Morphological differentiation among local trout (Salmo truttu) populations

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1 Biological Journal of the Linnean Society (2001), 72: With 5 figures doi: 10.1oos/bij , available online at httpj/m.idealibrary.com on I Morphological differentiation among local trout (Salmo truttu) populations SUSANNA PAKKASMAA* Integrative Ecology Unit, Division of Population Biology, Department of Ecology and Systematics, PO. Box 17 (Arkadiankatu 7), FIN University of Helsinki, Finland JORMA PIIRONEN Finnish Game and Fisheries Research Institute, Saimaa Fisheries Research and Aquaculture, Laasalan t ie 9, FIN , Enonkos k i, Fin land Received 8 February 2000; accepted for publication 11 September 2000 The trout (Salmo trutta) has been divided into three forms: sea-run trout, lake-run brown trout, and resident brown trout. They differ in their living environment, migratory behaviour, growth and appearance. As local trout populations are often isolated, and gene flow between them is minimal, differentiation between populations can be expected. The morphology of l-year-old trout from ten populations representing all three forms was studied in a commongarden experiment. The fish were reared under similar environmental conditions, and 20 morphometric characters were measured from each individual fish. Marked morphological differentiation was found, and differences between populations were greater than differences between forms. The results suggest that the differences have a genetic basis, and they are likely to indicate adaptation to local environmental conditions in the native habitat of the trout The Linnean Society of London ADDITIONAL KEYWORDS: local adaptation - morphological variation - geographical separation - salmonids. INTRODUCTION Populations of the same fish species often differ in phenotypic characters (e.g. Sandlund et al., 1992; Schluter & McPhail, 1992; Elliott, 1994). Morphological differences are expected to occur when the habitats require specific abilities, such as fast vs. slow swimming, or slow and precise manoeuvring vs. cruising, and the magnitude of these differences should be related to the degree of differences among habitats (Bourke, Magnan & Rodriguez, 1997). Differences have been suggested to reflect variation in, for example, migration distance, flow regime and feeding (Riddell & Leggett, 1981; Riddell, Leggett & Saunders, 1981; Skulason, Noakes & Snorrason, 1989; Sandlund et ul., 1992; Wimberger, 1992). * Corresponding author, present address: Department of Population Biology, Evolutionary Biology Centre, Uppsala University, Norbyvagen 18D, SE Uppsala, Sweden. susanna.pakkasmaa@ebc.uu.se /01/ $35.00/ Morphological differentiation can in principle result from two causes, genetic differences or environmental factors, or their interaction. Genetic differences and reproductive isolation between populations can lead to local adaptation, which is reflected in morphology, behaviour, physiology and life history traits (Taylor, 1991). Environmental factors, on the other hand, can produce phenotypic plasticity, which is the capacity of a genotype to produce different phenotypes in different environmental conditions (Steams, 1989; Scheiner, 1993). Phenotypic plasticity as a source of morphological variation has been widely studied in fish. For example, the deep body shape of the crucian carp (Carussius carassius) may have developed as an induced defence in the presence of predators (Bronmark & Miner, 1992; Bronmark & Pettersson, 1994) or as a result of good food conditions (Holopainen et al., 1997). Furthermore, variation can evolve as a result of diet variability (Day, Pritchard & Schluter, 1994), and specialization to different resource use may lead to subtle 2001 The Linnean Society of London

