Smoltification In Salmonids Research Articles

Changes in circulating insulin-like growth factor-1 and its binding proteins in yearling rainbow trout during spring under natural and manipulated photoperiods and their relationships with gill Na+, K+-ATPase and body size

Smoltification in salmonids occurs during spring in response to increasing photoperiod to prepare for marine life. Smoltification is associated with increased hypo-osmoregulatory ability and enhanced growth potential, mediated by growth hormone and insulin-like growth factor (IGF)-1. Rainbow trout is uniquely insensitive to the induction of smoltification-associated changes by photoperiod, such as the activation of gill Na+,K+-ATPase (NKA). We measured the circulating IGF-1 and IGF-binding protein (IGFBP)-2b levels in yearling rainbow trout exposed to natural and manipulated photoperiods during spring and correlated these with gill NKA activity and body size. Although the effect of photoperiod manipulation on body size and circulating IGF-1 and IGFBP-2b was negligible, they were positively correlated with gill NKA activity in fish under simulated natural photoperiod. We next pit-tagged yearling rainbow trout and fed them a restricted ration or to satiation under a natural photoperiod. In April, gill NKA activity was higher in the satiation group than in the restricted group and positively correlated with body size and growth rate. In addition, circulating IGFBP-2b was positively correlated with gill NKA, size and growth, whereas circulating IGF-1 was correlated only with size and growth. The relationship between circulating IGF-1 and growth intensified from May to June, suggesting that the IGF-1–growth relationship was disrupted in April when gill NKA was activated. Two additional IGFBPs were related to growth parameters but not to gill NKA activity. The present study suggests that circulating IGFBP-2b and IGF-1 mediate the size-dependent activation of gill NKA in yearling rainbow trout during spring.

Enhanced osmoregulatory ability marks the smoltification period in developing chum salmon (Oncorhynchus keta)

The freshwater (FW) life of chum salmon is short, as they migrate to the ocean soon after emergence from the substrate gravel of natal waters. The alevins achieve seawater (SW) acclimating ability at an early developmental stage and the details of smoltification are not clear. We examined the stage-dependent SW acclimating ability in chum salmon alevins and found a sharp increase in SW tolerance during development that resembles the physiological parr-smolt transformation seen in other salmonids. Perturbation of plasma Na+ after SW exposure was prominent from the hatched embryo stage to emerged alevins, but the plasma Na+ became highly stable and more resistant to perturbation soon after complete absorption of yolk. Marker gene expression for SW-ionocytes including Na/K-ATPase (NKA α1b), Na-K-Cl cotransporter 1a (NKCC1a), Na/H exchanger 3a (NHE3a), cystic fibrosis transmembrane conductance regulators (CFTR I and CFTR II) were all upregulated profoundly at the same stage when the alevins were challenged by SW, suggesting that the stability of plasma Na+ concentration was partly a result of elevated osmoregulatory capability. FW-ionocyte markers including NKA α1a and NHE3b were consistently downregulated independent of stage by SW exposure, suggesting that embryos at all stages respond to salinity challenge, but the increase in SW osmoregulatory capability is restricted to the developmental stage after emergence. We propose that the “smoltification period” is condensed and integrated into the early development of chum salmon, and our results can be extrapolated to the future studies on hormonal controls and developmental triggers for smoltification in salmonids.

Effects of different photoperiod regimes on the smoltification and seawater adaptation of seawater-farmed rainbow trout (Oncorhynchus mykiss): Insights from Na+, K+–ATPase activity and transcription of osmoregulation and growth regulation genes

