Effects of rearing conditions on growth and thyroid hormones during smolting of Atlantic salmon, Salmo salar L.

Aquaculture, 82 (1989) 29-38 Elsevier Science Publishers B.V., Amsterdam

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in The Netherlands

Effects of Rearing Conditions on Growth and Thyroid Hormones during Smolting of Atlantic Salmon, Sdmo salar L. GILLES BOEUF and JEAN-LOUIS IFREMER,

GAIGNON

Centre de Brest, B.P. 70,29263 Plouzank (France)

ABSTRACT Boeuf, G. and Gaignon, J.-L., 1989. Effects of rearing conditions on growth and thyroid hormones during smolting of Atlantic salmon, Salmo salar L. Aquaculture, 82: 29-38. Smolting Atlantic salmon, Salmo salar L., grew faster in large (22.5 m2) than in small tanks (4 m’). Their tri-iodothyronine (T3) levels were higher in small than in large tanks on 26 March and 2 April; and their thyroxine (T4) levels were higher in small than in large tanks from 26 March to 11 April, but surged simultaneously in all tanks. T4 appears related to fish length until the beginning of April, but not after the surge. The fish grew faster at low water velocity and flow (9 cm/s, and 20 l/min) than at high (40 cm/s, and 90 l/ min). Their T3 levels were consistently lower at high flows; and their T4 levels surged simultaneously under low, but earlier at high flows.

INTRODUCTION

Since the early reports of Hoar (1939), Fontaine et al. (1952) and Leloup and Fontaine (1960)) evidence for a role of the thyroid system in salmonid smolting has been developed and reviewed (Fontaine, 1975; Hoar, 1976; Folmar and Dickhoff, 1980, Boeuf, 1987). Studies on Pacific salmon (Oncorhynthus spp. ) have revealed a plasma thyroxine (T4) surge during smolting (Dickhoff et al., 1978, 1982; Nishikawa et al., 1979; Grau et al., 1981, 1982, 1985). Similar patterns have been found in Atlantic salmon, Sulmo s&r L., by Lindahl et al. (1983)) Youngson and Simpson (1984), Boeuf and Prunet (1985 ), Virtanen and Soivio (1985), Boeuf (1987), and Lin et al. (1988). In different experiments with Atlantic salmon, Fontaine and Leloup (1959)) Youngson and Simpson ( 1984 ), and Lin et al. (1988)) and with Pacific salmon, Grau et al. (1985), Nishioka et al. (1985) and Schreck et al. (1985) have all found that thyroid hormone levels could be affected by rearing conditions, such as current velocity, changes in freshwater, flow rates, and fish density. The present experiments were carried out to examine the influence of tank

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G. BOEUF AND J.-L. GAIGNON

volume, water current and fish density on plasma thyroid hormone levels and growth in freshwater during smolting. MATERIAL AND METHODS

Cultured Norwegian Atlantic salmon were reared in surface river water (pH 6.5-7) under natural photoperiod (48”N) and temperature regimes (5-15’ C; Fig. 1) in two experiments at Le Conquet hatchery, Brittany, from November 1984 to April and May 1985, respectively. The fish were fed daily on SSl IFREMER pellets (see Gaignon, 1987 for the formula) by automatic feeder. In the first experiment fish (113.5 ? 1.5 mm fork length) were stocked at 320/m3 in each of two small 2 x 2 m Ewos tanks, and at 310/m3 in each of two circular 22.5 m2 tanks. Water depth and turnover time was the same in all tanks, implying a higher volume flow (232 versus 42 l/min) in the larger tanks (Table 1) . In the second experiment 514 fish (size as in experiment 1) were stocked in each of three 2 x 2 m Ewos tanks, under differing water velocity regimes (Table 2). At high flow (90 l/min) the water velocity was maintained at 40 cm/s in

Le Conquet1984-85

9 w 15!!i z fi IO-

/

k :

/’ 5-

Jd

I J’F’M’A’M’J’J’DATE

Fig. 1. Freshwater temperature at the hatchery of Le Conquet. TABLE 1 Experimental conditions of the first experiment

Type of tank Turnover of water Water level (m) Water volume ( m3) Water flow (I/min) Density of fish in January

