Parr-smolt Transformation Of Atlantic Salmon Research Articles

Effects of short-term acid and aluminum exposure on the parr-smolt transformation in Atlantic salmon ( Salmo salar): Disruption of seawater tolerance and endocrine status

Episodic acidification resulting in increased acidity and inorganic aluminum (Al i) is known to interfere with the parr-smolt transformation of Atlantic salmon ( Salmo salar), and has been implicated as a possible cause of population decline. To determine the extent and mechanism(s) by which short-term acid/Al exposure compromises smolt development, Atlantic salmon smolts were exposed to either control (pH 6.7–6.9) or acid/Al (pH 5.4–6.3, 28–64 μg l −1 Al i) conditions for 2 and 5 days, and impacts on freshwater (FW) ion regulation, seawater (SW) tolerance, plasma hormone levels and stress response were examined. Gill Al concentrations were elevated in all smolts exposed to acid/Al relative to controls confirming exposure to increased Al i. There was no effect of acid/Al on plasma ion concentrations in FW however, smolts exposed to acid/Al followed by a 24 h SW challenge exhibited greater plasma Cl − levels than controls, indicating reduced SW tolerance. Loss of SW tolerance was accompanied by reductions in gill Na +,K +-ATPase (NKA) activity and Na +,K +,2Cl − (NKCC) cotransporter protein abundance. Acid/Al exposure resulted in decreased plasma insulin-like growth factor (IGF-I) and 3,3′,5′-triiodo- l-thyronine (T 3) levels, whereas no effect of treatment was seen on plasma cortisol, growth hormone (GH), or thyroxine (T 4) levels. Acid/Al exposure resulted in increased hematocrit and plasma glucose levels in FW, but both returned to control levels after 24 h in SW. The results indicate that smolt development and SW tolerance are compromised by short-term exposure to acid/Al in the absence of detectable impacts on FW ion regulation. Loss of SW tolerance during short-term acid/Al exposure likely results from reductions in gill NKA and NKCC, possibly mediated by decreases in plasma IGF-I and T 3.

Endocrine disruption of parr-smolt transformation and seawater tolerance of Atlantic salmon by 4-nonylphenol and 17β-estradiol

Sex steroids are known to interfere with the parr-smolt transformation of anadromous salmonids, and environmental estrogens such as nonylphenol have recently been implicated in reduced returns of Atlantic salmon in the wild. To determine the endocrine pathways by which estrogenic compounds affect smolt development and seawater tolerance, groups of juvenile Atlantic salmon were injected with one of five doses (0.5, 2, 10, 40 or 150 μg g −1) of branched 4-nonylphenol (NP), 2 μg g −1 of 17β-estradiol (E 2), or vehicle, during the parr-smolt transformation in April, and the treatment was repeated 4, 8, and 11 days after the first injection. Plasma was obtained for biochemical analysis 7 and 14 days after initiation of treatment. After 14 days of treatment, additional fish from each treatment group were exposed to seawater for 24 h to assess salinity tolerance. The E 2 treatment and the highest NP dose resulted in lower salinity tolerance and decreased plasma insulin-like growth factor I (IGF-I) levels, along with elevated levels of plasma vitellogenin and total calcium. Plasma growth hormone levels were elevated at intermediate NP doses only, and not affected by E 2. After 7 days, plasma thyroxine (T 4) levels decreased in a strong, dose-dependent manner in response to nonylphenol, but after 14 days, this suppressive effect of T 4 occurred at the highest NP dose only. Similarly, E 2 decreased plasma T 4 levels at 7, but not 14 days. Plasma 3,3′,5-triodo- l-thyronine was reduced by E 2 and the highest NP dose after 7 and 14 days of treatment. Plasma cortisol levels were not affected by any of the treatments. The results indicate that the parr-smolt transformation and salinity tolerance can be compromised by exposure to estrogenic compounds. Suppression of plasma IGF-I levels is a likely endocrine pathway for the effects of estrogenic compounds on hypo-osmoregulatory capacity, and the detrimental effects of E 2 and NP on thyroid hormone levels are also likely to compromise the normal parr-smolt transformation of Atlantic salmon.

Growth hormone endocrinology of Atlantic salmon (Salmo salar): pituitary gene expression, hormone storage, secretion and plasma levels during parr-smolt transformation.

