Movement and home range in relation to dominance;a telemetry study on brown trout Salmo trutta

J. HOJESJO*†, F. ØKLAND‡, L. F. SUNDSTROM*, J. PETTERSSON§AND J. I. JOHNSSON*

*Department of Zoology, Animal Ecology, University of Goteborg, Box 463, SE-405 30Goteborg, Sweden, ‡NINA, Norwegian Institute for Nature Research, Tungasletta 2,

NO-7485, Trondheim, Norway and §Institute of Freshwater Research, Swedish Board ofFisheries, Stangholmsvagen, SE-178 98 Drottningholm, Stockholm, Sweden

(Received 7 November 2005, Accepted 17 August 2006)

By combining behavioural observations on adult resident brown trout Salmo trutta in the

laboratory with radio telemetry studies in a natural stream, information on movement and

space use in relation to social status was obtained. Dominant individuals moved longer

distances and also tended to have larger home ranges than subordinates during the summer. In

general, home ranges were larger during daytime than at night. Fish were not strictly territorial

since the average overlap in interquartile range was 36% during the summer. During the

spawning period, the brown trout moved to specific spawning areas resulting in an increased

overlap (89%) in space use. Subordinate individuals now tended to increase both home range

and interquartile range and were also less frequently observed in spawning areas relative to

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Key words: brown trout; dominance; habitat preference; home range; movement; telemetry.

INTRODUCTION

Stream-living salmonids are believed to be relatively stationary (Heggenes, 1988;Hesthagen, 1988; Larsson, 1988; Garrett & Bennett, 1995; Bohlin et al., 2002;Hojesjo et al., 2002; Økland et al., 2004), where individual fishes compete forprofitable foraging sites (Elliott, 1994; Bagliniere & Maisse, 1999) generatingmosaic territories during the juvenile phase (Jenkins, 1969). Several studies ofsalmonids have demonstrated how dominant fishes occupy the most profitablefeeding station and especially larger relatively stationary brown trout Salmotrutta L. are known to be very territorial (Jenkins, 1969; Nakano, 1994). Inagreement, dominant fishes grow faster both under experimental conditions(Metcalfe, 1991) and in the wild (Nakano, 1994; Hojesjo et al., 2002) althoughHarwood et al. (2003) and Martin-Smith & Armstrong (2002) give a differentview. If so, subordinate individuals would be expected to be displaced fromprofitable feeding areas and dominant fishes to be less mobile than subordinate,

†Author to whom correspondence should be addressed. Tel.: þ 46 31 773 3636, fax: þ 46 31 416729;

email: johan.hojesjo@zool.gu.se

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doi:10.1111/j.1095-8649.2006.01299.x, available online at http://www.blackwell-synergy.com

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less aggressive individuals. Dominant aggressive fishes, however, are generallysuperior in interactions with conspecifics and might therefore stand a betterchance of establishing a new territory. Therefore, dominant individuals mightalso show a more pronounced exploratory behaviour (Sundstrom et al.,2004a). Further, the behaviour of dominants is probably affected by foodabundance. If food is scattered and unpredictable, a dominant and aggressivestrategy may be more costly compared with a situation where food is localizedand predictable (Grant, 1997). Therefore, under circumstances with good for-aging locations, dominant fishes would be expected to remain relatively station-ary whereas if food is more scattered, dominant fishes would be expected tomove more and longer. According to both these hypotheses subordinate anddominant individuals would be expected to be found in different positions.In agreement, several populations of stream living salmonids seem to bedivided into two components; one static and one smaller mobile component(Solomon & Templeton, 1976; Milner et al., 1979; Solomon, 1982; Harcupet al., 1984; Hesthagen, 1988; Heggenes et al., 1990; Bridcut & Giller, 1993;Smithson & Johnston, 1999). Information about space use of stream residentsalmonids in relation to dominance in the wild, however, is scarce.Most previous studies of movement in salmonids have used mark-recapture