Photoperiod is thought to be the main zeitgeber that induces smoltification in salmonids. However, its effects on the smoltification of rainbow trout (Oncorhynchus mykiss) are not fully understood and no published data documents the effects of the photoperiod regime currently used commercially, continuous light (LL). The present study compared the effect of four different photoperiod regimes (i.e. advanced phase photoperiod (APP), delayed phase photoperiod (DPP), LL and simulated natural photoperiod (SNP)) on the smoltification and growth of juvenile rainbow trout during their freshwater phase of winter-spring and the following summer post smolt phase. Smoltification was evaluated by monitoring gill Na+,K+–ATPase (NKA) activity and transcription of NKA α-subunit isoforms 1a and 1b, and Na+,K+,2Cl‐cotransporter 1a. Growth was measured as specific growth rate of both length and weight, and through molecular growth proxies such as the levels of circulating insulin-like growth factor 1 (IGF-I) in plasma and transcription of igf-I, igf binding protein 1b (igfbp1b), growth hormone receptor 1 (ghr1) and cathepsin L (ctsl) in the liver. Results indicate that APP induces a longer smolt window and higher levels of plasma IGF-I in both freshwater and seawater (two months post transfer), while DPP led to a shorter smolt window, lower plasma IGF-I levels in freshwater and seawater, an earlier decrease in liver igf-I and ctsl transcription in freshwater (as seen by modelling over time) and lower specific growth rate in freshwater. The transcription analysis of osmoregulatory genes complemented NKA activity and allowed for the detection of a transient response to light and of differences between the osmoregulatory capacity of parr and desmolted fish. Furthermore, an upregulation of the liver transcription of igf-I, ghr1 and ctsl was found in all treatments during the smolt window, which corresponded to the periods with highest growth. Finally, both plasma IGF-I and liver igf-I in seawater were found to be significantly correlated to fish growth in seawater. However, our data did not show that plasma IGF-I prior to seawater transfer could be used as a reliable predictor of growth in seawater. Overall, and especially when compared with other salmonid species, photoperiod seems to be a weaker inducer of smoltification in rainbow trout, according to the parameters that were tested, suggesting that other environmental cues might be more important drivers of this process.

Comparison of Migratory and Resident Populations of Brown Trout Reveals Candidate Genes for Migration Tendency.

Candidate genes associated with migration have been identified in multiple taxa: including salmonids, many of whom perform migrations requiring a series of physiological changes associated with the freshwater–saltwater transition. We screened over 5,500 SNPs for signatures of selection related to migratory behavior of brown trout Salmo trutta by focusing on ten differentially migrating freshwater populations from two watersheds (the Koutajoki and the Oulujoki). We found eight outlier SNPs potentially associated with migratory versus resident life history using multiple (≥3) outlier detection approaches. Comparison of three migratory versus resident population pairs in the Koutajoki watershed revealed seven outlier SNPs, of which three mapped close to genes ZNF665-like, GRM4-like, and PCDH8-like that have been previously associated with migration and smoltification in salmonids. Two outlier SNPs mapped to genes involved in mucus secretion (ST3GAL1-like) and osmoregulation (C14orf37-like). The last two strongly supported outlier SNPs mapped to thermally induced genes (FNTA1-like, FAM134C-like). Within the Oulujoki, the only consistent outlier SNP mapped close to a gene (EZH2) that is associated with compensatory growth in fasted trout. Our results suggest that a relatively small yet common set of genes responsible for physiological functions associated with resident and migratory life histories is evolutionarily conserved.

Is salmon smoltification an example of vertebrate metamorphosis? Lessons learnt from work on flatfish larval development

The terms metamorphosis and smoltification both describe developmental processes. However, the question on what specific criteria define these terms continues to engage scientists. At the same time, various views have been expressed on whether or not smoltification of anadromous salmonids should be regarded as an example of vertebrate metamorphosis. This short overview tries to summarize some of these discussions and starts by determining if smoltification meets any of the criteria used to define metamorphosis. In particular, it broadly compares the process of flatfish metamorphosis with that of salmonid smoltification from a morphological, endocrine, molecular and behavioral perspective. Tools and approaches developed and used in metamorphosis research which could be useful in continued work on smoltification are highlighted.

