Journal of Fish Biology (2002) 60, 000–000doi:10.1006/jfbi.2002.2179, available online at http://www.idealibrary.com on

Morphological variation between lake- and stream-dwellingrock bass and pumpkinseed populations

J. B* M. G. F†‡*Ontario Ministry of Natural Resources, Sudbury, Ontario, P3G 1E7, Canada and

†Environmental & Resource Studies Program and Department of Biology,Trent University, Peterborough, Ontario, K9J 7B8, Canada

(Received 11 March 2002, Accepted 22 October 2002)

Pumpkinseed Lepomis gibbosus and rock bass Ambloplites rupestris stream populations of bothsexes were significantly different in external morphology from lake populations in a centralOntario, Canada, watershed. The predictions that stream fishes would be more slender-bodied,and have a more anterior placement of lateral fins than lake fishes were generally supported.The prediction that stream fishes would have a more robust caudal peduncle was partiallysupported. The prediction that fin size would be larger in stream fishes was not supported, aslake rock bass generally had longer and wider fins than those from stream sites. The resultssuggest that in some species, smaller fins may be favoured in stream-dwelling individualsbecause the reduction of drag during swimming more than compensates for their reduced powerand propulsion efficiency in a current. Smaller fin size in stream-dwelling centrarchids may berelated to their body shape, or to their low usage of fast-moving water within the streams theyinhabit.

� 2002 Published by Elsevier Science Ltd on behalf of The Fisheries Society of the British Isles.

Key words: adaptation; body shape; centrarchids; fin size; running water.

‡Author to whom correspondence should be addressed. Tel.: +1 705 748 1011; fax +1 705 748 1569;email: mfox@trentu.ca

INTRODUCTION

Variation in the ecological strategies used by different populations of the samefish species, and by different species, are commonly observed at a variety ofgeographical scales. Evidence for these divergent strategies has been demon-strated across large geographical scales, including differences observed across acontinent (Gross, 1979), and among watersheds (Bodaly, 1979; Baltz & Moyle,1981; Hindar & Jonsson, 1982). Different ecological strategies can also beobserved at much smaller scales, such as those observed between populationsfrom different habitat types within a single lake (Hindar & Jonsson, 1982;Robinson et al., 1993), or stream (Beacham et al., 1989; McLaughlin & Grant,1994; McLaughlin & Noakes, 1998). Divergent morphological strategies havebeen documented in a number of fish species, including pumpkinseed Lepomisgibbosus (L.) (Robinson et al., 1993), bluegill Lepomis macrochirus Rafinesque(Ehlinger & Wilson, 1988), various sticklebacks (Gross, 1979; Lavin & McPhail,1993), Arctic charr Salvelinus alpinus (L.) (Hindar & Jonsson, 1982), brookcharr, Salvelinus fontinalis (Mitchill) (McLaughlin & Grant, 1994), andTrinidadian guppies Poecilia reticulata (Peters) (Robinson & Wilson, 1995).

10022–1112/02/000000+00 $35.00/0 � 2002 Published by Elsevier Science Ltd on behalf of The Fisheries Society of the British Isles.

2 . . .

While morphological divergence in many of these species has been docu-mented in either lake or stream environments, few studies have compared themorphology of lake and stream dwelling populations within a species. Streamenvironments offer greater variation in habitat type and structure, a lesspredictable frequency of catastrophic events (Baltz & Moyle, 1982; Ryder &Pesendorfer, 1989), and more arduous hydrodynamic conditions (Baltz &Moyle, 1982; McLaughlin & Grant, 1994) as compared to lakes. Gross (1979)compared the morphology of ninespine sticklebacks Pungitius pungitius (L.) from16 stream, lake and marine sites across Europe, but the comparisons wereprimarily based on meristic counts rather than morphometric measurements.Similarly, a study of tule perch Hysterocarpus traski Gibbons morphology (Baltz& Moyle, 1981) was based primarily on meristic traits (only four morphometricmeasures were used), and significant differences were found among threegeographically isolated watersheds, rather than between stream and lake sitesper se. Lavin & McPhail (1993) compared the morphology of threespinesticklebacks Gasterosteus aculeatus L. inhabiting streams and lakes in BritishColumbia using meristic traits and morphometric measures. Although thestream fish were observed to be smaller and deeper-bodied, many of themorphological differences were related to the feeding ecology of the fish, ratherthan to water flow. One comparative study that was more focused on mor-phology (rather than meristic traits) was of juvenile coho salmon Oncorhynchuskisutch (Walbaum) reared in stream and lake habitats (Swain & Holtby, 1989).This study related the observed morphological differences to factors other thanwater flow; in this case, schooling in the lakes and territoriality in the streams.

