Letter

Small-scale spatial variation in age and sizeat maturity of stream-dwelling brown trout,Salmo trutta

Un resumen en espanol se incluye detras del texto principal de este articulo.

Introduction

Freshwater fish often form reproductively isolatedpopulations across contrasting habitats, which makethem well suited for studies on local adaptation(Carvalho 1993). Many species show substantialinterpopulation variation in age at maturity (Leggett& Carscadden 1978; L’Abee-Lund et al. 1989; Hutch-ings & Jones 1998). In some cases, such life-historyvariation is known to have a genetic basis (Stearns1983; Reznick et al. 1990), but populations may alsodiffer in mean phenotype because of plasticity. Studieson brown trout (Salmo trutta, L.) have shown thatfaster growing individuals tend to mature before slowergrowing individuals (Alm 1959; Bagenal 1969). Thisobservation is in accordance with theoretical predic-tions (Stearns & Koella 1986). Optimal age at maturitywill also likely be influenced by prevailing mortalityregimes (Reznick et al. 1990; Hutchings 1993; Olsenet al. 2004). Low survival probability should select forearly maturation (Charlesworth 1994).

We have studied the life history of stream-dwellingbrown trout in a natural experimental setting(Diamond 1986) where impassable waterfalls blockthe upstream dispersal of fish within streams. Browntrout are present both below and above the waterfalls.The habitats above the waterfalls were probablycolonised before the isostatic uplift of landmassesfollowing the last ice age made them inaccessible. Asecond possibility is that man has carried the browntrout upstream. Several findings suggest that withinthese streams, brown trout above and below waterfallswill experience different selection regimes. First, amark–recapture study lasting for three successiveyears suggests that both survival rates and populationdensities are lower below the waterfalls than above thewaterfalls (Olsen & Vøllestad 2001a). Secondly,female brown trout from above the waterfalls haverelatively low fecundities and large eggs whencompared with females from below the waterfalls;after adjusting for differences in female body size(Olsen & Vøllestad 2003). Thirdly, juvenile brown

Ecology of Freshwater Fish 2005: 14: 202–208Printed in Singapore Æ All rights reserved

Copyright � Blackwell Munksgaard 2005

ECOLOGY OFFRESHWATER FISH

202 doi: 10.1111/j.1600-0633.2005.00094.x

Olsen EM, Vøllestad LA. Small-scale spatial variation in age and size atmaturity of stream-dwelling brown trout, Salmo trutta.Ecology of Freshwater Fish 2005: 14: 202–208. � BlackwellMunksgaard, 2005

Abstract – This study documents substantial small-scale spatial variationin age and size at maturity of brown trout (Salmo trutta) found either inallopatry (above major waterfalls) or in sympatry (below waterfalls) withthe Alpine bullhead (Cottus poecilopus) in forest streams in south-eastNorway. Within two streams, female brown trout above waterfalls tendedto delay the onset of sexual maturity, as compared with females fromneighbouring sites below the waterfalls. Four additional streams wererepresented with either an allopatric or a sympatric site. There wasconsiderable variation in age and size at maturity among these streams, butno consistent difference between allopatric and sympatric sites. It issuggested that the spatial variation in maturity responses is influenced bylocal opportunities for growth, and possibly also survival. Earlier studies inthese streams have linked spatial variation in brown trout behaviour anddemography to the presence or absence of the Alpine bullhead.

E. M. Olsen, L. A. VøllestadDepartment of Biology, Centre for Ecological andEvolutionary Synthesis, University of Oslo, Oslo,Norway

Keywords: age at maturity; brown trout; Salmotrutta; size at maturity; spatial variation

Esben Moland Olsen, Department of Biology,Centre for Ecological and Evolutionary Synthesis,University of Oslo, PO Box 1066 Blindern, N-0316Oslo, Norway; e-mail: e.m.olsen@bio.uio.no