A number of studies on the Atlantic salmon (Salmo salar), have reported changes in plasma GH during parr-smolt transformation, but there is a lack of information about the endocrinology of the GH system during this process. In order to elucidate the mechanisms underlying these changes in plasma GH levels during the parr-smolt transformation of Atlantic salmon, GH mRNA expression in the pituitary was studied together with total pituitary GH content, in vitro GH secretion rate and plasma GH and IGF-I levels. Atlantic salmon were kept in outside tanks, under natural condition from early February until late June. Approximately three times a month fish were killed and pituitaries and blood were sampled for investigation. Further, pituitaries were moved to the laboratory for in vitro GH secretion studies. The results show that the GH system is first activated by an increase in GH secretion rate, which leads to an increase in plasma GH levels and causes a drop in the total GH content of the pituitary. This drop in pituitary GH content is later reversed by an increased GH synthesis seen as an increase in GH mRNA expression. Maximal activation of the GH system is seen to occur in early May, when plasma IGF-I levels reach highest levels, after which a certain deactivation of the GH system takes place. The data show that plasma levels of GH are to a large extent regulated by the secretion rate from the pituitary, although changes in the GH clearance rate are also likely to take place and influence the plasma GH levels. The study further underlines the significant role that the GH-IGF-I axis plays in the parr-smolt transformation of the Atlantic salmon.

Parr-smolt transformation in Atlantic salmon: thyroid hormone deiodination in liver and brain and endocrine correlates of change in rheotactic behavior

We tested the hypothesis that metabolism of thyroid hormones by the brain of Atlantic salmon (Salmo salar) changes when rheotactic behavior reverses during parr-smolt transformation (PST). We measured brain and liver thyroxine (T4) and 3,5,3'-triiodo-L-thyronine (T3) outer-ring deiodination (ORD) and inner-ring deiodination (IRD) activities and plasma T4and T3levels in Atlantic salmon held under natural photoperiod in fresh water at 10°C in the spring of 1993 and 1994. We also measured changes in T4, T3, and cortisol levels during the change in rheotactic behavior. Condition factor decreased while salinity tolerance improved from mid-March to late April. The turbidity-induced transition from upstream to downstream swimming occurred in mid to late April. The main changes in brain deiodination were reduced T3IRD (1993 study) and elevated T4ORD (1994 study). In both years, a high ratio of T4ORD/T3IRD activities in the brain indicated an increased potential for T3production in the brain during advanced PST. Liver deiodination profiles differed between years, but during advanced PST the low T4ORD activity and low T4ORD/T3IRD activity ratio suggested a low potential for hepatic, and hence systemic, T3production. However, plasma T4was increased in downstream swimmers at 1 d (1993) and 4 h (1994) after the turbidity increase. Since at this time brain deiodination pathways were poised towards T3production, the surge in plasma T4would likely increase local T3formation in brain. We conclude that during PST there is no major change in hepatic deiodination and hence probably no major change in systemic T3availability. But deiodination properties in brain during late PST indicate the potential for local T3formation. This may be significant when plasma T4increases at the time of downstream migration.

Parr-smolt transformation in Atlantic salmon: thyroid hormone deiodination in liver and brain and endocrine correlates of change in rheotactic behavior

Parr-smolt transformation in Atlantic salmon: thyroid hormone deiodination in liver and brain and endocrine correlates of change in rheotactic behavior

Polyunsaturated fatty acid metabolism in Atlantic salmon (Salmo salar) undergoing parr-smolt transformation and the effects of dietary linseed and rapeseed oils.