In the present study, the morphology of lake and stream populations of twocentrarchid species were compared and related to the presence or absence offlowing water. It was hypothesized that stream populations will have morpho-logical characteristics that produce less drag on the fish and allow for strongerswimming in the current of lotic ecosystems. Four predictions developed fromhydrodynamic theory or from studies of stream populations (mainly ofsalmonids) were tested: (1) stream fishes will be more slender-bodied than theirlake counterparts to reduce drag when swimming into the current (Webb, 1984;McLaughlin & Grant, 1994; McLaughlin & Noakes, 1998); (2) stream fishes willhave longer and wider pelvic, pectoral, anal and dorsal fins to improve theirmanoeuvrability and stability in a current (Beacham et al., 1989; Swain &Holtby, 1989); (3) the caudal peduncle of stream fishes will be more robust(Webb, 1984), with a lesser depth (McLaughlin & Grant, 1994), but a greaterwidth to accommodate a greater muscle mass; (4) the lateral fins of stream fisheswill be more anterior in position than those of lake fishes to improve their abilityto orientate in current, and to assist with strong, steady swimming (Webb, 1984;Swain & Holtby, 1989).

The species used in this study were the pumpkinseed and the rock bassAmbloplites rupestris (Rafinesque). The pumpkinseed is native to east-centralNorth America. While it is often the most abundant species in small lakes,ponds and slow, quiet streams in Ontario (Scott & Crossman, 1973), it can alsobe found in streams with a moderate velocity (0·3–0·5 m s�1; pers. obs.). Therock bass is also a common species, and is also found in stream and lakeenvironments throughout east-central North America (Scott & Crossman, 1973).

3

Pumpkinseed and rock bass are gibbose in body form, in contrast to the fusiformsalmonids, upon which most studies of morphological adaptation to flowingwater are based. The gibbose form is more highly adapted to complexmanoeuvring in lentic environments than to swimming in a current (Webb,1998). The examination of morphological traits of gibbose fishes in flowingwater provides a broader understanding of the adaptation of fishes to hydro-dynamic forces. Furthermore, unlike many of the salmonid species used inmorphological studies, neither the pumpkinseed nor the rock bass are territorial,except for nesting males. Therefore, morphological differences between lake andstream populations of pumpkinseed and rock bass are more likely to be due tothe presence or absence of flow than to social differences that occur in thesedifferent habitats.

MATERIALS AND METHODS

STUDY DESIGN AND STUDY SITESDifferences in fish morphology between streams and lakes were assessed with a paired

stream–lake design, with fishes sampled from a given stream compared to those from anadjacent lake in the same watershed. By comparing fishes in proximal waterbodies withinthe same watershed, the potential effect of geographic distance (Gross, 1979), climate(Lotspeich, 1980) and watershed differences (Baltz & Moyle, 1981, 1982) on aspects of theecology and life history of the fishes were minimized.

The stream–lake pairs used in this study were Indian River (44�14� N; 78�9� W)–RiceLake (44�12� N; 78�8� W) and Eels Creek (44�36� N; 78�5� W)–Stony Lake (44�35� N;78�3� W) (Fig. 1). These waterbodies are part of the Kawartha Lakes region of centralOntario, Canada, located c. 110 km north-east of Toronto. The Kawartha Lakes arepart of the Trent–Severn Waterway, which connects Georgian Bay (Lake Huron) to theBay of Quinte in Lake Ontario.

Rice Lake is a shallow lake with a surface area of c. 100 km2, an average depth of 2·4 mand a maximum depth of 10·0 m. The lake can be considered eutrophic, with a meansummer chlorophyll a concentration of 14·6 �g l�1 and a mean Secchi disk depth of1·9 m (1995–96 data from Mercer et al., 1999). Most parts of the lake are covered bythick macrophyte beds that consist largely of Eurasian watermilfoil Myriophyllumspicatum, curley leaved pondweed Potamogeton crispus, waterweed Elodea canadensis,tapegrass Vallisneria americana and coontail Ceratophyllum demersum (Wile, 1974).Some nearshore areas of the lake are largely devoid of vegetation due to wave action andthe clearing of beaches.

The Indian River flows south into Rice Lake, entering the lake near the village ofKeene. Like many of the rivers entering the Trent–Severn system, flow is controlled bya number of dams along the length of the river. The predominant land use in thewatershed is agriculture, and runoff from the land makes this a relatively productive river.

Stony Lake has a surface area of c. 28·2 km2 and a mean depth of 5·9 m. The lake isdivided into two basins: a shallow, more productive west basin, and a deeper, lessproductive east basin. Stony Lake is less nutrient-enriched than Rice Lake, perhapsbecause the mixed deciduous–coniferous forests of the north shore are largely intact,agriculture is less predominant in the watershed, and the portion of the watershed northof the lake is located on the relatively nutrient poor soils of the Canadian Shield (Wile &Hitchin, 1976). A mean total phosphorus concentration of 10 �g l�1 was recorded forStony Lake in the summer of 1976, while the mean chlorophyll a concentration was3·9 �g l�1 and the Secchi disk depth was 4·3 m (Wile & Hitchin, 1976). More recentlimnological data for Stony Lake are unavailable, but the lake would probably beconsidered as mesotrophic at the time of study. The aquatic vegetation of Stony Lake ismore sparse than in Rice Lake, and the predominant species of submerged vegetation are

4 . . .

