor was accompanied by reduced plasma glucose and hepatic glycogen and increased plasma FFA. No consistent elevation of plasma GH occurred until after 8 weeks of fasting when plasma GH levels increased ninefold. No changes were observed in plasma total protein and AA until between weeks 13 and 17 when both were reduced significantly. When previously fasted fish resumed feeding, plasma glucose and FFA, and hepatic glycogen levels rapidly returned to control values and weight gain resumed. No significant changes in plasma cortisol levels, related to feeding regime, were evident at any point during the study and there was no evidence that SL played an active role in the response to fasting. The results suggest that overwinter fasting may not represent a significant nutritional stressor to rainbow trout and that energy mobilisation during fasting may be achieved without the involvement of GH, cortisol or SL.

Peripheral blood and head kidney leucocyte populations during out-of-season (0+) parr-smolt transformation and seawater transfer of Atlantic salmon (Salmo salar L.)

Using monoclonal antibodies (MAb) and flow cytometry, Atlantic salmon neutrophils and Ig+ cells in blood and head kidney were studied in under-yearling out-of-season (0+) smolts, and 2 and 4 weeks after transfer to seawater. The parr-smolt transformation was induced using a phase advanced simulated natural photoperiod regime, and sampling of four fish was performed at regular intervals, starting on the date of the photoperiod initiation. During the freshwater period the proportion of neutrophils in the head kidney leucocytes (HKL) remained quite stable and only gradual changes in Ig+ cells were observed. In the peripheral blood leucocytes (PBL), the proportion of neutrophils markedly increased during the last month prior to seawater transfer. The most notable changes in the proportions of MAb+ leucocytes were observed in PBL after seawater transfer, with a significant increase in Ig+ cells and a significant decrease in neutrophils after two weeks in seawater. In the freshwater samples, although there were fluctuations, a decrease in the numbers of total leucocytes per millilitre blood and per gram head kidney during parr-smolt transformation was observed. The number of MAb+ cells in blood appeared to be relatively stable, while the number in head kidney tended to decrease. Following seawater transfer, the numbers of total and MAb+ leucocytes in both blood and head kidney increased markedly. The results suggest that changes in both distribution and numbers of leucocytes in peripheral blood and head kidney take place during parr-smolt transformation, and that marked changes are associated with seawater transfer. Some mechanisms possibly involved are indicated.

Melatonin modulates secretion of growth hormone and prolactin by trout pituitary glands and cells in culture

In Teleost fish, development, growth, and reproduction are influenced by the daily and seasonal variations of photoperiod and temperature. Early in vivo studies indicated the pineal gland mediates the effects of these external factors, most probably through the rhythmic production of melatonin. The present investigation was aimed at determining whether melatonin acts directly on the pituitary to control GH and prolactin (PRL) secretion in rainbow trout. We show that 2-[125I]-iodomelatonin, a melatonin analog, binds selectively to membrane preparations and tissue sections from trout pituitaries. The affinity was within the range of that found for the binding to brain microsomal preparations, but the number of binding sites was 20-fold less than in the brain. In culture, melatonin inhibited pituitary cAMP accumulation induced by forskolin, the adenyl cyclase stimulator. Forskolin also induced an increase in GH release, which was reduced in the presence of picomolar concentrations of melatonin. At higher concentrations, the effects of melatonin became stimulatory. In the absence of forskolin, melatonin induced a dose-dependent increase in GH release, and a dose-dependent decrease in PRL release. Melatonin effects were abolished upon addition of luzindole, a melatonin antagonist. Our results provide the first evidence that melatonin modulates GH and PRL secretion in Teleost fish pituitary. Melatonin effects on GH have never been reported in any vertebrate before. The effects result from a direct action of melatonin on pituitary cells. The complexity of the observed responses suggests several types of melatonin receptors might be involved.

Atlantic halibut growth hormone: structure and plasma levels of sexually mature males and females during photoperiod-regulated annual cycles

The main objectives of this study were to obtain the amino acid sequence of Atlantic halibut (Hippoglossus hippoglossus) growth hormone (hhGH) and compare it with other teleost species, to establish a radioimmunoassay to assess plasma hhGH levels and thus to gain information about possible biological functions and regulation by photoperiod. The hhGH gene was cloned and its amino acid sequence deduced from the cDNA. The mature hhGH protein consists of 186 amino acids. Comparison with other flatfish species as well as a species from a different order, the pufferfish, reveals that the sequence similarities of the mature hhGH with that of the barfin flounder, the Japanese flounder, the sole and the pufferfish are 99.5, 81.7, 74.2, and 65.2%, respectively. The sequence similarities appear to correctly reflect the gross phylogenetic relationships among these teleost species. A specific GH-RIA was developed for measurements of Atlantic halibut GH levels. Assessment of plasma GH levels in adult halibut revealed large gender differences, with GH levels frequently being an order of magnitude higher in males than females. The mean (+/-SEM) plasma GH for males kept on normal annual photoperiod were 25.2+/-6.11 ngml(-1) and for females were 5.14+/-1.94 ngml(-1). It appears likely that plasma growth hormone levels in Atlantic halibut can be inversely correlated to growth and metabolism. Shifting of the annual photoperiod cycles demonstrated that photoperiod in not a regulator of plasma GH levels in the Atlantic halibut, but further research is needed to assess whether GH plays a role in the reproduction of this marine teleost species.