2 232 S. PAKKASMAA and J. PIIRONEN in morphology (e.g. Skulason et al., 1989; Wimberger, 1992; Snorrason et al., 1994; Robinson & Wilson, 1996). Local adaptation in morphology is less well studied, and the evidence is mainly obtained circumstantially from environment-phenotype correlations. For instance, Atlantic salmon (Salmo sular) reared under similar environmental conditions from the egg stage onwards demonstrated morphological differences despite a common rearing environment, showing that the differences must be of genetic origin (Riddell et al., 1981; Nicieza, 1995). The morphology of juvenile coho salmon (Oncorhynchus kisutch) displays a coastal-interior dichotomy with fish from coastal streams having a more robust body shape than those from interior stream areas (Taylor & McPhail, 1985). This variation was persistent among laboratory-reared fish, suggesting that morphological differences between fish from different areas are at least partially genetic, and thus likely to reflect local adaptation. Generally, body shape is considered to represent an adaptation to the prevailing environmental conditions (e.g. Riddell & Leggett, 1981), and it greatly affects the swimming performance of a fish. A streamlined shape is advantageous for sustained swimming or foraging in open waters, whereas a deeper body shape improves burst swimming and is advantageous when swimming in structurally complex habitats (Webb, 1984; Taylor & McPhail, 1985; Swain & Holtby, 1989; Robinson & Wilson, 1996). The water systems of Finland are inhabited by three different forms of trout: sea-run trout (Salmo trutta m. tmtta), lake-run brown trout (Salmo trutta m. lacustris) and resident brown trout (Salmo tmtta m. farzo). The most marked ecological difference among the forms is in their migratory behaviour. Both searun trout and lake-run brown trout migrate, whereas resident brown trout spend their whole life cycle in the same brook or stream. Sea-run trout reproduce in streams, but migrate to the sea to feed. Lake-run brown trout also reproduce in streams, but their feeding migrations are into lakes in the same water system. Because of differences in living environment, the forms differ considerably in size at maturity (Koli, 1990): sea-run trout and lake-run brown trout may weigh 15kg, whereas the growth of resident brown trout is slower and the fish usually remain rather small (25-30cm, <I kg). The forms also differ in colouring: adult sea-run trout and lake-run brown trout look rather similar having a silvery or dark colouring, though the local environment may modify it. Resident brown trout are more brownish and often have red spots on their sides throughout life, which are otherwise typical for juvenile fish. Environmental changes during recent decades have deteriorated the living conditions of many stocks in Finland, and therefore trout are now extensively reared in hatcheries (Kaukoranta et al., 1998). The morphology of the three trout forms was studied in a common garden experiment, in which fish from different populations were reared in a common hatchery environment. We thus specifically investigated morphological variation due to genetic effects in juvenile trout. MATERIALS AND METHODS The three trout forms were obtained from ten populations originating from different parts of Finland (Fig. 1). We had three sea-run trout populations originating from the Rivers Iijoki, Isojoki and Ingarskilajoki. The four studied lake-run brown trout populations originated from two rivers in northern Finland, the Rivers Kitkajoki and Kuusinkijoki, and from two water systems in Central Finland, the Rautalampi and Vuoksi water systems. Finally, the three populations of resident brown trout originated from the Rivers Ounasjoki and Kemijoki in northern Finland, and from the River Luutajoki in southern Finland. The hatchery histories of the study populations varied so that the fish represented first to fourth hatchery generations, and the number of parents used when creating each study was the maximum available (Table 1). The fish population were brought to the Saimaa Fisheries Research and Aquaculture hatchery (Fig. 1) as eyed-stage eggs in winter They were reared in similar standard hatchery conditions, with fish from each population in a separate l.lm2 circular tank. In early July 1998, the number of fish in each tank was adjusted to 200 individuals. The fish were fed automatically with commercial salmonid food. Water inflow was Ls-'. A random sample of 30 fish (total n=306) was taken from each population in April, 1999, when the fish were 1 year old. The fish were anaesthetized with a buffered tricaine solution (100 mg L-') and photographed in water first from above and then on their left side. The photographs were analysed with an image analysis programme, and altogether 20 morphometric measurements were taken from each fish. Body width was measured from four points from the top-view photographs (Fig. 2A). The following characters were measured from the sideview photographs (Fig. 2B): total body length, body length from behind the eye to the end of caudal fin, body length from behind the eye to the beginning of caudal fin, head length, eye diameter, snout length, antero-dorsal length from snout tip to the anterior edge of the dorsal fin, and antero-anal length from the snout tip to the anterior edge of anal fin. Additionally, the following height measurements were taken: head height, body height in front of dorsal fin, body height in front of anal fin and the height of the caudal part

3 MORPHOLOGICAL DIFFERENTIATION AMONG LOCAL TROUT POPULATIONS 233 Polar circle Iijoki P f Isojoki / Luutajoki + - Figure 1. Map of Finland showing the rivers and water systems from which the studied trout populations originated. Only those watercourses supplying study populations are shown on the map. Lake Paanajarvi into which the trout from the River Kuusinkijoki migrate to feed and Lake Kitkajarvi, into which the River Kitkajoki trout migrate to feed are also indicated. The location of the Saimaa Fisheries Research and Aquaculture hatchery where the experiment was performed is indicated with an asterisk. (narrowest point). The size of three fins was measured: pectoral fin length, dorsal fin base and caudal fin height. All the measurements were made by S. P. Some of the fish had a damaged upper jaw due to a Fluvobacterium infection, and upper jaw measurement could not be obtained from them. In the data set, the missing values (n=32) were replaced with the mean of the respective populations (Pimentel, 1979). The fish from the different populations differed in size (one-way analysis of variance, total length: F9,305