techniques and passive integrated transponders (PIT tags) for individual iden-tification purposes. In these studies with the recapture sites often being near theinitial tagging sites, there might be a bias towards detecting non-migratoryfishes more than migratory (Gowan et al., 1994; Gowan & Fausch, 1996). Fur-thermore, information collected with these techniques does not record move-ment between release and recapture. Radio telemetry allows detection ofvariation in movement on a much finer scale. In agreement, studies using radiotelemetry have revealed pronounced and variable movement patterns in stream-living salmonids (Meyers et al., 1992; Bunnell et al., 1998; Knouft & Spotila,2002; Økland et al., 2004).The aim of this study, was to record movement and habitat use of adult

stream living brown trout to examine (1) how dominance rank affects move-ment, habitat choice and space use, (2) temporal variation in movement and(3) spatial overlap in home range. In order to examine any potential changesin home range and habitat use during the breeding season, data were collectedbefore and during the spawning period.

MATERIALS AND METHODS

Adult resident brown trout [mean � S.E. mass: 152 � 19 g, range: 54�3–432�0 g andstandard length (LS): 234 � 13 mm, range: 170–360 mm] were caught in August 2001by electrofishing in the River Jorlanda, a small stream on the west coast of Sweden(57°589280 N; 11°569220 E, Fig. 1). The stream holds a mixed population of migratorybrown trout (sea trout) and resident (mainly males) brown trout. Only resident browntrout were used, these individuals generally have a darker body colouration with morepronounced spots on their body compared with sea trout (Dellefors & Faremo, 1988;Pettersson et al., 2001). All fish were measured for mass and LS, marked using PIT tagsand transported to the Department of Zoology in Goteborg. In the laboratory, the fishwere distributed randomly between four groups in aquaria (200 � 50 cm) with a waterdepth of 30 cm, six fish in each, landscaped to mimic a section of the natural river.Water was supplied through a closed circulatory system, and flow was generated using

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commercial aquarium pumps (Eheim). Water temperature was kept constant at 15° C.The fish were fed ad libitum daily with earthworms (Oligochaeta), always presented atthe same location in the aquarium. After acclimatization (7 days), the fish wereobserved for 50 min each day in order to record behavioural interactions, feeding, gen-eral activity and position (Hojesjo et al., 2002). Dominance rank was based on feedingposition and feeding order (Huntingford et al., 1993) as well as aggressive interactions(Hojesjo et al., 2002). The fish that fed first and held the position closest to the feedingstation was assigned dominant. The highest ranked fish in every aquarium was removedeach day until all six fish in every aquarium were ranked. Fish removed from eachaquaria were transferred individually to four holding tanks of similar size as the experi-mental aquaria, thereby keeping every group of ranked fish intact.

Before release, a radio transmitter [ATS; Advanced telemetry system, Model F1560,dimensions c.: length, 24 mm; width, 11 mm; height, 6 mm; mass, 2�5 g (in air); war-ranty life, 70 days; battery capacity, 140 days], transmitting at a unique frequency, wasinserted into its body cavity of each fish (Thorstad et al., 2000). After recovery from thesurgery, the four groups of fish were kept separated and released back to the stream.Due to technical problems, only 18 of the ranked fish (nine dominant and nine subor-dinate) were equipped with transmitters. The four groups of fish being released con-sisted of two intact groups with six individuals and two groups with only threeindividuals each (one group consisting of one high ranked and two low ranked individ-uals and one with two high ranked individuals and one low ranked) and were left toacclimatize in the stream over night. Each group was enclosed in perforated plasticboxes and released the following morning to reduce stress (Finstad et al., 2003). Thefour groups of fish were released at different locations, 100 m apart and mean � S.E.450 � 78 m from the catch site (range 9 to 900 m). In addition to the 18 ranked fishreleased into the stream (experimental fish), 10 fish were anaesthetized in the field (con-trol fish, mean � S.E. mass: 152�3 � 30�8 g, range: 78�7–393�0 g) and equipped withtransmitters using the same procedure as for experimental fish. This group was includedto compare movement and home range between fish that had been observed in the lab-oratory and fish that only had been tagged in the field. These control fish were releasedwithin a range of 350 and 650 m downstream of the nearest ranked group. The totalexperimental section was 1200 m and to facilitate an accurate tracking of the fishdivided into sections of 10 m. In the upstream end of the section was a waterfall thatprevented fish from moving further up the stream, while no barrier was present in thedownstream direction (Fig. 1).

