Journal of Fish Biology (2002) 60, 000–000
doi:10.1006/jfbi.2002.2179, available online at http://www.idealibrary.com on
Morphological variation between lake- and stream-dwelling
rock 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 both
sexes were significantly different in external morphology from lake populations in a central
Ontario, 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 partially
supported. The prediction that fin size would be larger in stream fishes was not supported, as
lake rock bass generally had longer and wider fins than those from stream sites. The results
suggest that in some species, smaller fins may be favoured in stream-dwelling individuals
because the reduction of drag during swimming more than compensates for their reduced power
and propulsion efficiency in a current. Smaller fin size in stream-dwelling centrarchids may be
related to their body shape, or to their low usage of fast-moving water within the streams they
inhabit.
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.
INTRODUCTION
Variation in the ecological strategies used by different populations of the same
fish species, and by different species, are commonly observed at a variety of
geographical scales. Evidence for these divergent strategies has been demonstrated across large geographical scales, including differences observed across a
continent (Gross, 1979), and among watersheds (Bodaly, 1979; Baltz & Moyle,
1981; Hindar & Jonsson, 1982). Different ecological strategies can also be
observed at much smaller scales, such as those observed between populations
from 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 have
been documented in a number of fish species, including pumpkinseed Lepomis
gibbosus (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), brook
charr, Salvelinus fontinalis (Mitchill) (McLaughlin & Grant, 1994), and
Trinidadian guppies Poecilia reticulata (Peters) (Robinson & Wilson, 1995).
‡Author to whom correspondence should be addressed. Tel.: +1 705 748 1011; fax +1 705 748 1569;
email: [email protected]
1
0022–1112/02/000000+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 documented in either lake or stream environments, few studies have compared the
morphology of lake and stream dwelling populations within a species. Stream
environments offer greater variation in habitat type and structure, a less
predictable 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.) from
16 stream, lake and marine sites across Europe, but the comparisons were
primarily 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 morphometric
measures were used), and significant differences were found among three
geographically isolated watersheds, rather than between stream and lake sites
per se. Lavin & McPhail (1993) compared the morphology of threespine
sticklebacks Gasterosteus aculeatus L. inhabiting streams and lakes in British
Columbia using meristic traits and morphometric measures. Although the
stream fish were observed to be smaller and deeper-bodied, many of the
morphological differences were related to the feeding ecology of the fish, rather
than to water flow. One comparative study that was more focused on morphology (rather than meristic traits) was of juvenile coho salmon Oncorhynchus
kisutch (Walbaum) reared in stream and lake habitats (Swain & Holtby, 1989).
This study related the observed morphological differences to factors other than
water 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 two
centrarchid species were compared and related to the presence or absence of
flowing water. It was hypothesized that stream populations will have morphological characteristics that produce less drag on the fish and allow for stronger
swimming in the current of lotic ecosystems. Four predictions developed from
hydrodynamic theory or from studies of stream populations (mainly of
salmonids) were tested: (1) stream fishes will be more slender-bodied than their
lake counterparts to reduce drag when swimming into the current (Webb, 1984;
McLaughlin & Grant, 1994; McLaughlin & Noakes, 1998); (2) stream fishes will
have longer and wider pelvic, pectoral, anal and dorsal fins to improve their
manoeuvrability 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 greater
width to accommodate a greater muscle mass; (4) the lateral fins of stream fishes
will be more anterior in position than those of lake fishes to improve their ability
to 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 bass
Ambloplites rupestris (Rafinesque). The pumpkinseed is native to east-central
North America. While it is often the most abundant species in small lakes,
ponds and slow, quiet streams in Ontario (Scott & Crossman, 1973), it can also
be found in streams with a moderate velocity (0·3–0·5 m s 1; pers. obs.). The
rock bass is also a common species, and is also found in stream and lake
environments throughout east-central North America (Scott & Crossman, 1973).
