Gill Raker Morphology and Feeding Ecology of Two Sympatric Morphs of European Whiteﬁ Sh (Coregonus Lavaretus)

Ann. Zool. Fennici 41: 291–300 ISSN 0003-455X Helsinki 26 February 2004 © Finnish Zoological and Botanical Publishing Board 2004

Gill raker morphology and feeding ecology of two sympatric morphs of European whiteﬁ sh (Coregonus lavaretus)

Per-Arne Amundsen*, Thomas Bøhn & Gry H. Våga

Norwegian College of Fishery Science, University of Tromsø, N-9037 Tromsø, Norway (*e-mail: [email protected])

Received 10 Sep. 2002, revised version received 31 Dec. 2002, Accepted 17 Feb. 2003

Amundsen, P.-A., Bøhn, T. & Våga, G. H. 2004: Gill raker morphology and feeding ecology of two sympatric morphs of European whiteﬁ sh (Coregonus lavaretus). — Ann. Zool. Fennici 41: 291–300.

European whitefi sh is a polymorphic species where different morphs often are identifi ed by differences in their gill raker numbers. Gill rakers may play an important role in food-particle retention, particularly with respect to zooplankton feeding. Possible associations between feeding ecology and gill raker number and morphology were studied in European whitefi sh in the Pasvik watercourse. The numbers of gill rakers exhibited a bimodal distribution pattern, demonstrating the presence of two sympatric forms, including a sparsely-rakered morph with 18–30 and a densely-rakered morph with 28–42 rakers. The morphology of the gill rakers was correlated to the raker number and exhibited distinct differences between the morphs; sparsely-rakered whitefi sh having shorter, thicker and less densely packed rakers than the densely-rakered morph. Differences in habitat choice and trophic ecology between the two morphs appeared to be correlated to gill raker number and morphology. Densely-rakered whitefi sh exhibited a diet dominated by zooplankton and other pelagic prey, whereas the sparsely-rakered morph mainly fed on zoobenthos. Within the morphs, however, the feeding ecology of individual fi sh was not correlated with number and morphology of the gill rakers. Thus, whereas gill raker number and morphology appear to be a reliable marker for identifying ecologically and genetically different European whitefi sh morphs, the functional role with respect to the feeding performance of individual fi sh is less obvious.