In order to allow time for recovery and acclimatization, tracking started on 4 Sep-tember, 5 days after release and lasted until 4 November. The manual tracking of the fishwas conducted during two periods, one during summer (4–21 September, mean � S.E.temperature 11�6 � 1�1° C) and one during spawning (29 October to 3 November, mean� S.E. temperature 8�5 � 1�2° C). During most of the summer period, fish were trackedonce a day but between the 10 and 15 September fish were located four times a day(c. 0100, 0700, 1300 and 2000 hours) in order to detect any circadian rhythm. Duringthe spawning season, the fish were located twice daily (0900 and 2100 hours).

For the entire experimental area a number of hydraulic measurements were taken(using a transect every second metre); depth, width, flow rate (1, calm, flat mirror sur-face of the water; 2, relatively fast moving water; 3, riffle section with fast moving waterand interrupted water surface), type of substratum (clay, pebbles and boulders), vege-tation (0, no vegetation; 1, <25%; 2, �50%; 3, >50% of bottom area covered with veg-etation) and degree of foliage cover (1, <25%; 2, �50%; 3, >50% of the stream sectioncovered by foliage). The streambeds were generally densely covered with vegetation(mainly meadow-sweet Filipendula ulmaria and blackberry Rubus sp.), which allowedtracking to be done near the stream without disturbing the fish. When a fish couldbe observed visually, it rarely moved but stayed motionless within the bed of vegetation.Therefore, the presence of the tracker did not cause any rapid movement of the fish, andthe telemetry data should be accurate within 2 m. Six discrete sections of the streamwere classified as spawning areas on basis of observed spawning activity and thepresence of spawning redds (both this season as well as previous seasons; pers. obs.).

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On 4 November, the fish were recaptured using electrofishing, and killed using an over-dose of 2 phenoxy-ethanol and a blow on the head. The fish were then weighed,measured (LS) and subsequently sexed in the laboratory.

TREATMENT OF DATA

The ranks obtained from the removal test were pooled so that higher ranked fish(one to three) were ascribed as dominants (D, n ¼ 9) whereas the lower ranked fish(four to six) were ascribed as subordinates (S, n ¼ 9). There were no significant differ-ences between high and low ranked individuals in either initial body mass (127�4 � 38�8and 177�3 � 32�4 g, mean � S.E. respectively) (Mann–Whitney U-test, n ¼ 18, P > 0�1)or LS (226�1 � 17�3 and 257�2 � 17�7 mm, mean � S.E., respectively) (Mann–WhitneyU-test, n ¼ 18, P > 0�1).

Habitat preference for depth, flow rate, bottom vegetation and foliage cover were an-alysed using a binomial test comparing the average habitat characteristics for the posi-tion utilized by each individual fish with the expected mean representing the averagehabitat in the stream.

Movement was calculated as the average change in position between successive loca-tions and to detect any difference in the direction of movement, the relative [includingupstream (þ) and downstream (�) movement] distance in metres were analysed. Inaddition, as a general measurement of activity, absolute movement was also analysed.Movement between the positions recorded at 0700, 1300 and 2000 hours were catego-rized as daytime movement whereas movement between the positions recorded at 2000,0100, and 0700 hours were categorized as night-time movement. The average positionwas calculated in a similar way but with location recorded at 0700 and 1300 hourscategorized as daytime position and the location recorded at 2000 and 0100 hours asnight-time position.