Effects of Melatonin Feeding on Smoltification in Masu Salmon (Oncorhynchus masou)

Influences of photoperiod on plasma melatonin profiles and effects of melatonin administration on long-day-induced smoltification in masu salmon (Oncorhynchus masou) were investigated in order to reveal the roles of melatonin in the regulation of smoltification in salmonids. Under light-dark (LD) cycles, plasma melatonin levels exhibited daily variation, with higher values during the dark phase than during the light phase. The duration of nocturnal elevation under short photoperiod (LD 8:16) was longer than that under long photoperiod (LD 16:8). Melatonin feeding (0.01, 0.1 and 1 mg/kg body weight) elevated plasma levels of melatonin in a dose-dependent manner for at least 7 h but not for 24 h. When masu salmon reared under short photoperiod were exposed to long photoperiod (LD 16:8) and fed melatonin (1 mg/kg body weight) 7 hours before the onset of darkness, a significantly smaller proportion of smolts appeared in the melatonin-fed group after 32 days than in the control group. However, after 59 days of the treatment, there was no difference in the proportion of smolts between the control and melatonin-treated groups. Thus, melatonin feeding mimicked the effects of short photoperiod, which delays but does not completely suppress smoltification. These results indicate that the day length is transduced into changes in the duration of nocturnal elevation in plasma melatonin levels, and that artificial modification of the plasma melatonin pattern possibly delays the physiological processes of smoltification induced by long-day photoperiodic treatment.

The importance of smolt development to salmon conservation, culture, and management: perspectives from the 6th International Workshop on Salmonid Smoltification

The 6th International Workshop on Salmonid Smoltification was held in Westport, Ireland, during September 3–7, 2001. Perhaps more so than at past workshops, this meeting took a ‘salmon life cycle’ approach to smoltification. Results presented at the workshop illustrated the variation in the smoltification process among species, populations and individuals, suggesting that within-species variation can be of adaptive significance. Workshop presentations also demonstrated the importance of environmental factors and culture conditions. Salmon conservation and restoration programs should work to restore aquatic systems that allow for the full suite of smolt phenotypes, ensuring their full range of adaptations. A future challenge to anadromous salmonids, which depend on both marine and freshwater ecosystems, is global warming. The rate of change in, e.g., temperature, rainfull, runoff, and ocean climate is critical to the performance of smolts and their ability to cope with change. It is essential to ensure that the genetic variation and diversity in wild stocks are not reduced, so that the capacity to adapt remains. Climatic changes will likely further compound the

Juvenile coho salmon locomotion and mosaic muscle are modified by 3',3',5'-tri-iodo-L-thyronine (T3).

Studies of maximum aerobic swimming performance in smolting juvenile salmonids indicate that these animals may be aerobically compromised during downstream migration. To test our hypothesis that hyperthyroid status contributes to decreased swimming performance through modification of muscle contractility in juvenile (112 mm mean total length) coho salmon (Oncorhynchus kisutch), we measured swimming performance and isolated muscle bundle contractility of fish implanted with 3',3',5'-tri-iodo-l-thyronine (T3) pellets, of fish implanted with sham pellets and of fish with no pellet implantation (control group). After 3 weeks (N=12-13), critical swimming speeds (maximum aerobic swimming speed or U(crit)) were measured. Muscle bundles (N=15-16) were dissected from the hypaxial musculature and stimulated to measure the force and velocity of an isometric twitch and tetani. T3-treated fish demonstrated visible morphological changes associated with smoltification. Mean values of U(crit) were significantly decreased and the prolonged contraction (tetani) and twitch rates of contraction, relaxation and maximum force were significantly increased by T3 treatment compared with both the sham and control fish. Hematocrit, body mass and body length were not significantly affected by T3 treatment. In conclusion, we suggest that the reported decrease in U(crit) during salmonid smoltification may be mediated by endogenous T3-induced contractile modification of mosaic muscle fibers.

