さけ・ます資源管理センター
研究報告 第 6 号 13-21頁
2004年3月

Bulletin of the National Salmon
Resources Center No. 6: 13-21
March 2004

Participation of Thyroxine in Smoltification of
Sockeye Salmon (Oncorhynchus nerka)
Masatoshi Ban
Research Division, National Salmon Resources Center,
2-2 Nakanoshima, Toyohira-ku, Sapporo 062-0922, Japan(dukeban @salmon.affrc.go.jp)
Abstract. − In order to describe effects of thyroxine on smoltification of sockeye salmon(Oncorhynchus nerka), seasonal patterns of smoltification were investigated. Factors assessed
include seawater tolerance, fin margin blackening, condition factor and changes in plasma thyroxine
concentration (plasma-T4). Juvenile sockeye salmon reared under natural photoperiod graduallydeveloped seawater tolerance from January to May in two consecutive years (1991 and 1992). Fin
margin blackening and decreases in condition factor were observed in May. Plasma-T4 showed significant elevations in concert with an increase in photoperiod from January to March, followed by
rapid decreases. Temporary elevation of plasma-T4 was also induced by switchover of artificial
photoperiod from short day (8/16 h light/dark cycle) to long day (16/8 h). With this hormonal surge,
fish developed seawater tolerance and transformed to smolt out-of-season. However, a single
administration of thyroxine supplemented in a commercial diet for twenty days did not induce seawater tolerance. These results suggest that thyroxine has a role as an enhancer of smoltification insockeye salmon and operates at an early phase of this process indirectly.
Key words : sockeye salmon, smoltification, photoperiod, thyroxine

Introduction
Smoltification in anadromous salmonids occurs
during their freshwater phase and is a complex,
developmental phenomenon (Hoar 1988). Thistransformation involves morphological, physiological and behavioral changes, which adapt them forseaward migration and the marine habitat (Wedemeyer et al. 1980). As smoltification progresses, anincreased seawater tolerance is accompanied by activation of gill Na+, K+-ATPase as a physiologicalchange, and fin margin blackening as well as reduction of condition factor as morphological changes.
Smoltification also occurs in association with thedevelopment of endocrine systems. Hoar (1939) was
the first to report a more developed thyroid gland in
smolt than in parr for Atlantic salmon (Salmo salar).
Dickhoff et al. (1978) observed a distinct peak inplasma thyroxine (T4) levels during smoltification incoho salmon (Oncorhynchus kisutch). Ban et al.
Contribution A No. 35 from the National Salmon
Resources Center.
© 2004 National Salmon Resources Center(1987) found hypertrophy of thyroid follicles insmolt of masu salmon (O. masou). There are various
other studies supporting the relationship between
smoltification and thyroid hormones (Alexander etal. 1998; Hutchison and Iwata 1998; Kelly andWood 2001). These studies indicate that T4 plays
important roles in smoltification in salmonid fish.
However, seasonal change in plasma T4 concentration seems not to be uniformity among salmonid
species. Masu salmon show a sharp and dramaticincrease in plasma T4 in May, while amago salmon(O. rhodurus) have a plateau and blunt T4 peak from
November to February (Yamauchi et al. 1984).
Dickhoff et al. (1978) reported that duration and timeof onset of the T4 surge varies in different stocks ofcoho salmon. Furthermore, serum T4 levels are correlated with the stream discharge rate (Youngson andSimpson 1984). Hence, secretion of T4 can be influenced by factors such as aquatic environments,
strains or habitat.
Release trials for sockeye salmon (O. nerka) havebeen performed by the National Salmon Resources
Center for a fisheries resource in northern Japan. In
order to boost both the survival of released juveniles

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さけ・ます資源管理センター研報 Bull. Natl. Salmon Resources Center, No. 6, 2004

and the return of adults, data on the physiologicalcharacteristics of smoltification and improvement ofstock enhancement techniques are really needed.
This paper describes the seasonal pattern of smoltificaton including development of seawater tolerance
and changes in plasma T4 concentration in juvenile
sockeye salmon reared under natural photoperiod,
effect of artificial photoperiodic control on smoltification and secretion of T4, and effect of T4 administration on smoltification.