Tank 4

Tank 3

Tank 2

Tank 1

Ewos2X2m 1 each 31.5 min 0.32 1.3 42 320/m3

Ewos2x2m 1 each 3 1.5 min 0.32 1.3 42 320/m3

Circular 22.5 m2 1 each 32 min 0.32 7.4 232 310/m3

Circular 22.5 m2 1 each 32 min 0.32 7.4 232 310/m3

REARING CONDITIONS EFFECTS ON SMOLTING SALMO SALAR

31

TABLE 2 Experimental conditions of the second experiment

Type of tank Water flow (l/min) Approximate current velocity (cm/s) till 11 April Current velocity after 11 April Number of fish at the beginning of the experiment

Tank 15

Tank 16

Tank 18

Ewos2X2m 90 6 40 514

Ewos2X2m 90 40 40 514

Ewos2X2m 20 9 9 514

tank 16, and at6 cm/s in tank 15 until 11 April, when this was increased also to 40 cm/s. In tank 18, a low flow of 20 l/min and velocity of 9 cm/s was maintained throughout. In tanks 16 and 18 the current velocity depended chiefly on the flow and the position of the inlet tube. In tank 15 the change in velocity depended only on the diameter of the inlet tube. The velocities were only a means of comparing the tanks. They were checked at the surface, opposite the inlet, and at the same distance from the tank wall. At 09.00-10.00 h on each sampling date 65-85 fish were measured, and blood was taken from the posterior aorta of 20-30 fish with a heparinized syringe. After centrifuging, the plasma was stored at - 28’ C. Plasma concentrations of tri-iodothyronine (T3) and thyroxine (T4) were subsequently determined by radioimmunoassay by the method of Chopra (1978)) modified by McKenzie et al. (1978) (see Boeuf and Prunet, 1985). All samples were measured in the same assay for each experiment, and the intra-assay coefficient of variation was < 7%. All physical data were expressed as mean ? standard error. One-way analysis of variance and SNK classification tests were used to assess the significance of length differences, and Kruskall-Wallis and Friedman tests for changes in thyroid hormone levels.

RESULTS

Experiment 1 Mortality was low (l.l-2.0% ) throughout, with no differences between tanks (Table 3). Growth (except during April) was greater in large than in small tanks (Table 3 and Fig. 2), Plasma T3 levels (Fig. 3) were similar until 15 March, but on 26 March levels were significantly higher (PC 0.01) in fish in the small tanks. Final T3 levels did not differ. T3 levels were unrelated to fish length. Similarly, plasma T4 levels (Fig. 4) were similar until 15 March, but on 26 March and 11 April levels were significantly higher (PC 0.001) in fish in the small tanks. Final T4 levels did not differ. A clear surge occurred in all tanks

32

G. BOEUF AND J.-L. GAIGNON

TABLE 3 Results of first experiment Small tanks

Large tanks Tank 1 Total mortality (% ) Percentage of small fish on 22 April ( < 127 mm and not smolts) Mean length (mm) on 28 March (SD)

1.98

,50_

-

tank

.-

tank2

1.54

1.40

1.40

143.6

143.1

(7.9)

160 -

Tank 2

(9.3)

Tank 3 1.07 0

139.5 (7.6)

Tank 4 1.90 1.70

134.7 (6.6)

I

.---.

tank 3

C--4

tank4

I.20 -

110 -

100

O’N’D’J’F’M’A’M

Fig. 2. Growth curves in the first experiment. Same letters: no significant differences. Different letters: significantly different. Figures: density in kg/m3.

on 11 April. Until 26 March T4 levels increased directly with fish length, but the correlation disappeared after that date (Fig. 5 ) . Experiment 2 Mortality was low throughout, but differed significantly (P-C 0.005) between tanks, being highest under consistently high water velocities (Table 4). Growth, proportion smolting and condition index were lowest under consistently high water velocity conditions, and highest under consistently low velocities. However, specific growth rate between 11 and 22 April was highest

REARINGCONDITIONSEFFECTSONSMOLTING

33

SALMOSALAR

IO-

15/01

01/02 07/02

26102

15/03 26/0302/04 II 18/04

Fig. 3. Blood plasma tri-iodothyronine changes of fishes in tanks 1,2,3 and 4 (first experiment). Strong dotted line: small tank (3); fine dotted line: small tank (4); strong line: big tank (2); fine line: big tank (1). Vertical lines indicate standard error of the mean.

