effects of the sea louse lepeophtheirus salmonis on ... et al.pdf · groups of mature (5+ year old)...

Journal of Fish Biology (2010) 76, 2318–2341

doi:10.1111/j.1095-8649.2010.02636.x, available online at www.interscience.wiley.com

Effects of the sea louse Lepeophtheirus salmonison temporal changes in cortisol, sex steroids, growth

and reproductive investment in Arctic charrSalvelinus alpinus

H. Tveiten*†, P. A. Bjørn*, H. K. Johnsen‡, B. Finstad§and R. S. McKinley‖

*Nofima Marin, N-9291, Tromsø, Norway, ‡Norwegian College of Fishery Science, Universityof Tromsø, N-9037 Tromsø, Norway, §Norwegian Institute of Nature Research, N-7485

Trondheim, Norway and ‖Centre for Aquaculture and the Environmental Research,The University of British Columbia, West Vancouver, BC, V5Z 1M9 Canada

(Received 15 May 2009, Accepted 3 February 2010)

Groups of mature (5+ year old) Arctic charr Salvelinus alpinus held in sea water were exposedfor 34 days to either a high (mean ± s.e. 0·15 ± 0·01 sea lice Lepeophtheirus salmonis g−1

fish mass) (HI), medium (0·07 ± 0·00 sea lice g−1 fish mass) (MI) or no [control (C)] sea-liceinfection during early stages of gonad development (June to July). Infection with sea lice resultedin increased plasma cortisol concentrations and this was related to intensity of infection; femalestended to have higher cortisol concentrations than males at high infection intensities (HI group:female c. 130 ng ml−1; male c. 80 ng ml−1). Plasma osmolality (C c. 330, MI c. 350 and HI c. 415mOsm) and chloride concentrations (C c. 135, MI c. 155 and HI c. 190 mM) increased significantlywith infection intensity, indicating osmoregulatory problems in infected fish. A strong positiverelationship between plasma osmolality and cortisol concentration was recorded. Plasma sex-steroidconcentrations were influenced negatively by sea-lice infection, particularly in the HI group, andwere inversely related to plasma cortisol concentrations. The most heavily infected fish postponedthe initiation of reproductive development until exposed to fresh water and timing of ovulationtended to be delayed in these fish. Growth rate and condition were negatively influenced by sea-liceinfection and growth rate was inversely related to plasma cortisol concentrations. Sea-lice infectionresulted in mortality among females in the HI group, and the proportion of maturing females waslower in the MI group (46%) than in the controls (85%). Egg production in the MI and HI groupswas c. 50 and 30% of the C group. Egg size, embryonic survival and fry mass did not differacross groups. Sea lice influence reproductive development and egg production in S. alpinus, andconsequently these parasites may influence populations via sublethal effects on broodfish, affectinggrowth and condition, and their reproductive output. © 2010 The Authors

Journal compilation © 2010 The Fisheries Society of the British Isles

Key words: condition; ectoparasite; egg production; growth; stress.

INTRODUCTION

In fishes, stress may have the potential to affect reproductive development, invest-ment and gamete quality (Schreck et al., 2001). There is evidence that physiological

†Author to whom correspondence should be addressed. Tel.: +47 77629063; fax: +47 77629100;email: [email protected]

2318© 2010 The Authors

Journal compilation © 2010 The Fisheries Society of the British Isles

S E A L I C E A F F E C T S A LV E L I N U S A L P I N U S R E P RO D U C T I O N 2319

stress exerts inhibitory effects on reproductive processes in teleosts through modi-fied endocrine function (Pankhurst & Van Der Kraak, 1997), mediated specificallythrough changes in plasma sex-steroid concentrations (Pankhurst & Van Der Kraak,2000). Also, studies involving in vivo administration of cortisol do show inhibitoryeffects of cortisol on reproductive endocrine processes and development (Carragheret al., 1989; Foo & Lam, 1993). As in mammals, an elevation of plasma cortisolis the main indicator of stress in fishes (Wendelaar-Bonga, 1997). Cortisol is themain corticoid steroid produced by the interrenals and plays the role of both glucoand mineralo-corticosteroid as seen in higher vertebrates (Mommsen et al., 1999),and stress may influence growth physiology and energy accumulation (Barton, 1997).Since cortisol may have wide-ranging metabolic effects (Pankhurst & Van Der Kraak,1997; Mommsen et al., 1999), it is difficult to discern whether there is a directeffect of cortisol on steroid synthesis or indirect effect through affecting nutritionalstatus of the fishes which is important for the initiation or progression of reproduc-tive development (Leatherland, 1999). Pankhurst & Van Der Kraak (2000) providedcompelling evidence that cortisol does have an effect on plasma sex-steroid concen-trations without altering nutritional status and, thus, may act at different levels toinhibit reproductive development.

In salmonid, infection with the sea louse Lepeophtheirus salmonis induces anintegrated stress response. In sea trout (anadromous brown trout) Salmo trutta L. forexample, plasma cortisol concentrations are elevated within a few days of infection,when the sea lice still are at an early stage of development, without inducing anyosmoregulatory problems (Bjørn & Finstad, 1997; Finstad et al., 2000). A severeosmoregulatory problem, which is associated with an additional increase in plasmacortisol concentrations, occurs when the sea lice reach their pre-adult and adult stages,usually 20–25 days post-infection (Bjørn & Finstad, 1997; Finstad et al., 2000).Thus, the stress response to sea-lice infection may be considered to be chronic in itsnature and no acclimation seems to occur. Under experimental conditions, reducedgrowth and fish mortality may be induced by high sea-lice infection (0·5–1·5 sealice g−1 fish) (Bjørn & Finstad, 1997), which is likely also to occur under naturalconditions (Birkeland, 1996; Bjørn et al., 2001). Although increased mortality wouldobviously reduce reproductive output in a population, the long-term, sublethal, stresseffect of sea-lice infection on growth and reproductive processes is little studied.

Wild, anadromous Arctic charr Salvelinus alpinus (L.) undertake annual migra-tions to sea during the summer months, and usually spend 40–50 days in sea waterbefore returning to fresh water to overwinter (Jobling et al., 1998). In winter, foodintake is very limited but during their short sea migration the fish grow rapidlyand accumulate body energy at a high rate which is subsequently allocated tomeet energy demand for reproduction and metabolism during overwintering (Joblinget al., 1998).