Materials and Methods
Seasonal pattern (experiment-1)
Juvenile sockeye salmon (brood year 1989) werereared in an indoor pond supplied with spring waterfrom February to July in 1990 at the Chitose Hatchery, National Salmon Resources Center in Hokkaido.
They were subsequently removed to an outdoor pond
until May 1991 with spring and/or river water. Fivehundred fishes were then transferred into an indoor
fiberglass tank with 1.5 metric ton capacity andreared in spring and/or river water until July 1992.
They were usually fed a commercial diet at 2% bodyweight/day, however this feeding ratio was switched
to 1% body weight/day between November and thesubsequent March. Changes in water temperatureand photoperiod throughout the experiment are
shown in Fig. 1.
The following sampling procedures were conducted monthly. At each sampling ten fish werecaught randomly and measurements of fork length(FL, mm) and body weight (BW, g) were made after
anesthesia with tricaine methanesulfonate (MS222,
100 mg/liter). A condition factor (CF) was calculated from BW x 103/FL3. Blood samples were collected from the severed caudal vein of each fish using a
micro tube. Then, plasma was obtained after centrifugation of the blood at 2,000 g for 15 min andstored at -40°
C until assays for thyroxine concentration (plasma-T4). The gill filaments were excisedfrom the same fish and frozen with homogenizing
solution (250 mM sucrose, 6 mM EDTA 2Na, 20
mM imidazole, pH 6.8) for later analysis of Na+, K+ATPase activity (gill-ATPase).
To determine seawater tolerance, twenty fish wererandomly taken from the tank and transferred directly
into artificial seawater at 33 ppt. At the 24 h after thistransfer, plasma samples were obtained via the method
above and stored at -40°
C until assay for sodium concentration (plasma-Na) could be conducted.
Effects of photoperiod (experiment-2)
Juvenile sockeye salmon (brood year 1996) weremoved from an outdoor pond to an indoor fiberglasstank (1 metric ton capacity) at the Chitose Hatcheryon August 5, 1997, and divided into two groups with
400 individuals each. Photoperiod was controlled byartificial light (2 x 40 w), and kept at a 8/16 hlight/dark cycle from August 5 to October 1. One
group (8L-group) was maintained at this constantshort daylight cycle until November 27. The photoperiod of the other group (16L-group) was
switched to a 16/8 h light/dark condition from Octo-

Fig. 1. Changes in natural photoperiod (solid line) and water temperature (circles) during Experiment-1 from February, 1990 toJuly, 1992.

Ban − Participations of thyroxine on smoltificationber 2 to November 27. The natural photoperiod atthe beginning of this experiment (August 5) was
15/9 h. Both groups were fed a commercial diet at
2% body wt/day, and supplied recirculating water at
8°C throughout the experiment.
Samples (n=5 each) were collected at two-weekintervals from October 2 to November 27. At eachsampling, plasma and gill samples were acquired
from ten fish of both groups for the assay of plasmaT4 and gill-ATPase. In addition, twenty additionalfish were used for a seawater challenge test, andtheir blood was collected for assays of plasma-Na.
Effects of T4 (experiment-3)
Juvenile sockeye (brood year 1995) were transferred from the outdoor pond at the Chitose Hatcheryto indoor plastic tanks with 30 l capacity on November 10, 1996, and divided into two groups with 30
individuals each. One group (T4 group) was given
thyroxine (T4, Sigma chem. Co.) as a daily supplement to the commercial diet until November 30 for
twenty days. The administered dosage of this hormone for each specimen was at 20 μg per day. The
other group was given only the commercial diet(control group). The photoperiod was natural andtanks were supplied with well water at 10°C.
On December 1, gill and plasma samples weretaken from ten fish for both groups, and anothertwenty fish from each group were transferred to artificial seawater at 33 ppt. At 24 h after transfer, plasma samples were obtained. Plasma-Na and gill-