Specifying divergent migrations in the concept of stock: the contingent hypothesis

The current concept of stock does not sufficiently accommodate the effects of migration behavior on population dynamics. New insight on the prevalence and ecological significance of divergent migrations within stocks has arisen through advancing technologies in tracking fishes. In particular, otolith elemental analysis may provide unique perspective on divergent migrations within stocks. Using otolith elemental fingerprints of Hudson River striped bass ( Morone saxatilis), I evaluated Clark’s (Clark, J.,1968. Trans. Am. Fish. Soc. 97, 320–343.) contingent hypothesis of intrapopulation migratory groups. Similar to the results of Clark’s earlier tagging study, I observed three spatially discrete cohorts of Hudson River striped bass. An intriguing question is, if such contingents exist, then how are they regulated? Using frameworks for understanding smoltification in salmonids and ontogenetic niche shifts, I hypothesized that early life decisions regarding energy allocation lead to later divergence in habitat shifts (migration). This mechanism results in the maintenance of “retentive” and “exploratory” behaviors and contributes to features of population regulation including persistence, range contraction and expansion, and colonization. Support for the contingent hypothesis, i.e. divergent migration tactics, exists for a diverse array of fish taxa. Assignment of contingents should allow improved specification of factors which affect a population’s accessibility to human impacts. Consideration of stock structure should incorporate a hierarchy of biological levels which evaluate both lineage (sub-population–population–meta-population–sub-species–species) and accessibility (individual–brood–school/shoal–year class–contingent).

Influence of bacterial kidney disease on smoltification in salmonids: is it a case of double jeopardy?

We investigated the effects of a chronic, progressive infection with Renibacterium salmoninarum (Rs), the causative agent of bacterial kidney disease (BKD), on selected aspects of smoltification in yearling juvenile spring chinook salmon ( Oncorhynchus tshawytscha). After experimentally infecting fish with Rs using an immersion challenge, we sampled them every two weeks to monitor changes in gill Na +, K +-ATPase (ATPase), cortisol, infection level, mortality, growth, and other stress-related physiological factors during the normal time of parr–smolt transformation in fresh water (i.e., from winter to spring). A progressively worsening infection with Rs did not alter the normal changes in gill ATPase and condition factor associated with smoltification in juvenile chinook salmon. The infection did, however, lead to elevated levels of plasma cortisol and lactate and depressed levels of plasma glucose, indicating that the disease is stressful during the later stages. A dramatic proliferation of BKD was associated with maximal responses of indicators of smoltification, suggesting that the process of smoltification itself can trigger outbreaks of disease. Our results suggest mechanisms that probably influence the reported inability of Rs-infected fish to successfully adapt to sea water.

Regulation of Metamorphosis-Associated Changes in the Lipid Metabolism of Selected Vertebrates

SYNOPSIS. The life histories of many vertebrates include complex, postembryonic developmental pathways that involve morphological and physiological changes that adapt juveniles to a new habitat. A survey of such developmental pathways, including lamprey metamorphosis, salmonid smoltification, and anuran metamorphosis, reveals a common strategy of lipid metabolism consisting of two distinct phases. The first phase is characterized by lipid accumulation in storage sites and resultsfrom lipogenesis prevailing over lipolysis. The second phase is characterized by lipid depletion from storage sites and results from lipolysis prevailing over lipogenesis. Regulation of lipid deposition and lipid mobilization is essential for ensuring availability of lipid during times of need. Lipogenesis is promoted by insulin and, in lamprey and anurans, also by thyroid hormones. Lipolysis is promoted by a number of hormones, including prolactin, growth hormone, adrenocorticotropic hormone, corticosteroids, somatostatins, and thyroid hormones. The coordinate regulation of development-associated changes in lipid metabolism results from interactions among hormones and other internal and environmental cues.

