Acclimation of Atlantic salmon (Salmo salar) parr to seawater in autumn: stimulatory effect of a long photoperiod

Aquaculture, 103 ( 1992) 341-358

Elsevier Science Publishers B.V., Amsterdam

341

arr to seawate

J. Duston and J.D.E. Knox’ Connors Bros., Ltd., Aquaculture Division, Blacks Ha&our, N,B., Canada

(Accepted 30 September 1991)

ABSTRACT Duston, J. and Knox, J.D.E., 1992. Acclimation of Atlantic salmon (Saltno salt-v)parr to seawater in autumn: stimulatoty effect of a long photoperiod. Aquaculture, 103: 341-358. Starting on 29 September, 1+ year-old Atlantic salmon parr (ca. 70 g) were maintained under either a long (LD 18 : 6 ) or simulated natural photoperiod (LDN; 45 “N) for a 40-day period during which groups were either acclimated to seawater (SW, ca. 32 parts per thousand, ppt) by increasing salinity by 5-10 ppt every 5 days, or remained in fresh water (control). The increase in salinity and photoperiod independently resulted in a significant increase in gill Na+K+ATPase activity and high salinity (37 ppt) tolerance, with a combination of the two treatments beingadditive. Parr maintained mean plasma osmolality between 291 and 319 mosmol kg-’ during acclimation, indicating a maintenance of iono/osmotic balance. On 5 November SW acclimated parr from LD18 : 6 and LDN groups were individually tagged and then maintained for 11 months in a s’?gle tank supplied with SW under an LDN photoperiod. During winter (SW temperature = l-2°C) overall cumulative mortality of parr acclimated under LD18 : 6 and LDN was 10%and 60% respectively, with the number of sexually mature male mortalities being disproportionately high. Death appeared due to breakdown in hypoosmoregulatoty ability. Parr surviving the winter in SW completed smoltification in spring as judged by a decline in condition factor and change in appearance. The. results support the hypothesis that an increase in photoperiod during autumn can improve the acclimation of Atlantic salmon parr to SW, and indicate the feasibility of transferring such fish to commercial marine facilities in autumn.

INTRODUCTION

Atlantic salmon (Salmo s&r) parr reside one or more years in fresh water (FW) before undergoing a seasonal physiological change, known as smoltification, which in spring time culminates with migration to sea ( Correspondence to: Dr. J. Duston, Connors Bros., Limited, Aquaculture Division, Blacks Harbour, New Brunswick, BOG 1HO, Canada. ‘Present address: Department of Fisheries and Oceans, Biological Sciences Branch, Biological Station, St. Andrews, New Brunswick, EOC 2X0, Canada.

0044-8486/92/$05.00

0 1992 Elsevier Science Publishers B.V. All rights reserved.

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3. DUSTON AND J.D.E. KNOX

1988; Wedemeyer et al., 1980; McCormick and Saunders, 1987). Completion of smoltification includes a preadaptive development of hypoosmoregulatory mechanisms which remain functional for approximately 6 $weeksin the spring (Duston and Saunders, 1990; Duston et al., 199 1) and allows cultured smelts to withstand direct transfer to seawater (SW) with only a sh-t-term h-uption of iono/osmotic balance (Boeuf et al., 1978; Stagg et al., 1989; Nance et al., 1990). The ability of juvenile salmonids to hypoosmoregulate outside this smelt period is reduced (Stagg et al., 1989), and transfer to SW can result in retarded growth (stunting) and high mortality (Clarke and Nagahama, 1977; Folmar et al., 1982; Bjomsson et al., 1988). Current hatchery practice in Eastern Canada results in approximately 60% of the cultured Saint John River stock completing smoltification as 1+ year-olds (S 1‘s), the remaining 40% smaller fish smolting the following year as S2’s (Bailey et al., 1980; Kristinsson et al., 1985 ). S2 smolts are commercially less desirable than S l’s because during their additional year in the hatchery they can reach > 100 g in size, requiring tank space and feed disproportionate to their monetary value. Salmon farmers in New Brunswick (N.B. ) have attempted to overcome this inefficiency by introducing large (ca. 2 70 g) 1.5 + year-old parr to SW in the autumn, 6 months prior to completion of smoltification, similar to studies in Norway (Bergheim et al., 1990). However, reports of survival and growth in N.B. have been highly variable and poorly documented, necessitating the present quantitative study. Although certain stocks of Atlantic salmon in nature have been reported to exhibit some smolt characteristics in the auiumn (see Cunjak et al., 1990), salinity tolerance tests and analyses of gill Na+K+ATPase activity have indicated that the Saint John River stock under culture conditions in N.B. exhibits this preadaptation to SW only in the spring (J. Duston and R.L. Saunders, unpublished observations). Therefore, the ability of parr to survive transfer to SW in the autumn would appear to be an acclimation response to increased salinity, similar to non-smolting species such as rainbow trout (Landless, 1976; Kaushik et al., 1977; Macleod, 1977; Bath and Eddy, 1979a,b; Leray et al., 198 1; Johnston and Cheverie, 1985 ). The ability of salmonids to tolerate transfer to SW is influenced by a number of factors including species (Parry, 1960; Hoar, 1976 ), body size ( Parry, 1960; Jackson, 198 1; Johnsson and Clarke, 1988; Bjerknes et al., 1992), gonadal maturation (Ikuta et al., 1987; Lundqvist et al., 1989), SW temperature (Byrne et al., 1972; Saunders et al., 1975; Virtanen and Oikari, 1984; Finstad et al., 1988; Sigholt and Finstad, 1990) and rate of increase in salinity (Kaushik et al., 1977 ). Moreover, increases in photoperiod in autumn and winter can stimulate the development of hypoosmoregulatory mechanisms in Atlantic salmon parr (Duston et al., 1989; Duston and Saunders, 1990). The present study tests the hypothesis that an increase in photoperiod in autumn stimulates the ac-