During its short sea migration, S. alpinus may be heavily infected by sea-lice(Bjørn et al., 2001). Although the physiological response to sea-lice infection is lit-tle studied in S. alpinus, the sea lice appear to have a similar developmental rate,host distribution and pathogenicity to that found in sea trout S. trutta (Bjørn &Finstad, 1998). Sea-lice infection, by inducing physiological stress to the fish, maytherefore have the potential to influence growth performance, reproductive devel-opment and gamete investment in S. alpinus. To that end, the effect of different

© 2010 The AuthorsJournal compilation © 2010 The Fisheries Society of the British Isles, Journal of Fish Biology 2010, 76, 2318–2341

2320 H . T V E I T E N E T A L .

sea-lice infection intensities on stress physiology, reproductive endocrine homeosta-sis and how these relate to growth, reproductive development, gamete productionand embryonic survival in S. alpinus were studied.

MATERIALS AND METHODS

F I S H A N D H O L D I N G C O N D I T I O N S

The study was conducted at the Aquaculture Research Station, Karvika, northern Norway(70◦N) during the period April 2000 to May 2001. The fish used in the study were mature5+ year-old (Hammerfest strain) S. alpinus of the 1995 year class. On 17 November 1999,all fish were anaesthetized in benzocaine (50 ppm) in order to remove gametes and confirmreproductive status. To simulate feeding conditions experienced by wild S. alpinus (Dutil,1986; Boivin & Power, 1990), fish were not fed during winter (until early June). Fish wasthen fed (Skretting, 6 mm; www.skretting.no) until 27 July by means of automatic discfeeders. Feed supply was in excess based on calculations of feed requirement using a growthmodel for S. alpinus (Jobling, 1983). On 26 April 2000, three groups of fish with 33–34fish in each group were established by selecting fish from a group of c. 300. The fish wereweighed (M , mean + s.e. 731 ± 40 g), fork length, LF, measured (42·5 ± 0·8 cm) andtagged individually with FT-69 fingerling tags (Floy Tag & Mfg.; www.floytag.com), andrandomly distributed into three circular tanks (500 l). Until 26 April, fish were held at ambientphotoperiod, thereafter all groups were exposed to a simulated natural photoperiod accordingto Johnsen et al. (2000). Briefly, each tank was shielded by a black, light-proof plastic canopy,and lighting was provided at an intensity of c. 80 lx at the water surface. Photoperiod wascontrolled by automatic timers without a twilight period.

Tanks were supplied with fresh water of ambient temperature (0·1–3·2◦ C) until 16 June.Then, during a 3 day period, the fish were gradually transferred to full strength sea water(salinity 33). On 23 June, two of the groups were infected with either a medium or a highdose of sea-lice copepodids, and all groups were then maintained in sea water for 34 days(27 July) which approximate the length of a natural sea migration of this strain of S. alpinus(Jobling et al., 1998). When kept in sea water, temperature increased slowly from 6·6 to8·1◦ C. In June, wild fish of this strain of S. alpinus migrate to sea (Rikardsen et al., 1997)and no mortality was recorded after transfer. After residence in sea water (on 27 July), fishwere transferred, during a 3 day period, back to fresh water at ambient temperature (10·2◦ C).From August to the beginning of the spawning season (20 October), temperature decreasedslowly from c. 10 to 5◦ C. Since temperature may influence timing of ovulation and eggquality in salmonids, including S. alpinus (Gillet, 1991; Taranger et al., 2000), all tanks weremaintained at a temperature close to 5◦ C throughout the spawning season.

S E A - L I C E I N F E C T I O N

On 23 June, two of the groups were infected with either a medium (MI) or a high (HI)dose of sea lice copepodids, whereas control fish were sham infected. Infection was performedaccording to Bjørn & Finstad (1998) with either c. 120 or 240 copepodid larvae per fish inthe MI and HI groups, respectively, to attain levels of c. 50 and 100 lice per fish based on65% infection success and 65% lice survival. This procedure resulted in 100% prevalenceof sea lice-infected fish and a final (end of seawater residence) sea-lice infection intensity of72 ± 4 and 140 ± 5 sea lice per fish (mean ± s.e.), which was equivalent to 0·07 ± 0·00and 0·15 ± 0·01 sea lice g−1 (mean ± s.e.) of fish in the MI and HI groups, respectively.Calculation of final sea-lice infection intensity in the HI group was based on recordings fromsurviving fish. As predicted from previous experiments (Bjørn & Finstad, 1998), the sea licehad reached pre-adult stages (c. 260 degree-days of development) at the end of seawaterresidence.



S A M P L I N G

All fish from each of the three groups were sampled for blood, LF and M on 26 April,19 May, 16 June, 27 July, 9 September, 21 September, 17 October and 29 November. Tominimize handling stress fish were sedated with benzocaine (70 ppm) by adding it to thetank through a tube, carefully placed in the tank through a hole in the canopy wall. Watersupply was then closed, and the fish were left undisturbed until immobilized. All fish wereimmobilized within 3 min and was then transferred to an oxygenated sedation bath of lowerbenzocaine concentration (35 ppm) and maintained there until sampling. The whole samplingprocedure was terminated within 22–25 min. Under normal conditions (in fresh water), thissampling procedure results in cortisol levels c. 20 ng ml−1, 20–25% lower than that recordedwhen conventional netting is used (pers. obs.).

At each sampling, the fish were weighed (c. 0·5 g) and LF measured (c. 0·1 cm). Conditionfactor (K) was calculated as: K = 100 M LF

−3 (M in g and LF in cm). Mass data wereused to calculate specific growth rates (G) for individual fish according to the formula:G = 100 (lnM2 − lnM1) (t2 − t1)−1, where M1 and M2 are fish masses at time (days) t1 andt2, respectively.

Blood (1–2 ml) was sampled from the caudal vasculature using vacutainer tubes (Vacu-tainer; www.bd.com) containing 57·2 U.S.P. units Li+-heparin. The samples were held on iceuntil centrifugation at 3800 g for 10 min at 0–2◦ C and plasma was stored at −80◦ C untilassayed for cortisol and sex steroids.

P L A S M A A NA LY S E S

Plasma concentrations of oestradiol-17β (E2), testosterone (T) and 11-ketotestosterone (11-KT) were measured by means of radioimmunoassay (RIA), according to Schulz (1985).Validations of the assays for S. alpinus plasma, and cross-reactivities of the E2 and T anti-serum, have been previously examined (Frantzen et al., 2004). Cross-reactivities of the 11-KTantiserum are given by Schulz (1985).

Briefly, steroids were extracted from 200 μl plasma with 4 ml diethyl ether under vigorousshaking for 4 min. The aqueous phase was frozen in liquid nitrogen, whereas the organic phasewas transferred to a glass tube, evaporated in a water bath at 45◦ C and then reconstitutedby addition of 600 μl assay buffer and then assayed for E2, T and 11-KT.