15

ATPase were measured.
Assay of plasma-T4, plasma-Na and gill-ATPase
Plasma-T4 (ng/ml) was measured by using a
DELFIA thyroxine reagent kit with a DELFIA Fluorometer (Wallac) in experiment-1, and by radioimmunoassay according to the method of Suzuki and
Suzuki (1981) in experiment-2.
Plasma-Na (mM) was measured by using an
Atomic Absorption and Flame Emission Spectrometer (Shimadzu, AA-640-13). At the time of analysis,
defrosted serum was diluted 1,000 times with distilled water.
Gill-ATPase was determined according to themethod of Ban and Yamauchi (1991). Defrosted gill
filaments were homogenized in the homogenizing
solution and centrifuged at 2,000 g for 5 min at 4°C.
The supernatant was incubated for 20 min at 37°
C.
Phosphorus (Pi) produced from the incubation wasmeasured according to the method of Goldenberg
and Fernandez (1966). Protein concentrations were
determined by the method of Lowry et al. (1951).
The enzyme activity was expressed as micromoles Pi
per milligram protein per h (μ mols Pi/mg pro /h).
Statistical analysis
The data were subjected to a one-way analysis ofvariance (ANOVA) followed by a Student's t-test to
determine significant differences in plasma T4 andplasma Na, and gill-ATPase. A probability level ofless than 0.05 was considered significant.

Fig. 2. Changes in fork length (solid circles) and condition factor (open circles) for juvenile sockeye salmon reared in Experiment-1 from February, 1990 to July, 1992.

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さけ・ます資源管理センター研報 Bull. Natl. Salmon Resources Center, No. 6, 2004

Fig. 3. Changes in gill Na+, K+-ATPase activity (open circles) and plasma thyroxine concentration (solid circles), and plasma sodium concentration (triangles) 24 h after transfer into seawater for juvenile sockeye salmon observed in Experiment-1 from February, 1990 to July, 1992. Vertical bars indicate SE. Some SE are hidden by symbols. Asterisks above points show significant differences (p<0.05).

Results
Seasonal pattern
Fork length of juvenile sockeye salmon increased
from 25.1 mm in February 1990 to 280.5 mm in July
1992 (Fig. 2). Mean monthly elongation of fork
length ranged between 2.7 - 3.7 mm from November
to the subsequent March, and 9.3 - 13.4 mm in othermonths. The CF changed from 5.6 in February 1990
to 11.4 in July 1992 (Fig. 2). Remarkable gains in
CF were recognized from February to June in 1990
and from July to November in 1991. In April and
May in 1991 and March and April in 1992, CF
showed a temporary reduction. Fin margin blackening was observed in July and September in 1990,
May and July in 1991, and May and July in 1992.
Plasma-T4 increased significantly (p<0.05) in
January and March in 1991 and 1992 (Fig. 3). Dramatic elevations of plasma-T4 were also observed
from July to October in 1990 and from June to September in 1991. The levels of plasma-T4 decreased
and reached their lowest levels during June - July
every year. For February to June 1990 samples,
analyses of this hormone were not conducted because

plasma volumes were insufficient for an assay.
Plasma-Na showed significant (p<0.05) declines
from May to July in 1990 and from January to May inboth 1991 and 1992, but the lowest concentrations(approximately 157 mM) were obtained during May
and June in 1991 (Fig. 3). In other months, plasmaNa was higher levels more than 190 mM. There wereno data from February to April in 1990, because all of
the fish transferred to seawater have died within 24 h.
Gill-ATPase displayed a dramatic increase andreached at approximate 15 μ moles Pi/mg pro/h inJuly - August 1990, and in May - June 1991, following rapid decreases (Fig. 3). This enzyme activity
also showed a rapid increase in March 1992, and subsequently reaching a plateau at a level of approximate
11 μ mols Pi/mg pro/h until July. In almost all othermonths, gill-ATPase was lower than 5 μ mols Pi/mg
pro/h.
Effects of photoperiod
There was no significant difference in fork lengthand condition factor of fish between the 8L and 16L
groups (Table 1). Plasma-T4 in the 16L-group
showed a temporary increase in plasma-T4 on October 30 and reached 6.1 ng/ml (Fig. 4). Other than