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climation of 1.5+ year-old parr to SW, and quantifies long-term survival, growth and completion of smoltification. MATERIALS AND METHODS

Atlantic salmon (Saint John River stock) were raised from fry at Lake Utopia hatchery (Connors Bros., Ltd. ) in southern N.B. under natural day length (lat. $5 ON) and temperature, ranging between 19°C in summer and 1“C in winter. Approximately 60% of the fish completed smoltification as Sl’s and were transferred to sea-cages in the spring of 1989. The other 40% potential S2’s remained in the hatchery until 29 September 1989 when a random sample of 650 fish estimated to range between 60 and 80 g were transported to either of two SW acclimation sites (Site A and Site B). Site .4 (Connors Bros., Ltd., Beaver Harbour, N. .) Groups of n= 80 fish were randomly allocated to one of four treatments: Group

Photoperiod

Salinity

FW(LDN) FW(LD18:6) FW-SW( LDN) FW-SW(LD18:6)

LDN LD18:6 LDN LD18:6

Fresh Fresh Fresh Fresh

water water water water

only (Control) only increased to full seawater increased to full seawater

Fish were held in lightproof 2-m square tanks maintained on either a si ulated natural photoperiod (LDN, 45 ON) or a long photoperiod ( 18 h light : 6 h dark; LD 18 : 6) for the 40-day acclimation period. At the start of the experiment all four groups were supplied with a flow-through supply of fresh well water (8-9 OC). In two groups the salinity was increased step-wise by 5 or 10 parts per thousand ( ppt ) every 5 days (Fig. 1) by progressively opening a valve allowing in SW (temperature at 12- 13 OC ) . Flow in all four tanks was maintained at about 15 1min- ‘. Site B (D.F.O. Biological Station, St. Andrews, N.B. ) A total of 330 fish were randomly divided between two lightproof l-m2 tanks (volume = 0.3 m3) maintained under either LDN or LD 18 : 6. Limited tank space at Site B prevented full replication of the treatments at Site A. In both groups the salinity was increased according to the protocol at Site A (Fig. 1). Following acclimation at Site B, 50 fish from each group were individually identified by a combination of clipping and Panjet marking of fins ( erbinger et al., 1990), eter tank (volume = 2.9 m3) with flowand transferred to a single 2-m 29-32 ppt ) at ambient temperature (Fig. through (20 1mix? ) supply of 6 ) and simulated natural day length (L N) to assess long-term growth and survival. Fish in all groups were hand fed to satiation three times daily with a

J. DUSTON AND J.D.E. KNOX

Fig. 1, Experimental changes in salinity and photoperiod. Arrow indicates date of transfer of fish from hatchery to acclimation sites.

commercial moist pellet (Connors Bros., Ltd. ). Photoperiod in all tanks was without a twilight period and was provided by a single fluorescent strip giving 30 lux at the water surface. Sampling and analytical techniques At ca. IO-day intervals throughout the acclimation, a random sample of eight fish was removed from each of the six groups, anaesthetised ( I% tertiary amyl alcohol; Fisher), gill filaments trimmed from the first and second gill arches on one flank and a blood sample taken from the caudal vessel with a heparinised syringe. Gill filaments were frozen in buffer at - 80°C and subsequently assayed for Na+K+ATPase activity using the modified method of Zaugg ( 1982), as described by McCormick et al. ( 1987) except that gill homogenates were incubated at 20°C rather than 37 “C. Enzyme activity was expressed as rate of production of inorganic phosphorus per unit protein (pmol Pi mg protein’ ’ h- ’ ) . Blood was centrifuged ( 2000 rpm, 6 min ) and plasma osmolality measured using a vapour pressure osmometer (Wescor Inc., 5 100~).Also at ca. 1 O-day intervals a random sample of 10 fish was removed from each group and challenged In a 96-h salinity tolerance test which involved transferring fish directly to a plastic box containing 65 1 aerated sea-

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water maoe up to 37 ppt by addition of Instant Ocean (Aquarium Systems, Mentor, Ohio). Boxes were checked daily, dead fish removed and the number of fish alive at 96 h was expressed as percent survival. At approximately monthly intervals fork length (FL, nearest mm) and body weight 0.1 g) of fish in the 2-m tank were recorded and condition factor ( Ot; FL3) and specific growth rate were calculated using the equation: (ln BWZ-ln BWl) x loo T2-T,

where BWI and BW2 are body weights on days T, and T2, with T2-T, the number of days between measurements. Growth measurements were not taken either before or during acclimation as the stress of netting and handling during this period can result in high mortality (J. Duston and J.D.E. Knox, unpublished observations). Two-way analysis of variance followed by one-way analysis of variance (ANOVA) incorporating Duncan’s multiple range test to compare inter- and intrawith a significance level set at PIO.05 was activity, plasma osmolality group differences in mean gill Na+K+AT wnlee, 1960) was used to levels and somatic growth. rownlee’s tes compare the percent survival between groups subjected to the salinity tolerance tests, and mortality following acclimation. RESULTS

Gill Nd K+ATPase activity (Fig. 2) Site A. Mean (standard error, s.e.; n = 8 ) enzyme activity of the F controls exhibited no significant change, remaining between 0.7 (0.1) and groupgill 1.4 (0.3 ) pm01 Pi mg protein- * h-’ (units). In the FW(LD18:6) Na+K+ ATPase activity increased significantly from 1.5 (0.3) on 3 1 October to 2.4 (0.3) units on 7 November. In the FW-SW (LDN) group Na+K+ATPase activity increased significantly from 1.4 (0.2) on 19 October to 3.2 (0.4) units on 31 October, which was significantly higher than both FW groups. The largest increase in Na+K+ATPase activity was observed in the FW-SW (LD 18 : 6) group, increasing significantly from 1.7 (0.3 ) on 19 October to 5.1 (0.4) on 31 October and 6.0 (0.4) on 7 November, significantly higher than all other groups.