25-

15101

15102

Fig. 4. Blood plasma thyroxine

changes

15103

15104

(legend as in Fig. 3).

under high velocity, and lowest in those fish which experienced a sharp increase in velocity after 11 April. Plasma T3 levels (Fig. 6) differed between tanks, fish under consistently high water velocity having significantly (P
34

G. BOEUF AND J.-L. CAIGNON

4 30-

II April

tank

T4

I

II

April

. j;

.

c

.

l.

tank

. l

. 9;.

3

=.*

l5-

135 LENGTH

I70

(mm)

Fig. 5. Relationships between the length of salmon and the thyroxine level before smolting (January and February) and at smolting time (April).

TABLE 4 Results of second experiment (on

22

April)

Characteristics

Tank

15

changing current velocity Weight (g) Condition index Cumulativemortality rate (%) Growth rate (% per day) Percentageof small fiih ( < 127 mm and not smelts) Averagelength (mm) of smoke ( > 130 mm ) Growth rate (% per day) till 10 April Growth rate (% per day) between 10 and 22 April

Tank 16

Tank 18

strong current high water velocity

weak current low water velocity

37.2 1.121

33.8 1.112

43.0 1.127

0.19 0.55

3.29 0.48

0.39 0.65

8 150.1” (1~6.4

13.6 145.5b (~~7.2

1.9 155.0” az6.6

0.51

0.39

0.57

0.99

1.53

1.46

“~b*cSignificantlydifferent values. 0, standard deviation.