4 234 S. PAKKASMAA and J. PIIRONEN Table 1. Hatchery history of the studied trout populations. Parents refer to the number of females and males used in fertilizations for the studied populations. Hatchery generation refers to the generation of the experimental fish Form Population Parents Hatchery generation Females Males Sea-run trout Iijoki Sea-run trout Isojoki /4 Sea-run trout Ingarskilajoki Lake-run brown trout Kitkajoki Lake-run brown trout Kuusinkij oki Lake-run brown trout Rautalampi <4 Lake-run brown trout Vuoksi Resident brown trout Ounasjoki Resident brown trout Kemijoki Resident brown trout Luutajoki A BW4 B CF UJ Figure 2. Measurements taken from the fish, (A) from the top-view photographs and (B) from the side-view photographs. Abbreviations: (A) BW =body width (four points), (B) TL= total length, PO =post-orbital length, BL=body length (used in standardization), AD = antero-dorsal length, AA = antero-anal length, HL =head length, ED =eye diameter, SL = snout length, HH =head height, UJ =upper jaw length, BH =body height (three points), PF = pectoral fin, DF = dorsal fin, CF=caudal fin. TL = 32.77, P<O.OOl), and therefore all morphometric caused by damaged snout or caudal fin, The regression measurements were size-adjusted. Because the re- residuals, or shape variables, were used as response lationship between body length and the other meas- variables in the analyses. We used principal component urements was linear (Fig. 3), the size-adjustment was analysis (PCA) to reduce the initial morphometric varimade by calculating linear regressions for each variable ation to uncorrelated principal components, that all inseparately against body length. Body length was used clude variation in several initial variables. The instead of total length, thus avoiding the possible error principal components were further analysed with nes-

5 MORPHOLOGICAL DIFFERENTIATION AMONG LOCAL TROUT POPULATIONS Body length o Sea-run trout * Lake-run trout Resident trout Figure 3. Example of the relationship between body length and three morphometric characters for each of the three forms. All measurements are in mm. ted analysis of variance, in which the populations were nestedwithinforms. The componentswithineach forms were analysed with multivariate analysis of variance. RESULTS General variability in morphometric measurements among all populations is illustrated in Figure 4. Principal component analysis resulted in six components with eigenvalues higher than 1.00 (e.g. Chatfield & Collins, 1983). However, a clear interpretation could be found for the first three components, which explained c. 50% of the morphological variation. According to the PCA, most of the morphological variation among the fish occurred in head size, body height and body width (Table 2). The first principal component (PC1) was composed of body height, body width, antero-dorsal length, antero-anal length, pectoral fin size and total length. These characterized the overall body shape of the fish. The second component (pc2) consisted of body width, as well as the third component (PC3), which also included variation in dorsal fin size. The three principal components all characterized the build of the fish and their slenderness or robustness, characters essentially related to swimming ability. Fin sizes are also related to the swimming ability; pectoral fins in particular are important to maintaining position in the stream. PCA reflected both the differences between the trout forms (Fig. 5A), and among populations (Fig. 5B). In terms of body shape, the lake-run brown trout populations from the Vuoksi and Rautalampi water systems were close to each other, as were also the northern lake-run brown trout populations from the Rivers Kitkajoki and Kuusinkijoki. The northern resident brown trout populations (the Rivers Kemijoki and Ounasjoki) were also close to each other. Nested analysis of variance revealed that the differences between the forms on PC1-3 were not statistically significant (PC1: F2,? = 2.37, P= 0.164; PC2: F2,, = 2.80, P= 0.128; PC3: F2,7 = 1.30, P= 0.332). The principal components (PC1-3) for each form were analysed with multivariate analysis of variance to examine within-form variation (Table 3). In lake-run brown trout and resident brown trout the differences between populations were satistically significant, whereas there were no significant differences between the three sea-run trout populations. DISCUSSION Northern Europe has gone through dramatic climatic changes in the past, and the last ice age ended about years ago. The glacial period has affected the geographical distribution of intraspecific polymorphism among freshwater and anadromous fish species in Europe (Nesb~ et d., 1999, and references therein). For example, the initial genetic composition of local brown trout populations can be a result of colonization, and thus it could be a founder effect. Population diversification may have increased with time as a result of random events or locally varying selection pressures. In the present study, most of the morphological variation among the fish from the different populations occurred in body height and width. These characterize the overall body build, and robustness or slenderness of the fish, which are of considerable importance ~DZthe swimming performance (Webb, 1984). Differences were also found in fin sizes. Long pectoral fins are related to slow and precise manoeuvring, whereas shorter fins are associated with cruising movements