Site B. Both groups exhibited a small decline in mean Na+ +ATPase activity increasing signific over the first 13 days of 18 : 6) groups Na+ after. In the FW-SW( L (0.3) on 19 October to 2.5 (0.;; activity increased from 1.1 (0. and 3.9 (0.9) respectively on 31 October. On 7 November, mean

J. DUSTON AND J.D.E. KNOX

346

-

Site

A

-----

Site

6

FW

(LD18:8)

Fig. 2. Effect of experimental increases in salinity and photoperiod on mean (n = 8; f s.e. ) gill Na+K+ATPase activity in I + year-old Atlantic salmon Parr. Arrow indicates date of transfer of fish from hatchery to acclimation sites.

Na+K+ATPase activity of the FW-SW(LD18:6) group was 5.9 (0.8), significantly higher than the FW-SW(LDN), 2.8 (0.3). Plasma osmolality (Fig. 3) Site A. All four groups exhibited only minor fluctuations in plasma osmolal-

ity, with mean (s.e. ) values ranging between 29 1 (8 ) and 3 19 (4) mosmol kg- I. The FW-SW(LDN ) and FW-SW(LD 18: 6) groups exhibited a signifiicant increase in plasma osmolality from 29 P ( 8 ) mosmol kg- * on 26 September to 3 19 (4) and 308 (5) mosmol kg-’ on 11 October with no significant changethereafter. In the FW (LDN ) group, osmolality increased significantly from 293 (2) on 11 October to 302 (2) on 7 November. Site B. The FW-SW (LDN) group exhibited a significant increase in mean

osmolality levels from 296 (2 ) mosmol kg- ’ on 19 October to 3 17 ( 7 ) mosmol kg-’ on 31 October, declining to 309 (6) mosmol kg-’ on 7 November. The PW-SW(LD18 : 6) group exhibited no significant change in mean osmolality. The high mean of 320 mosmol kg- l on 3 1 October was attributable to a small ( 30 g) sexually mature male fish (477 mosmol kg-’ ), the other fish having values ranging between 292 and 305 mosmol kg- *.

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OF ATLANTIC

SALMON PARR TO SEAWATER

347

360 r -

Site

A

-----

Site

B

,r”

320

ILDN)

G ‘j E

310

(LDN)

ii 2

300

ii h 290 I I

I 1

2eoL. 25

I

I

I

I

I

5

10

15

20

25

Sept

Ott

I 30

I 1

I 5 Nov

I 10

Fig. 3. Effect of experimental increases in salinity and photoperiod on mean (n = 8; f s.e. ) plasma osmolality in 1 + year-old Atlantic salmon Parr. Arrow indicates date of transfer of fish from hatchery to acclimation sites. TABLE 1 Effect of acclimation treatments on percent survival of groups (n = 10) of 1+ year-old Atlantic salmon parr subjected for 96 h to 37 ppt salinity tolerance tests Date of test

11 Oct. 19Oct. 3ooct. 6 Nov.

Site B (Biological Station)

Site A ( Beaver Harbour ) Fw (LDN)

Fw (LD18:6)

Fw-SW (LDN)

Fw-SW (LD18:6)

0 0 0 0

0 0 30 60

0 0 50 100

0 10 90 100

Fw-SW (LD18:6)

W-SW (LDN) 0

0

(3 Nov.) 9: (8 Nov.) 60

;: 100

96-h 37ppt salinity tolerance tests (Table 1) Site A. The FW (LDN ) controls exhibited 100% mortality throughout the acclimation period. In the other three groups salinity tolerance increased signif186) grou icantly between 19 October and 30 er survival than bot exhibited 90% survival on 30 Octobe (LD 18 : 6) groups which were not significantly the FW-SW (LDN) and different from each other on that date. On 6 November bot groups accli-

348

J. DUSTON AND J.D.E. KNOX

mated to seawater exhibited 100% survival, significantly higher than the 60% survival in the FW (LD 18 : 6 ) group.

Site B. Salinity tolerance increased significantly in both groups between the 20 October and 3 November tests. In the final test on 8 November, survival in the FWSW(LD18: 6) group was lOO%,significantly higher than the FWSW(LDN) group that showed only 60% survival, a significant decline from the 90% survival on the 3 November test.

EfSectofsexual maturation on acclimation to seawater Amongst those fish sampled, the proportion of sexually mature males was 17O/b, compared to 27% immature males and 56% females. Because of this low incidence of mature males there were insufficient data within the respective treatments to conduct a meaningful statistical comparison with immature fish. However, pooling the gill Na+K+ATPase and plasma osmolality data from groups at both sites indicated that both mature and immature parr responded similarly to the increase in salinity. The mean (s.e.) gill Na+K+ATPase activity of mature males (n= 5) and immature fish (n = 57) sampled on 3 1 October and 7 November in full SW was the same, 4.1 (0.46 ) and 4.1 (0.26). Plasma osmolalities between mature males and immature fish were also similar; with the exception of one fish (see above) levels ranged between 289322 mosmol kg- I (mean, s.e. = 306,6 ), and 275-367 mosmol kg- ’ (mean, s.e. = 308,2 ) respectively.

Fig. 4. Cumulative mortalityof Atlantic salmon parrfollowing acclimation to seawater under either LDN or LD 18: 6 photoperiod.