Duplicate groups of Atlantic salmon parr were fed diets containing either fish oil (FO), rapeseed oil (RO), linseed oil (LO) or linseed oil supplemented with arachidonic acid (20:4n-6; AA) (LOA) from October (week 0) to seawater transfer in March (week 19). From March to July (weeks 20–34) all fish were fed a fish oil-containing diet. Fatty acyl desaturation and elongation activity in isolated hepatocytes incubated with [1-14C]18:3n-3 increased in all dietary groups, peaking in early March about one month prior to seawater transfer. Desaturation activities at their peak were significantly greater in fish fed the vegetable oils, particularly RO, compared to fish fed FO. Docosahexaenoic acid (22:6n-3:DHA) and AA in liver and gill polar lipids (PL) increased in all dietary groups during the freshwater phase whereas eicosapentaenoic acid (20:5n-3; EPA) increased greatly in all groups after seawater transfer. The AA/EPA ratio in tissue PL increased up to seawater transfer and then decreased after transfer. AA levels and the AA/EPA ratio in gill PL were generally higher in the LOA group. The levels of 18:3n-3 in muscle total lipid were increased significantly in the LO, LOA and, to a lesser extent, RO groups prior to transfer but were reduced to initial levels by the termination of the experiment (week 34). In contrast, 18:2n-6 in muscle total lipid was significantly increased after 18 weeks in fish fed the diets supplemented with RO and LO, and was significantly greater in the FO and RO groups at the termination of the experiment. Gill PGF production showed a large peak about two months after transfer to seawater. The production of total PGF post-transfer was significantly lower in fish previously fed the LOA diet. However, plasma chloride concentrations in fish subjected to a seawater challenge at 18 weeks were all lower in fish fed the diets with vegetable oils. This effect was significant in the case of fish receiving the diet with LOA, compared to those fed the diet containing FO. The present study showed that during parr-smolt transformation in Atlantic salmon there is a pre-adaptive increase in hepatocyte fatty acyl desaturation/elongation activities that is controlled primarily by environmental factors such as photoperiod and temperature but that can also be significantly modulated by diet. Feeding salmon parr diets supplemented with rapeseed or linseed oils prevented inhibition of the desaturase activities that is induced by feeding parr diets with fish oils and thus influenced the smoltification process by altering tissue PL fatty acid compositions and eicosanoid production. These effects, in turn, had a beneficial effect on the ability of the fish to osmoregulate and thus adapt to salinity changes.

Timing of parr-smolt transformation in Atlantic salmon ( Salmo salar): effects of changes in temperature and photoperiod

Water temperature modified the development of smolt characteristics and their duration after an abrupt increase in daylength. A temperature change alone was not a sufficient environmental cue to induce a similar development. To complete parr-smolt transformation after an abrupt increase in daylength, a thermal sum of approximately 400 degree-days (°d) seemed to be necessary. Three groups of Atlantic salmon (1 +) were reared under continuous light (LL) from hatching and then exposed to a 7-week period of short days (LD 8.15:15.45) which ended April 2. In parallel, three groups were exposed to continuous light alone. All six groups were then split into two subgroups of equal numbers. A subgroup from the short-day regime were then mixed with a subgroup from the continuous light regime, to make it possible to continue the experiment with only six tanks and still use duplicates in each combination of light and temperature regime. Thereafter, three temperature regimes were applied: 7.7°C (mean) (4.5–10.8) (range), 10.7°C (7.6–13.7) or 13.7°C (10.5–17.3). Mean temperature from January 31 to April 2 was 9.1°C. The medium temperature regime included no abrupt temperature shift. The short-day regime caused a lower thermal-growth coefficient (TGC) than the continuous regime until the increase in daylength. After this increase and in the rest of the experiment, the TGC was still lowest ( P<0.05) in the groups previously exposed to short days. The short-day regime caused a reduction in the condition factor ( P<0.001) and an increase in gill Na–K-ATPase ( P<0.001) after the increase in daylength in contrast to the continuous light regime. Silvery colour and the ability to regulate plasma Cl − level in a 24-h seawater challenge test developed under both light regimes. The decrease in condition factor was not significantly affected by temperature. Silvering and Na–K-ATPase activity developed more rapidly at the high temperature regime ( P<0.001), but no difference was found between the medium and low temperatures. An elevated gill Na–K-ATPase activity and low plasma Cl − levels in the 24-h seawater challenge test were found for a shorter period at higher temperatures. Plasma Cl − levels below 150 mM were found for approximately 1, 2 or 6 weeks in the high, medium and low temperature regime, respectively. In the continuous light regime, the temperature regime had no significant effect on the development of silvering, condition factor or gill Na–K-ATPase activity, but the ability to regulate plasma Cl − was affected by temperature ( P<0.05).

Effects of l-thyroxine on brain monoamines during parr-smolt transformation of Atlantic salmon ( Salmo Salar L.)