0 5 10 km

Rice Lake

Indian River

Eels Creek

Stony Lake

KeeneOtonabee R.

Ouse R.

HastingsPeterborough

Lakefield

Clear LakeDrummer Lake

N

F. 1. Location of waterbodies used in this study. Symbols refer to sites where pumpkinseed only (�),rock bass only (�) or both species (�) were sampled.

pondweeds (Potamogeton spp.), Eurasian watermilfoil, coontail, muskgrass (Chara spp.),waterweed (Elodea spp.), tapegrass and pickerelweed Pontederia cordata (Wile, 1974).

Eels Creek flows in a southerly direction into the east basin of Stony Lake. Thewatershed is located on the granite of the Canadian Shield, and the surroundingvegetation is predominantly composed of coniferous and mixed forests. The flow of EelsCreek is largely unregulated in comparison to other rivers in the Kawarthas, which,combined with the impermeable underlying rocks, causes periodic severe spring floodevents. Such an event occurred in the spring of 1998 prior to field sampling (K. Irwin,pers. comm.). The productivity of Eels Creek is substantially lower than that of IndianRiver because of the differences in watershed characteristics. The soils of the Eels Creekwatershed are thin and relatively infertile, and the extensive forests are largely intact.

5

FISH COLLECTION AND HABITAT ASSESSMENTAll fishes were sampled in the late spring and early summer of 1998, except for the

Indian River pumpkinseed population, most of which were collected in August 1997(Table I). In the spring of 1998, pumpkinseeds were collected from vegetated andunvegetated sites on Rice Lake to compare morphological differences between thesehabitat types. While these samples could be used to make the morphological comparisonwith Indian River, a sufficient number of pumpkinseeds from Eels Creek could not becollected to make the second stream–lake comparison. For this reason, no pumpkinseedswere collected from Stony Lake, and the stream–lake comparison for this species wasrestricted to a single set of paired waterbodies.

The location of all sample sites is indicated on Fig. 1. Sites were selected usingfour criteria: abundance of pumpkinseed and rock bass, suitable water velocity, siteaccessibility and the presence of similar physical habitat in the stream–lake pairs. Allstream fishes were sampled from locations with flow rates >0·25 m s�1 (i.e. riffle and runhabitat types). Although stream-dwelling lepomids and rock bass are most frequentlyfound in pool habitats (Probst et al., 1984; Schlosser, 1987), sampling in flow areasensured that the individuals used in the study were making some use of riffles and runs(presumably for foraging). Sampling sites were between 50 and 100 m of stream channellength or lake shoreline length. Stream sites were separated from their paired lake bya distance of at least 1 km and at least one rapid or waterfall. This would make themigration of centrarchids between habitats unlikely, although some stream juvenilescould have been swept downstream into the lakes. Wire funnel traps (100 cmlength�60 cm diameter, 1 cm mesh) were used to collect all stream samples. Lake fisheswere collected from the nearshore littoral zone using a combination of funnel traps andbeach seines (15·0 m�1·5 m, 6 mm mesh).

Physical habitat parameters from each site are described in Table II. Water velocitywas measured using a hand held Pygmy Gurly current meter at a location immediatelyadjacent to set wire funnel traps. Vegetation density was estimated as per cent cover ina 1 m�1 m quadrant that was randomly selected in each site. Substratum size wasmeasured and classified according to Stanfield et al. (1996).

Captured fishes were sacrificed in carbon dioxide saturated water and stored on ice fortransport back to the laboratory. All fishes were frozen within 8 h of being sacrificed.Fishes were not treated with fixatives or preservatives to avoid distortions that couldaffect their morphological traits.

T I. Fish collection dates and sample sizes

Species Site Dates collected Sample Size Size rangeLS (mm)

Pumpkinseed Indian River 11–27 August 1997 23 14 50·9–102·65–12 June 1998 3 1 72·5–102·0

Rice Lake—unvegetated 25–26 May 1998 34 50 58·1–138·2Rice Lake—vegetated 26–28 May 1998 31 49 61·8–137·7

Rock Bass Indian River 5–12 June 1998 39 40 62·8–189·0Rice Lake 28 May–4 June 1998 44 42 70·8–155·6Eels Creek 15–18 June 1998 43 46 64·5–139·4Stony Lake 18–26 June 1998 42 27 63·8–152·8

MORPHOMETRIC MEASURESMorphology of adult fishes was analysed using a modification of the box truss design

(Strauss & Bookstein, 1982) that is similar to the trusses used in other studies ofcentrarchid morphology (Ehlinger, 1991). The truss design included 14 inter-landmarkdistances based on eight homologous points (Fig. 2). This method offers more complete