ACCLIMATION OF ATLANTIC SALMON PARR TO SEAWATER

FW-SW

349

(LD16:6)

1.20-

r, 1.16-

1

~____L___L_--I_

.~oL_J_--L__ Nov

Dee

Jan

Feb

Mar

Apr

May

I Jun

Jul

Aug

1 Sep

.-I Ott

Fig. 5. Changes in mean ( f s.e.) condition factor (BW 100/FL3) of 1 + year-old Atlantic salmon following acclimation to seawater under either LDN or LD18 : 6 photoperiod.

Mortality (Fig. 4) The FW-SW (LDN ) group at Site B had an overall cumulative mortality of 60%, significantly higher than the 10% mortality in the group. In all cases death was associated with a decrease in FL (ca. 3%). In the FW-SW (LDN) group 15% of the fi Qctober and 16 November of which 70°h were sexual1 tween mid-November and late-December few deaths occurred tive mortality rate increased reaching 60% by mid-February. in the FW-SW (LDN) group decreased from late-January onwards with no deaths occurring after 20 February. In the the fish died during acclimation, rising to were sexually mature males, with no mortalities ased on 8 November measurements of immature fish from th

J. DUSTON AND J.D.E. KNOX

350 1.4

Growth F

Rate

I-

‘;

iiT 1.2 0 8

l.O-

;

\

0.8-

.c i

0.6-

$ ” ._ c

0.4-

::z

0.2-

\

\

\

\

\

- 600

\ - 500

-400 O-

s Y 2 a, ._ ;

-300

Body

Weight FW-SW

FW-SW ,s 16 L

al

I-,-P F

Nov

p” li

(LDN) 200

(LD18:81 1”’ c-

.

100

Dee

Jan

Feb

Mar

Apr

May

Jun

Jul

Aug

Sep

Ott

Fig. 6. Mean ( Z!Is.e.) body weight and specific growth rate of 1 + year-old Atlantic salmon following acclimation to seawater under either LDN or LD 18: 6 photoperiod. Daily seawater temperature is also shown.

SW(LDN) group, there was no significant difference in mean (se. ) BW of those fish that subsequently died during the winter (68.0 g, 2.7) and those fish that survived (75.8 g, 3.6). Sexuallymature males constituted 36%of the overall mortalities, but made up only 17%of the original population. Condition factor (Fig. 5) and growth (Fig. 6)

Analysis of growth included only those fish that survived the duration of the experiment. At the end of the acclimation period at Site B there was no significant difference in the mean (s.e.) BW between the two groups. The mean specific growth rate (s.e. ) of the FW-SW (LD 18: 6 ) group during November was 1.12 (0.04)0/oday-‘, significantly higher than in the FW-SW

ACCLIMATION

OF ATLANTIC

SALMON PARR TO SEAWATER

351

(LDN) group (0.29 (0.05)% day-’ ), resulting in a signific on 6 December of 111.5 (4.0 compared to 76.6 (5.4) g. hibited no significant change i W between December and early growth rate of the FW-SW ( 8 : 6 ) and FW-SW (LDN ) groups increased significantly from early March and April respectively, reaching peaks of 1.24 (0.03) and 1.44 (0.03)% day-’ respectively during July. from 26 September onwards, the growth rates of the two groups were nificantly different, but between 7 March and 28 August the FW-SW group had a significantly higher growth rate than the FW-SW ( Mean BW of the FW-SW (LD18 : 6) group was significantly FW-SW (LDN) group from December until 28 August, after which there was no significant difference between the two groups, reaching 644 (23 ) and 630 (28) g respectively by the end of October. At the end of the experiment none of the fish exhibited sexual maturation. The mean (s.e. ) condition factor of the FW-SW (LDN) group on 8 November was 1.134 (0.015) and it did not change significantly until between ay when it declined significantly, May. Mean condition factor of the increased significantly between November and early March, but did not differ significantly from the FW-SW (LDN) group during this period. Mean condition factor of the FW-SW (LD18: 6) group exhibited no significant change during March and April, then declined significantly during reaching a nadir of 1.068 (0.009) on 30 May. Thereafter both groups exhibited a significant increase in condition factor during June and July, a significant decline during August, and then a significant increase again until the end in August of the experiment. The decline in growth rate an r. S. Lall, was associated with the fish suffering a vitami D.F.O., Halifax, N.S., personal communication, 1990 1 ‘lvhichwas remedied by adding a vitamin supplement (Fundy Choice ‘Vitamin 90, Corey Feed, Fredricton, N.B.) to the feed. DISCUSSION

The results show that Atlantic salmon parr (60-80 g) are euryhaline and can be acclimated to full SW in autumn. However, over the winter 60% of the parr acclimated to SW under LDN photoperiod died in association with the low SW temperatures, indicating that adaptation to SW was limited. trast, in those fish acclimated under LD 18 : 6, cumulative mortality lo%, supporting the hypothesis that a long photoperiod stimulates adaptation to SW. The long photoperiod treatment may represent a simple means of’ proving the commercial viability of transferring Atlantic salmon Parr to in the autumn. Parr surviving in SW through the winter cotqpieted smoltification in the spring, as judged by changes in appearacc;e and condition factor.