Introduction 1979, Lindsey 1981, 1988, Bergstrand 1982, Amundsen 1988a, Sandlund & Næsje 1989, European whitefi sh (Coregonus lavaretus) and Bernatchez et al. 1996), and the differences its North-American sibling, the lake whitefi sh have also been shown to have a genetic basis (Coregonus clupeaformis), are highly poly- (Svärdson 1952, 1979, Bernatchez et al. 1996). morphic species. Different morphs are often Frequently two or more morphs live sympatri- separated by differences in gill raker numbers cally in the same lake (Svärdson 1952, 1957, (Svärdson 1952, 1957, 1979, 1998, Bodaly 1979, Bodaly 1979, Bergstrand 1982, Amund- 292 Amundsen et al. • ANN. ZOOL. FENNICI Vol. 41 sen 1988a, 1988b, Bernatchez et al. 1996), pared with respect to gill raker number and mor- and co-existing morphs often exhibit profound phology to scrutinize the assumed differences differences in habitat and food selection (e.g. in length, breadth and distance of the rakers. It Nilsson 1958, Lindström & Nilsson 1962, was hypothesized that the two morphs exhibit Amundsen 1988a, Bernatchez et al. 1999). The distinct quantitative differences with respect feeding habits of sympatric whitefi sh morphs to gill raker morphology. Habitat choice and have commonly been found to correlate with the feeding ecology were further compared with number of gill rakers. Morphs with the lowest gill raker number and morphology both at the gill raker numbers are usually benthivorous population (i.e. morph) and individual levels, whereas those with high gill raker numbers hypothesizing that fi sh with more numerous and are planktivorous (Svärdson 1952, 1957, 1979, densely packed gill rakers exhibit a stronger ten- Nilsson 1978, Bodaly 1979, Bergstrand 1982, dency for pelagic and planktivorous behaviour. Amundsen 1988a, Bernatchez et al. 1999). It Thus, the high-rakered morph was expected to is suggested that morphs with high gill raker be predominantly planktivorous, and the low- numbers are better adapted to zooplankton feed- rakered form mainly benthivorous. Similarly, ing because dense gill raker spacing is assumed within each of the two morphs, the individuals to be more effi cient for retaining small prey in with more numerous rakers were expected to be the mouth cavity (O’Brien 1987, MacNeill & more pelagic and planktivorous than those with a Brandtt 1990, Link & Hoff 1998). Exceptions lower raker number. to this pattern do however exist (e.g. Svärdson 1950, Kliewer 1970, Chouinard et al. 1996), and the role of gill rakers in zooplankton feed- Study area and fi sh communities ing has been questioned (Seghers 1975, Wright et al. 1983, Langeland & Nøst 1995). The Pasvik watercourse belongs to three coun- In lakes in northern Norway, two sympatric tries. It originates from Lake Inari (1102 km2) morphs of the European whitefi sh have been in Finland, runs into Russia and then defi nes the identifi ed from a bimodal distribution of the gill border between Norway and Russia for a length raker numbers; one mode ranging from approx. of about 120 km. The Norwegian–Russian part 20–30 and the other from approx. 30–40 gill of the river system has a total area of 142 km2, a rakers (Amundsen 1988a, Amundsen et al. catchment area of 18 404 km2 and a mean annual 1997, 2002, 2004, and unpubl. data). Individu- water fl ow of about 175 m3 s–1. There are alto- als of the two morphs may also be separated gether seven water impoundments in the water- visually by examining the morphology of the gill course. Most rapids and waterfalls have disap- rakers; the morph with the lowest raker numbers peared, and lakes and reservoirs currently domi- having shorter, thicker and more widely spaced nate the former river system. The water level rakers, as compared with the longer, slender and fl uctuations are small, usually less than 80 cm. more densely spaced rakers of the other morph The ice-free season in the lakes and reservoirs (Amundsen 1988a). This subjective classifi cation lasts from late May or early June, to the end of of morphological differences has, however, pre- October or early November. The lakes and res- viously not been examined and tested quantita- ervoirs in the watercourse are oligotrophic with tively. The two morphs appear to have a diet and some humic impact; the Secchi-depth ranges habitat choice correlated to their gill raker num- from 2 to 6 m. The geology in the region is dom- bers: the sparsely-rakered morph predominantly inated by bedrock, mainly containing gneiss. having a benthic habitat and food choice; and The catchment area is dominated by birch- and the densely-rakered morph mainly being pelagic pinewoods intermingled with stretches of bogs. and planktivorous (Amundsen 1988a, Bøhn & Annual mean air temperature is low (–0.3 °C) Amundsen 1998, 2001, Amundsen et al. 1999). and minimum and maximum monthly mean In the present study, two sympatric morphs temperatures are –13.5 °C and +14.0 °C, respec- of European whitefi sh in the Pasvik watercourse, tively. The precipitation in the area is low, with northern Norway and Russia, have been com- an annual mean of 358 mm. ANN. ZOOL. FENNICI Vol. 41 • Gill raker morphology and feeding ecology of two morphs of whitefi sh 293