6 I 236 S. PAKKASMAA and J. PIIRONEN BODY HEAD HEIGHT Total length Head length Body height 1 Post-orbital length Eye diameter Body height Antero-dorsal length Snout length Body height 3 s Antero-anal lenah Head height WIDTH Bodv width 1 Bodv width 2 Body width 3 Body width FINS Pectoral fin Dorsal fin Caudal fin 90 ' Umer iaw " 90 Figure 4. General variability in morphometric measurements in all populations: (0) sea-run trout, (m) lake-run brown trout, (a) resident brown trout populations. The measurements are standardized to a body length of 80 mm, which corresponds approximately to a total length of 100 mm. Vertical axis indicates the proportion (in Yo) that each individual measurement is from the maximum value of all populations. (Webb, 1984; Ehlinger, 1990). Large fins are also effective in maintaining a fish's position in the river (Riddell & Leggett, 1981). Notable fin damage was not observed among the studied fish. The populations of two of the three forms displayed morphological variation, which was associated with geographical separation. In lake-run brown trout, the northern populations (Kuusinkijoki and Kitkajoki) were different from the fish from the Central Finland populations (the Vuoksi and Rautalampi water systems). The Vuoksi and Rautalampi trout were larger and more robust in their build than the fish from the northern populations, and they also had larger pectoral fins. This may partially reflect the differences in the original habitats, because the Vuoksi and Rautalampi water systems are both large complexes of separate lakes (Fig. 1) connected by short rapids where the brown trout reproduces. Additionally, both Vuoksi and Rautalampi trout have been extensively stocked in the recent decades. The Kuusinkijoki and Kitkajoki populations live in the same water system in small rivers running into a common main lake, Lake Paanajarvi in Russia, which runs into the White sea. The Kuusinkijoki fish migrate downstream into Lake Paanajarvi to feed, whereas the Kitkajoki fish migrate upstream into Lake Kitkajarvi to feed. The northern resident brown trout populations from the Rivers Kemijoki and Ounasjoki were different from the fish from the Luutajoki river located in southern Finland. The Rivers Kemijoki and Ounasjoki belong to the same water system, and have similar environmental conditions with short growing season, cool water and rather low resource availability as compared to the southern river. The River Luutajoki is smaller than the northern rivers and has dark, humic water and higher summer water temperature. The Luutajoki fish were slimmer than those from the northern populations. The morphological differences were minimal between the three sea-run trout populations that originated from different parts of Finland. This may be because the searun trout populations migrate to the Baltic sea to feed andthe common feeding areamay partially homogenize