352

J. DUSTON AND J.D.E. KNOX

The gradual increase in salinity stimulated gill Na+K+ATPase activity and the development of hypoosmoregulatory mechanisms, similar to previous studies on anadromous salmonids not in the smolt stage (Boeuf and Harache, 1982; Johnsson and Clarke, 1988; Bergheim et al., 1990), non-smelting species of trout (Lahlou et al., 1975; Landless, 1976; Kaushik et al., 1977) and Arctic charr, Salvelinusalpinus (Finstad et al., 1989a,b). The gradual acclimation enabled the parr in all groups to maintain stable plasma osmolality (Fig. 3), avoiding the ‘crisis period’ of iono/osmotic imbalance observed when non-smolt salmonids are transferred directly to full SW (Boeuf et al., 1978; Leray et al., 198 1; Stagg et al., 1989). However, the FW-SW(LDN) group exhibited poor growth rate in SW during November and December compared to the FW-SW (LD 18 : 6) group, and an overall cumulative mortality of 60%, similar to previous studies on juvenile S. salar (Bjornsson et al., 1988) and Oncorhynchuskisutch (Clarke and Nagahama, 1977; Folmar et al., 1982; Young et al., 1989) transferred to SW prior to the completion of smoltification. Mortality of the FW-SW (LDN) group occurred in two phases: 15% died in a 2-week period following the increase in salinity to full SW in late October; the other 45% died from late-December onwards in association with the low SW temperatures. A disproportionately large number of the November mortalities in the FW-SW (LDN) group and all the mortalities in the FW-SW (LD 18: 6) group were small ( < 42 g at death), sexually mature males. The results support previous findings that adaptation of salmonids to SW can be limited by both small body size (Parry, 1960; Jackson, 198 1) and gonadal maturation (Ikuta et al., 1987; Lundqvist et al., 1989). The gill Na+K+ATPase and plasma osmolality data indicate that sexually mature males are as competent as immature parr to acclimate to SW, but for some reason their survival in SW is tenuous, and over the long term they are more likely to die. Increase in the mortality rate in the FW-SW(LDN) group in late-December was associated with a decline in SW temperature below 3 “C and the short winter daylength and, by contrast with the mortalities in November, was not correlated with body size or sexual maturation. Death was associated with a 10~sin body weight indicating tissue dehydration, supporting previous findings that mortalities at low SW temperatures are associated with impaired hypoosmoregulatory ability (Byrne et al., 1972; Virtanen and Oikari, 1984; Sigholt and Finstad, 1990). The limited ability of the FW-SW (LDN ) parr to tolerate SW at low temperatures during winter appears similar to other facultative anadromous salmonids such as rainbow trout, Oncorhynchusmykiss (Saunders et al., 1975; Johnsson and Clarke, 1988), brook trout Salvelinus fontinalis (Saunders et al., 1975; McCormick et al., 1985 ) and Arctic charr (Finstad et al., 1989a,b). In nature the latter two species reside only temporarily in SW during the summer, migrating back into FW for the winter (Mathisen and Berg, 1968; Castonguay et al., 1982).

ACCLIMATION OF ATLANTIC SALMON PARR TO SEAWATER

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Cumulative mortality in the FW-SW (LD 18 : 6 ) group was only 10% cornpared to 60% mortality in the FW-SW (LDN) group (Fig. 4). The SW (LD 18 : 6 ) treatment stimulated mean gill Na+K+ ATPase activity to 5-6 units, significantly higher than the FW-SW (LDN) groups (2-3 units), and comparable to levels found in smolts in spring (Duston et al., 1989, 1991). The FW (LD 18 : 6 ) group also exhibited a significant increase in salinity tolerance and gill Na+K+ATPase activity, confirming that an increase in photoperiod alone can stimulate salinity tolerance in salmonids (Duston et al., 1989; Finstad et al., 1989b). The 40-day long photoperiod treatment in autumn subsequently resulted in the FW-SW( LD 18 : 6 ) group exhibiting an increase in appetite and growth rate in winter compared to the FW-SW (LDN ) group (Fig. 6). It is speculated that the effects of extended daylength were at least partially mediated by growth hormone. Bjomsson et al. ( 1989) showed that increases in photoperiod and somatic growth are associated with increased serum growth hormone titres in Atlantic salmon; and independently of its effects on growth, growth hormone stimulates adaptation to SW in salmonids (Collie et al., 1989; Boeuf et al., 1990). The efficacy of a long photoperiod to stimulate adaptation to SW may be dependent on season, as an acclimation experiment performed in September using the same protocol resulted in a cumulative mortality over winter of 70% in both LDN and LD 18: 6 acclimated groups (J. Duston and J.D.E. Knox, unpublished observations). Juvenile salmonids appear to require a period of ‘short daylengths’ before an increase in photoperiod will stimulate development of salinity tolerance (Clarke et al., 1989; Okumoto et ak., 1989). Out-of-season increases in photoperiod can stimulate SW adaptation in juvenile salmonids by advancing the timing of smoltification (Wagner, 1974; Clarke e ton and Saunders, 1990). Jt is suggested that the FW-SW ( ment stimulated the completion of the hypoosmoregulatory aspects of smoltification. This would account for the low mortality in the FWSW (LD18 : 6) group over winter. However, the treatment did not cause the full completion of smoltification, as the fish retained parr marks, a greenish coloration and high condition factor until spring. The results support the hypothesis that high salinity tolerance can be dissoci.ated from the other aspects of smoltification (Folmar and Dickhoff, 198 I). It is speculated that the completion of smoltification is a collection of changes that normally occur coincidentally in the spring, but are not necessarily causally related, and may be cued/entrained by different environmental stimuli. From January the mortality rate in the FW-SW (LDN) gro spite the fact that SW temperatures remained <2.0°C until provement in tolerance to low SW temperatures was associ sonal increase in photoperiod (Finstad et al., 1989b), ston and Saunders, full completion of smoltification (Clarke et al., 1985; ay the parr ma s disappeared and the fish de1990). Between