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F. 2. Location of nine homologous landmarks used in the morphological analysis of pumpkinseed androck bass (illustrated). Landmarks 1 to 8 are used to form the truss network from which thecentroid was calculated. The measures include: (1-2) predorsal, (1-3) prepelvic, (1-4) preanal, (2-3)body depth, (2-4) anterior dorsal–anterior anal, (2-5) dorsal fin base, (3-4) anterior pelvic–anterioranal, (4-5) anterior anal–posterior dorsal, (4-6) anal fin base, (5-6) depth at anterior of caudalpeduncle, (5-7) length of caudal peduncle (dorsal plane), (6-7) caudal peduncle truss, (6-8) length ofcaudal peduncle (ventral plane), (7-8) depth at posterior of caudal peduncle, (1-9) prepectoral,pectoral fin length (length of 2nd pectoral fin ray), pectoral fin width (length from end of 1st ray toend of last ray), pelvic fin length (length of 1st soft pelvic fin ray, i.e. ray next to pelvic fin spine),pelvic fin width (length from end of pelvic spine to end of last pelvic ray), dorsal fin height (lengthof 1st soft dorsal ray), anal fin length (length of 1st soft anal fin ray—i.e. ray next to anal fin spine),interorbital (width from orbital bone to orbital bone), width at insertion of pectoral fins (i.e. widththrough the body at 9 above), width at anterior of caudal peduncle (i.e. width at 5-6 above), andhorizontal gape (width at the posterior terminus of the maxillary bones).

coverage of the biological form of a fish, especially in terms of depth, than traditionalmorphometric measures (Strauss & Bookstein, 1982; Winans, 1984; Bookstein et al.,1985). As well, trusses are able to compensate for random measurement errors that mayoccur, and errors are more readily identified than with traditional morphometricmeasures (Bookstein et al., 1985). In addition, several traditional morphometricmeasures were used to represent the girth (width) of the fish (five measurements), fin sizes(six measurements), the position of the pectoral fins (one measurement), standard length(LS) and total length (LT). Typically, measures such as girth are not well represented intruss designs.

A modified centroid was calculated from the sum of the squares of all external bodymeasures on the fish (i.e. measurements 1-2, 1-3, 2-5, 3-4, 4-6, 5-7, 6-8 and 7-8 in Fig. 2).The modified centroid was compared to the traditional centroid measure (which includesinterior measures 2-3, 4-5, and 6-7 in Fig. 2) used by other authors (Strauss & Bookstein,1982; Ehlinger, 1991; Robinson et al., 1993, 1996). The two measures were highlycorrelated in all seven populations (r>0·997, P<0·0001 in all cases).

All morphometric measures were taken with Ultra-Cal Mark III digital calipers (Fred.V. Fowler Co., Inc.) on the left side of fishes that were pinned to a white styrofoambackground. The measurements were electronically input into a computer spreadsheetusing the software ExCaliper version 2.00 (Palmer, 1994). When one of the left fins of themid-lateral pairs was damaged, a measurement from the intact right fin was used in itsplace.

Repeatability of the 28 morphometric measurements was determined by measuring asubsample of 15 fish a second time. The difference between the two sets of measurementswas generally <5%, which is within acceptable limits for morphometric analyses (Winans,1984). The greatest difference occurred on the width of the caudal peduncle at theinsertion of the caudal fin in rock bass. This is a small measure for both species

9

(4·1–5·9 mm in rock bass; 4·1–5·4 mm in pumpkinseed). Because of the high per centdifference (8·5% in rock bass), this measure was excluded from further analyses.

DATA ANALYSISPrior to morphometric analysis, the populations were tested for sexual dimorphism, a

phenomenon that has been shown to occur in bluegill (Ehlinger, 1991). Sexualdimorphism was tested for the 25 morphometric variables using ANCOVA, with eachmorphometric measure as the dependent variable and the modified centroid as thecovariate. All variables were first ln-transformed to linearize the relationship with thecovariate. This was necessary because the modified centroid is a sum of the squares ofindividual linear measurements.

Differences between stream and lake fishes (habitat dimorphism) were also assessedwith ANCOVA. Only the 15 measures that related directly to the original hypotheseswere included in this analysis. Bonferroni corrections were employed to provide aguaranteed individual probability in these multiple paired comparisons.

To further examine differences among stream and lake fishes, canonical discrimi-nant function analysis (DFA) was performed on ln-transformed morphometric data.Differences in body size were removed statistically prior to running the DFA by takingthe residuals from the regression of the ln-transformed morphometric variables againstthe ln transformed modified centroid (Ehlinger, 1991; Robinson et al., 1993; Robinson &Wilson, 1995, 1996). The residuals were used in the subsequent DFA for analysing shapeindependent of size. The resultant discriminant function scores were not correlated withthe modified centroid or LS (r<0·01, P>0·90). To determine the separation of thesamples in multivariate space, the Mahalanobis distance (D2) and associated F statisticwere calculated between all pairs of samples.