Two different lakes in the watercourse were Material and methods investigated, Ruskebukta in the upper part and Skrukkebukta in the lower. Ruskebukta Fish sampling (69°13Ń, 29°14É; 52 m a.s.l.) has an area of 5.3 km2, and a maximum depth of 15 m. Sampling for the present study was carried out Skrukkebukta (69°33Ń, 30°7É; 21 m a.s.l.) in lakes Ruskebukta and Skrukkebukta during has an area of 6.6 km2, and a maximum depth of 12–18 June, 5–11 August and 15–21 September 19 m. Both lakes are dimictic and oligotrophic in 1998. In both lakes, the same patterns were with humic impacts. revealed with respect to gill raker number and Altogether 15 different fi sh species have been morphology and feeding ecology and habitat recorded in the Pasvik watercourse, but the most choice. Data from the two localities have there- commonly occurring species in the lakes are fore been integrated in the result presentation. European whitefi sh (Coregonus lavaretus), Eura- European whitefi sh were sampled both in benthic sian perch (Perca fl uviatilis), northern pike (Esox and pelagic habitats using gillnets with bar mesh lucius), burbot (Lota lota), 9-spined sticklebacks sizes from 10 to 45 mm (knot to knot). Each fi sh (Pungitius pungitius) and brown trout (Salmo was subjectively classifi ed as belonging to the trutta). Vendace (Coregonus albula) has recently sparsely or densely-rakered morph from a visual invaded the Pasvik River system, after being evaluation of the gill raker morphology (see introduced to lake Inari in the 1960s (Amund- Amundsen 1988a). The fi sh were measured for sen et al. 1999, Bøhn & Amundsen 2001). Prior fork length and weight, and gills and stomachs to the invasion of vendace, European whitefi sh were sampled and preserved in 96% ethanol. was the dominant fi sh species in the pelagic, Only fi sh larger than 140 mm were used in the profundal and littoral habitats of the lakes and analyses, since gill raker development appears reservoirs in the Pasvik watercourse (Amundsen to be incomplete and raker number may increase et al. 1999). The European whitefi sh consists of with increasing size in juvenile fi sh (Todd 1998, two different morphs, differentiated by the mor- Sandlund et al. 2002, Yu. S. Reshetnikov pers. phology and number of gill rakers, and referred comm.). In the present study, the gill raker to as densely- and sparsely-rakered whitefi sh number was independent of fi sh size at lengths (Amundsen et al. 1997, 1999), or, with reference > 140 mm (Linear regression; p > 0.05). to their predominant food and habitat choice, as pelagic and benthic whitefi sh (Amundsen 1988a, 1988b, Amundsen et al. 2002), respectively. In Analyses of gill raker number and the pelagic zone, the densely-rakered whitefi sh morphology on average constituted > 95% of the total catches prior to the vendace invasion, whereas sparsely- In the laboratory, the fi rst left branchial arch was rakered whitefi sh, brown trout, Eurasian perch cut off from the rest of the gill, and the number of and northern pike occasionally were caught gill rakers (including rudiments) counted under a (Amundsen et al. 1997, 1999, Bøhn et al. 2002). dissecting microscope. The gill arch was further After the invasion of vendace, the European mounted with the gill rakers perpendicular to the whitefi sh has been partially displaced from the base of the arch, and a digital photographic image pelagic zone where vendace has now become was taken under the microscope. From the digi- the dominant species (Bøhn & Amundsen 1998, talised images were measured: (1) the gill raker 2001, Amundsen et al. 1999, unpubl. data). In the length from tip to base of the longest and the next profundal zone, both densely- and sparsely-rak- two ventral rakers, (2) the distances between these ered whitefi sh frequently occur and dominate the rakers, and (3) the breadth at the base of the long- catches, which also include burbot and Eurasian est and the next ventral raker. The parallel meas- perch. Densely- and sparsely-rakered whitefi sh urements of raker length, distance and breadth are, together with Eurasian perch, also the domi- exhibited strong correlations (Linear regression; nant species in the littoral, but northern pike and p < 0.001, r2 ≥ 0.87, n = 278 for all tests), and burbot are also regularly caught. the average measurements for each individual fi sh 294 Amundsen et al. • ANN. ZOOL. FENNICI Vol. 41

30 2.0 6

20 1.5

4 Raker length (mm)

10 * 1.0 *

Number of observations 0 15 18 21 24 27 30 33 36 39 42 45 2 Number of gill rakers 0.5