354

J. DUSTON AND J.D.E. KNOX

veloped silvery flanks, steel-grey colouration to the dorsal surface and darkened fins, characteristic of smolts (Gorbman et al., 1982; Hoar, 1988). During the same period both groups exhibited a temporary decline in condition factor (Fig. 5 ), another characteristic of the completion of the Parr-smolt transformation (Farmer et al., 1978; Duston and Saunders, 1990). The timing of the decline in condition factor in the FW-SW (LD 18 : 6 ) group was delayed by approximately 1 month compared to the FW-SW (LDN) group in agreement with Saunders et al. ( 1989) who showed that subjecting Atlantic salmon parr in FW to 2-3 months of extended photoperiod in the autumn subsequently caused a delay in the completion of smoltification the following spring. The pattern of growth between the FW-SW (LDN ) and FW-SW (LD 18 : 6 ) groups exhibited significant differences (Fig. 6) despite the fact that they were maintained in the same tank from November onwards under LDN and fed to satiation daily. The long photoperiod treatment was followed by a significant increase in growth rate similar to previous studies (see Dustan and Saunders, 1990)) resulting in the FW-SW (LD 18 : 6 ) group having a significantly higher body weight compared to the FW-SW (LDN) group from December until the following August. However, from May until late-August the FW-SW (LDN) group exhibited a significantly higher growth rate than the FW-SW (LD18 : 6) group, and by the end of the experiment there was no significant difference in body weight between the groups. These data suggest that increase in body weight in Atlantic salmon is under homeosta.tic control (Brody, 1964), and that temporary perturbations in growth rate are subsequently corrected by compensatory growth (Quinton and Blake, 1990; Skilbrei, 1990). In conclusion, Atlantic salmon parr can be acclimated to SW in autumn. Subsequent mortalities during winter can be significantly reduced if the fish are acclimated under a long photoperiod. The enhancement of salinity tolerance by an increase in photoperiod does not occur in non-smolting brook trout (McCormick and Naiman, 1984) and rainbow trout (Johnsson and Clarke, 1988 ), and may represent an important difference between smolting and nonsmolting salmonids. Those fish surviving through winter appear ;o fully camplete smoltification in the spring, supporting the hypothesis that SW adaptation and smoltification are dissociable physiological events (Folmar and Dickhoff, 198 1). ACKNOWLEDGEMENTS

are indebted to Dr. R.L. Saunders for use of laboratory and computing facilities at D.F.O. Biological Station, St. Andrews. Thanks also go to I? Harmon and M. Stewart for technical assistance, F. Cunningham and VU.McMullon (all D.F.O. St. Andrews) for preparation of figures. The work was

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supported by an Industrial Research Assistance Program ( ) grant from the National Research Council Canada to Connors ros., Limited.

REFERENCES Bailey, J.K., Saunders, R.L. and Buzeta, M.I., 1980. Influence of parental smolt age and sea age on growth and smolting of hatchery-reared Atlantic salmon (Saltizo s&r). Can. J. Fish. Aquat. Sci., 37: 1379-I 386. Bath, R.N. and Eddy, F.B., 1979a. Salt and water balance in rainbow trout (Sulmo guirdneri) rapidly transferred from fresh water to sea water. J. Exp. Biol., 83: 193-202. Bath, R.N. and Eddy, F.B., 1979b. Ionic and respiratory regulation in rainbow trout during rapid transfer to seawater. J. Comp. Physiol., 134: 35 l-357. Bergheim, A., Kroglund, F., Vatne, D.F. and Rosseland, B.O., 1990. Blood plasma parameters in farmed Atlantic salmon (Sulmo s&r L. ) transferred to sea cages at eight to ten months. Aquaculture, 84: 159-I 65. Bjerknes, V., Duston, J., Knox, D. and Harmon, P., 1992. Importance of body size for acclimation of underyearling Atlantic salmon parr (S&no sulur L. ) to seawater. Aquaculture, in press. Bjornsson, B.T., Ogasawara, T., Hirato, T., Bolton, J.P. and Bern, H.A., 1988. Elevated growth hormone levels in stunted Atlantic salmon, Salma s&r. Aquaculture, 73: 275-28 1. Bjornsson, B.T., Thorarensen, H., Hirano, T., Ogasawara, T. and Kristinsson, J.B., 1989. Photoperiod and temperatur< affect plasma growth hormone levels, growth, condition factor and hypoosmoregulatory ability of juvenile Atlantic salmon (S&no s&r) during Parr-smolt trs%formation. Aquaculture, 82: 77-9 1. Boeuf, G. and Harache. Y., 1982. Criteria for adaptation of salmonids to high salinity seawater in France. Aquaculture, 28: 163-I 76. Boeuf, G., Lasserre, P. and Harache, Y., 1978. Osmotic adaptation of i%corhync!ws kisutch Walbaum. II. Plasma osmotic and ionic variations, and gill Na”K+ATPase activity of yearling coho salmon transferred to sea water. Aquaculture, 15: 35-52. Boeuf, G., Prunet, P. and LeBail, P.Y., 1990. Un traitement a l’hormone de croissance peut-il stimuler la smoltification du saumon atlantique? C.R. Acad. Sci. Paris. 3 IOa (series III ): 75-80. Brody, S., 1964. Homeostasis and organismic theory. In: S. Brody (Editor), Bioenergetics and Growth. Hafner Publishing Co., New York, NY, pp. 243-263. Brownlee, K.A., 1960. Statistical Theory and Methodology of Science and Engineering. Wiley, New York, NY, 570 pp. Byrne, J.M., Beamish, F.W.H. and Saunders, R.L., 1972. Influence of salinity, temperature, and exercise on plasma osmol&ty and ionic concentration in Atlantic salmon (Suln~osulur). J. Fish. Res. Board Can., 29: 12 17-I 220. Castonguay, M., Fitzgerald, G.J. and Cote, Y., 1982. Life history and movements of anadromous brook charr, Salvelinusfontinulisin the St. Jean River, Gaspe, Quebec. Can. J. ZOO].,60: 3084-309 I. Clarke, W.C. and Nagahama, Y., 1977. Effect of premature transfer to sea water on growth and morphology of the pituitary, thyroid, pancreas, and interrenal in juvenile coho salmon (Oncorhynchuskisutch). Can. J. Zoo]., 55: 1620- 1630. Clarke, W.C., Lundqvist, H. and Eriksson, L.O., 1985. Accelerated photoperiod advances seasonal cycle of seawater adaptation in juvenile Baltic salmon, Salvo sulur L. J. Fish Biol., 26: 29-35. Clarke, W.C., Shelbourn, J.E., Ogasawara, T. and Hirano, T., 1989. Effect of initial daylength