RESULTS

SEXUAL DIMORPHISMSignificant sexual dimorphism was exhibited in 14 of 26 variables in the

pumpkinseed and in 18 of 26 variables in the rock bass. Even when theprobability values were Bonferroni corrected, three of the 26 variables inthe pumpkinseed still showed sexual dimorphism (length of snout to anal fin,base of anal fin and interorbital width) and six of 26 variables showed sexualdimorphism in the rock bass (length of snout to anal fin, base of anal fin,interorbital width, body depth, depth of the caudal peduncle and distancebetween the pelvic and anal fins). Therefore, the sexes were analysed separatelyfor habitat differences in morphology.

HABITAT DIMORPHISM (UNIVARIATE ANALYSIS)A few significant differences were found between pumpkinseeds sampled from

the vegetated and unvegetated habitats in Rice Lake. In both sexes, themaximum depth of the anal fin was significantly larger, and in females, pectoralfins were significantly wider in the unvegetated sample than in the vegetatedsample. The caudal peduncle was also significantly wider in females from thevegetated site relative to those from unvegetated sites. Because of thesesignificant differences, Rice Lake pumpkinseeds from vegetated and unvegetatedhabitats were not pooled.

Pumpkinseed of both sexes from Indian River had longer pectoral fins thanthose from either unvegetated or vegetated habitats in Rice Lake [supportsPrediction 2; Table III(a)]. In three of the four habitat comparisons, the IndianRiver population had a more robust (i.e. less deep, but wider) caudal peduncle

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12 . . .

(supports Prediction 3), although there were no significant differences betweenthe Indian River and Rice Lake vegetated females. Generally, Rice Lake fishhad a greater body depth (supports Prediction 1), longer anal fins (inconsistentwith Prediction 2) and more posteriorly placed pelvic fins (supports Prediction4); although these trends were not significant for the comparison of Indian Riverand vegetated Rice Lake females. The dorsal fin height of unvegetated RiceLake females was greater than that of Indian River females (inconsistent withPrediction 2). In males, there was also tendency for dorsal fin height of RiceLake pumpkinseeds to be greater than that of Indian River pumpkinseeds, butthe difference was not significant in the comparison between individuals caughtin the river and those caught in vegetated sites in the lake. In the comparisonbetween river males and males caught in unvegetated sites in the lake, theregression slopes of the two groups were significantly different, but there wasalmost no overlap in the best-fit lines over the size range of pumpkinseedssampled from these sites.

Rock bass of both sexes sampled from Rice and Stony Lakes had longerpectoral fins than the fins of stream populations within the same sub-watershed(inconsistent with Prediction 2), although this difference was not significant forthe comparison of Rice Lake and Indian River females [Table III(b)]. Rock bassfrom Rice Lake had longer and wider pelvic fins, longer and wider anal fins, and,for females only, taller dorsal fins than those from the Indian River (inconsistentwith Prediction 2). The pectoral and pelvic fins of males were more posteriorlyplaced in Rice Lake (supports Prediction 4) and Rice Lake males had a greaterbody depth than those of the Indian River (supports Prediction 1). There werefewer significant differences between the rock bass populations of Eels Creek andStony Lake. As noted previously, the pectoral fins of the Stony Lake fish werelonger than those of the Eels Creek fish. Also, the body of stream fish at thepoint of the insertion of the pectoral fins was wider than that of lake fish in allfour comparisons, but the difference was only significant between female rockbass in Eels Creek and Stony Lake.

HABITAT DIMORPHISM (MULTIVARIATE ANALYSIS)Significant body shape differences were detected among populations in

pumpkinseeds of both sexes (females: Wilk’s �=0·152, F50,120=3·8, P<0·001;males: Wilk’s �=0·133, F50,148=5·2, P<0·001). With the DFA, 89% of thefemales and 82% of the males were correctly classified back to their a priorigroups (Table IV).

Indian River females were well separated in multivariate space from thefemales captured in unvegetated and vegetated habitats in Rice Lake (D2=12·0and 12·5 respectively, Fig. 3). Female pumpkinseeds from these lake habitatswere separated from one another to a lesser extent (D2=5·8), but theMahalanobis distance between them was statistically significant (P=0·003).Similar results were found with the pumpkinseed males, with the river fish wellseparated in multivariate space from the lake fish (D2=36·1 and 25·8 inunvegetated and vegetated habitats, respectively), and the lake males from thetwo habitats significantly separated from one another (D2=3·7, P=0·003).