Ban − Participations of thyroxine on smoltification

17

Table 1. Changes in fork length and condition factor of 8Land 16L-groups at the start and end of Experiment-2.
Fork length (mm) Condition factor

Date

Group

August 58L74.3±0.9*

8.5±0.1(start)16L

76.5±1.4

8.6±0.1

November 278L96.6±1.6

10.7±0.1

(end)

16L

95.5±2.0

10.1±0.1* Mean±SE

Table 2. The mean fork length (FL, mm), condition factor(CF), gill Na+, K+-ATPase activity (gill-ATPase, μmols Pi/mg
pro/h) and plasma sodium concentration 24 h after seawater
challenge of control group and T4 treated group in sockeye
salmon at the end of the Experiment-3 (December 1, 1996).
Group

FL

CF

control 101.3±1.9* 8.6±0.3
T4101.2±2.4

8.8±0.3

gill-ATPaseplasma-Na

4.9±0.4

201.5±6.1

5.0±0.3

194.5±3.0

Fig. 5. Changes in plasma sodium concentration 24 h after
transfer to seawater for 8L-group (open circles) and 16Lgroup (solid circles) fish in Experiment-2. Vertical bars indicate SE. Asterisks in the graph show significant differences
between the 8L- and 16L-groups (p<0.05).

* Mean±SE

Fig. 6. Changes in gill Na+, K+-ATPase activity for 8L-group(open circles) and 16L-group (solid circles) fish in Experiment-2. Vertical bars indicate SE. Some SE are hidden bysymbols. Asterisks above the symbols show significant differences between the 8L- and 16L-groups (p<0.05).
Fig. 4. Changes in plasma thyroxine concentration for 8Lgroup (open circles) and 16L-group (solid circles) fish in
Experiment-2. Vertical bars indicate SE. Some SE are hidden
by symbols. Asterisks in the graph show significant differences (p<0.05).

this increase, both groups showed similar reductions
in plasma-T4 from 5.3 - 5.5 ng/ml on August 5 to 0.7- 1.6 ng/ml on November 27. Differences in plasmaT4 between October 16 and October 30, and the 8Lgroup and the 16L-group on October 30 were significant. Plasma-Na of both groups maintained higher
levels of 200 mM from August 5 to October 16, followed by decrease to 174.2 mM in the 16L-groupand 190.9 mM in the 8L-group on November 27(Fig. 5). Significant differences (p < 0.05) in plasma-Na between the two groups were observed from

October 30 to November 27. Gill-ATPase of both
two groups declined from 6 μ mols Pi/mg pro/h on
August 5 to 2 μ mols Pi/mg pro/h on October 2.
Subsequently, gill-ATPase increased to 8 μ mols
Pi/mg pro/h in the 16L-goup and 5 μ mols Pi/mg
pro/h in the 8L-group on November 27 (Fig. 6).
Gill-ATPase activity for the 16L-group was significantly higher than that of the 8L-group from October
16 to November 27.
Effects of T4
There was no significant difference in thesesmoltification characters (FL, CF, gill-ATPase, andplasma-Na) between two groups (Table 2). However, fin margin blackening was observed only in theT4-group.