Fig. 1. Distribution of gill raker numbers of European Raker distance and breadth (mm) * whitefi sh from the Pasvik watercourse. The two morphs are indicated with different colouring based on the 0.0 0 visual classifi cation of gill raker morphology. SR DRSR DRSR DR Distance Breadth Length have therefore been used in the further analyses. Fig. 2. Comparisons of gill raker distance, breadth and All three measurements increased with increasing length between the sparsely-rakered (SR) and densely- rakered (DR) morphs. Error bars represent standard length of the fi sh (linear regression: p < 0.001), deviation and arrows indicate the total range of the dis- and the measurements were therefore standard- tributions. Signifi cant differences between the morphs ized to a fi sh of 200 mm fork length using the are indicated by asterisks (p < 0.001). equation:

b M200 = M(FL200/FL) , Diet overlap was quantifi ed using the percentage overlap index (Krebs 1999): where M is the original morphological measure, S M200 the standardized measure, FL the original D = min(Aij, Aik), fork length of the fi sh, FL200 the standardized fork length (200 mm), and b the slope in a linear where D is the diet overlap, and Aij and Aik are regression between log M and log FL (Ihssen et the prey abundance of prey i for predator j and al. 1981, Adams et al. 1998). The standardized k, respectively. The overlap is considered to be morphological measurements were independent signifi cant when the index value exceeds 60% of fi sh length (linear regression: p > 0.05). (Wallace 1981).

Stomach analyses Statistical methods

Stomachs were opened and the percentage Liliefors test (Zar 1999) revealed no signifi - degree of total fullness was determined, rang- cant differences from the normal distribution ing from empty (0%) to full (100%). Food items both for number, distance, length and breadth were further identifi ed and their contribution to of the gill rakers. Statistical comparisons of the total fullness estimated. The proportion of means were performed with Student’s t-test. each diet category was expressed in percent as The Dunn-Sidak adjustment was used with prey abundance (Ai): multiple testing (Sokal & Rohlf 1994). Linear regression was used for examining relation- S S ¥ Ai = ( Si / St) 100, ships between the morphometric parameters and fi sh size and gill raker numbers. Mul- where Si is the stomach fullness composed by tivariate discriminant analysis was used to prey i and St the total stomach fullness of all prey determine if the a priori (i.e. by eye) grouping categories (Amundsen et al. 1996). of the two morphs could be distinguished on ANN. ZOOL. FENNICI Vol. 41 • Gill raker morphology and feeding ecology of two morphs of whitefi sh 295

6 abSparsely rakered Sparsely rakered Densely rakered Densely rakered 40 4

30 2

0 20 –2 Discriminant score No. of observations 10 –4

0 –6 –5 –4 –3 –2 –1 0 1 2 34 5 10 20 30 40 50 Discriminant score No. of gill rakers