Significant body shape differences were also evident in both sexes of rock bass(females: Wilk’s �=0·0697, F75,374=7·2, P<0·001; males: Wilk’s �=0·0982,

13

F75,344=5·4, P<0·001). The DFA was able to correctly classify 90% of thefemales and 85% of the males in the four waterbodies (Table IV). Females andmales from all of these populations were separated by significant Mahalanobisdistances (Fig. 4), with the Indian River–Rice Lake pair more widely separatedthan the Eels Creek–Stony Lake pair (females: D2=14·2 and 5·3; males: D2=18·7and 4·8, respectively).

T IV. Percentage of pumpkinseed and rock bass correctly classified to their a priorigroups based on the discriminant function analysis

Species Sex A priori groups % correctlyclassified

Samplesize

Pumpkinseed Females Indian River 92·3 26Unvegetated 81·8 33Vegetated 92·9 28Pooled females 88·5 87

Males Indian River 100 14Unvegetated 81·6 49Vegetated 76·3 38Pooled males 82·2 101

Rock Bass Females Indian River 91·4 35Rice Lake 92·7 41Eels Creek 84·6 39Stony Lake 92·1 38Pooled females 90·2 153

Males Indian River 86·1 36Rice Lake 90·0 40Eels Creek 85·4 41Stony Lake 76·9 26Pooled males 85·3 143

DISCUSSION

Morphological differences between stream and lake fish were evident in bothpumpkinseeds and rock bass, though they were not always consistent amongpopulations or species. Stream–lake differences in body form were evident fromthe DFA, despite some overlap among the populations (Figs 3 and 4). Thedegree of overlap on the canonical axes and the few fish at the extreme ends ofthe axes suggest that pumpkinseed and rock bass do not fall into two discretecategories (i.e. a discrete stream morph and a discrete lake morph). A lack ofdiscrete morphological types was also found by Robinson & Wilson (1996) intheir study of trophic dimorphism in pumpkinseeds. They found that pumpkin-seeds formed a unimodal distribution of pelagic and littoral morphs, andthat the general body shape of most individual fish was located at an inter-mediate position on this distribution somewhere between the extreme pelagicand the extreme littoral forms. Robinson et al. (1993, 1996) correctly assigned

14 . . .

4

Axis 1

Axi

s 2

–7

0

–40 7

(b)

5

Axi

s 2

–5

0

–50 5

(a)

F. 3. Distribution of (a) pumpkinseed female and (b) pumpkinseed male canonical scores from thediscriminant function analysis (DFA) with 50% ellipsoids about the centroid of each group plottedon the first two canonical axes. The DFA was performed on the residuals after regressing eachof the morphological variables against the centroid developed from the truss network shown inFig. 2. Pumpkinseeds collected from the Indian River (�), Rice Lake unvegetated habitat (�) andRice Lake vegetated habitat ().

pumpkinseeds to their trophic group 81–89% of the time with DFA. Similarly,82–90% of the present fishes were correctly classified to their a priori group by thesame statistical technique.

Prediction 1, that stream fishes would be more slender-bodied than lake fishes,was generally supported by this study. In both species, the body was less deep inthe stream populations, but in most comparisons, there was no significantdifference between stream and lake populations in width through the body. Thestream–lake differences in body depth are consistent with the findings of othermorphological studies. Fishes inhabiting ecosystems with more arduous hydro-dynamic conditions, such as streams, tend to be more slender-bodied to reducethe drag induced by the current (Webb, 1984; McLaughlin & Grant, 1994; Ryder& Pesendorfer, 1989). Fishes with a more gibbose body shape suffer higher dragpenalties when swimming. Bronmark & Miner (1992) found that crucian carpCarassius carassius (L.) with deeper bodies had a 32% increase in drag at a

15

5

Axis 1

Axi

s 2

–5

0

–50 5

(b)

5

–6

0

–50 6

(a)

F. 4. Distribution of (a) rock bass female and (b) rock bass male canonical axis scores from thediscriminant function analysis (DFA) with 50% ellipsoids about the centroid of each group plottedon the first two canonical axes. The DFA was performed on the residuals after regressing eachof the morphological variables against the centroid developed from the truss network shown inFig. 2. Rock bass collected from the Indian River (�), Rice Lake (�), Eels Creek (�) and StonyLake ().

swimming speed of 10 cm s�1 in small ponds. The added drag penalty woulddecrease the swimming performance of the fish, but minimization of resistancedoes not appear to be important in the type of low-speed manoeuvringperformed by fishes that forage in complex lake environments (Webb, 1998).

With the increased burden of swimming in the stream due to the hydro-dynamic conditions, selection pressures should favour the development ofmechanisms that allow sustained swimming to be maintained. Fishes selected forsustained swimming ability are generally more slender-bodied, rounder incross-section and have a greater proportion of red muscle tissue, whereas more

16 . . .

sedentary lake fishes are generally more gibbose, more laterally compressed (oroblong in cross-section) and have a higher percentage of white muscle tissue(Ryder & Pesendorfer, 1989).