During spring, seaward migrating juvenile Atlantic salmon ( Salmo salar) undergo parr-smolt transformation (PST) which involves changes in physiology, including one or two peaks in plasma thyroxine (T 4). To investigate if changes in plasma T 4 influence neural function, we measured levels of dopamine (DA) and its metabolites, 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA), and also measured serotonin (5-hydroxytryptamine, 5-HT) and its metabolite, 5-hydroxyindoleacetic acid (5-HIAA) in brain regions of two groups of Atlantic salmon parr on an 8:16 h light/dark photoperiod. One group was treated with ambient T 4 to simulate the natural smolt peak in plasma T 4. T 4 treatment depressed DOPAC levels as well as DOPAC/DA and 5-HIAA/5-HT ratios in the olfactory system but with no changes in the optic tectum. We conclude that during PST monoaminergic functions in specific brain regions of juvenile Atlantic salmon are affected by T 4 treatment.

Effects of dietary vitamin C on growth and parr-smolt transformation in Atlantic salmon, Salmo salar L.

Atlantic salmon (Salmo salar) parr were fed fish meal based pelleted diets supplemented with graded levels of ascorbate-2-monophosphate (AP), equivalent to 0, 20, 60 and 1000 mg ascorbic acid (AA) kg”1 throughout smoltification on a continuous light regime from February to June. No differences were observed in growth rate and body length distribution between the dietary regimes. The condition factor and the hepatosomatic index were somewhat elevated in fish fed no vitamin C throughout the smoltification, which can reflect changes in lipid metabolism in fish with suboptimal vitamin C nutrition. Sea water challenge tests (exposure to sea water with salinity of 34 gL−1 and ambient temperature for 24 h) performed monthly did not reveal differences attributed to the vitamin C status, as measured by mortality, serum chloride and cortisol concentrations, haematological parameters and liver and head kidney ascorbate concentrations after 24 h. Elevated serum cortisol concentrations most probably reflected stress in the challenge tests, and some lower concentrations in fish fed high vitamin C levels may indicate a certain stress-ameliorating effect. The present results do not, however, support the anticipation of increased requirement of dietary vitamin C above the minimum requirement during smoltification in Atlantic salmon.

Comparison between parr and smolt Atlantic salmon (Salmo salar) α subunit gene expression of Na+/K+ ATPase in gill tissue

Increases in branchial Na(+)/K(+) ATPase activity during seawater adaptation of euryhaline fish species, have been well documented. During the parr-smolt transformation of salmonids this activity increases two to five fold and is used as an indicator of the transformation. In order to improve the understanding of differences in enzyme activity found between Atlantic salmonSalmo salar parr and smolt fish, we investigated the gene expression of the Na(+)/K(+) ATPase α-subunit(s) in gill tissue. Gill mRNAs were analyzed and quantified at distinct time points using Northern and Dot blot techniques. We amplified by PCR, a conserved region of the cDNA encoding the Na(+)/K(+) ATPase α-subunit of the rainbow troutOncorhynchus mykiss. The PCR products (670 bp) were cloned and all independent clones showed a sequence corresponding to the α subunit of the Na(+)/K(+) ATPase. The fragments obtained appeared as a heterogenous population of three sequences showing, when compared between each other, 86 to 93% identity. This suggests that different allelic forms of the α-subunit are expressed in gill tissue. Hybridization studies performed with these PCR probes revealed two mRNA species, a major 3.7 kb transcript and a minor transcript of 1.8 kb. Enhanced 3.7 kb transcript levels are concurrent with elevated enzyme activity in smolts during the March and April parrsmolt transformation of Atlantic salmon. Interestingly, our study disclosed that smolt fish only displayed a two-fold increase in transcript levels when compared to parr whereas enzyme activity showed a 4 to 5 fold increase. This suggests that the increase in the 3.7 kb mRNA content of gill tissue is probably not the only mediator leading to the rise in enzyme activity during parr-smolt transformation.