    
3
Pumpkinseed and rock bass are gibbose in body form, in contrast to the fusiform
salmonids, upon which most studies of morphological adaptation to flowing
water are based. The gibbose form is more highly adapted to complex
manoeuvring in lentic environments than to swimming in a current (Webb,
1998). The examination of morphological traits of gibbose fishes in flowing
water provides a broader understanding of the adaptation of fishes to hydrodynamic forces. Furthermore, unlike many of the salmonid species used in
morphological studies, neither the pumpkinseed nor the rock bass are territorial,
except for nesting males. Therefore, morphological differences between lake and
stream populations of pumpkinseed and rock bass are more likely to be due to
the presence or absence of flow than to social differences that occur in these
different habitats.
MATERIALS AND METHODS
STUDY DESIGN AND STUDY SITES
Differences 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 an
adjacent lake in the same watershed. By comparing fishes in proximal waterbodies within
the same watershed, the potential effect of geographic distance (Gross, 1979), climate
(Lotspeich, 1980) and watershed differences (Baltz & Moyle, 1981, 1982) on aspects of the
ecology and life history of the fishes were minimized.
The stream–lake pairs used in this study were Indian River (4414 N; 789 W)–Rice
Lake (4412 N; 788 W) and Eels Creek (4436 N; 785 W)–Stony Lake (4435 N;
783 W) (Fig. 1). These waterbodies are part of the Kawartha Lakes region of central
Ontario, Canada, located c. 110 km north-east of Toronto. The Kawartha Lakes are
part of the Trent–Severn Waterway, which connects Georgian Bay (Lake Huron) to the
Bay 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 m
and a maximum depth of 10·0 m. The lake can be considered eutrophic, with a mean
summer chlorophyll a concentration of 14·6 g l 1 and a mean Secchi disk depth of
1·9 m (1995–96 data from Mercer et al., 1999). Most parts of the lake are covered by
thick macrophyte beds that consist largely of Eurasian watermilfoil Myriophyllum
spicatum, 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 and
the clearing of beaches.
The Indian River flows south into Rice Lake, entering the lake near the village of
Keene. Like many of the rivers entering the Trent–Severn system, flow is controlled by
a number of dams along the length of the river. The predominant land use in the
watershed 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 is
divided into two basins: a shallow, more productive west basin, and a deeper, less
productive east basin. Stony Lake is less nutrient-enriched than Rice Lake, perhaps
because 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 north
of 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 for
Stony Lake in the summer of 1976, while the mean chlorophyll a concentration was
3·9 g l 1 and the Secchi disk depth was 4·3 m (Wile & Hitchin, 1976). More recent
limnological data for Stony Lake are unavailable, but the lake would probably be
considered as mesotrophic at the time of study. The aquatic vegetation of Stony Lake is
more sparse than in Rice Lake, and the predominant species of submerged vegetation are
4
.   . . 
Eels Creek
N
Stony Lake
Drummer Lake
Clear Lake
Lakefield
Indian River
Ouse R.
Peterborough
Hastings
Keene
Otonabee R.
Rice Lake
0
5
10 km
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. The
watershed is located on the granite of the Canadian Shield, and the surrounding
vegetation is predominantly composed of coniferous and mixed forests. The flow of Eels
Creek is largely unregulated in comparison to other rivers in the Kawarthas, which,
combined with the impermeable underlying rocks, causes periodic severe spring flood
events. 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 Indian
River because of the differences in watershed characteristics. The soils of the Eels Creek
watershed are thin and relatively infertile, and the extensive forests are largely intact.
    
5
T I. Fish collection dates and sample sizes
Species
Site
Dates collected
Pumpkinseed Indian River
Rock Bass
Sample Size
Size range
LS (mm)
11–27 August 1997
5–12 June 1998
Rice Lake—unvegetated 25–26 May 1998
Rice Lake—vegetated
26–28 May 1998
23
3
34
31
14
1
50
49
50·9–102·6
72·5–102·0
58·1–138·2
61·8–137·7
Indian River
Rice Lake
Eels Creek
Stony Lake
39
44
43
42
40
42
46
27
62·8–189·0
70·8–155·6
64·5–139·4
63·8–152·8
5–12 June 1998
28 May–4 June 1998
15–18 June 1998
18–26 June 1998
FISH COLLECTION AND HABITAT ASSESSMENT
All 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 and
unvegetated sites on Rice Lake to compare morphological differences between these
habitat types. While these samples could be used to make the morphological comparison
with Indian River, a sufficient number of pumpkinseeds from Eels Creek could not be
collected to make the second stream–lake comparison. For this reason, no pumpkinseeds
were collected from Stony Lake, and the stream–lake comparison for this species was
restricted to a single set of paired waterbodies.