The use of current refuges in the stream (e.g. boulders or submerged logs) mayallow the fish to reduce its need for unsteady swimming and allow it to maintainits position with steady swimming at a slower rate. In laboratory tests, steadyswimming was two to four times less energetically costly than unsteady swim-ming (Webb, 1991), so this type of locomotion should be favoured. Similarly, infield situations, the use of even small-scale current refuges reduced swimmingcosts in brook trout by 10% on average, while foraging ability was not affected(McLaughlin & Noakes, 1998). The use of such habitat structure providedindividuals with an energetic advantage. Pumpkinseed and rock bass inhabitingstreams undoubtedly use such refuges and backwater areas to reduce swimmingcosts. It is likely that stream centrarchids only utilize the faster flowing waterwhen they are feeding on invertebrates caught drifting in the current, and thisfeeding would occur from sheltered locations whenever possible.

Contrary to Prediction 1, stream and lake fishes generally did not show asignificant difference in body width. While this dimension has not beenmeasured in most previous studies of morphological differences in flowing waters(Bodaly, 1979; Baltz & Moyle, 1981; Beacham et al., 1989; McLaughlin & Grant,1994), it was expected that having a reduced body depth might have an influenceon the body width of the fish. According to Ryder & Pesendorfer (1989), morefusiform fishes are typically rounder in cross-section than gibbose fishes. Arounder cross-section, however, can be generated by a reduction in body depth,without necessarily increasing body width.

Prediction 2, that stream fishes will have longer and wider fins, was notsupported. It was expected that stream centrarchids would have larger pairedlateral fins for holding position and for orientating themselves in the current, andthat larger dorsal and anal fins would be used for stability in flowing water.While stream pumpkinseed pectoral fins were longer than those of lake pump-kinseed, in most other cases the length and width of the fins was greater in thelake fish. Anal and dorsal fin heights were greater in lake fishes of both species,and all fin sizes (pectoral, pelvic, anal and dorsal) were larger in the lake dwellingrock bass.

According to Webb (1984), fishes adapted to prolonged steady swimmingshould have a larger fin area relative to body size. Chinook salmonOncorhynchus tshawytscha (Walbaum) inhabiting areas with faster currentvelocities did have larger lateral fins (Beacham et al., 1989). Brook charr usetheir fins to maintain an upstream orientation in the faster, often turbulent flow(McLaughlin & Noakes, 1998). Larger fins should move a greater volume ofwater and may reduce energy expenditures from additional fin beats. Inaddition, larger fins may be used by stream fishes in conjunction with steadyswimming, a propulsive mechanism observed for stream fishes in the field (Webb,1991; McLaughlin & Noakes, 1998). The dorsal and anal fins of stream-dwellingcoho salmon are larger than those of lake-dwelling coho, although this may bedue to increased territorial behaviour in the stream (Swain & Holtby, 1989).Territorial fishes use larger dorsal and anal fin margins to create the illusion ofincreased body size.

17

Larger fins can create a greater drag potential in flowing water. The use oflateral fins increases the surface area of the fish when it is viewed head-on. If afish is oriented in an upstream direction to forage on aquatic insects driftingin the current, the increase in surface area exposed to the current would resultin a greater drag coefficient. This would reduce the distance covered pertail beat, or alternatively, it would mean that a fish would have to increase itstail beat frequency to hold position in the current (Webb, 1991; McLaughlin& Noakes, 1998). Similarly, an increase in the surface area of the dorsal andanal fins would be sub-optimal when a fish is not oriented precisely in anupstream direction, as the fins would catch the current as a sail catches the wind.Optimal fin size may be a trade-off between the use of large fins for orientationand maintaining position, against the extra drag that is created by these largerfins. The optimal solution to this trade-off may vary in species that differ inoverall body shape, or which use the fast current of a stream to a differentdegree.

Prediction 3, that stream fishes will have a more robust caudal peduncle thanlake fishes (reduced depth but greater width), was partly supported. In pump-kinseed, stream fish had a more shallow depth at the anterior end of the caudalpeduncle (although this difference was only significant for males) and stream fishhad a wider caudal peduncle (although this result was only significant forfemales). There were few significant differences, however, in the width or depthof the caudal peduncle in rock bass.

For prolonged, constant speed swimming, stream fishes require a caudalpeduncle that is muscular, yet capable of large amplitude beats to increase theforward thrust power of the fishes (Webb, 1984). To allow for faster, morepowerful swimming, stream fishes also need to be able to make these largeamplitude caudal peduncle displacements at high frequencies. A shallow caudalpeduncle with a large muscle mass (i.e. increased width) allows the fish tomaximize thrust while reducing energy lost in recoil (Webb, 1984, 1998;McLaughlin & Noakes, 1998). McLaughlin & Grant (1994) found that juvenilebrook charr collected from sites with faster current had a shallower caudalpeduncle than those collected from sites in the same watershed with a slowercurrent velocity. Although the width of the caudal peduncle was not measuredby McLaughlin & Grant (1994), it would probably be greater in stream fish tocontain the increased muscle mass necessary for prolonged steady swimming(Webb, 1984).