Photoperiod Regulation of Plasma Growth Hormone Levels during Parr-Smolt Transformation of Atlantic Salmon: Implications for Hypoosmoregulatory Ability and Growth

The influence of photoperiod on plasma growth hormone (GH) levels during parr smolt transformation was studied in Atlantic salmon, using a specific salmon GH radioimmunoassay; in addition, the impact of endogenous changes in GH levels on hypoosmoregulatory ability and growth was assessed using seawater challenge tests and measurements of growth rates. Groups of fish were kept on simulated natural photoperiod or continuous light, and subgroups were subjected to reciprocal transfers between these two light regimes at different times. For Atlantic salmon on simulated natural photoperiod, GH levels increased during the parr-smolt transformation, from 0.6 ng ml-1 in January-March to 6.6 ng ml-1 in June, while the corresponding change in fish on continuous light was from 0.4 to 1.0 ng ml-1. The study demonstrates that photoperiod is a major zeitgeber for the increased GH levels during the parr-smolt transformation of Atlantic salmon. The data further support the view that exposure to continuous light into fall and winter causes a "free running" of an endogenous rhythm governing smolting and a subsequent phase-delay of the parr-smolt transformation-related increase in plasma GH levels. A strong positive nonlinear correlation was found between specific growth rate and GH levels, with growth rate increasing rapidly with increasing GH levels up to 2-3 ng ml-1 at which point near maximal growth rate is reached. revealing that relatively small increases in GH levels may be of great importance for an increased specific growth rate. A positive nonlinear correlation was also found between GH and hypoosmoregulatory ability, leading to the conclusion that a regime of continuous light has adverse effects on seawater tolerance and that a period of short daylength is necessary for the normal smolt-related physiological improvement in seawater adaptability.

Effects of photoperiod and temperature on growth and parr-smolt transformation in Atlantic salmon ( Salmo salar L.) and subsequent performance in seawater

Potential Atlantic salmon ( Salmo salar L.) 1 + smolts were reared during late winter and spring in a factorial design combining elevated, 12–13°C ( e), or ambient ( a) temperature with four photoperiod regimes: simulated natural photoperiod, 60°N (LN60, control); simulated natural photoperiod, 70°N (LN70); simulated natural photoperiod, 1 month phase delayed (LD60); and continuous light (LL). Growth rate in fresh water was enhanced by elevated water temperature and continuous light. No consistent differences were seen among the LN70 and LN60 groups at the two temperature regimes, whereas the LD60 e group had a lower growth rate than LN60 e. Condition factor decreased between February and April on LL under both thermal regimes. For the other groups on elevated temperature, condition factor decreased from March onwards, while no clear reduction was seen among the LN60 a, LN70 a or LD60 a groups. Based on morphological changes, increase in hypo-osmoregulatory ability and gill Na +,K +-ATPase activity, the LN60 e, LN70 e and LD60 e groups developed smolt characters in April or early May, while corresponding groups on ambient temperature did not complete smoltification until late May or early June. Fish were transferred to seawater in mid-June and measured again in mid-November. Groups reared on ambient temperature had higher growth rates in seawater than corresponding groups from elevated temperature. Smolts from LD60 had lower growth rate than did those from LN60 and LN70, indicating that the delay in photoperiod influenced the completion and/or timing of smolting. The LL e group had the lowest growth rate in seawater, suggesting that the continued exposure to LL and elevated temperature interfered with the normal course of parr-smolt transformation.

Thyroid Hormone Deiodination in Brain, Liver, Gill, Heart and Muscle of Atlantic Salmon (Salmo salar) during Photoperiodically-Induced Parr-Smolt Transformation, I. Outer- and Inner-Ring Thyroxine Deiodination