Accepted for publication February 4, 2005

trout grow more rapidly below the waterfalls thanabove the waterfalls (Olsen & Vøllestad 2003). In onestream, this population difference in growth wasconfirmed when offspring from wild parent fish werehatched and reared in a common laboratory setting(Olsen & Vøllestad 2001b). This study also foundsignificant additive genetic variation for most earlylife-history traits, meaning that they are open forselection (Olsen & Vøllestad 2001b). Lastly, a studyon the diet of juvenile brown trout in this same streamindicate that brown trout below the waterfall feed moreon invertebrate drift and from the surface than browntrout above the waterfall, while invertebrate commu-nities are similar (Holmen et al. 2003). Several studiessuggest that these differences in life history and dietamong brown trout from above and below waterfallscould be influenced by the Alpine bullhead (Cottuspoecilopus Heckel), which is present only below thewaterfalls (Vøllestad et al. 2002; Hesthagen & Heg-genes 2003; Holmen et al. 2003; Hesthagen et al.2004).Here, we present field data on variation in age and

size at maturity in some of these brown troutpopulations, found either in allopatry (above water-falls) or in sympatry (below waterfalls) with theAlpine bullhead. We discuss how the results corres-pond to earlier findings and theoretical predictions.

Materials and methods

Brown trout were sampled from seven forest sites insouth-east Norway (for map of study sites, see: Olsen& Vøllestad 2001a). In the streams Søre Osa andNordre Bjøraa (hereafter Osa and Bjøraa), we fishedboth above and below a major waterfall. In addition,we sampled brown trout below a waterfall in thestream Gjesa, and above waterfalls in the streamsUlvaa and Bellbekken. The upstream habitat in Gjesais apparently not inhabited by the brown trout (E. M.Olsen, personal observation). In Ulvaa and Bellbekkenthe waterfalls are found at the outlet of the streams,and thus no below-waterfall sites are available. Allsites are found within a radius of 70 km. The lowerand upper sites within Osa and Bjøraa are found2–3 km apart.The streams differ in size, Osa being the largest

(20–40 m wide) and Gjesa the smallest (2–5 m).Bottom substrate and canopy cover varies substan-tially among streams while there is less variationwithin streams in these characteristics (Olsen &Vøllestad 2001a). Temperature also varies amongstreams. Gjesa and Osa have the warmest temperatureregimes (up to about 18 �C during summer), Ulvaaand Bjøraa are more intermediate (up to about 15 �Cduring summer), while Bellbekken has the coldesttemperature regime (up to about 12 �C during sum-

mer) (Vøllestad et al. 2002). Water flow and temper-atures will often change rapidly because of rain andflooding (Lund et al. 2003). Osa is a regulated river,with a more stable water flow (Lund et al. 2003).Brown trout from all study sites appear to be residentfish (Olsen & Vøllestad 2001a), with some localdispersal into and out of study sites (Lund et al.2003). Brown trout and Alpine bullhead are the twomost common fish species in the study streams, butthe Alpine bullhead is only present below thewaterfalls. The minnow (Phoxinus phoxinus L.) isrelatively common at the two Osa sites and occasion-ally captured at the two Bjøraa sites and in Gjesa. Pike(Esox lucius L.) is a potential predator on brown trout,and we captured a few small individuals at the lowersite in Bjøraa and in Gjesa. We also sporadicallyfound perch (Perca fluviatilis L.) in Osa, Ulvaa andGjesa.

Brown trout were sampled with an electrofishingapparatus during the 1997–1999 spawning seasons,from mid-September to mid-October (Table 1). Mostof the data were collected in 1999, but all data fromthe two Osa sites were collected in 1998. Age-0 fish(identified from length) were released, as brown troutwill not mature at this young age. All other fish werekilled with an overdose of benzocaine. Fork lengthwas measured to the nearest mm and mass to thenearest g. Sex and maturity state (juvenile or mature)was determined for each individual by visual inspec-tion of the gonads (Sømme 1941). Age was estimatedfrom otoliths (Jonsson 1976; DeVries & Frie 1996).