Plasma concentrations of cortisol were measured using the same RIA procedure as de-scribed for measurement of sex steroids. The detection limit for the assay was 0·6 ng ml−1.The cortisol antiserum, raised in New Zealand white (NZW) rabbits, gave 14·8% cross-reaction with 17,21-dihydroxy-4-pregnene-3,20-dione (11-deoxycortisol), 8·8% with 17,21-dihydroxy-4-pregnene-3,11,20-trione (cortisone), 4·7% with 11β,17-dihydroxy-4-pregnene-3,20-dione (21-deoxycortisol), 1·9% with 5β-pregnane-3α,17,20β-triol, 1·2% with 5β-pregnane-3α,17,20β-triol sulphate, 1·0% with 3β-hydroxy-5-pregnen-20-one (pregnenolone),0·7% with 5β-pregnane-3β,17,20β-triol, 0·6% with 17,20β,21-trihydroxy-4-pregnen-3-one,0·6% with 17,20α-dihydroxy-4-pregnen-3-one and <0·1% with 4-pregnene-3,20-dione (pro-gesterone), 17-hydroxy-4-pregnen-3,20-dione (17-hydroxyprogesterone), 17,20β-dihydroxy-4-pregnen-3-one (17,20β-P) 17β-hydroxy-4-androstene-3,11-dione (11-ketotestoster-one), 17β-hydroxy-4-androsten-3-one (testosterone) and 1,3,5(10)-estratriene-3,17β-diol(oestradiol-17β).

To validate the cortisol assay for S. alpinus, a plasma pool was divided into four aliquots,two of which were stripped of endogenous steroids by charcoal treatment [45 mg char-coal (Sigma-Aldrich; www.sigmaaldrich.com) and 4·5 mg Dextran T 70 (Pharmacia Biotech;www.apbiotech.com) per ml plasma, incubated for 1 h at room temperature under continu-ous shaking]. Half of stripped and untreated plasma samples were then spiked with 150 ngcortisol (Sigma-Aldrich) per ml plasma to give four preparations (untreated, untreated andspiked, stripped, and stripped and spiked). To test extractions of steroids, these preparationswere subjected to ether extraction as described above. Products resulting from the differenttreatments were then assayed by the cortisol-RIA at two dilutions. Steroid recovery afterether extraction was 87%. Cortisol values were corrected for recovery losses. The inter andintra-assay coefficients of variation (c.v.) for the cortisol assay were 12·7% (n = 28) and


2322 H . T V E I T E N E T A L .

7·7% (n = 9), respectively. Analysis of plasma cortisol concentrations was carried out forsamplings before (16 June), at the end (27 July) and after (21 September) sea-lice infection.

Osmolality [mosmol kg−1 (mOsm)] was measured by Fiske ONE-TEN Osmometer (FiskeAssociates; www.aicompanies.com), and chloride (Cl−) concentrations were determinedusing a Corning 925 chloride titrator (CIBA Corning Diagnostics; www.novartis.co.uk). Anal-ysis of plasma osmolality and Cl− concentrations was carried out for the same samplings asindicated for cortisol.

There was not always sufficient plasma to carry out all the analysis indicated above, andthe number of fish analysed for the different variables may deviate slightly from the totalnumber of fish in the different groups.

G A M E T E C O L L E C T I O N

From 3 October onwards, the fish were anaesthetized and examined for spermiation andovulation at 5–7 days intervals. Spermatocrit (S) was assessed from milt transferred to haema-tocrit tubes (75 mm × 1·2 mm), spun at 4500 g for 10 min and calculated according to theformula: S = 100HC H−1

T , where HC is the height of the cell layer in the haematocrit tubeand HT is the total height of the seminal fluid including the cell layer. Spermatocrit was mea-sured at the beginning (just after the first ovulating female), mid (c. 50% ovulated females)and at the end (all females had ovulated) of the spawning season.

Eggs were collected from ovulated females and the total volume was recorded. The numberof eggs in a sample of 25 ml was counted, and total fecundity (total number of eggs)and relative fecundity (number of eggs kg−1) were calculated. Egg diameters were deter-mined as the average from measurement of 75–100 freshly ovulated eggs from each female.Eggs were fertilized with milt (in excess) pooled from several males from the same group.Fertilized eggs were then incubated, in triplicate samples (c. 1200 eggs), in an upwellingincubator. Each incubator (diameter. 12 cm) had a perforated bottom plate through whichwater was delivered at a rate of c. 0·5 l min−1 at a temperature of 3·95 ± 0·02◦ C(mean ± s.e.).

Fertilization rate was estimated by examining at least 75 eggs c. 2 days after fertilization.Eggs were cleared in a solution of glacial acetic acid and saline (1:20 v/v), examined undera binocular microscope, and cleaved eggs were classified as fertilized. Eggs from 11, six andfour females in the control, MI and HI groups, respectively, were fertilized and incubated.Dead eggs were removed and counted at regular intervals during incubation. Egg survival tothe eyed stage was determined after 300 ± 8 degree-days (mean ± s.e.) of incubation, andthe proportion of eggs that survived to the eyed stage was assessed in relation to the numberof fertilized eggs.

Egg production by each female was assessed as relative egg mass (MRE) calculated accord-ing to the formula: MRE = 100ME M−1

B , where ME is the wet mass of the eggs with ovarianfluid and MB is the mass of the female after stripping.

F I R S T F E E D I N G A N D E A R LY G ROW T H

Approximately 100 degree-days after hatching, 500 fry from each female in each groupwere transferred to first-feeding trays (10 cm × 15 cm; water depth 5 cm), and feedingwas initiated at 165 ± 8 degree-days (mean ± s.e.) after hatch when c. 0·33 of the yolk sacremained. First feeding was carried out at 4·05 ± 0·03◦ C (mean ± s.e.) and under conditionsof continues dim light and food supply (Skretting, Nutra Starter, particle size was increasedgradually from 0·3 to 1·0 mm). At hatch, and 3 and 6 weeks after initiation of first feeding,fry LF (mm) and M (mg) were recorded (n = 10) for each batch.

The experiments and procedures described here have been conducted in accordance withthe laws and regulations controlling experiments and procedures with live animals in Norway,i.e. the Animal Welfare Act of 20 December 1974, No. 73, chapter VI, sections 20–22 andthe Regulation on Animal Experimentation of 15 January 1996.