It is curious that in the present study, the depth of the anterior caudal pedunclewas smaller in stream fishes, whereas the depth at the posterior of the caudalpeduncle was greater than that of lake fishes. While it was expected that theentire caudal peduncle would be less deep in stream fishes, energy losses in recoilwould be reduced as long as the lateral surface area of the caudal peduncle issmaller in a relative sense. The posterior depth of the caudal peduncle wasmeasured from the dorsal insertion of the caudal fin to the ventral insertion ofthe caudal fin. If the base of the caudal fin was larger in stream fishes, then thedepth of the caudal peduncle at the insertion of the fin would also be greater.Although caudal fin dimensions were not measured in the current study,McLaughlin & Grant (1994) report that brook charr captured from fasterflowing water had larger caudal fins.

18 . . .

Prediction 4, that the lateral fins of stream fishes would have a more anteriorplacement than in lake fishes was partly supported. In the pumpkinseed, thepelvic fins of both sexes were more anterior in stream populations, but thepectoral fins were not. In the rock bass, both the pelvic and the pectoral finswere generally more anterior in stream populations, but this result was onlysignificant for males in one of the two habitat comparisons.

Although published data on the placement of the lateral fins are sparse, thereis some evidence that the pectoral and pelvic fins of stream fishes are located ina more anterior position than those of lake fishes (Swain & Holtby, 1989). Themore anterior insertion of the lateral fins allows for additional manoeuvrabilityin fishes (Webb, 1984), an adaptation that is necessary for stream fishes that mustorient and maintain their position in flowing water.

It is evident from the above discussion that the observed morphologicaldifferences in the current study were not always consistent between the twospecies studied, nor were the observed results always consistent with otherstudies. Although the results for many of the variables were consistent betweenthe two species (i.e. if for a certain trait, stream pumpkinseed were larger thanlake pumpkinseed, then stream rock bass would also be larger than lake rockbass), there were a few exceptions. For example, the length of the pectoral finsand the length of the dorsal fin base were greater in stream pumpkinseed and inlake rock bass.

There are a number of examples of conflicting results in the literature. Forexample, Bodaly (1979) found two morphological forms of lake whitefishCoregonus clupeaformis (L.), a benthic morph and a pelagic morph, in fivedifferent Yukon lakes. Although the two morphs could be consistently distin-guished within a single lake, the sets of differences that distinguished them werenot consistent among the five lakes. Bodaly (1979) suggested that the among-lakedifferences might have occurred because the fish were not only adapting to thetwo niches available in each lake, but also to environmental differences that existamong the lakes. Similar mechanisms may be influencing the morphology of thefishes in the present study.

One question that cannot be answered from the study is whether the observedmorphological differences between stream and lake populations are the result ofgenetic differentiation or phenotypic plasticity. Evidence from stream–lakestudies of other species and from studies of pumpkinseed differentiation in lakessuggest that both mechanisms may contribute to stream–lake differentiation inpumpkinseed morphology. In threespine stickleback evidence from rearingstudies indicates that the morphological traits of stream and lake morphs areinherited (Lavin & McPhail, 1993), and subsequent mtDNA analysis of popula-tions from a stream and an adjacent lake indicate that the gene pools of the twoare distinct (Thompson et al., 1997). Similarly, juvenile coho salmon from a lakeand an adjoining stream showed differences in body depth, fin placement andcolouration even after 2 months of rearing in a common environment (Swain &Holtby, 1989). In contrast, the variation in juvenile brook charr body depth,caudal peduncle depth and caudal fin height between individuals inhabiting slowand fast flowing sites within a stream appears to be due mainly to phenotypicplasticity, as the morphological differences apparent in one year did not persistseveral years later (Imre et al., 2001). Furthermore, in a rearing experiment with

19

littoral and pelagic forms of one of the test species, Robinson & Wilson(1996) estimated that 53% of the observed dimorphism in pumpkinseeds wasattributable to phenotypic plasticity, whereas genetic differences accounted foronly 14% of the variation. A combination of genetic analysis and controlledrearing experiments would be required to identify the relative importance ofphenotypic plasticity and divergent selection.

We wish to thank M. Allen, S. Bobrowicz, S. Bowman, K. Brodribb, K. Caldwell,M. Duffy, L. Gatzke, K. Ovens, A. Todd, V. Vaughan and M. Wilson for their assistancein the field. As well, thanks are extended to B. Robinson and K. Somers for their helpfuladvice during this study, and to N. Mandrak and T. Whillans for their advice andcomments on an earlier version of the manuscript. Financial support for this study wasprovided by a Natural Sciences and Engineering Research Council of Canada grant toM. Fox and an Ontario Graduate Scholarship to J. Brinsmead.

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