Estimates of maturity at age and size were obtainedfrom multiple logistic regression models (Hosmer &Lemeshow 1989), with maturity state (juvenile ormature) as response variable. Model selection wasbased on the AICC criteria, selecting the model withthe lowest AICC value (Hurvich & Tsai 1989;Burnham & Anderson 1998). In addition, we giveP-values from type III likelihood ratio tests. First, wetested whether the populations and genders haddifferent maturity responses by applying age, siteand gender as predictor variables. Age was modelledas a continuous variable while site and gender weremodelled as factors. We also tested models where thesite factor was reduced to two levels: below and abovewaterfalls (corresponding to Alpine bullhead presenceor absence). From the selected model, we thencalculated age at 50% probability of being mature(for details on the method, see: Haugen 2000). We didadditional logistic regression analyses on the twostreams where we had data both above and below awaterfall within the same stream (Osa and Bjøraa).Here, we included age, stream and waterfall aspredictor variables. Again, age was modelled as acontinuous variable while stream and waterfall weremodelled as factors. The factor ‘waterfall’ denotes

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whether the site was located below or above thewaterfall. These analyses were run separately for eachgender. Lastly, we did a similar set of analyses withbody length, instead of age, as a continuous predictorvariable.

Results

Age-based maturity patterns differed among sites andsexes (Page*site*gender ¼ 0.020). This most complexmodel where maturity is described independently foreach site and gender could not be simplified withoutincreasing the AICC value. The age when 50% of thefish were mature varied from 1.8 to 4.4 years,depending on site and gender (Table 2). The maleswere generally mature at a younger age than thefemales, and the youngest mature fish were found inthe Gjesa stream (Table 2). When comparing age-based maturity patterns of females in Osa and Bjøraaonly (where we had sites both above and below awaterfall within the same stream), the waterfall effectwas retained in the best model (Page*waterfall ¼ 0.029).The maturity response was steeper below the water-falls than above the waterfalls (Fig. 1). In addition,there was a stream effect where females from Osa

tended to be mature at younger ages than females fromBjøraa (Pstream ¼ 0.0048, Table 2). The waterfalleffect was not retained in the analysis of male age-based maturity patterns in these two streams. Asimpler model containing only age and stream aspredictor variables had the lowest AICC value(Page*stream ¼ 0.0033).

Maturity patterns based on body length differedamong sites and genders (Pbody length*site < 0.0001,Pbody length*gender < 0.0001). This model where all sites

Table 1. Samples (N) of brown trout collectedfrom sites with sympatric populations of theAlpine bullhead (C. poecilopus) (below waterfalls)or in allopatry (above waterfalls) in streams insouth-east Norway during the spawning seasonsof 1997–1999.

Stream Site Gender N Length (mm) Weight (g) Age (years)

Bellbekken Allopatric Females 79 124 (82–197) 26 (6–97) 2.7 (1–5)Males 60 121 (74–219) 24 (5–110) 2.4 (1–6)

Bjøraa Sympatric Females 77 147 (78–270) 44 (5–218) 3.1 (1–7)Males 79 150 (81–220) 47 (8–128) 3.0 (1–6)

Bjøraa Allopatric Females 95 155 (89–226) 47 (9–133) 4.0 (1–8)Males 106 156 (99–221) 49 (10–128) 3.6 (1–7)

Gjesa Sympatric Females 57 127 (91–225) 25 (9–92) 1.5 (1–4)Males 85 134 (89–253) 33 (8–197) 1.6 (1–4)

Osa Sympatric Females 76 184 (100–365) 85 (12–478) 3.2 (1–8)Males 66 181 (100–390) 92 (12–692) 2.9 (1–9)

Osa Allopatric Females 89 144 (74–213) 40 (5–105) 3.1 (1–6)Males 97 142 (83–240) 41 (7–158) 2.9 (1–7)

Ulvaa Allopatric Females 45 182 (100–262) 72 (12–192) 3.5 (1–5)Males 37 178 (104–260) 69 (12–180) 3.4 (1–6)

Length, weight and age are given with arithmetic mean and range of observations (sampling years pooled).Individual age was estimated as the number of annuli in the otoliths, assumed to correspond to the numberof winter periods of slow growth that the fish had experienced up until capture (Jonsson 1976). A fish wasconsidered to be age 0 during the time from hatching (in spring) until the next 1 January, age 1 during theperiod between 1 January of its second year of life and 1 January of its third year of life, and so on (DeVries& Frie 1996).