FIG. 1. The geographical location of the stream (Jorlandaan) and a sketch of the section used in this study.

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The home range, both for the summer and the spawning period, was calculated fromthe distance between the furthest upstream and downstream position recorded for eachindividual. The interquartile range was based on all positions recorded during eachperiod. Interquartile range was used as a more biologically relevant measurement of spaceuse since the larger home range generally reflects a prolonged movement during only oneor two occasions. The proportion of recordings in any of the discrete spawning areas wascalculated for each fish, both for the summer period (summer index) and the spawningperiod (spawning index). Seasonal differences in utilization of the spawning areas werethen tested using Wilcoxon signed rank test. To compensate for any change in habitatbetween day and night, only the first scan on every day was used. For each fish, thisyielded a total of 18 scans during the summer and seven during spawning. Specific growthrate (G) (Ricker, 1979) was calculated using the formula G ¼ log10ðS1S�1

0 Þ100d�1, whereS0 and S1 were the initial and final mass (g) and d (days) is the elapsed period betweenmeasuring initial and final masses. Non-parametric statistics were used [Mann–WhitneyU-test for independent data and Wilcoxon signed ranked test for dependent/paired data(SPSS 13)] since the data did not conform to a normal distribution.

The alpha value was set to 0�05 for all tests and to simplify presentation of data teststatistics are only given for P < 0�10.

RESULTS

RECAPTURE RATE AND GROWTH

In total, 28 fish (18 ranked þ 10 controls) were equipped with transmitters andreleased into the stream. Contact was lost with one individual (ranked) almostimmediately whereas the other 27 individuals were successfully monitoredthroughout the summer period. Another 12 fish were lost during the periodin-between summer and spawning (six ranked fish and six controls). After thelast monitoring, 10 ranked and five control fish were recaptured; two femalesand 13 males. The average growth rate for these individuals was slightly negative(mean � S.E. G ¼ �0�023 � 0�023) with no difference in G between control fish(G ¼ �0�047 � 0�061) and fish that were ranked in the laboratory (G ¼�0�012 � 0�019). Recaptured fish, four high ranked individuals (G ¼ �0�026 �0�038) and six low ranked individuals (G ¼ 0�002 � 0�021) looked healthy andthe wound from surgery had healed properly with no signs of fungal growth.Due to the low sample size no statistical tests were performed to test for differencein recapture rate or growth between high and low ranked individuals.

SUMMER PERIOD (4–21 SEPTEMBER)

Habitat use and positionThe fish were located in water with mean � S.E. depth of 40�4 � 1�4 cm,

deeper than the average depth of the stream (36�4 � 1�7 cm), (binomial test,n ¼ 19, P < 0�001). The fish were found in areas with slow flow (mean �S.E. flow rate index ¼ 1�29 � 0�11), but no difference compared to average flowrate (flow rate index ¼ 1�38 � 0�10) was observed. Generally, fish were posi-tioned in areas with little vegetation on the bottom (mean � S.E. vegetationindex ¼ 1�10 � 0�19), but no difference compared to average bottom vegetation(vegetation index ¼ 1�58 � 0�23) was detected. The foliage cover over usedhabitat (mean � S.E. foliage cover index ¼ 2�24 � 0�55), however was higherthan the average foliage cover (foliage index ¼ 1�67 � 0�17) (binomial test,

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n ¼ 20, P < 0�05). At night, fish were generally positioned downstream fromtheir daytime position (Wilcoxon, n ¼ 27, P < 0�05). The distribution of sub-ordinate and dominant fish did not differ either in depth, flow rate, vegetationor overhang (Wilcoxon, P > 0�1 all cases).Overall, dominant individuals tended to be positioned further downstream

(mean � S.E. 533�6 � 121 m) compared with subordinates (270�0 � 66�6 m)(Mann–Whitney U-test, n ¼ 17, P ¼ 0�068). A trend that was similar at day-time (528�9 � 119�4 m and 267�5 � 65�6 m respectively) (Mann–WhitneyU-test, n ¼ 17, P ¼ 0�068) and at night (560�5 � 130�1 m and 284�3 � 73�1m respectively) (Mann–Whitney U-test, n ¼ 17, P ¼ 0�10).