Statistical differences

P< 0.005

P-zO.05 P
REARING CONDITIONS EFFECTS ON SMOLTING SALMO SALAR

I ‘T 12F

T3

,-

15

i ----

16 ,6

35

Le Conquet 1985

~~~~

02/04

0

, 15101

I 26102

I l5/03

I 26103

18

II

l5/04

I

25 , I 22 3O/@J

Fig. 6. Blood plasma T3 changes in tanks 15, 16 and 18 (see Table 2) (second experiment), mean + standard error. 15, high flow rate with current velocity changes; 16, high flow rate, high current velocity; 18, low flow rate, low current velocity.

-

16

Le Conquet

1985

02/w

15/01

26/02

15/03

II

Es/o3

Fig. 7. Blood plasma T4 changes during the second experiment

18

15/w

25 22 3OK4

(legend as in Fig. 6).

36

G. BOEUF AND J.-L. GAIGNON

DISCUSSION

On the evidence of other physiological changes associated with smolting, Boeuf et al. (1989) concluded that Parr-smolt transformation was completed in the present groups of fish during the first 3 weeks of April. The role of thyroid hormones (TH) during this phase of life of salmonid fishes has been well studied and the recent reviews of Grau (1987) and Boeuf (1987) insist on involvement in both growth and seawater preadaptation. The results of the first experiment showed a possible influence of tank size on T3 levels at the time just prior to completion of smolting. Very little information exists in the literature on T3, and in Pacific species the levels do not change consistently in the freshwater phase. In Salmo s&r, we have observed T3 fluctuations regularly since 1982. In the same hatchery, the salmon reared in four tanks showed a maximal T4 surge simultaneously, independent of rearing conditions. But the higher levels in fish in small tanks could indicate an influence of tank size or of flow. Alternatively, density, which changed during these experiments, might have influenced T4 levels, as fish in the small tanks at a final lower density, had higher plasma T4 levels during the surge. Schreck et al. (1985) found a similar effect of low density in coho salmon (0. kisutch), but our final densities were very low (2.4-3.7 kg/m3). Fontaine and Leloup (1959)) Youngson and Simpson ( 1984)) and Youngson et al. (1986) have all suggested that water velocity may induce high T4 levels. In the present experiment the velocity was not higher in the small tanks, but the fish showed significantly higher T4 levels. Although T3 was not correlated in these experiments with salmon length, there was an allometric correlation with T4 until the end of March (Fig. 5). This relationship involving thyroid hormone in the length growth of Atlantic salmon disappeared when the thyroxine increased suddenly. T4 could be involved in other mechanisms at this time. The second experiment tasted the effect of a sudden water velocity increase on TH levels. On 11 April this change had no influence, the T4 surge occurring on the same day (just before the change) in two tanks (15 and 18) and had already begun in the other. T3 did not respond. The three experimental rearing conditions did not influence T4 levels differentially, except that under high velocity plasma T4 began to increase sooner, and fish grew least. T3 was lower in this tank throughout the experiment, very significantly so compared to the other two tanks. Increasing the current velocity resulted in decreased growth during the days following, probably indicating some disturbance, and rearing salmon in tanks with a high water velocity and a high flow rate produced unfavourable results: lower survival, growth, and smolting rates. These conditions increased plasma T4 levels earlier than in other tanks and this is in agreement with previous studies showing high T4 levels in fish exposed to faster currents.

REARING CONDITIONS EFFECTS ON SMOLTING SALMO SALAR

37

From these two experiments we confirm that a circulating T4 surge occurs during smolting, in spite of considerable changes in rearing conditions (tank size, flow rate, current velocity). T3 and T4 levels cannot be linked only to the intense growth phase of salmon at this time. At the end of smolting they must be involved in other functions such as migration preadaptation, acquisition of euryhalinity, metabolic changes or imprinting. From the results of these experiments we conclude that: (1) growth was faster in large than in small tanks, and under low water velocities; (2 ) plasma T4 level increased with fish size until early April, but this size relationship disappeared after that; (3 ) plasma T4 peaks occurred simultaneously independent of tank volume, or flow conditions; (4) plasma T4 level increased under the influence of high water velocity, and surged sooner and for longer under these conditions; (5) in one experiment T3 appeared associated with growth rate of the fish; and (6) T3 and T4 levels were increased at smaller tank size. Clearly better growth results are obtained by rearing salmon in big tanks and under low water velocity and flow rate. But better results in terms of activated T4 in the plasma are obtained by rearing them in small tanks at lower densities, or using higher currents and flow rates. Information on seawater performances is needed to choose the best way to rear this species in freshwater prior to migration. ACKNOWLEDGEMENTS