Table 2. Predicted age and body length at 50%maturity (25th and 75th percentiles in parenthesis)for female and male brown trout sampled fromsites with sympatric populations of the Alpinebullhead (C. poecilopus) (below waterfalls) or inallopatry (above waterfalls) in streams in south-east Norway.

Stream Site

Males Females

Age (years) Length (mm) Age (years) Length (mm)

Bellbekken Allopatric 3.1 (2.8–3.4) 142 (137–146) 3.5 (3.2–3.7) 147 (139–154)Bjøraa Sympatric 3.2 (2.4–4.0) 159 (134–184) 4.0 (3.5–4.6) 168 (158–178)Bjøraa Allopatric 3.2 (1.8–4.6) 141 (116–166) 4.4 (3.6–5.2) 169 (158–179)Gjesa Sympatric 1.8 (1.4–2.2) 148 (134–162) 2.5 (2.2–2.8) 174 (160–188)Osa Sympatric 3.3 (2.8–3.8) 187 (179–195) 3.8 (3.5–4.2) 188 (185–192)Osa Allopatric 3.6 (3.0–4.3) 162 (152–172) 4.2 (3.8–4.7) 164 (156–171)Ulvaa Allopatric 2.6 (1.8–3.3) 172 (151–193) 3.5 (3.0–4.1) 169 (159–179)

1 2 3 4 5 6 7 8

0.0

0.2

0.4

0.6

0.8

1.0

1 2 3 4 5 6 7 8

Bjøråa

P (

mat

ure)

Age (years)

Osa

Fig. 1. Observed and predicted probabilities of being mature as afunction of age for female brown trout living either abovewaterfalls (open circles) or below waterfalls (filled circles) in thestreams Osa and Bjøraa.

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were treated separately could not be simplified to amodel where the sites were grouped according towhether they were located above or below waterfalls.This would increase the AICC value by at least10 units. The body length where 50% of the fish weremature varied from 141 to 188 mm, depending on siteand gender (Table 2). Brown trout from Bellbekkentended to be mature at the smallest body length whilebrown trout from the lower site in Osa tended to bemature at the largest body length (Table 2). Whencomparing brown trout from Osa and Bjøraa only,length-based maturity patterns differed among streamsfor both genders (Pbody length*stream < 0.05). In addi-tion, there was an effect of the waterfall variable, thatdepended on the stream (Pstream*waterfall < 0.01). InOsa, females from below the waterfall tended to bemature at a greater body length, when compared withfemales from above the waterfall, while in Bjøraamaturity responses were more similar (Table 2). Malebrown trout tended to be mature at a greater bodylength below the waterfall in both streams (Table 2).

Discussion

This study documents small-scale spatial variation inage and size at maturity of stream-dwelling browntrout (Salmo trutta). Within streams, female browntrout from sites above waterfall barriers delayed theonset of sexual maturation, when compared withconspecifics from neighbouring sites below the water-falls. We suggest that this pattern of variation isinfluenced by biotic factors that are possibly linked tothe presence or absence of the Alpine bullhead (Cottuspoecilopus). This species is present only below thewaterfalls.Within both Osa and Bjøraa, brown trout from