Silvering and gill “mitochondria-rich” cells in the eel, Anguilla anguilla

Gills of typical yellow and silver ells, Anguilla anguilla L., were examined by light and electron microscopy. In both eel types, “mitochondria-rich” cells were located in the epithelium covering the primary lamellae and consisted of“chloride cells” and “accessory cells”. As compared to yellow eels, the primary gill epithelium of silver eels was thicker and contained larger and more numerous chloride cells with enlarged mitochondria. The accessory cells also increased in number but did not show significant modifications in their size or ultrastructural features. These observations indicate that, as far as mitochondria-rich cells are concerned, the silvering process in eels would be equivalent to smoltification in salmonids. It corresponds to a preparation for seawater life and is probably controlled by hormonal factors.

Insulin-like Growth Factor I Gene Expression during Parr-Smolt Transformation of Coho Salmon

The status of insulin-like growth factor I (IGF-I) mRNA in the liver and gill of coho salmon (Oncorhynchus kisutch) during the parr-smolt transformation (smoltification) was examined in relation to changes in plasma levels of thyroxine, cortisol, growth hormone and somatolactin. Smoltification coincided with silvering and an increase in gill Na /K -ATPase activity occurred during mid-April and May [22]. The levels of IGF-I mRNA in the liver rose steadily from the first sampling date in February to maximum levels in April. Increases in IGF-I mRNA in the gill occurred later in smoltication. Plasma cortisol, thyroxine and growth hormone increased during the latter half of April and May. Plasma somatolactin declined in April. These results demonstrate that IGF-I gene expression in both liver and gill increased during smoltification and suggest that IGF-I expression in both organs may be involved in salmonid smoltification.

Regulation of lipid metabolism in poikilothermic vertebrates

Patterns of lipid storage and utilization reflect the special life histories of the animal group. Poikilothermic vertebrates (fish, amphibians, reptiles) have evolved a number of life history patterns (smoltification, metamorphosis, hibernation) which present problems for lipid storage regulation. A general theme among poikilotherms is to store lipids among several sites (mesenteric fat, liver, muscle) as opposed to a single depot type (adipose tissue) as in homeotherms; lipids are stored primarily as triacylglycerols, but various other lipid classes also are stored and constituent fatty acids tend to be more complex (longer chain, polyunsaturated). Lipid storage is influenced by de novo lipid synthesis and by lipid deposition from various plasma lipoproteins. Lipid accumulation generally occurs in most poikilotherms during periods of feeding when plasma insulin levels are elevated. Lipid mobilization is controlled by an intracellular lipase enzyme. Lipid depletion occurs during transitional and non-feeding periods and is especially apparent during salmonid smoltification and amphibian metamorphosis.

Gill (Na + + K +)-ATPase involvement and regulation during salmonid adaptation to salt water

1. 1. The involvement of gill (Na ++ K +)-ATPase in salmonid adaptation to salt water (SW) is discussed. 2. 2. Gill (Na ++ K +)-ATPase increase during SW adaptation is mainly related to the increased number and complexity of chloride cells deputed to salt extrusion. 3. 3. The temporal relationships between serum peaks of thyroid hormones, Cortisol, growth hormone, prolactin and gill (Na + + K +)-ATPase rise during salmonid smoltification, suggest a hormonal involvement in the enzyme stimulation and thus in the acquirement of SW tolerance. 4. 4. Literature on gill (Na + + K +)-ATPase response to hormonal treatments is reviewed. The effects produced on gill (Na + + K +)-ATPase and chloride cells by exogenous hormones point out a complex inter-relationship between the hormones considered. The mechanisms involved in hormonal regulation of the enzyme remain a matter of debate.

A relationship between adult recoveries of chinook salmon ( Oncorhynchus tshawytscha) and lunar phase at time of their release from a hatchery on the Rakaia River, New Zealand

Studies on chinook salmon ( Oncorhynchus tshawytscha) juveniles held at the Glenariffe Salmon Research Station on the Rakaia River, New Zealand, have shown cyclical surges in plasma thyroxine, apparently associated with lunar phase. Since thyroid secretions are implicated in salmonid smoltification, data from batches of tagged chinook salmon released from Glenariffe were analysed to compare percent adult recovery with time on the lunar cycle when they were released as smolts. Results from a total of 136 batches were examined, covering 7 year classes released between 1979 and 1985. A relationship was found between survival to adulthood and lunar phase at the time of smolt release. A mathematical model, utilising both smolt weight at release and time of release within the lunar cycle, was developed to fit the data. According to this model, theoretical maximum recoveries would come from releases made 3–6 days before new moon.