S TAT I S T I C A L A NA LY S E S

All data were first tested for normal distribution by the non-parametric Kolmogorov–Smirnov test (Lilliefors distribution). A two-way ANOVA was used to investigate possi-ble effects of time, sea-lice infection and their interaction on the variables studied. Whenappropriate, data were square-root, log10 or arcsin transformed to obtain normality. When thetwo-way ANOVA revealed significant effects, a one-way ANOVA, followed by a Tukey posthoc test, was used to identify where differences occurred between groups at each samplingdate and within groups between sampling dates. All values are given as mean ± s.e. Possiblerelationships between variables were assessed using Pearson correlation analysis (Zar, 1999).Proportions of maturing fish were compared using χ2 analyses of frequency data (Zar, 1999).A probability level of P < 0·05 was considered significant in all tests. All computations wereperformed with Systat 9.2 (www.systat.com).

RESULTS

S E A - L I C E I N F E C T I O N

At the end of seawater residence, sea-lice infection intensities differed significantlybetween all treatment groups (Fig. 1) but did not differ between male and femalefish within treatments (P > 0·05).

M O RTA L I T Y A N D M AT U R AT I O N R AT E S

Sea-lice infection had a significant effect on fish survival. In the HI group, 40%of the fish died during the period of sea-lice infection, whereas corresponding valuein the control and MI groups was 6% (Fig. 2). In the control group, for unknownreasons, another five fish died after re-entry to fresh water, with mortality reachinga total of 21% in this group. The proportions of maturing and not re-maturing fishwas also significantly affected by lice infection, with only about half the number ofthe females re-maturing in the MI group compared with that of the control (Table I).In the HI group, although not statistically different, the total number of maturingfemales was reduced by what appeared to be sex-specific mortality (Table I). Thisresulted in a skewed female-to-male ratio (30 v. 70%) in the HI group, although notstatistically different from that of the other groups (P > 0·05). There was no effectof sea-lice infection on the proportion of maturing males (Table I).

Table I. Frequency (%) of sexual maturation in male and female Salvelinus alpinus exposedto no (control), medium and high sea-lice infection for 34 days during gonad recrudescence(23 June to 27 July). Number in parentheses indicates number of fish. Different superscriptlowercase letters within columns indicate significant differences (P < 0·05) between groups

Category of fish

Treatment Mature females Immature females Mature males Immature males

Control 84·6 (11)a 15·4 (2)a 92·3 (12)a 7·7 (1)a

Medium infection 46·2 (6)b 53·8 (7)b 94·2 (16)a 5·8 (1)a

High infection 66·7 (4)a 33·3 (2)a 85·7 (12)a 14·3 (2)a


2324 H . T V E I T E N E T A L .

C O RT I S O L A N D S E X S T E RO I D S

There was a significant effect of sea-lice infection on plasma cortisol concentra-tions, and plasma cortisol concentrations increased in a dose–response-like manner(Fig. 1), with cortisol tending although not significantly (P > 0·05) to be higher infemales than in males at high infection intensity. The two-way ANOVA revealed,however, a significant overall higher plasma cortisol concentration in maturing fe-males compared with maturing males. Also at the individual level, there was a sig-nificant positive relationship between sea-lice infection intensity and plasma cortisolconcentrations with this relationship being stronger in maturing females (R2 = 0·86,n = 16) than in maturing males (R2 = 0·33, n = 40).

Before seawater transfer and sea-lice infection, cortisol concentrations were15–20 ng ml−1 in all groups both in males and females [Fig. 3(a), (b)]. In the con-trol group, plasma cortisol concentrations did not change statistically with time,and both in maturing males and females these concentrations were largely unaf-fected by seawater exposure and season [Fig. 3(a), (b)]. In the MI group, sea-liceexposure resulted in male and female plasma cortisol concentrations of c. 60 and70 ng ml−1, respectively [Fig. 3(a), (b)], with female values being significantly higherand lower, respectively, compared with corresponding values in the control and HI

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Fig. 1. Relationship between mean ± s.e. sea-lice infection intensity and plasma cortisol concentrations in male( , , ) and female ( , , ) Salvelinus alpinus exposed to no [control ( , )], medium [MI ( , )]and high [HI ( , )] sea-lice infection during gonad recrudescence (23 June to 27 July). Differencesin plasma cortisol concentrations are indicated by different lowercase letters (P < 0·05). Differences insea-lice infection intensity are indicated by different uppercase letters (P < 0·05). Fish were sampled atthe end of sea-lice and seawater exposure (fish still in sea water). Strength of relationships (R2) betweenvariables is calculated from individual data.



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Fig. 2. Per cent mortality in groups of Salvelinus alpinus exposed to no (control), medium (MI) and high (HI)sea-lice infection during gonad recrudescence (23 June to 27 July). Different lowercase letters abovebars indicate significant differences (P < 0·05) between treatments.

groups. Cortisol values in MI males did not differ significantly from other groups,but only marginally (P > 0·05) differ from males in the control group. Plasma cor-tisol concentration in females that did not re-mature in the MI group was 48·5 ±10·8 ng ml−1, values being not different from that of maturing females in the samegroup. In the HI group, male and female plasma cortisol concentrations were close to80 and 130 ng ml−1, respectively, at the end of sea-lice infection, and female valueswas significantly higher compared with corresponding values in the control and MIgroups [Fig. 3(a), (b)]. Cortisol values in HI males were significantly different fromthat of the control, but did not differ from male MI values. In the autumn (Septem-ber), plasma cortisol concentrations were only slightly higher than those recordedbefore lice infection [Fig. 3(a), (b)] and did not tend to differ between sexes.

In late spring, low sex-steroid concentrations were recorded in all groups [Figs 4(a)and 5(b)]. From June onwards, plasma sex-steroid concentrations rose rapidly inboth male and female fish in the control group. More specifically, in males T peakedin September, whereas peak values of 11-KT were recorded in October [Fig. 4(a),(b)]. Thereafter, sex-steroid concentrations decreased throughout the spawning season[Fig. 4(a), (b)]. In female fish, E2 reached highest concentrations during September,and then declined c. 1 month earlier than did T to reach minimal concentrations inNovember [Fig. 5(a), (b)]. Sea-lice infection had a significant influence on temporalchanges in plasma sex-steroid concentrations both in maturing males and females.Elevation in plasma sex-steroid concentrations, both in the MI and HI groups, wassignificantly delayed compared with the control group, the exception being E2 in theMI group [Figs 4(a) and 5(b)]. After transferring back to fresh water, concentrationsrose rapidly in the MI and HI groups, and timing of peak plasma sex-steroid concen-trations were similar in all groups [Figs 4(a) and 5(b)]. In the HI group, however,sex-steroid concentrations did not fully reach the levels of those in the control and MIgroups, of which was particularly evident for 11-KT in male fish [Figs 4(a) and 5(b)].