Fig. 3. — a: Frequency distribution of discriminant function scores of sparsely and densely-rakered whitefi sh from analysis of gill raker distance, length and breadth. The discriminant function analysis was highly signifi cant for the differences between the two morphs (Wilks’ l = 0.2446, p < 0.001). — b: Plot of discriminant function score against gill raker number. the basis of the quantitative morphometric gill distribution of the discriminant function scores raker data. was highly bimodal with only a minor overlap between the two morphs (Fig. 3a), and in total, 98% of the fi sh had been correctly classifi ed with Results respect to morphological differences of the gill rakers (Table 1a). Furthermore, plotting discri- Gill raker number and morphology minant scores against gill raker numbers, the points were separated into two distinct groups The European whitefi sh from the Pasvik water- representing each of the two morphs (Fig. 3b). course exhibited a bimodal distribution of gill Similarly, expanding the discriminant analysis raker numbers (Fig. 1), confi rming the presence also to include raker numbers, 100% of the fi sh of two different morphs in the watercourse. had been categorized correctly from the a priori According to the visual classifi cation of the two visual classifi cation (Table 1b). morphs based on the morphological feature of the gill rakers, the sparsely-rakered morph had from 18 to 30 gillrakers (mean 23.3) and the Table 1. Classifi cation of European whitefi sh morphs by jackknife discriminant analyses of (a) gill raker dis- densely-rakered morph from 28 to 42 rakers tance, length and breadth, and (b) gill raker number in (mean 33.7). addition to the three parameters used in part a. SRM The two morphs exhibited large and sig- = Sparsely-rakered morph, DRM = Densely-rakered nifi cant differences both in length, distance and morph. breadth of the gillrakers (Fig. 2). The densely- Discriminant A priori grouping rakered morph had longer and narrower rakers analysis with a shorter distance between the rakers as grouping SRM DRM Total compared with those of the sparsely-rakered morph. The differences were highly signifi cant a for all three measures (t-test: p ≤ 0.001), but SRM 125 0 125 DRM 6 147 153 were most profound and with the least overlap % Correctly assigned 95 100 98 for length and distance. A discriminant analysis of length, distance and breath of the gill rakers b showed that the a priori visual classifi cation SRM 131 0 131 of the two morphs was strongly in accordance DRM 0 147 147 % Correctly assigned 100 100 100 with the morphometric measurements. The 296 Amundsen et al. • ANN. ZOOL. FENNICI Vol. 41

Habitat distribution (D = 70.7%), but the overlap decreased towards autumn and was 54.5% and 31.0% in August and In the samples of European whitefi sh from the September, respectively (Fig. 6). benthic habitat, the sparsely-rakered morph had Comparisons of the diet within each of the a higher prevalence than the densely-rakered two morphs between fi sh with respectively low morph (Table 2). The densely-rakered morph, and high raker number (18–23 versus 24–30 in contrast, totally dominated the pelagic habi- rakers for the sparsely-rakered morph, and tat, where only a few sparsely-rakered fi sh were 28–34 vs. 35–42 for the densely-rakered morph) caught. There were no signifi cant differences in revealed a high diet similarity within each of the gill raker number and morphological measures morphs (Fig. 6; Diet overlap always > 60%), for the sparsely-rakered morph between individ- suggesting that the number of gill rakers did uals caught in the benthic and pelagic habitats, not have any signifi cant impact on intra-morph respectively (Fig. 4a). For the densely-rakered variations in prey choice. Furthermore, the con- whitefi sh small but signifi cant differences were tribution of zooplankton and pelagic prey to the observed; the number and length of the gill diet did not exhibit any increase with increasing rakers were signifi cantly larger in fi sh caught in gill raker number within the morphs, whereas a benthic habitats, whereas the distance between distinct increment was observed in the transition the rakers was smallest in the pelagic caught between the two morphs, i.e. going from 28 to fi shes (Fig. 4b). 30 gill rakers (Fig. 7). Finally, comparisons were also carried out for number, distance and length of gill rakers within the two morphs between: Diet (1) individuals that had a predominantly pelagic respective to a predominantly benthic diet, and The diet of the sparsely-rakered morph was dom- (2) individuals that had a predominance of small inated by zoobenthos such as molluscs, benthic respective to large prey in their stomachs. None crustaceans and insect larvae throughout all sam- of these comparisons revealed any signifi cant pling periods and exhibited moderate seasonal differences (t-test: p > 0.05), supporting the con- variations (Fig. 5a). Zooplankton was almost clusion that intra-morph diet choice was not cor- absent from the diet in June and August and gave related to gill raker number or morphology. only a minor contribution (9.7%) in September. The densely-rakered whitefi sh had in contrast a diet dominated by pelagic prey types (zooplank- Discussion ton, chironomid pupae and surface insects), but zoobenthos also gave a signifi cant contribution The data presented here on gill raker number to the diet (Fig. 5b). Chironomid pupae were the and morphology confi rms the existence of two most important prey in June, whereas zooplank- discrete European whitefi sh morphs in the Pasvik ton exhibited an increasing importance towards watercourse. The two morphs exhibit distinct autumn and dominated in September, constitut- differences both in gill raker number, distance, ing 73.9% of the diet. In June, the two morphs length and breadth; the densely-rakered morph exhibited a high percentage overlap in the diet having the longest, most closely packed and narrowest rakers. Furthermore, it is shown that Table 2. Relative composition (%) of the sparsely and a visual classifi cation based on gill raker appear- densely-rakered morphs in European whitefi sh catches ance successfully distinguishes between individu- from the benthic and pelagic habitats in the Pasvik als of the two morphs. Thus, the two morphs are watercourse. easily and precisely separated by a brief in situ Sparsely-rakered Densely-rakered n examination of the gills, and laborious gill raker morph morph counting is not a necessity for morphotype identifi cation. The two morphs recorded in the Pasvik Benthic catches 59.2 40.8 439 watercourse closely resemble the European Pelagic catches 3.5 96.5 85 whitefi sh morphs that have been described from ANN. ZOOL. FENNICI Vol. 41 • Gill raker morphology and feeding ecology of two morphs of whitefi sh 297