Variation in the degree of silvering of wild coho salmon, Oncorhynchus kisutch, smolts migrating seaward from Carnation Creek, British Columbia

Degree of silvering (colouration) is often used as an index of the degree of smoltification in salmonids. To judge the reliability of silvering as an indicator of migratory readiness, we examined the silvering of wild coho salmon, Oncorhynchus kisutch, smolts upon entry into the Carnation Creek, British Columbia, estuary. Silvering of migrants was greater in larger fish and increased over the course of the migratory period. Photoperiod appeared to account for most of the observed increase in silvering over time; increased silvering was not correlated with temperature or lunar phase. Overall, 50% of seaward migrants were completely silvered, 45% were in a transitional phase, and 5% still retained colouration characteristics of parr. Colouration thus appears to have limited utility as a reliable indicator of migratory readiness. Our findings suggest that estuarine residence is important for completion of parr‐smolt transformation and that acclimation of hatchery smolts in brackish water prior to sea water entry may enhance their survival.

Rhythmic changes in plasma thyroxine concentrations in hatchery‐reared chinook salmon,Oncorhynchus tshawytscha

Abstract Plasma concentrations of throxine (T4) in chinook salmon, Oncorhynchus tshawytscha, rearing at a New Zealand salmon hatchery, were measured by radioimmunoassay. Blood samples for assay were collected at approximately weekly intervals during the period February‐August (southern late summer‐winter). Plasma T4 levels varied in a cyclical fashion, with maximum concentrations occurring near the time of each new moon. Within the lunar month, the mean date of the recorded T4 peak was at 6.4 + 3.9 days before new moon (95% confidence level). Since thyroid hormones have been implicated in the process of salmonid smoltification, we suggest that the planning of hatchery release times for chinook salmon should include a component which takes account of the apparent link between lunar phase and circulating T4 levels in these fish.

Development-associated changes in thyroxine kinetics in juvenile salmon.

Smoltification is a transformation that occurs in some species of salmon, during which solitary fish in fresh water become schooling fish and migrate to the sea. This process is accompanied by large increases in plasma T4. T4 secretion rate and other parameters of T4 metabolism in juvenile coho salmon were estimated by applying kinetic analyses to measurements of the disappearance of injected T4 radiotracer from plasma. Studies were performed at the beginning (March) and end (May) of the increase in T4 concentration in fresh water and seawater. Early and intensive sampling permitted characterization of a very fast initial component of the T4 disappearance curve when analyses included a zero time datum derived from an independent estimate of plasma volume. The plasma volume, equal to 1.77% of body weight, was obtained by measuring the disappearance of radiolabeled albumin from the plasma in two other groups of animals in fresh water and seawater. There were 3- to 7-fold changes in T4 production, distribution, and metabolism between March and May, whereas environment (fresh water vs. seawater) had relatively minor effects on T4 kinetics. In fresh water, the T4 secretion rate was 4.48 ng/h in March and 1.50 ng/h in May. The total T4 pool size was 37.8 ng in March and 12.2 ng in May. Plasma-tissue T4 fluxes were 3- to 7-fold greater in May. Relatively less T4 was distributed in tissue in May (63% vs. 83%), and T4 spent much less time in tissue in May than in March during each pass through the tissue space (11 min vs. 3.1 h). We propose that the difference in secretion rate and a redistribution of T4 between blood and tissues contribute to both the rise and fall in the plasma T4 concentration between March and May. Changes in T4 kinetics during salmonid smoltification resemble those occurring during amphibian metamorphosis and mammalian gestation and neonatal life, and may reflect an increased requirement and an important role for thyroid hormones during periods of rapid development in vertebrates in general.