The location of all sample sites is indicated on Fig. 1. Sites were selected using
four criteria: abundance of pumpkinseed and rock bass, suitable water velocity, site
accessibility and the presence of similar physical habitat in the stream–lake pairs. All
stream fishes were sampled from locations with flow rates >0·25 m s 1 (i.e. riffle and run
habitat types). Although stream-dwelling lepomids and rock bass are most frequently
found in pool habitats (Probst et al., 1984; Schlosser, 1987), sampling in flow areas
ensured 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 channel
length or lake shoreline length. Stream sites were separated from their paired lake by
a distance of at least 1 km and at least one rapid or waterfall. This would make the
migration of centrarchids between habitats unlikely, although some stream juveniles
could have been swept downstream into the lakes. Wire funnel traps (100 cm
length60 cm diameter, 1 cm mesh) were used to collect all stream samples. Lake fishes
were collected from the nearshore littoral zone using a combination of funnel traps and
beach seines (15·0 m1·5 m, 6 mm mesh).
Physical habitat parameters from each site are described in Table II. Water velocity
was measured using a hand held Pygmy Gurly current meter at a location immediately
adjacent to set wire funnel traps. Vegetation density was estimated as per cent cover in
a 1 m1 m quadrant that was randomly selected in each site. Substratum size was
measured and classified according to Stanfield et al. (1996).
Captured fishes were sacrificed in carbon dioxide saturated water and stored on ice for
transport 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 could
affect their morphological traits.
MORPHOMETRIC MEASURES
Morphology 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 of
centrarchid morphology (Ehlinger, 1991). The truss design included 14 inter-landmark
distances based on eight homologous points (Fig. 2). This method offers more complete
Elevation (m)
Mean July air temperature ( C)a
(climate station)
Sample size (n)
Water velocity (ms 1)
Water depth (m)
Water temperature ( C)
Vegetation density
Dominant substratum
9
0·310·02
0·880·08
19·60·2
<10%
Gravel/small cobble
(10–100 mm)
20·3
(Trent University,
Peterborough)
200–210
Indian River
6
0
1·160·11
20·00·0
<10%
Gravel/small cobble
(10–100 mm)
20·5
(Peterborough Sewage
Treatment Plant)
190
Rice Lake—unvegetated
20·5
(Peterborough Sewage
Treatment Plant)
190
3
0
0·950·10
20·00·0
50–70%
Sand (0·5–1·0 mm)
Rice Lake—vegetated
T II. Summary of aquatic habitat parameters for the systems studied. All data were collected in May and June 1998 (see Table I for
dates). Data for water velocity, water depth and water temperature are means.. (n=3). (a) Data for sites where pumpkinseed were
collected. (b) Data for sites where rock bass were collected
(a)
a
Taken from Environment Canada, 1993.
Elevation (m)
Mean July air temperature ( C)a
(climate station)
Sample size (n)
Water velocity (ms 1)
Water depth (m)
Water temperature ( C)
Vegetation cover (%)
Dominant substratum
200–210
6
0·290·02
0·980·10
19·50·2
<10%
Gravel/small cobble
(10–100 mm)
20·3
(Trent University)
Indian River
6
0
1·410·22
20·50·2
<10%
Gravel/small cobble
(10–100 mm)
20·5
(Ptbo. Sewage
Treatment Plant)
190
Rice Lake
T II. (b)
245–260
19·5
(Apsley)
6
0·360·05
1·050·13
19·00·4
<10%
Cobble (100–300 mm)
Eels Creek
240
6
0
1·170·26
19·30·3
<10%
Gravel/small cobble
(10–100 mm)
19·6
(Apsley)
Stony Lake
8
.   . . 