2326 H . T V E I T E N E T A L .

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Fig. 3. Plasma cortisol concentrations in maturing (a) male and (b) female Salvelinus alpinus before (16 June,in fresh water), at the end (27 July, in sea water) and post (21 September, in fresh water) sea-liceinfection. The fish was exposed to no (control, ), medium ( ) and high ( ) sea-lice infection duringgonad recrudescence (23 June to 27 July). Differences between treatments within sampling dates areindicated by different lowercase letters (P < 0·05). Differences across sampling dates within treatmentsare indicated by different uppercase letters (P < 0·05).

At the end of sea-lice infection, there was a significant negative relationshipbetween plasma concentrations of T and cortisol both in male and female fish (Fig. 6).Taking the data together, this negative relationship was apparent also at the individ-ual level, but stronger for females (R2 = 0·65, n = 17) than for males (R2 = 0·40



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Fig. 4. Temporal changes in plasma (a) testosterone and (b) 11-ketotestosterone concentrations of male Salveli-nus alpinus exposed to no [control ( )], medium ( ) and high ( ) sea-lice infection during gonadrecrudescence (23 June to 27 July). Different letters indicate significant differences (P < 0·05) withinsampling dates. Horizontal bars indicate timing of sea-lice and seawater exposure, and the breedingperiod, respectively.

n = 41). Similar relationships were obtained for the other sex steroids measured(female E2: R2 = 0·57 and male 11-KT: R2 = 0·41).

P L A S M A O S M O L A L I T Y A N D C H L O R I D E C O N C E N T R AT I O N S

Plasma osmolality and chloride concentrations are shown in Fig. 7(a), (b), respec-tively. There was no overall effect of sex on plasma osmolality and chloride concen-trations, neither was there any interaction between sex and treatment. In fresh water,


2328 H . T V E I T E N E T A L .

0

20

40

60

80

100

120

140

160 (a)

21/4 21/5 20/6 20/7 19/8 18/9 18/10 17/11 17/12

Tes

tost

eron

e (n

g m

l−1)

Sea-lice and seawater exposure Breeding period

a

aa

a

b

a

a

bab

a

a

aa

a

a

a

a

aa

aa

a

aa

0

10

20

30

40

50

60 (b)

21/4 21/5 20/6 20/7 19/8 18/9 18/10 17/11 17/12

Date (day/month)

Oes

trad

iol (

ng m

l−1)

Sea-lice and seawater exposure Breeding period

a

aa a

b

b

a

aa

a

b

ab

a

a

a

a

aa

a

aa a

a

a

Fig. 5. Temporal changes in plasma (a) testosterone and (b) oestradiol-17β concentrations of maturing femaleSalvelinus alpinus exposed to no [control ( )], medium ( ) and high ( ) sea-lice infection during gonadrecrudescence (23 June to 27 July). Different letters indicate significant differences within sampling dates(P < 0·05). Horizontal bars indicate timing of sea-lice and seawater exposure, and the breeding period,respectively.

just before sea-lice infection, plasma osmolality and chloride concentrations werec. 310 mOsm and 140 mM, respectively, in all groups. In the control group, seawa-ter exposure resulted in a slight, but significant, increase in plasma osmolality (c.330 mOsm), whereas chloride concentrations remained largely unaffected comparedwith freshwater values [Fig. 7(a), (b)].



0

5

10

15

20

25

30

0 50 100 150 200

Cortisol (ng ml−1) 27 July (end of seawater exposure)

Tes

tost

eron

e (n

g m

l−1)

R2 = 0·40 (male)

R2 = 0·65 (female)

Fig. 6. Relationship between plasma cortisol and testosterone concentrations in male ( , , ) and female( , , ) Salvelinus alpinus exposed to no [control ( , )], medium ( , ) and high ( , ) sea liceduring gonad recrudescence (23 June to 27 July). Fish were sampled at the end of sea-lice and seawaterexposure (fish still in sea water). Strength of relationships (R2) between variables is calculated fromindividual data.

Sea-lice infection resulted in a significant increase in plasma osmolality and chlo-ride concentrations. The effect was most apparent in the HI group where plasmaosmolality and chloride concentrations reached levels of c. 410 mOsm and 200 mM,respectively [Fig. 7(a), (b)]. Also in the MI group, plasma osmolality (c. 350 mOsm)and chloride concentrations (c. 155 mM) were influenced by sea-lice infection andwas statistically higher and lower, respectively, compared with the control and HIgroups [Fig. 7(a), (b)]. Plasma osmolality and chloride concentration in females thatdid not re-mature in the MI group were 338·1 ± 5·3 mOsm and 144·3 ± 1·4 mM,respectively, values not being significantly different from that of maturing fish in thesame group. Osmolality and chloride values returned to pre-infection values aftertransfer to fresh water [Fig. 7(a), (b)].

Also at the individual level there was a strong positive relationship between sea-lice infection intensity and plasma osmolality (Fig. 8), with no apparent differencebetween males (R2 = 0·67, n = 35) and females (R2 = 0·71, n = 18). Furthermore,plasma cortisol concentration was significantly and positively related to plasma osmo-lality (Fig. 9). At the individual level, this relationship was stronger for females(R2 = 0·77, n = 17) than for males (R2 = 0·47, n = 37).

G ROW T H A N D C O N D I T I O N

After initiation of feeding in early June, growth rate and condition increasedrapidly in all groups. Growth and condition were, however, significantly affected in


2330 H . T V E I T E N E T A L .

280

300

320

340

360

380

400

420

440 (a)

(b)

Pre-infection End-infection Post-infection

Osm

olal

ity (

mO

sm)

aaa

a

c

b

aa

aA

A

B

AA

B

A

A

B

130

140

150

160

170

180

190

200

210

Pre-infection End-infection Post-infection

Time of sampling

Chl

orid

e (m

mol

l−1)

aa a

a

c

b

aaa A

A

A A

A

B

AA

B

Fig. 7. Plasma (a) osmolality and (b) chloride concentrations in maturing male and maturing female Salvelinusalpinus before (16 June, in fresh water), at the end (27 July, in sea water) and after (21 September, infresh water) sea-lice infection. The fish was exposed to no [control ( )], medium ( ) and high ( ) sea-lice infection during gonad recrudescence (23 June to 27 July). Differences between treatments withinsampling dates are indicated by different lowercase letters (P < 0·05). Differences across sampling dateswithin treatments are indicated by different uppercase letters (P < 0·05).