356

J. DUSTON AND J.D.E. KNOX

on growth, seawater adaptability and plasma growth hormone levels in underyearling coho, chinook, and chum salmon. Aquaculture, 82: 5 l-62. Collie, N.L., Bolton, J.P., Kawauchi, H. and Hirano, T., 1989. Survival of salmonids in seawater and the time frame of growth hormone action. Fish Physiol. Biochem., 7: 3 15-32 I. Cunjak, R.A., Saunders, R.L. and Chadwick, E.M.P., 1990. Seasonal variations in the smelt characten:tics ofjuvenile Atlantic salmon (Salmu s&r) from estuarine and riverine environments. Cdn. J. Fish. Aquat. Sci., 47: 8 13-820. Duston, J. and Saunders, R.L., 1990. The entrainment role of photoperiod on hypoosmoregulatory and growth related aspects of smolting in Atlantic salmon (S. su/ur). Can. J. ZOOI., 68: 707-7 15. Duston, J., Saunders, R., ;!armon, P. and Knox, D., 1989. Increases in photoperiod and temperature in winter advancr aspects of smoltilication in Atlantic salmon. Bull. Aqua. ASSOC. Can., 3: 19-2 1. Duston, J., Saunders, R.L. and Knox, D.E., 199 I. Effects of increases in freshwater temperature on loss of smolt characteristics in Atlantic salmon (Sulmo s&r). Can. J. Fish. Aquat. Sci., 48: 164-169. Farmer, G.J., Ritter, J.A. and Ashfield, D., iP78. Seawater adaptation and Parr-smolt transformation ofjuvenile Atlantic salmon, Sulmo slur. J. Fish. Res. Board Can., 35: 93-100. Finstad, B., Staurnes, M. and Reite, O.B., 1988. Effect of low temperature on sea-water tolerance in rainbow trout, Salmo guirdneri. Aquacuhzue, 72: 3 19-328. Finstad, B., Nilssen, K.J. and Gulseth, O.A., 1989a. St+water tolerance in freshwater resident Arctic charr (Salvelinus ulpinus). Comp. Biochem. Pnvsiol., 92A: 599-600. Finstad, B., Nilssen, K.J. and Arnesen, A.M., 1989b. Seasoual changes in sea-water tolerance of Arctic charr (Salvelinus ulpinus). J. Comp. Physiol B., 155‘.37 l-378. Folmar, L.C. and Dickhoff, W.W., 198 1. Evaluation of some physiological parameters as predictive indices of smoltification. Aquaculture, 23: 309-324. Folmar, L.C., Dickhoff, W.W., Mahnken, C.V.W. and Waknitz, F.W., 1982. Stunting an,d parr reversion during smoltification of coho salmon (Oncorhynchus kisxrch). Aquaculture, 28: 91-104. Gorbman, A., Dickhoff, W.W., Mighell, J.L., Prentice, E.F. and Waknitz, F.W., 1982. Morphological indices of developmental progress in the Parr-smolt coho salmon, Oncurhynchus kiwtch. Aquaculturc, 28: I-1 9. iierbinger, CM., Newkirk, G.F. and Lanes, S.T., 1990. Individual marking of Atltintic salmon: evaluation of cold branding and jet injection of Alcian Blue in several fin locations. J. Fish _ Bial., 36: 99-101. Hoar, W.S., 1976. Smolt transformation: evolution, behaviour and physiology. J. Fist, Res. Board Can., 33: 1234-1252. Hoar, W.S., 1988. The physiology of smolting salmonids. In: W.S. Hoar and D.J. Randall (Cditors), Fish Physiology, Vol. 1lB. Academic Press, New York, NY, pp. 275-343. Ikuta, K., Aida, K., Okumoto, N. and Hanyu, I., 1987. Effects of sex steroids on smoltiflcation of masu salmon, Oncorhynchus masou. Gen. Comp. Endocrinol., 65: 99- 110. Jackson, A.J., 198 1. Osmotic regulation in rainbow trout (Salmo guirdneri) following transfer to sea water. Aquaculture, 24: 143- 15 1. Johnsson, J. and Clarke, W.C., 1988. Development of seawater adaptation in juvenile steelhead trout (Suhrro gairdneri) and domesticated rainbow trout (Sulmo gairdneri) - effects of Size, temperature and photoal*riod. Aquaculture, 7 1: 247-263. Johnston, C.E. and Cheverie J.C., 1985. Comparative analysis of ionoregulation in rainbow trout (&-ho gairdm?) of different sizes following rapid and slow salinity adaptation, Can. 9. Fish. Aquat. Sci., 42: 1994-2003. Kaushik, S., Harache, Y. and Luquet, P., 1977. Variations in the total free amino acids level in