below the waterfalls have higher juvenile growth rateswhen compared with brown trout from above thewaterfalls (Olsen & Vøllestad 2003). This spatialvariation in growth could explain the observedvariation in age at maturity (Alm 1959; Stearns &Koella 1986; Lobon-Cervia et al. 1997; Morita et al.2000; Morita & Morita 2002). Faster growth belowwaterfalls when compared with above waterfalls couldbe linked to per capita resource availability. In bothstreams, brown trout population density is higherabove the waterfalls (Olsen & Vøllestad 2001a),suggesting that intraspecific competition could bemore intense in these habitats. In addition to the spatialvariation in growth, mark–recapture data stronglysuggests that brown trout survival rates are higherabove waterfalls than below waterfalls in these streams(Olsen & Vøllestad 2001a). Hence, the parallel within-stream variation in female brown trout age at maturityalso support the theoretical prediction stating that highmortality in the range of ages around potential ages of

maturation should favour early maturation, while lowmortality should favour a postponement of maturation(Charlesworth 1994).

It is uncertain to what extent the within-streamspatial variation in female maturity responses reflectsgenetic differences (i.e. local adaptations), environ-mental effects (i.e. phenotypic plasticity), or acombination of the two. In Osa, females from belowthe waterfall tended to mature at both a greater sizeand a younger age when compared with females fromabove the waterfall. This indicates that the spatialvariation in age at maturity can be explained simply byphenotypic plasticity mediated through faster growthbelow the waterfall (Stearns & Koella 1986; Olsenet al. 2004). In Bjøraa, however, growth rates arefaster below the waterfall while brown trout tends tomature at similar lengths both above and below thewaterfall. This suggests that the spatial variation in ageat maturity is not solely the result of growth-mediatedphenotypic plasticity, but that plasticity not related togrowth and possibly also local adaptations could beinvolved (Stearns & Koella 1986; Olsen et al. 2004).Note also that growth and mortality effects need not beindependent. Higher mortality below the waterfallscould explain why population densities are lower inthese sites. Faster growth could therefore be anindirect plastic effect of higher mortality whereresource abundance is increased for the survivors(see: Reznick et al. 2001). In addition, growth ratesmay have a heritable component, and differentmortality regimes select for different growth rates(Conover & Munch 2002). When brown trout fromOsa were hatched and reared under identical condi-tions in the laboratory, fry from below the waterfallgrew faster than fry from above the waterfall (Olsen &Vøllestad 2001b). This indicates evolved differencesin growth rates among the populations.

While both abiotic and biotic factors may influencespatial variation in life histories of stream-dwellingfishes, neighbouring populations living above andbelow major waterfalls which serve as barriers to theupstream dispersal of fish are good candidates foranalysing effects of biotic interactions (Reznick et al.1990). In the present case, we suggest that the Alpinebullhead – which is present only below the waterfallsin our study streams – could influence brown troutmaturity responses through either growth rates, survi-val rates, or both. Freshwater sculpins are known tointeract with sympatric salmonids through predationon salmonid eggs and juvenile stages and also throughcompetition for food and space (Andreasson 1980;Foote & Brown 1998; Gabler & Amundsen 1999;Hesthagen & Heggenes 2003). In the region where thepresent study was conducted, several studies suggestthat brown trout and Alpine bullhead are interacting.First, a mark–recapture study comparing somatic

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growth of brown trout from above and below water-falls in several streams found a significant negativeeffect of the presence of Alpine bullhead on browntrout growth, after adjusting for differences in tem-perature, season, fish mass and brown trout populationdensity (Vøllestad et al. 2002). Secondly, experimentsin artificial stream channels – with fish from a nearbystream – documented that when in allopatry, juvenilebrown trout and Alpine bullhead have a similarpreference for coarse substrate, while in sympatrylarge Alpine bullheads will displace the brown troutfrom their preferred habitats and may also prey onsmall brown trout (Hesthagen & Heggenes 2003). Thisfinding corresponds well to what has been shown in astudy on the diet of juvenile brown trout above andbelow a waterfall in the Osa stream. Here, brown troutbelow the waterfall (in sympatry with the Alpinebullhead) feed more on invertebrate drift and from thesurface than brown trout above the waterfall(in allopatry), while invertebrate communities aresimilar (Holmen et al. 2003). Thirdly, the Alpinebullhead and juvenile brown trout have similar diets,although the Alpine bullhead feed less on surfaceinsects (Holmen et al. 2003; Hesthagen et al. 2004).Fourthly, there is an inverse relationship between thedensity of brown trout and the density of Alpinebullhead in a river where the two species are foundin sympatry at several localities (Hesthagen et al.2004).