both sea-lice-infected groups, with the effect being most pronounced in the HI group[Fig. 10(a)–(d)]. Before sea-lice infection, K in females that did not re-mature in theMI group was significantly lower compared with that of their maturing counterparts[Fig. 10(d)]. During late summer and autumn, after withdrawal of food, it appeared,



300

320

340

360

380

400

420

440

460

480

0·00 0·05 0·10 0·15 0·20 0·25 0·30

Sea-lice infection (sea lice g−1 fish 27 July (end of seawater exposure)

Osm

olal

ity (

mO

smol

)

R2 = 0·68

Fig. 8. Relationship between sea-lice infection intensity and plasma osmolality in maturing male and maturingfemale Salvelinus alpinus exposed to no [control ( )], medium ( ) and high ( ) sea-lice infectionduring gonad recrudescence (23 June to 27 July). Fish were sampled at the end of sea-lice and seawaterexposure (fish still in sea water).

however, that K decreased at a slower rate in the sea-lice-infected groups, andat the end of the experiment there were no differences in K between treatments[Fig. 10(b), (d)]. Growth rate during seawater and sea-lice exposure was significantlyand negatively related to plasma cortisol concentrations (Fig. 11). This relationshipwas also apparent at the individual level but again being stronger for females (R2 =0·75, n = 17) than for males (R2 = 0·25, n = 40).

OV U L AT I O N A N D G A M E T E P RO D U C T I O N

Eleven, six and four females ovulated and were stripped of eggs in the control, MIand HI groups, respectively. In the control group, the first ovulation was recorded on20 October, c. 10 days earlier than in the MI and HI groups, but median ovulationtime did not differ significantly between groups (Fig. 12). Fertilization rates werebetween 98 and 100% and did not differ between treatments.

Low number of maturing females in the MI and HI groups resulted in reducedreproductive output in sea-lice-infected fish, and egg production in the MI and HIgroups was only c. 50 and 30%, respectively, of that in the control group (Table II).Relative and absolute fecundity also tended to be reduced among HI fish, althoughvalues were not statistically different (Table II). Egg size and mortality until hatchwere very similar between treatments, as well as fry mass at hatch and after 6 weeksof start feeding (Table II).


2332 H . T V E I T E N E T A L .

Tab

leII

.In

dice

sof

repr

oduc

tive

outp

ut,e

ggan

dem

bryo

nic

qual

ity

infe

mal

eSa

lvel

inus

alpi

nus

expo

sed

tono

(con

trol

),m

ediu

man

dhi

ghse

a-lic

ein

fect

ion

for

34da

ysdu

ring

gona

dre

crud

esce

nce

(23

June

to27

July

).V

alue

sar

em

ean

±s.

e.T

here

wer

eno

sign

ifica

ntdi

ffer

ence

sbe

twee

ntr

eatm

ents

(P>

0·05)

Fish

mas

sbe

fore

stri

ppin

g(g

)

Rel

ativ

eeg

gm

ass

(%)

Rel

ativ

efe

cund

ity(e

ggs

kg−1

)

Abs

olut

efe

cund

ity(e

ggs

fem

ale−1

)

Egg

diam

eter

(mm

)

Tota

leg

gpr

oduc

tion*

(num

ber

ofeg

gs)

Egg

mor

tali

tyto

hatc

h(%

)

Lar

val

mas

sat

hatc

h(m

g)

Lar

val

mas

s6

wee

ksaf

ter

star

tfe

edin

g(m

g)

Con

trol

1014

±10

015

·2±

1·744

96±

361

3756

±47

04·3

0±

0·03

4133

25·7

±1·9

39·9

±0·8

54·7

±2·3

Med

ium

infe

ctio

n96

2±

8713

·4±

1·544

12±

335

3500

±35

74·2

4±

0·09

2099

85·4

±2·0

39·4

±0·8

48·8

±1·4

Hig

h infe

ctio

n84

0±

131

11·2

±2·1

4173

±70

530

82±

839

4·25

±0·0

612

327

5·0±

3 ·439

·6±

1·952

·5±

5·0

*Tot

aleg

gpr

oduc

tion

was

not

subj

ect

tost

atis

tical

eval

uatio

n.



Spermatocrit tended to be reduced in sea-lice-infected fish compared with controlfish at the beginning of the spawning season (control: 13·1 ± 1·1; MI: 9·5 ± 1·0;HI: 10·6 ± 1·0), but values were not statistically different. In sea-lice-infected fish,spermatocrit increased somewhat with time and at the end of the spawning seasononly small differences in spermatocrit were recorded between treatments (control:13·0 ± 1·8; MI: 13·1 ± 1·4; HI: 11·4 ± 1·0).

DISCUSSION

The present study has shown that sea-lice infection during early stages of gonadgrowth of S. alpinus has the potential to elicit a significant reduction in reproductiveoutput. In sea-lice-infected fish, reproductive output was reduced mainly via thenumber of maturing females and increased fish mortality, whereas quality of thegametes that were produced appeared to be little influenced. The observed effects ofsea lice appear to be mediated through influences on stress physiology, alterations inreproductive endocrine homeostasis and fish body condition at the time of infection.

Compared with studies on other salmonid species, the relative sea-lice infectionintensity (sea lice g−1 fish) used in this study is considered to be within the sub-clinical range where physiological effects are likely to occur, whereas in absoluteterms (sea lice fish−1) they may be considered as clinical infections (Wagner et al.,2008). Infection intensities are also comparable with those that are encountered under

0

20

40

60

80

100

120

140

160

180

300 350 400 450 500

Osmolality (mOsmol) 27 July (end of seawater exposure)

Cor

tisol

(ng

ml−1

)

R2 = 0·47 (male)

R2 = 0·77 (female)

Fig. 9. Relationship between plasma osmolality and cortisol concentrations in male ( , , ) and female( , , ) Salvelinus alpinus exposed to no [control ( , )], medium ( , ) and high ( , ) sea-liceinfection during gonad recrudescence (23 June to 27 July). Fish were sampled at the end of sea-lice andseawater exposure (fish still in sea water). Strength of relationships (R2) between variables is calculatedfrom individual data.