F. 2. Location of nine homologous landmarks used in the morphological analysis of pumpkinseed and
rock bass (illustrated). Landmarks 1 to 8 are used to form the truss network from which the
centroid 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–anterior
anal, (4-5) anterior anal–posterior dorsal, (4-6) anal fin base, (5-6) depth at anterior of caudal
peduncle, (5-7) length of caudal peduncle (dorsal plane), (6-7) caudal peduncle truss, (6-8) length of
caudal 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 to
end 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 (length
of 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. width
through the body at 9 above), width at anterior of caudal peduncle (i.e. width at 5-6 above), and
horizontal gape (width at the posterior terminus of the maxillary bones).
coverage of the biological form of a fish, especially in terms of depth, than traditional
morphometric measures (Strauss & Bookstein, 1982; Winans, 1984; Bookstein et al.,
1985). As well, trusses are able to compensate for random measurement errors that may
occur, and errors are more readily identified than with traditional morphometric
measures (Bookstein et al., 1985). In addition, several traditional morphometric
measures 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 in
truss designs.
A modified centroid was calculated from the sum of the squares of all external body
measures 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 includes
interior 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 highly
correlated 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 styrofoam
background. The measurements were electronically input into a computer spreadsheet
using the software ExCaliper version 2.00 (Palmer, 1994). When one of the left fins of the
mid-lateral pairs was damaged, a measurement from the intact right fin was used in its
place.
Repeatability of the 28 morphometric measurements was determined by measuring a
subsample of 15 fish a second time. The difference between the two sets of measurements
was generally <5%, which is within acceptable limits for morphometric analyses (Winans,
1984). The greatest difference occurred on the width of the caudal peduncle at the
insertion 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 cent
difference (8·5% in rock bass), this measure was excluded from further analyses.
DATA ANALYSIS
Prior to morphometric analysis, the populations were tested for sexual dimorphism, a
phenomenon that has been shown to occur in bluegill (Ehlinger, 1991). Sexual
dimorphism was tested for the 25 morphometric variables using ANCOVA, with each
morphometric measure as the dependent variable and the modified centroid as the
covariate. All variables were first ln-transformed to linearize the relationship with the
covariate. This was necessary because the modified centroid is a sum of the squares of
individual linear measurements.
Differences between stream and lake fishes (habitat dimorphism) were also assessed
with ANCOVA. Only the 15 measures that related directly to the original hypotheses
were included in this analysis. Bonferroni corrections were employed to provide a
guaranteed individual probability in these multiple paired comparisons.
To further examine differences among stream and lake fishes, canonical discriminant function analysis (DFA) was performed on ln-transformed morphometric data.
Differences in body size were removed statistically prior to running the DFA by taking
the residuals from the regression of the ln-transformed morphometric variables against
the ln transformed modified centroid (Ehlinger, 1991; Robinson et al., 1993; Robinson &
Wilson, 1995, 1996). The residuals were used in the subsequent DFA for analysing shape
independent of size. The resultant discriminant function scores were not correlated with
the modified centroid or LS (r<0·01, P>0·90). To determine the separation of the
samples in multivariate space, the Mahalanobis distance (D2) and associated F statistic
were calculated between all pairs of samples.
RESULTS
SEXUAL DIMORPHISM
Significant sexual dimorphism was exhibited in 14 of 26 variables in the
pumpkinseed and in 18 of 26 variables in the rock bass. Even when the
probability values were Bonferroni corrected, three of the 26 variables in
the pumpkinseed still showed sexual dimorphism (length of snout to anal fin,
base of anal fin and interorbital width) and six of 26 variables showed sexual
dimorphism in the rock bass (length of snout to anal fin, base of anal fin,
interorbital width, body depth, depth of the caudal peduncle and distance
between the pelvic and anal fins). Therefore, the sexes were analysed separately
for 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, the
maximum depth of the anal fin was significantly larger, and in females, pectoral
fins were significantly wider in the unvegetated sample than in the vegetated
sample. The caudal peduncle was also significantly wider in females from the
vegetated site relative to those from unvegetated sites. Because of these
significant differences, Rice Lake pumpkinseeds from vegetated and unvegetated
habitats were not pooled.