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さけ・ます資源管理センター研報 Bull. Natl. Salmon Resources Center, No. 6, 2004

Discussion
Sockeye salmon in their first year (underyearling) and second year (yearling) after hatching transform to smolt in early summer and spring, respectively (Ban and Yamauchi 1991). Shrimpton et al.
(2000) also reported the similar smoltification occurring in two consecutive years on Atlantic salmon. In
the present experiment, three age-classes (underyearlings, yearlings and 2-year olds) of sockeye salmon
displayed obvious smolting characteristics. Anadromous salmonids likely repeat smoltification regularly
if they are continually kept in fresh water. However,the smoltification, which occurred at each age
showed different patterns (Fig. 3). Seawater tolerance, as indicated by plasma-Na and gill-ATPase,
reached a peak during July - August in underyealings, May - June in yearlings and May - July in 2year olds. Peaks in the gill-ATPase of underyearlings and yearlings were sharp, though that of 2-year
olds was lower and at a plateau. Yearling smolt
showed lower plasma-Na than fish in the other twoclasses. The CF of underyearlings continuously
increased, while CF in yearlings and 2-year olds fish
decreased during smoltification. Fin margin blackening was observed in every age class' smolt. Thus
the quality of smolt was not definite during eachyear. Yearly variations in smoltification seem to bemodulated by growth, threshold size, metabolic andendocrine conditions, and/or environmental factors
as pointed out by Hoar (1988). Since physiologicalquality of juvenile fish is assumed to affect on earlylife mortality in nature, we should correctly assess
the quality of smolt at the time of their release. In
order to improve the stock enhancement programsfor sockeye salmon, accurate estimations of effectsof smolt quality on long term survival (i.e., after seaward migration) is also necessary.
Given that smoltification occurs seasonally, thisphenomenon can be correlated with periodical
changes in the endocrine systems. Thyroxine is oneof the major hormones involved in smoltification(Hoar 1939; Dickhoff et al. 1978; Youngson andSimpson 1984). In the present study, long-termchanges in plasma-T4 of juvenile sockeye salmon
reared under natural photoperiod were examined inorder to define the role of thyroxine on smoltification. The data shows that plasma-T4 has a repeatedsemi-annual variation over two years (Fig. 3). The

level of plasma-T4 uniformly reached peaks in
March and September, October, and bottomed out in
June, July and December. These months essentiallycorrespond to the switchover time of natural photoperiod (equinox) in Japan. Such changes in plasma-T4 according to photoperiod also reappeared
under artificial light conditions in the present experiment. Decreasing photoperiod from 15 h to 8 h
reduced the plasma-T4, while lengthening photoperiod to a constant 16 h elevated plasma-T4 levels.
These phenomena are very close to the plasma-T4
profile under a natural photoperiod from October to
May the following year in experiment-1. From theseresults, it can be predicted that a change in photoperiod from maximum or minimum daylight stimulates
secretion of T4. Effects of water temperature andrearing conditions on T4 secretion, however, were
not recognized as of significant importance in comparison with the photoperiod. On the other hand, it
is known that plasma-T4 levels surge during newmoon in coho salmon (Grau et al. 1981). Similarchanges in T4 levels coinciding with the new moon
have also been demonstrated in masu salmon(Yamauchi et al. 1984) and amago salmon (Fujioka
et al. 1990). Although, the correspondence of plasma-T4 increased with the lunar cycle occurred during a limited period at the peak of smoltification.
The present study suggests that the photoperiod is
the most important environmental factor controlling
the long-term annual cycle of T4 secretion in sockeye salmon.
Periodic changes in plasma-T4 and the annual
cycle of smoltification were not parallel (Fig. 3).
However, significant increases in plasma T4 anddecreases in plasma-Na were synchronized during
January and March, accompanied by a lengthening
daylight period. This phenomenon was repeated inboth 1991 and 1992. The decline of plasma-Na
commenced with a temporal elevation in plasma-T4
which was induced by the lengthened photoperiod in
the present experiment. Positive effects of longer
photoperiod on seawater tolerance have already beenreported in Atlantic salmon (Sigholt et al. 1998;
Berrill et al. 2003). The present results further indicate that T4 is strongly involved with the progress ofseawater tolerance in sockeye salmon, although plasma-T4 was reduced to its lowest levels when seawater tolerance reached peaks in both 1990 and 1991.
Furthermore, an increase plasma-T4 was observed
only at the beginning of the developmental stage ofBan − Participations of thyroxine on smoltification