Outer-ring deiodinase (ORD) and inner-ring deiodinase (IRD) pathways for l -thyroxine (T4) were examined in the microsomes of tissues of 20-month-old Atlantic salmon induced to undergo parr-smolt transformation (PST) in late February and March by imposing a 16-hr photoperiod. Smolt external characteristics, decline in condition factor, increases in brainsomatic index, cardiac-somatic index, gill-somatic index, gill Na+/K+ ATPase activity, and food consumption developed progressively during the 5-week study. Plasma T4 (4.1 ng/ml, Week 1) rose to 13.5 and 12.5 ng/ml (Weeks 3 and 4) and fell to 7.5 ng/ml (Week 5). Plasma T3 rose from 2.1 ng/ml (Week 1) to 3.8 ng/ml (Week 2) and fell to 2.4 ng/ml (Week 5). T4ORD activity occurred in liver, heart, gill, brain, and skeletal muscle, increasing in liver and heart between Weeks 1 and 2, and in brain between Weeks 4 and 5. Only liver T4ORD activity was correlated (r = + 0.946) with plasma T3 concentrations, suggesting that hepatic T4ORD may determine the plasma T3 concentration. For hepatic T4ORD the mean Km was 0.42 nM and the mean Vmax 1.2 pmols T4 converted · hr-1 · mg microsomal protein-1. HPLC analysis revealed 3,3′,5-T3(rT3) and 3.3′-diiodo-l-thyronine, as evidence of T4IRD activity, but only in liver and brain. The lower bound estimates of T4IRD activities to generate rT3 were 28% (liver) and 50% (brain) of the respective T4ORD activities. Brain T4IRD increased progressively during PST. In post-smolts in later October, plasma T3 concentrations and both hepatic and brain T4ORD and T4IRD activities were low, but some rT3 was produced. We conclude that during induced PST in Atlantic salmon (i) T4ORD activity increases in liver, heart, and brain, but not in gill or skeletal muscles; (ii) the hepatic T4ORD is highly correlated with plasma T3 and may therefore determine plasma T3; and (iii) there are changes in T4ORD and T4IRD in brain, which may reflect regulation of intracellular thyroid hormone metabolism in that tissue. However, it remains to be determined how changes in deiodinase activity relate to the surge in plasma T4 and the role of thyroid hormones daring PST.

Growth and parr-smolt transformation of Atlantic salmon ( Salmo salar L.) under different light intensities and subsequent survival and growth in seawater

Groups of potential Atlantic salmon ( Salmo salar L.) smolts were reared under a simulated natural photoperiod from November 1987 to May 1988. Three different light intensities were used: 27, 335 or 715 lux. Growth rate was not significantly influenced by differences in light intensity. All groups developed the silvery external appearance, increased hypo-osmoregulatory ability and decreased condition coefficient associated with smolting. Smolts from all groups were successfully acclimatized to seawater in early May. The subsequent seven months of rearing in seawater did not reveal any differences in post-smolt performance of the groups. The incidence of early maturation during the first autumn was similar in all groups. We conclude that within the limits of the light sources used in the present experiment, light intensity had no significant effect on the growth and smolting in juvenile Atlantic salmon.

Investigation of hepatic cytosolic proteins during parr-smolt transformation of Atlantic salmon ( Salmo salar)

The diverse metabolic functions of the teleost liver and previous studies of hepatic mitochondria implicate this organ in smolting. To investigate hepatic involvement further, cytosolic proteins were examined in Atlantic salmon during parr-smolt transformation. Two separate studies were conducted in which juvenile salmon were reared with 6-month advanced (6MA), 8-month advanced (8MA) and simulated natural (SNP) photoperiod regimens. The two phase-shifted photoperiods were utilized in an attempt to produce non-smolting controls for discrimination between proteins related to ontogeny and proteins specific to parr-smolt transformation. Branchial Na + K + - ATPase activities, condition factors and salinity tolerances indicated that SNP fish underwent smolting during a normal time period. Numerous changes in cytosolic proteins were observed in SNP, 6MA and 8MA fish. Seven polypeptides exhibited qualitative or quantitative changes between December and May in SNP fish during the two investigations. However, when these differences were compared with the contemporary controls in the 6MA and 8MA studies, the changes did not appear to be specific to smolting.

Plasma prolactin levels during smolting in Atlantic salmon, Salmo salar

To obtain more information on a possible involvement of prolactin during parr-smolt transformation of Atlantic salmon, Salmo salar, parr and smolt populations of the same age were sampled from February to June. A clear separation of the two populations was observed according to their size and gill (Na +K +)-ATPase activity. Comparison of plasma prolactin (PRL) levels between these two populations indicated a significant decrease from May to early July in smolts whereas, in parr, a steady increase was observed. Diel variations in plasma PRL levels at three different dates were also studied in fish undergoing smolting. Sampling fish over a 24-h period on 11 March, 5 May and 15 July did not indicate the presence of a diel rhythm in PRL release. These data support the hypothesis that the PRL decrease is associated with smolting in Atlantic salmon and that such a decrease cannot be explained by a change in diel rhythm of plasma PRL levels.