MovementMean � S.E. relative movement between successive locations was 3�6 � 1�3 m.

During the 4 days of more intense monitoring, mean � S.E. daytime movementstended to be larger (3�6 � 2�0 m) than night-time movements (1�0 � 0�88 m)(Wilcoxon, n ¼ 27, P ¼ 0�068).Mean � S.E. absolute movement was 19�4 � 3�1 m. During the days of more

intense monitoring, daytime movement was larger (23�6 � 4�2 m) than duringnight (9�7 � 4�2 m) (Wilcoxon, n ¼ 27, P < 0�001).In terms of relative movement dominant fish moved more than subordinate

individuals (Mann–Whitney U-test, n ¼ 17, P < 0�05) [Fig. 2(a)]. When thedata were split into day and night-time observations, dominant fish moved lon-ger during daytime (Mann–Whitney U-test, n ¼ 17, P < 0�05) whereas no dif-ference between the groups was detected at night. The absolute movement,however, did not differ between dominant and lower ranked fish during anytime period (Mann–Whitney U-test, P > 0�1 all cases) [Fig. 2(b)]. Further,the mean relative distance moved was lower in the control group (�0�6 �1�1 m) compared with movement in the ranked group (6�1 � 1�8 m) (Mann–Whitney U-test, n ¼ 27, P < 0�05) whereas no difference could be detectedin absolute movement (Mann–Whitney U-test, P > 0.1).

Home rangeMean home range (i.e. river stretch used) was equal to daytime home range

(217�4 � 46�4 m) and larger than the home range used at night (22�6 � 4�39 m)(Wilcoxon, n ¼ 27, P < 0�001). The interquartile range was 17�5 � 4�7 m withdaytime range being larger (20�6 � 6�8 m) compared with night-time (7�5 � 1�5 m)(Wilcoxon, n ¼ 27, P < 0�05). The average overlap in interquartile range was36�4 � 0�1%. Forty-three per cent of the fish had exclusive territories (not usedby any other tagged fish) and 16�7% of the fish had an interquartile range thatwas completely within that of other fish [Fig. 3(a)].The mean home range was 222�8 � 54 m for subordinate fish and 371�1 �

125�9 m for dominant fish. Interquartile range was 14�1 � 3�8 m for subordinatefish and 30�7 � 14�5 m for dominant fish. No significant effects of dominancestatus on home range or interquartile range could be detected (Mann–WhitneyU-test, P > 0.1 both cases). Further, the home range was smaller in the controlgroup (89�5 � 25�8 m) compared with home range in the ranked group (292�6 �66�2 m) (Mann–Whitney U-test, n ¼ 27, P < 0�05) whereas no difference couldbe detected for interquartile range (Mann–Whitney U-test, P > 0.1) (Fig. 3).

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SPAWNING PERIOD (29 OCTOBER TO 3 NOVEMBER)

Habitat use and positionWithin the experimental section, six well defined spawning areas were iden-

tified (Fig. 3). During the spawning period (29 October to 3 November) thebrown trout visited these spawning areas significantly more than during thesummer period (Wilcoxon, n ¼ 17, P < 0�001). Higher ranked fish tended tooccupy these spawning grounds more often than lower ranked individuals(Mann–Whitney U-test, n ¼ 11, P ¼ 0�053) [Fig. 3(b)]. Also, the preferred hab-itats were not different from the available habitats in the river for the recordedvariables (depth, flow rate, bottom vegetation and overhanging vegetation).The brown trout, however, tended to prefer habitat with faster flow in thespawning period (spawning index ¼ 1�75 � 0�24 v. summer index ¼ 1�29 �0�11) (Wilcoxon, n ¼ 7, P ¼ 0�071).In contrast to the summer period, subordinate individuals now tended tobeposi-