ACCLIMATION

OF ATLANTIC

SALMON

PARR TO SEAWATER

357

rainbow trout muscle and blood during its adaptation to sea water. Ann. Hydrobiol., 8: 145151. Kristinsson, J.B., Saunders, R.L. and W&s, A.J., 1985. Growth dynamics during the development of bimodal length-frequency distribution in juvenile Atlantic salmon (Sulmo s&r L. ). Aquaculture, 45: l-20. Lahlou, B., Crenesse, D., Bensahla-Talet, A. and Porthe-Nibelle, J., 1975. Adaptation de la truite d’elevage g l’eau de mer. J. Physiol. (Paris), 70: 593-603. Landless, P.J., 1976. Acclimation of rainbow trout to sea water. Aquaculture, 7: 173-179. Leray, C., Colin, D.A. and Florentz, A., 198 1. Time course of osmotic adaptation and gill energetics of rainbow trout (Salmo gairdncri R. ) following abrupt changes in external salinity. J. Comp. Physiol. B, 144: 175- I8 1. Lundqvist, H., Borg, B. and Berglund, I., 1989. Androgens impair seawater adaptability in smolting Baltic salmon (Safmo salar). Can. J. Zool., 67: 1733-l 736. Macleod, M.G., 1977. Effects of salinity on food intake, absorption and conversion in the rainbow trout, Salmo gairdneri. Mar. Biol., 43: 93-l 02. Mathisen, O.A. and Berg, M., 1968. Growth rates of the char Salvelinusalpinus (L.) in the Vardnes River, Troms, northern Norway. Rep. Inst. Freshwater Res. Drottningholm, 48: 177-186. McCormick, S.D. and Naiman, R.J., 1984. Osmoregulation in the brook trout, Salvelinusfonhnalis. II. Effects of size, age and photoperiod on seawater survival and ionic regulation. Comp. Biochem. Physiol., 79A: 17-28. McCormick, S.D. and Saunders, R.L., 1987. Preparatory physiological adaptations for marine life of salmonids: osmoregulation, growth and metabolism. Am. Fish. Sot. Symp., 1: 21 I229. McCormick, S.D., Naiman, R.J. and Montgomery, E.T., 1985. Physiological smolt characteristics of anadromous and non-anadromous brook trout (Salvelinusfontinalis) and Atlantic salmon (Salmo salar). Can. J. Fish. Aquat. Sci., 42: 529-538. McCormick, S.D.. Saunders, R.L., Henderson, E.B. and Harmon, P.R., 1987. Photoperiod control of Parr-smolt transformation in Atlantic salmon (Salmo salar): changes in salinity tolerance, gill Na+,K+-ATPase activity, and plasma thyroid hormones. Can. J. Fish. Aquat. Sci., 44: 1462- 1468. Nance, J.M., Bornancin, M., Sola, F., Boeuf, G. and Dutil, J.D., 1990. Study of transbranchial Na” exchange in Salmosalarsmolts and post-smolts directly transferred to sea water. Comp. Biochem. Physiol., 96A: 303-308. Okumoto, N., Ikuta, K., Aida, K., Hanyu, 1. and Hirano, T., 1989. Effects of photoperiod on smolting and hormonal secretion in masu salmon, Oncorhynchusmasou. Aquaculture, 82: 63-76. Parry, G., 1960. The development of salinity tolerance in the salmon, Salmo salar ( L. ) and some related species. J. Exp. Biol., 37: 425-434. Quinton, J.C. and Blake, R.W., 1990. The effect of feed cycling and ration level on the compensatory growth response in rainbow trout, Oncorhynchusmykiss.J. Fish Biol., 37: 33-41. Saunders, R.L., Muise, B.C. and Henderson, E.B., 1975. Mortality of salmonids cultured at low temperature in sea water. Aquaculture, 5: 243-252. Saunders, R.L., Specker, J.L. and Komourdj& M.P., 1989. Effects of photoperiod on growth and smolting in juvemle Atlantic salmon (Salmo salar). Aquaculture, 82: 103-l 17. Sigholt, T. and Finstad, B., 1990. Effect of temperature on seawater tolerance in Atlantic salmon (Salmo salar) smolts. Aquaculture, 84: 167-l 72. Skilbrei, O.T., 1990. Compensatory sea growth of male Atlantic salmon, Safmo safar L., which previously mature as Parr. J. Fish Biol., 37: 425-435. Stagg, R.M., Talbot, C., Eddy, F.B. and Williams, M., 1989. Seasonal variations in osmoregu-

358

1.DUSTON AND J.D.E.

KNOX

latory and respiratory responses to seawater exposure of juvenile Atlantic salmon (Salvo s&r) maintained in freshwater. Aquaculture, 82: 2 19-228. Virtanen, E. and Oikari, A., 1984. Effects of low acclimation temperature on salinity adaptation in presmolt salmon, Sufmo sulur L. Comp. Biochem. Physiol., 78A: 387-392. Wagner, H.H., 1974. Photoperiod and temperature regulation of smolting in steelhead trout (Sulmo gairdneri). Can. J. Zool., 52: 2 19-234. Wedemeyer, G.A., Saunders, R.L. and Clarke, W.C., 1980. Environmental factors affecting smoltification and early marine survival of anadromous salmonids. Mar. Fish. Rev., 42: l14. Young, G., Prunet, P., Ogasawara, T., Hirano, T. and Bern, H.A., 1989. Growth retardation (stunting) in coho salmon: plasma levels in stunts in seawater and after transfer to fresh water. Aquaculture, 82: 269-278. Zaugg, W.S., 1982..A simplified preparation for adenosine triphosphatase determination in gill tissue. Can. J. Fish. Aquat. Sci., 39: 215-217.