We thank B. Petton and Y. Normant for fish maintenance, A. Le Roux and A. Severe for laboratory assistance, and S. Gros and N. Soun for graphic and typing assistance. We are indebted to J. Thorpe (Freshwater Fisheries Laboratory, Pitlochry) for reviewing the manuscript. REFERENCES Boeuf,G., 1987.ContributionB1’6tudedeI’adaptationgI’eaudemerchezlesPoissonsSalmonides Determination de critkres de smoltification par mesures de l’activitk (Na+K+ )-ATPasique des microsomes de la branchie et des hormones thyroidiennes plasmatiques. These de Doctorat d’Etat, Universitd de Bretagne Occidentale, 370 pp. Boeuf, G. and Prunet, P., 1985. Measurements of gill (Na+-K+)-ATPase activity and plasma thyroid hormones during smoltification in Atlantic salmon (Salmo salar L. ). Aquaculture, 45: 111-119. Boeuf, G., Le Bail, P.Y. and Prune& P., 1989. Growth hormone and thyroid hormones during Atlantic salmon, Salmo salar L., smolting, and after transfer to seawater. Aquaculture, 82: 257268. Chopra, I.J., 1978. Radioimmunoassay of iodothyronines. In: G. Abraham (Editor), Handbook of Radioimmunoassay. Dekker, New York, NY, pp. 679-703. Dickhoff, W.W., Folmar, L.C. and Gorbman, A., 1978. Changes in plasma thyroxine during smoltification of coho salmon (Oncorhynchus kisutch). Gen. Comp. Endocrinol., 36: 229-232. Dickhoff, W.W., Folmar, L.C., Mighell, J.L. and Mahnken, C.W., 1982. Plasma thyroid hormones during smoltification of yearling and underyearling chinook salmon and steelhead trout. Aquaculture, 28: 39-48.

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G. BOEUF AND J.-L. GAIGNON

Folmar, L.C. and Dickhoff, W.W., 1980. The Parr-smolt transformation (smoltification) and seawater adaptation in salmonids. A review of selected literature. Aquaculture, 21: l-37. Fontaine, M., 1975. Physiological mechanisms in the migration of marine and amphihaline fish. Adv. Mar. Biol., 13: 241-355. Fontaine, M. and Leloup, J., 1959. Influence de la nage B contre-courant sur le metabolisme de l’iode et le fonctionnement thyroidien chez la truite arc en ciel (Salmo gairdneri Richardson). C. R. Acad. Sci. Paris, 249: 343-347. Fontaine, M., Leloup, J. and Olivereau, M., 1952. La fonction thyroidienne du jeune salmon (Salmo salar L.), parr et smolt et son intervention possible dans la migration d’avalaison. Arch. Sci. Physiol., 6: 83-104. Gaignon, J.L., 1987. L’elevage des juveniles de saumon atlantique (Salmo salar). Piscicult. Fr., 90: 5-57. Grau, E.G., 1987. Thyroid hormones. In: P. Pang and M.P. Schreibman (Editors), Vertebrate Endocrinology: Fundamentals and Biochemical Implications, Vol. 2. Regulation of Water and Electrolytes. Academic Press, New York, NY, pp. 85-98. Grau, E.G., Dickhoff, W.W., Nishioka, R.S., Bern, H.A. and Folmar, L.C., 1981. Lunar phasing of the thyroxine surge preparatory to seaward migration of salmonid fishes. Science, 211: 607609. Grau, E.G., Specker, J.L., Nishioka, R.S. and Bern, H.A., 1982. Factors determining the occurrence of the surge in thyroid activity in salmon during smoltification. Aquaculture, 28: 49-58. Grau, E.G., Fast, A-W., Nishioka, R.S., Bern, H.A., Barclay, D.K. and Katase, S.A., 1985. Variations in thyroid hormone levels and in performance in the seawater challenge test accompanying development in coho salmon raised in Hawaii. Aquaculture, 45: 121-132. Hoar, W.S., 1939. The thyroid gland of the Atlantic salmon. J. Morphol., 65: 257-295. Hoar, W.S., 1976. Smolt transformation: evolution, behavior and physiology. J. Fish. Res. Board Can., 33: 1233-1252. Leloup, J. and Fontaine, M., 1960. Iodine metabolism in lower vertebrates. Ann. N.Y. Acad. Sci., 86: 316-353. Lin, R.J., Cross, T.F., Mills, C.P.R., Nishioka, R.S., Grau, E.G. andBern, H.A., 1988. Changes in plasma thyroxine levels during smoltification in hatchery-reared one-year and two-year Atlantic salmon (Salmo salar). Aquaculture, 74: 369-378. Lindahl, K., Lundqvist, H. and Rydevik, M., 1983. Plasma thyroxine levels and thyroid gland histology in Baltic salmon (Salmo salar L.) during smoltification. Can. J. Zool., 61: 1954-1958. McKenzie, D.S., Licht, P. and Papkoff, H., 1978. Thyrotropin from amphibian (Rana catesbeiana) pituitaries and evidence for heterothyrotropic activity of bullfrog luteinizing hormone in reptiles. Gen. Comp. Endocrinol., 36: 566-574. Nishikawa, K., Kirashima, T., Suzuki, T. and Suzuki, M., 1979. Changes in circulating L-thyroxine and L-triiodothyronine of the masu salmon (Oncorhynchus masou) accompanying the smoltification, measured by radioimmunoassay. Endocrinol. Jpn., 26: 731-735. Nishioka, R.S., Young, G., Bern, H.A., Jochimsen, W. and Hiser, C., 1985. Attempts to intensify the thyroxin surge in coho and king salmon by chemical stimulation. Aquaculture, 45: 215225. Schreck, C.B., Patino, R., Pring, G.K., Winton, J.R. and Holway, J.E., 1985. Effects of rearing density on indices of smoltification and performance of coho salmon (Oncorhynchus kisutch). Aquaculture, 45: 345-358. Virtanen, E. and Soivio, A., 1985. The patterns of T,, Tq, cortisol and Na+-K+-ATPase during smoltification of hatchery-reared Salmo salar and comparison with wild smolts. Aquaculture, 45: 97-109. Youngson, A.F. and Simpson, T.H., 1984. Changes in serum thyroxine levels during smolting in captive and wild Atlantic salmon (Salmo salar L.). J. Fish Biol., 24: 29-39. Youngson, A.F., McLay, H.A. and Olsen, T.C., 1986. The responsiveness of the thyroid system of Atlantic salmon (Salmo salar L.) smolts to increased water velocity. Aquaculture, 56: 243255.