tioned further downstream (509�5� 166�4m) than dominant fish (244�7� 206�9m)(Mann–Whitney U-test, n ¼ 11, P ¼ 0�100). A similar trend was also detected dur-ing daytime (518�8 � 165�9 m and 246�5 � 208�1 m respectively) (Mann–WhitneyU-test, n ¼ 11, P ¼ 0�100) as well as during the night (393�7� 134�8 m and 193�0�164�0 m respectively) (Mann–Whitney U-test, n ¼ 11, P ¼ 0�100).

MovementMean � S.E. relative distance moved and absolute movement were 11�6 �

9�3 m and 58�8 � 18�8 m, respectively, with no significant difference comparedto the summer period (Wilcoxon, P > 0.1).In contrast to the summer period, there were no differences between subor-

dinate and dominant individuals in movement (relative or absolute). Further,no significant difference between ranked and control fish could be detectedin either mean distance moved (4�4 � 10�1 m and 25�0 � 19�1 m respectively)nor absolute movement (42�2 � 13�9 m and 89�3 � 46�9 m respectively)(Mann–Whitney U-test, P > 0�1 all cases).

FIG. 2. Mean � S.E. (a) relative movement and (b) absolute movement at day ( ) and night ( ) during the

summer for high ranked individuals (H, n ¼ 9) and low ranked individuals (L, n ¼ 9). In (a) (relative

movement), downstream movement generated negative distances and upstream movement generated

positive distances whereas in (b) (absolute movement), all movements generated positive distances.

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Home rangeThe mean � S.E. home range was 281�5 � 77�6 m, and interquartile range

was 90�4 � 41�1 m. The home range used did not differ between the spawningperiod and the summer, whereas the interquartile range was larger during thespawning period (Wilcoxon, n ¼ 19, P ¼ 0�044). The average overlap ininterquartile range was now more than twice as high, 89�0 � 0�05% comparedwith the summer. None of the fish had exclusive territories, and 78% of the fishhad a home range that totally overlapped with other fish [Fig. 3(b)].

FIG. 3. Home range for each individual fish (L, low ranked individuals, H, high ranked individuals and C,

controls) during the (a) summer and (b) spawning period with the location of the defined spawning

locations marked. The boundary of the box indicates the range between the 25th and 75th percentile

(interquartile range) and the line within the box marks the median. The error bars above and below

the box indicates the 90th and 10th percentiles and the points represents the outliers.

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There was no significant difference in the home range utilized in summer com-pared with the home range utilized during the spawning season, for either sub-ordinate or dominant individuals (Wilcoxon, n ¼ 17, P > 0�1 and P ¼ 0�080,respectively). The interquartile range, however, was significantly larger duringthe spawning period compared with the summer period for the subordinate indivi-duals (Wilcoxon, n ¼ 17, P ¼ 0�046) whereas no such difference could be detectedfor the dominant individuals (Fig. 3). No significant difference between rankedand control fish could be detected in either home range (264�9 � 100�3 m and311�8 � 131�9 m respectively) nor interquartile range (58�6 � 21�8 m and 144�9 �107�2 m respectively) (Mann–Whitney U-test, P > 0.1 all cases).

DISCUSSION

During the summer period dominant individuals in this study generallymoved more than subordinate fish. This is in agreement with Sundstromet al. (2004a), who found that exploratory and bold behaviour is correlatedwith dominance. This result supports the second hypothesis that dominant in-dividuals are more exploratory and express a more pronounced movement thansubordinates. Dominants also tended to be positioned further downstream thansubordinates indicating a status-dependent habitat use where subordinate indi-viduals might avoid costly interactions with dominant individuals. Regardlessof social status and consistent with previous findings (Armstrong et al.,2003), fish preferred relatively complex habitat with slow currents and over-hanging vegetation. This probably provides protection against predators andacts as a visual barrier, thereby decreasing the scope for intra- and interspecificaggressive interactions (Kalleberg, 1958).Overall, the brown trout in this studywere relatively stationarywith only limited

movement within a small interquartile range; on average <20 m in the summerand <100 m during the spawning season (Fig. 3). This is in agreement with pre-vious studies using PIT tags and mark-recapture techniques on younger browntrout parr in the same river system (Bohlin et al., 2002; Sundstrom et al., 2004b).Contrary to what has been reported in previous studies (Kalleberg, 1958;