ab 100 45 6 45 6 a Distance and length (mm) Distance and length (mm) 80 40 5 40 * 5 * 60 35 4 35 4 ns 30 3 30 3 40 ns

No. of gillrakers 25 2 No. of gillrakers 25 2 20 ns 20 1 20 * 1 0

15 0 15 0 100 B PPB P B BBBPPP No.Dist. Length No.Dist. Length b 80 Fig. 4. Comparisons of gill raker numbers, distance Prey abundance (%) and length between individuals caught in benthic (B) 60 and pelagic (P) habitats. — a: Sparsely-rakered morph, — b: Densely-rakered morph. Error bars = S.D., * = 40 signiﬁ cant differences (p < 0.05), ns = no signiﬁ cant differences. 20

0 100 June Aug. Sep. Other Trichoptera larvae

80 Molluscs Chironomid pupae Benthic crustaceans Surface insects Other insects Zooplankton 60 Fig. 5. Comparison of the diet of (a) sparsely-rakered and (b) densely-rakered whiteﬁ sh morphs. 40 Diet overlap (%) 100

20 Pelagic prey Zooplankton 80 0 June Aug. Sep. 60 Intramorph overlap SR Intermorph overlap SR vs. DR

Intramorph overlap DR 40 Fig. 6. Intra-morph diet overlap between individuals with low and high raker numbers, respectively Contribution to diet (%) (sparsely-rakered morph (SR): 18–23 vs. 24–30 rakers; 20 and densely-rakered morph (DR): 28–34 vs. 35–42 rakers), and inter-morph diet overlap between the 0 sparsely and densely-rakered morphs. 18 20 22 24 26 28 30 32 34 36 38 40 42 No. of gillrakers other lake localities in northern Norway (Amund- Fig. 7. The relationship between number of gill rakers sen 1988a, 1988b, Amundsen et al. 2002, 2004). and the contribution of zooplankton and total pelagic Identifi cation of different whitefi sh morphs from prey to the diet. Error bars = standard error. the numerical distribution of gill rakers has commonly been accomplished (e.g. Svärdson 1950, The two European whitefi sh morphs in the 1957, 1979, 1998, Lindsey 1981, 1988, Heinonen Pasvik watercourse are ecologically dissimilar. 1988, Sandlund & Næsje 1989, Bernatchez et al. The densely-rakered morph dominated in the 1996), but this is to our knowledge the fi rst docu- pelagic zone, whereas the sparsely-rakered mentation of distinct inter-morph differences in whitefi sh was most prevalent in the benthic gill raker morphology. habitat. Similarly, the densely-rakered morph 298 Amundsen et al. • ANN. ZOOL. FENNICI Vol. 41 had mostly been feeding on zooplankton, espe- has also been suggested from other studies of cially in the autumn, or other pelagic prey like both whitefi sh (e.g. Heikinheimo-Schmid 1985, surface insects and chironomid pupae in early Amundsen 1988a, Bodaly et al. 1992, Bernatchez summer. For the sparsely-rakered morph, in et al. 1999) and other fi sh species (e.g. Lavin & contrast, typical benthic prey such as molluscs, McPhail 1986). A link between morphology and Trichoptera larvae and benthic crustaceans resource-use specialization (see e.g. Wikramana- dominated the diet. A similar resource-partition- yake 1990, Schluter 1995, Wainwright & Barton ing pattern has frequently been found in other 1995, Wainwright 1996) may, therefore, also systems with sympatric whitefi sh morphs (e.g. apply to gill rakers and zooplankton feeding. Nilsson 1978, Bodaly 1979, Amundsen 1988a, Gill raker number and morphology may thus Bernatchez et al. 1999). The diet and habitat be adaptive traits mediating the resource use of segregation between the two sympatric morphs sympatric morphs through a functional role of in the Pasvik watercourse was however less gill rakers in zooplankton retention, and resource distinct than observed in other