2334 H . T V E I T E N E T A L .

−0.4

−0·20

0·2

0·4

0·6

0·81

1·2

1·4

(a)

10/5

9/6

9/7

8/8

7/9

7/10

6/11

6/12

Gw (% day−1)

Sea-

lice

and

seaw

ater

exp

osur

eB

reed

ing

peri

od

ab b

ac b

ac bab ab

ab abab b

ab b

0·8

0·91

1·1

1·2

1·3

1·4

1·5

(b)

15/4

15/5

14/6

14/7

13/8

12/9

12/1

011

/11

11/1

2

K

Sea-

lice

and

seaw

ater

exp

osur

eB

reed

ing

peri

od

ab ab

ab a

ab ab

ab ab

ab ab

aa a

aa aaa a

−0·8

−0·6

−0·4

−0·20

0·2

0·4

0·6

0·81

1·2

1·4

1·6

(c)

(d)

10/5

9/6

9/7

8/8

7/9

7/10

6/11

6/12

Gw (% day−1)

Sea-

lice

and

seaw

ater

exp

osur

eB

reed

ing

peri

od

ab abcb ab

ac b

ab bab ab

aa a

aa a

ac

bc

0·75

0·85

0·95

1·05

1·15

1·25

1·35

1·45

15/4

15/5

14/6

14/7

13/8

12/9

12/1

011

/11

11/1

2D

ate

(day

/mon

th)

K

Sea-

lice

and

seaw

ater

exp

osur

eB

reed

ing

peri

od

a

ac b

ab b

aa a

ab ab

aa aaa a

aa aab ab b

ab

Fig.

10.

Tem

pora

lch

ange

sin

(a),

(c)

spec

ific

grow

thra

tein

mas

s(G

w)

ofm

atur

ing

(a)

mal

ean

d(c

)fe

mal

ean

d(b

),(d

)co

nditi

onfa

ctor

(K)

ofm

atur

ing

(b)

mal

ean

d(d

)fe

mal

eSa

lvel

inus

alpi

nus

expo

sed

tono

[con

trol

()]

,m

ediu

m(

)an

dhi

gh(

)se

a-lic

ein

fect

ion

duri

nggo

nad

recr

udes

cenc

e(2

3Ju

neto

27Ju

ly).

(c),

(d),

Gw

and

Kof

not

re-m

atur

ing

fem

ales

befo

rean

daf

ter

sea-

lice

expo

sure

inth

em

ediu

mgr

oup

are

indi

cate

d(

).H

oriz

onta

lba

rsin

dica

tetim

ing

ofse

a-lic

eex

posu

rean

dth

ebr

eedi

ngpe

riod

,re

spec

tivel

y.D

iffe

renc

esbe

twee

ntr

eatm

ents

with

insa

mpl

ing

date

sar

ein

dica

ted

bydi

ffer

ent

low

erca

sele

tters

(P<

0·05)

.



0

0·2

0·4

0·6

0·8

1

1·2

1·4

1·6

0 50 100 150 200

Cortisol (ng ml−1) on 27 July (end of seawater exposure)

Gw

(%

day

−1 1

6/06

to 2

7/07

)

R2 = 0·25 (male)

R2 = 0·75 (female)

Fig. 11. Relationship between plasma cortisol at the end of sea-lice exposure (27 July) and specific growthrate in mass (Gw 16 June to 27 July) of maturing male ( , , ) and female ( , , ) Salvelinusalpinus exposed to no [control ( , )], medium ( , ) and high ( , ) sea-lice infection during gonadrecrudescence (23 June to 27 July). Strength of relationships (R2) between variables is calculated fromindividual data.

natural conditions (Bjørn et al., 2001). The different sea-lice intensities used cantherefore be considered as relevant for studying physiological effects of sea-liceinfection also in S. alpinus, even though few studies have been carried out for thisspecies. Generally, however, the present observations corroborate earlier studies onAtlantic salmon Salmo salar L. and brown trout S. trutta (Bjørn & Finstad, 1997;Finstad et al., 2000; Bjørn et al., 2001; Fast et al., 2006) and indicate that S. alpinusdisplay similar stress-related physiological changes and suits of responses to sea-liceinfection to that of other salmonids.

Sea-lice infection resulted in a reduction in egg production by 50 and 70% in theMI and HI groups, respectively, compared with that of the control. In the HI group,reduced egg production was caused mainly by a combination of increased mortalityand a reduced proportion of maturing females, whereas in the MI a reduction inproportion of maturing females was the main cause. Since, as far as is known, thereare no previous reports on the effect of sea-lice infection on reproductive performancein any salmonid species, comparison is difficult, but a reduction in egg productionby 50 and 70% after sea-lice exposure must be considered to be substantial.

There is compelling evidence that cortisol has an inhibitory effect on plasma sex-steroid concentrations (Pankhurst & Van Der Kraak, 2000), and, thus, this steroidmay influence or inhibit reproductive development. The close negative association


2336 H . T V E I T E N E T A L .

0

20

40

60

80

100

15/10 20/10 25/10 30/10 4/11 9/11 14/11 19/11

Date (day/month)

Cum

ulat

ive

ovul

atio

n (%

)

Fig. 12. Influence of sea-lice infection during gonad recrudescence (23 June to 27 July) on cumulative per-centage of ovulated female Salvelinus alpinus over time. Fish were exposed to no [control ( )], medium( ) and high ( ) sea-lice infection during gonad recrudescence (23 June to 27 July).

between plasma cortisol and sex-steroid concentrations especially that of T, found inthe present study, supports this notion. It may therefore be hypothesized that elevatedcortisol concentrations in sea-lice-infected fish have suppressed plasma sex-steroidconcentrations to the level that reproductive development was inhibited as seen inthe MI and HI groups.

There are, however, a few problems with the latter interpretation. Although infor-mation about the temporal changes in cortisol concentration during sea-lice exposurein the different groups is lacking, cortisol concentrations in females that did not re-mature in the MI group were the same as maturing fish at the end of sea-lice exposure.Neither osmolality nor chloride concentrations differ. Also, re-maturing females inthe HI group had much higher plasma osmolality, cortisol and chloride concentra-tions than non-maturing females in the MI group. It is therefore possible that highplasma cortisol or osmoregulatory problems may not alone have resulted in suppres-sion of ovarian development. Since there is evidence that energy acquisition andnutritional status are important for mediating cessation or triggering of reproductivedevelopment in salmonids (Rowe & Thorpe, 1990; Rowe et al., 1991; Kadri et al.,1996; Silverstein et al., 1998; Shearer & Swanson, 2000), condition of the fish mayalso be taken into account. Although there was no difference in female conditionbetween groups before sea-lice infection (P > 0·05), sea-lice-infected females thatdid not re-mature had significantly lower condition before sea-lice infection than



their maturing counterparts (i.e. they were from the lower part of the distribution)[Fig. 10(d)]. Since there were no differences in growth rate, osmolality, plasma cor-tisol or chloride concentrations in females with high and low K , the reason for thedifferent effects of sea-lice infection on maturation may be related to differences innutritional status.