Jenkins, 1969; Elliott, 1994), however, only half of the fish in this study had exclu-sive territories (interquartile ranges) in summer. The real territorial overlap be-tween individuals may have been even higher due to the presence of untaggedbrown trout not included in the study. Previous studies using mark and recapturetechniques might therefore have underestimated the extent of territorial overlap.In agreement, using radio telemetry, Økland et al. (2004) showed that Atlanticsalmon Salmo salarL. (>3þ years) during the summer hadahome range that partlyoverlapped with other radio-tagged individuals. These results indicate a morecomplex social structure as opposed to a rigid defence of a fixed territory.Interestingly, at the onset of the spawning season, the interquartile range

increased and none of the fish had exclusive territories. The main reason forthis was probably that resident males now moved upstream to the spawningareas. The results suggest that dominant individuals moved to the spawningground during the autumn and maintained a smaller home range there,whereas the lower ranked individuals were forced to move downstream but

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with regular visits to the spawning ground reflected in an increased interquar-tile range for subordinate fish (Fig. 3).Further, in agreement with Schulz &Berg (1992), themaximumhome range and

interquartile range used during daytime was larger than at night. As suggested byFraser et al. (1993) andMetcalfe et al. (1997), this might be correlated with increas-ing food availability during the day, probably attributed to a reduced sensorycapacity at night. Several studies have also indicated that activity is greater duringthe day than at night (Eriksson&Alanara, 1992). Contrary to the present findings,however, Valdimarsson & Metcalfe (2001) reported that brown trout were moreactive at night and Orpwood et al. (2006) have shown a preference for Atlanticsalmon parr to forage outside the shelter at night relative to daytime foraging.These experiments were performed during winter and in semi-natural environ-ments, and the discrepancy might therefore be explained by a seasonal or envi-ronmental effect. In agreement, Heggenes et al. (1993) showed that brown troutbecame increasingly nocturnal at lower temperature. The present study, however,did not indicate any shift to nocturnal activity later in the season. Further, innatural rivers, an increased activity of potential prey such as hatching of variousinsect larvae during night may decrease the amount of swimming needed forforaging and therefore an increase in activity can be difficult to detect.To summarize, this study is the first to show differences in home range and

movement patterns between dominant and subordinate brown trout in a naturalstream. The results from the summer period verify the findings from previousstudies that brown trout are territorial with a limited home range. In contrastto what has been reported in previous studies (Kalleberg, 1958; Jenkins, 1969;Elliott, 1994), the average territorial overlap (interquartile ranges) in this studywas relatively large and home range sizes highly variable among individuals.There was also an overall preference for more complex habitats with a highdegree of foliage cover. In addition, dominant fish were more mobile comparedwith subordinate individuals. Further, the use of spawning areas and territorialoverlap increased during autumn. Subordinate individuals now tended toincrease both home range and interquartile range relative to dominants. Subor-dinates were also less frequently observed in spawning areas. These findingscan aid in understanding population structure and habitat requirements in sal-monids and can be useful for stream management purposes.

J.H. was financial supported by Helge Ax:sson Johnsson foundation and Vitterhetssam-hallet. Valuable assistance in the field and with behavioural observation were provided byB. Pakrooh, M. Persson, M. Ohlsson, I. Moum and A. Asp. E. Thorstad and an anony-mous referee provided valuable comments on an earlier version of the manuscript.

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