When comparing brown trout populations fromdifferent streams, we could find no consistent differ-ence in maturity responses between allopatric sitesabove waterfalls and sympatric sites below waterfalls.Among streams there is considerable variation in otherenvironmental characteristics such as water tempera-ture and canopy cover (Olsen & Vøllestad 2001a),which are likely to influence the life histories ofstream-dwelling fishes (Elliott & Hurley 1999; Gretheret al. 2001; Reznick et al. 2001). Study sites aboveand below waterfalls within streams are much morealike with respect to these environmental factors(Olsen & Vøllestad 2001a).

There was no evidence for within-stream differ-ences in age at maturity of male brown trout in Osaand Bjøraa. Both above and below the waterfall inBjøraa, and also in some of the other populations, wefound some particularly small and young maturemales. These individuals may have been sneakers,males that sneak rather than fight for access to females.This phenomenon has been documented in severalbrown trout populations (Bohlin 1975; L’Abee-Lundet al. 1990). Jonsson & Sandlund (1979) also foundsome very small mature males in Osa and two of itstributaries.

In conclusion, this study documents significantvariation in age and size at maturity among neigh-

bouring populations of stream-dwelling brown trout ata very small geographical scale. It is suggested thatwithin-stream differences in maturity responses areinfluenced by biotic factors that could be linked to thepresence or absence of the Alpine bullhead.

Resumen

1. Este estudio describe variaciones espaciales a pequena escalaen la edad y tamano en la primera madurez de Salmo trutta encondiciones de alopatrıa por encima de una cascada y ensimpatrıa con Cottus poecilopus en localidades vecinas, pordebajo de la catarata.2. Mostramos que las hembras por encima de la catarata tiendena retrasar el inicio de la madurez sexual en comparacion a lashembras del mismo rıo que se localizan por debajo de lacatarata. Sugerimos que este patron de variacion esta influenci-ado por factores abioticos posible relacionados a la presencia oausencia de Cottus poecilopus. Estudios anteriores en estosmismos rıos mostraron que por debajo de las cataratas lasdensidades de poblaciones son relativamente menores mientrasque las tasas de crecimiento y mortalidad son mas altas encomparacion a las localidades localizadas por encima de lascataratas. Tambien, estudios anteriores en rıos de la regionsugirieron que C. Poecilopus compiten con los juveniles deS. trutta por el aliento y por el espacio y puede depredar sobreestos juveniles. AL comparar las poblaciones de S. trutta devarios rıos no encontramos diferencias consistentes en respu-estas a la madurez entre localidades por encima y por debajo delas cataratas. Entre rıos, hay una variacion considerable en lascaracterısticas ambientales tales como la temperatura del agua yla cubierta vegetal mientras que las localidades de estudio porencima y por debajo de las cataratas dentro del mismo rıo sonmas similares respecto de estas caracterısticas. Los machos deS. trutta tendieron a madurar siendo mas jovenes que lashembras no apareciendo diferencias consistentes en repuestas ala madurez por encima y por debajo de las cataratas. La edadoptima y el tamano en la primera madurez en los machos esmenos facil de predecir ya que los machos pueden adoptartacticas de emparejamiento alternativas. SI encontramos algu-nos machos particularmente pequenos y jovenes maduros quebien pueden ser ‘‘sneakers’’.

Acknowledgements

This study was financed by the Norwegian Research Council.We thank Lars Flodmark and Thrond O. Haugen for valuablecomments on the manuscript, and Arild Engen, Finn Gregersen,Johannes Holmen, Jørn Lima, Hans Petter Rømme and ThomasWestly for their skilful assistance in capturing the fish.

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