7 0.08 MORPHOLOGICAL DIFFERENTIATION AMONG LOCAL TROUT POPULATIONS 237 Table 2. Principal component analysis for the whole data. The analysis is computed from the shape variables, and it is based on the correlation matrix. Largest correlation coefficients (r) for each variable are indicated in bold Variable pc1 r Pc2 r Pc3 r Body width 1 Body width 2 Body width 3 Body width 4 Total length Post-orbital length Head length Eye diameter Snout length Antero-dorsal length Antero-anal length Pectoral fin Dorsal fin Head height Body height 1 Body height 2 Body height 3 Caudal fin Upper jaw Eigenvalues Yo of variance Cum. YO of variance ~ between-population differences (Nicieza, 1995). Furthermore, anadromous populations generally show less genetic variation than non-anadromous populations (Gyllensten, 1985), at least partially as a consequence of straying, which may explain the relative similarity of the studied sea-run trout populations. The difference between different trout populations in Finland may be caused by different initial colonization routes. Alternatively, locally varying selection pressures associated with the climatic conditions or the size of the river could cause morphological differentiation. The size of the river has been observed to affect the body morphology of chum salmon (Oncorhynchus keta), so that fish from large rivers had larger heads, thicker caudal peduncles as well as larger fins than fish from small rivers (Beacham & Murray, 1987). The distance between the southern andnorthern trout populations was several hundred kilometres, which also means considerable variation in the length of growing season, duration of ice cover, and resource availability. Different climatic conditions can cause varying selection pressures, and also influence fish growth or morphology. In the wild, juveniles of all trout forms live in streams (Koli, 1990), but the streams where resident brown trout live are in general smaller than those inhabited by the other forms. The trout prefer rather shallow areas and slow-flowing water (Heggenes, 1988; Maki- Petays et al., 1997), and choose focal points on the basis of water velocity and food supply to maximize net energy gain (Fausch, 1984). The habitat preferences change with fish growth and season (Maki-Petays et ae., 1997). Juvenile trout feed on aquatic insects and other invertebrates. Head morphology reflects the feeding habits of a species, and trophic structures are often subject to strong selection pressures and environmental modification (Skulason et al., 1989). We found differences in head dimensions between the populations, and the differences probably reflect genetic differentiation as a consequence of subtle differences in the natural environment and diet. The shared hatchery environment has probably partly levelled off the growth differences expressed in natural environments (Frier, 1994), although the sea-run trout were still on average larger than the other forms. In this study, the resident brown trout did not differ very much in size from the other forms suggesting that they may have potential to grow, but in the wild the growth may be resource-limited. The studied populations had hatchery histories of variable length, which may partially have affected our results. The trout in Finland is a widely stocked species (Kaukoranta et al., 1998), and even wild populations may have some kind of hatchery background.

8 238 S. PAKKASMAA and J. PIIRONEN 3 2 v * i o 0 a g -1 U -2 A 7 Thble 3. Multivariate analysis of variance for principal components (PC1-3) for the three trout forms separately; the components are computed for the whole dataset. Degrees of freedom: sea-run trout 2,88; lake-run brown trout 3,120 and resident brown trout 2,88 throughout the analyses. Treatment and error mean squares (MS), F- statistic and P-values are indicated. The multivariate statistic Wilks' lambda tests the equality of group means of the variables in the univariate tests m b U g o 0 p. g -1 U -2-3 Sea-run trout Lake-run trout Resident trout B Figure 5. Means of principal component scores for each form (A) and population (B). (m) PC1; (0) PC2; (n) PC3. This study provides additional confirmation that the trout is a polymorphic species. The studied fish were imported to the common hatchery as eggs, after which they were reared in similar standard hatchery conditions. This common garden study design minimized the role of phenotypic plasticity in creating morphological differentiation. The finding that the populations differ morphometrically when reared in a similar environment suggests that the differentiation is of genetic origin. The differences between the populations were greater than differences between the forms. We suggest that the observed morphological differences reflect local adaptation to the environmental conditions of the native habitat of the studied trout populations. ACKNOWLEDGEMENTS The fish populations were obtained from the following hatcheries of the Finnish Game and Fisheries Research Institute: Taivalkoski, Kuusamo, Laukaa and Saimaa. The fish were reared at Saimaa Fisheries Research and Aquaculture, Enonkoski, Finland. E. Ranta and I Sea-run trout PC / PC2 2.05/ PC Wilks' lambda=0.897, FG,lil= 1.607, P=0.148 Lake-run brown trout PC / Pc / PC3 8.7w Wilks' lambda=0.391, F9.,,;= , P<O.001 Resident brown trout PC / Pc2 9.19/ PC / Wilks' lambda=0.509, FG,liz= , P< < <0.001 < <0.001 A. Laurila provided valuable comments on the manuscript and M. Jones kindly checked the language. This study was funded by The Graduate School in Evolutionary Ecology of The Finnish Ministry of Education and The Academy of Finland (S. P.). REFERENCES Beacham TD, Murray CB Adaptive variation in body size, age, morphology, egg size, and developmental biology of chum salmon (Oncorhynchus keta) in British Columbia. Canadian Journal of Fisheries and Aquatic Sciences Bourke P, Magnan P, Rodriguez MA Individual variations in habitat use and morphology in brook charr. Journal of Fish Biology 51: Bronmark C, Miner JG Predator-induced phenotypical change in body morphology in crucian carp. Science Bronmark C, Pettersson LB Chemical cues from piscivores induce a change in morphology in crucian carp. Oikos Chatfield C, Collins AJ Introduction to multivariate anazysis. London: Chapman & Hall. Day T, Pritchard J, Schluter D A comparison of two sticklebacks. Evolution 48:

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