Replenishment of somatic reserves, which seems necessary before gonad growthcan be initiated (Rowe et al., 1991), together with increased energy requirementsfor maintaining osmoregulatory functions may therefore have resulted in limitedenergy available for reproductive growth in low K individuals. A rapid increase inplasma sex-steroid concentrations in maturing high K females (even in the HI group)after loss of the stressor (fish devoid of food), indicating that overall energy storeswere monitored as sufficient for reproductive development, supports this notion.These observations, however, also suggest that endocrine signalling of nutritionalstatus (which are likely to be multi-factorial) from the somatic to the reproductiveneuroendocrine axis may be influenced (blocked) or differently translated when thefish are under stressful conditions.

Although dead fish were not sexed, it appears that most of the fish that died werefemales which also had the lowest condition before sea-lice infection. In line with theobserved female-specific reduction in maturation rate and survival, females appearto have a stronger and more closely regulated physiological response to sea-liceinfection compared with that of males. In females, there was always a stronger (R2

of 0·65–0·86) relationship than for males (R2 of 0·25–0·47), between cortisol andall the variables investigated (infection intensity, osmolality, growth rate and sexsteroids). In studies on brown trout S. trutta and O. mykiss, significant differencesin plasma cortisol concentrations between males and females in response to stresshave also been detected, with females displaying higher concentrations than males(Campbell et al., 1992, 1994; Pottinger & Carrick, 1999; Clements et al., 2002).The reason for this is not clear but may be related to sexual differences in plasmasex steroids as indicated for mammals (McQuillan et al., 2003). Sexual differencesin susceptibility to stress may also explain why the proportion of maturing maleswas not influenced by sea-lice infection. On the other hand, before sea-lice infectionK was somewhat higher in maturing males than in maturing females [Fig. 10(b),(d)], and a possible difference in nutritional status may also have influenced thesex-specific effect of sea-lice exposure on maturation. Definitively, further studieson the influence of stress on reproduction should take both sex and body conditionsinto consideration.

In the fish that did mature, reproductive physiology was also influenced by sea-liceexposure. First, in the control group, temporal changes in plasma sex-steroid con-centrations, both in males and females, were very similar to what has been recordedpreviously for this strain of S. alpinus when fish are held in fresh water (Tveitenet al., 1998; Frantzen et al., 2004). In sea-lice-exposed fish, however, plasma sex-steroid concentrations were reduced, and in females the effect was found to be moreevident for T than for that of E2. In the MI group, where cortisol was moderatelyelevated, plasma T concentrations were depressed compared with controls, whereasthose of E2 were not. Unaffected E2 concentrations, despite depressed plasma Tconcentrations may, however, indicate that T synthesis was sufficient (T should beavailable for aromatization) for ample E2 synthesis. This may indicate that cortisolexerts its effect, in part, on suppression of pathways leading to the production of T,


2338 H . T V E I T E N E T A L .

rather than on aromatase activity. This is in agreement with results from studiesof O. mykiss (Pankhurst & Van Der Kraak, 2000) and with what was observed inthe HI group, where a further reduction in plasma T concentrations was associatedwith a significant reduction also in E2. Also, after return to fresh water, plasmaT concentrations were significantly lower in the HI group without the effect beingthat apparent for E2. Furthermore, the close negative relationship between plasmacortisol and sex-steroid concentrations together with the rapid increase in sex-steroidconcentrations after re-entry to fresh water may indicate a direct effect of stressand cortisol on sex steroid synthesis. It appears that the delay in commencement ofgonad development induced by sea-lice infection tended to delay, but did not greatlyinfluence, the timing of ovulation.

In male fish also, plasma sex-steroid concentrations were reduced by sea-liceinfection, but apparently not to the extent that reproductive development was com-promised in terms of proportions of maturing individuals. In the sea-lice-infectedgroups there was a tendency towards reduced density of spermatozoa during the firstpart of the spawning season. If this reduction would cause any decline in fertilizationsuccess under natural condition, where spermatozoa density may be a limiting factor,is not known.

Gamete quality and reproductive investment was little affected in the sea-lice-infected fish. Studies on brown trout S. trutta and O. mykiss showed that prolongedstress, associated with elevated plasma cortisol concentrations, had a significant effecton egg and progeny survival (Campbell et al., 1992, 1994). Based on sex steroidprofiles, stress in the present experiment was applied during early stages of gonadgrowth (Frantzen et al., 1997; Tveiten et al., 1998), which is different from studieson S. trutta and O. mykiss (Campbell et al., 1992, 1994) where stress was main-tained throughout gonad development. The apparent lack of stress on gamete qualityin the present study may be due to the fact that in sea-lice-infected fish gonaddevelopment was delayed until after return to fresh water and, thereby, loss of thestressor.

Although sea-lice infection intensities in the range used in this study are encoun-tered in sea trout S. trutta and S. alpinus populations in the wild (Bjørn et al., 2001),extrapolation of the present results to what may occur under natural conditions isdifficult. Seen in the light that condition at the time of sea-lice exposure may becritical for initiation and progression of maturation, there may be some reason forconcern. For example, in the wild, K of descending S. alpinus of the Hammerfeststrain is c. 0·7 (Rikardsen et al., 1997; Rikardsen & Elliott, 2000), which is actuallyslightly below the condition in not re-maturing females in the present study. Sea-liceinfection in the wild may therefore elicit a larger reduction in maturing fish, or theremay be a reduction in maturation to lower infection intensities, than seen in thisstudy. A similar scenario of lost feeding opportunity as indicated in this study mayalso apply under natural conditions, where sea lice-infected fish re-enter fresh waterat an earlier stage than non-infected fish, losing opportunity for energy accumula-tion as seen in sea trout S. trutta (Tully et al., 1993; Birkeland, 1996; Birkeland &Jakobsen, 1997).

Finally, the apparent female-specific effect of sea-lice infection on maturationand mortality would certainly also have implications for the long-term reproductivepotential of a given population via a continuous decrease in female to male ratioamong spawning fish.



This study forms part of project no. 149187/730 supported by the Norwegian ResearchCouncil. The technical assistance of A. B. Tennøy, B.-S. Sæther, M. Frantzen, J. Espen TauStrand, M. Johnsen and T. Lexau Hanebrekke is highly appreciated. R.S.M acknowledgesfinancial support from AquaNet project EI 4 – Risks and consequences of sea lice infestations.Thanks to R. W. Shulz, University of Utrecht, The Netherlands for providing the 11-KTantiserum.

References

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