Mitochondrial enzyme and Na+, K+-ATPase activity, and ion regulation during parr-smolt transformation of Atlantic salmon (Salmon salar)

Atlantic salmon (Salmo salar) exposed to either simulated natural photoperiod (SNP) or continuous light (L24) were used to examine developmental changes in the presence and absence, respectively, of the parrsmolt transformation. Plasma osmolarity and ion concentrations were unaffected by photoperiod treatment. Gill Na(+), K(+)-ATPase specific activity increased 150% between February and June in SNP fish and was low and unchanged in L24 fish. Kidney Na(+), K(+)-ATPase specific activity varied within similar, narrow limits in both groups. Citrate synthase of liver, gill and kidney, expressed as specific activity or activity/g total body weight (relative activity), increased 25-60% between March and June in SNP fish. With the exception of kidney relative activity, citrate synthase activity declined to initial (March) levels by August. Liver, gill and kidney cytochrome c oxidase activity of the SNP group underwent similar though less marked changes. Liver, gill and kidney citrate synthase and cytochrome c oxidase activities of the L24 group remained relatively constant between March and August, and where significant differences occurred, they were lower than those of the SNP group. These results indicate that respiratory capacities of the liver, gill and kidney increase in smolls concurrent with preparatory osmoregulatory changes, and subsequently decline. The findings are consistent with a hypothesized transient increase in catabolic activity during the parr-smolt transformation that may be due to the metabolic demands of differentiation.

Smoltification and seawater adaptation in Atlantic salmon ( Salmo salar): Plasma prolactin, growth hormone, and thyroid hormones

To obtain more information on the role of prolactin and growth hormone during the parr-smolt transformation of Atlantic salmon, a population of fish in fresh water was sampled from January to June during two consecutive years. Gill Na+,K+-ATPase activity increased steadily during smoltification and a plasma thyroxine peak was observed 2-3 weeks before the gill Na+,K+-ATPase peak. On the basis of these two parameters, smoltification was considered complete in our populations in April 1985 and May 1986. Two peaks in plasma growth hormone levels occurred in 1986, one in mid-April and the second in mid-May. In both cases, these peaks coincided with a peak in plasma triiodothyronine and preceded the thyroxine peak by 1-2 weeks. Moreover, the second peak which lasted for 1 month coincided with maximal gill Na+,K+-ATPase activity. A decrease in plasma prolactin levels was observed during smoltification of Atlantic salmon in 2 consecutive years. During this period of decreasing and low plasma prolactin levels, gill Na+,K+-ATPase activity increased to its highest values. Atlantic salmon smolts were also directly transferred into seawater. After 2 days or more in seawater, plasma prolactin levels were not significantly different from those on Day 0, whereas in fresh water they showed large fluctuations. All these data indicate that growth hormone may play an important role in the development of hypoosmoregulatory activity. Increased hypoosmoregulatory ability also appears to be associated with low prolactin levels.

The interrelationship of cortisol, Gill (Na + K) ATPase, and homeostasis during the Parr-Smolt transformation of atlantic salmon ( Salmo salar L.)

Serum cortisol concentrations were measured in juvenile Atlantic salmon ( Salmo salar L.) undergoing the parr-smolt transformation in fresh water, at either 1 year (S1 population) or 2 years (S2 population) after hatching. Serum cortisol levels were generally low (<10 ng ml −1), but during smoltification became significantly elevated in both populations. In addition, the S2 population showed a small cortisol peak in the autumn prior to smoltification. Simultaneous measurement of gill (Na + K) ATPase activity and serum cortisol concentrations in S2 salmon juveniles revealed that both features rose during smoltification in fresh water. The rise in gill (Na + K) ATPase activity was independent of cortisol levels, and preceded the rise in cortisol titer by approximately 1 month. After seawater transfer, gill enzyme levels remained high while cortisol titers fell sharply. Serum cortisol levels, but not gill (Na + K) ATPase activities, were progressively reduced by acclimation of smolts to increasing salinities. Linear regression studies indicated that, at any one level of gill (Na + K) ATPase, cortisol titer increased with increasing surface area: volume ratio. Extra-cellular fluid volume (sodium space) was found to decline with increasing gill (Na + K) ATPase activity, and to increase with serum cortisol titers. These results indicate that high serum cortisol levels represent a secondary response caused by the development of hypoosmoregulatory ability while still resident in fresh water. Cortisol does not appear to directly stimulate gill (Na + K) ATPase activity in Atlantic salmon smolts.