lake systems in competition may suggestively be a driving northern Norway (Amundsen 1988a, Amund- force in the phenotypic divergence between sen et al. 2004, and unpubl. data), because the morphs. On the other hand, given a strong link densely-rakered whitefi sh also utilised benthic between gill raker characteristics and the ability habitats to a large extent and fed on typical ben- for zooplankton feeding, a distinct intra-morph thic prey. This may partly be related to the riv- variation in resource use could also be expected erine characteristics of the Pasvik watercourse in relation to the variability in gill raker counts that has a morphometry dominated by fairly and morphology. In the present study, however, narrow and shallow lakes with a short distance despite distinct ecological differences between between pelagic and benthic habitats. However, the two morphs, no effect of gill raker number and the main reason for the benthic component in the morphology was found with respect to the feeding ecology of the densely-rakered morph seems to ecology within each morph. The role of gill be the recent invasion of vendace in the water- rakers in the feeding performance of individual course (Amundsen et al. 1999). The vendace fi sh therefore remains dubious. It should on the invasion has relegated European whitefi sh from other hand be kept in mind that both gill raker the pelagic zone into benthic habitats (Bøhn number and morphometric measurements exhibit & Amundsen 2001), whereas prior to the ven- a strong bimodality, and the discriminant analysis dace invasion the segregation between the two further demonstrated a distinct segregation in European whitefi sh morphs was more profound. morphological traits between the two morphs. However, even in the presence of the zooplank- This might illustrate the presence of adaptive tivore specialist vendace in the pelagic zone, peaks (see e.g. Eldredge 1989) with respect there was a strong segregation between the two to gill raker morphology and the utilization European whitefi sh morphs in autumn, when the of respectively pelagic and benthic resources, food resources are known to be most limited in and potentially also involving disruptive sub-arctic lake systems (Nilsson 1967, Amund- selection and a low fi tness of individuals with sen & Klemetsen 1988, Amundsen 1989). At an intermediate gill morphology (see e.g. Lu & this time, zooplankton totally dominated the Bernatchez 1998). Hence, large differences in the diet of the densely-rakered morph, whereas the ecological performance should mainly be found sparsely-rakered whitefi sh mainly consumed between morphs, and not between individuals zoobenthos. within the morphs. But these aspects are however The resource polymorphism observed in still open to speculation. Thus, in conclusion, the European whitefi sh in the Pasvik watercourse gill raker number and morphology appear to be appears to be related to the phenotypic reliable markers for identifying ecological and differentiation in gill raker number and genetically different European whitefi sh morphs, morphology of the two morphs; the morph with whereas the functional role with respect to the the most numerous and densely-spaced gill feeding performance of individual fi sh is less rakers being planktivorous. A similar relation obvious. ANN. ZOOL. FENNICI Vol. 41 • Gill raker morphology and feeding ecology of two morphs of whitefi sh 299

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