seawater tolerance in Experiment-2. From theseresults, it appears that T4 acts at an early phase ofsmoltification as an enhancer of this process.
On the other hand, increases in plasma T4 during
June - October did not cause smoltification. Moreover, administration of T4 had no significant effecton the plasma-Na and gill-ATPase in Experiment-3.
These results indicate that T4 alone cannot lead toseawater tolerance. Recently, evidence of the effectof growth hormone (GH) and cortisol on hypoosmoregulation in salmonids has been accumulating.
Plasma GH and cortisol levels increase duringsmoltification (Specker and Schreck 1982; Sweeting
et al. 1985; Stefansson et al. 1991). Administrationof GH and/or cortisol develops seawater tolerance inAtlantic salmon and sockeye salmon (McCormick
1996; Ban 2002). Furthermore, lengthening the photoperiod induces smoltification, accompanied withincreases in plasma GH and cortisol in both Atlantic
salmon and sockeye salmon (Björnsson et al. 2000;
Ban unpublished data). In experiment-3, fin margin
blackening was observed in the T4-group but not in
the control group. This feature is more marked when
T4 is combined with GH (Miwa and Inui 1983,1985). These findings suggest that T4 promotes
smoltification in cooperation with GH and/or cortisol, which also show a periodic cycle with photoperiod. Future endocrine researches with the aim ofenhancing our knowledge of the relationshipbetween photoperiod and smoltification in sockeye
salmon are highly needed.
Duncan and Bromage (1998) observed smoltification in juvenile Atlantic salmon under a constant,short photoperiod condition. In Experiment-2, the
8L-group also gradually developed seawater tolerance, while the levels of plasma-Na and gill-ATPase
were significant lower than that of the 16L-group.
The constant short daylight period may have similareffect as a lengthened photoperiod, as a result of theaccumulation of daylight. There may be a thresholdin the daytime as a trigger for smoltification. Therefore, a transformation to smolt probably commences
and progress faster in 16L-group than in 8L-group.
In conclusion, the present study reveals a periodic pattern for smoltification and changes in plasmaT4. Smoltification commences in accordance withthe elevation of plasma T4, which is induced by alengthening of the photoperiod. However, T4 alone
does not lead to seawater tolerance. The present
findings suggest that T4 acts as an enhancer of19smoltification in sockeye salmon and that it has asignificant impact, albeit indirectly, at an early phaseof this process.

Acknowledgements
I wish to express my appreciation to the staff ofthe Chitose Branch, National Salmon Resources
Center for their efforts in keeping the fish in goodcondition. Thanks also to Dr. Michael Arvedlund
and Dr. Akihiro Takemura at Sesoko Station Tropical Biosphere Research Center University of the
Ryukyu, and Dr. G. Curt Fiedler at Maryland University College, Asia for revision of this manuscript.

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チロキシンがベニザケのスモルト化に果たす役割
伴真俊

チロキシン（T4）がベニザケのスモルト化に果たす役割を明らかにするため，海水適応能，つま黒，肥満度から判断されるスモルト化と血中T4濃度の周年変化を調べた．自然日長下で飼育されたベニザケ幼魚は，1月から5月にかけて徐々に海水適応能を高めた．つま黒の発現と肥満度の減少は5

Ban − Participations of thyroxine on smoltification

月に認められた．血中T4濃度は1月から3月にかけて有意な上昇を示した．一方，人為的に日長を短日（LD 8：16）から長日（LD 16：8）へ切り換えることで，一時的な血中T4濃度の上昇と，それに続くスモルト化を誘起することができた．しかし，

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T4の単独投与はスモルト化を促さなかった．これ
らの結果は，日長の増加にともなって上昇するT4が，ベニザケのスモルト化を間接的に誘起することを示唆している.