Pumpkinseed of both sexes from Indian River had longer pectoral fins than
those from either unvegetated or vegetated habitats in Rice Lake [supports
Prediction 2; Table III(a)]. In three of the four habitat comparisons, the Indian
River population had a more robust (i.e. less deep, but wider) caudal peduncle
Body depth
Width at pectoral insertion
Pectoral fin length
Pectoral fin width
Pelvic fin length
Pelvic fin width
Anal fin length
Dorsal fin height
Dorsal fin base
Anal fin base
Depth at anterior peduncle
Depth at posterior peduncle
Width at anterior peduncle
Prepelvic
Prepectoral
(a)
Measure
1
1
2
2
2
2
2
2
2
2
3
3
3
4
4
Prediction
38·47*
14·69
27·19*
14·72
17·78
11·81
14·40*
14·88*
40·00*
17·71
16·15
11·18*
7·78*
34·81*
27·72
Indian
River
40·21*
14·22
25·69*
14·85
17·53
11·99
15·85*
16·09*
38·86*
17·31
16·59
10·53*
7·01*
36·20*
27·74
RL Unveg.
36·82*
N/A
26·76*
14·00
16·89
11·06
14·04*
N/A
38·21
17·25
15·39
10·53*
7·21
33·58
27·06
Indian
River
37·68*
N/A
24·31*
14·03
17·12
11·48
15·43*
N/A
38·86
16·86
15·91
10·12*
6·95
34·09
26·60
RL Unveg.
Indian River v. Rice Lake unvegetated
Females
Males
36·23
N/A
26·15*
12·72
16·05
10·75
13·87
14·03
37·64
17·00
N/A
9·90
6·92
33·15
25·36
Indian
River
36·93
N/A
24·31*
12·71
15·60
10·59
14·57
14·72
37·11
16·48
N/A
9·57
6·73
33·62
25·76
RL Veg.
34·19*
13·69
24·48*
13·24
16·05
10·92
13·16*
13·50
N/A
15·93
14·45*
9·57
7·10
31·69*
25·36
Indian
River
35·52*
13·30
23·20*
12·71
15·60
11·28
13·85*
14·07
N/A
15·44
15·27*
9·57
6·93
32·56*
25·64
RL Veg.
Indian River v. Rice Lake vegetated
Females
Males
T III. Results of univariate tests (ANCOVA) for habitat dimorphism in (a) pumpkinseed, and (b) rock bass. Values are
back-transformed adjusted means (mm). *Significant differences between the two populations being compared (Bonferroni corrected,
P<0·0033). N/A, the ANCOVA assumption of parallel slopes was violated
Body depth
Width at pectoral insertion
Pectoral fin length
Pectoral fin width
Pelvic fin length
Pelvic fin width
Anal fin length
Dorsal fin height
Dorsal fin base
Anal fin base
Depth at anterior peduncle
Depth at posterior peduncle
Width at anterior peduncle
Prepelvic
Prepectoral
Measure
1
1
2
2
2
2
2
2
2
2
3
3
3
4
4
Prediction
46·02*
19·26
24·63
19·28
21·46*
14·40*
21·67*
22·24*
48·18
29·52*
N/A
14·27*
8·54
44·52
N/A
Indian
River
48·13*
19·11
25·15
19·73
22·51*
15·44*
23·31*
23·31*
48·67
31·06*
N/A
14·91*
8·28
44·84
N/A
Rice Lake
48·33*
20·23
25·23*
20·21
22·44*
15·30*
22·94*
23·78
50·50*
31·41*
21·71
15·36
8·82
45·88*
41·02*
Indian
River
50·20*
19·89
25·89*
20·55
23·55*
16·15*
24·19*
24·24
51·16*
32·69*
21·54
15·43
8·67
46·71*
42·27*
Rice Lake
Indian River v. Rice Lake
Females
Males
T III. (b)
42·01
17·44*
22·38*
18·19
20·07
13·79
19·81
20·05
44·70
27·55
17·92
N/A
7·71
40·89
37·04
Eels Creek
41·35
16·79*
23·57*
18·69
20·15
13·61
20·21
20·15
45·06
27·39
17·60
N/A
7·64
41·43
37·26
Stony Lake
40·81
16·76
21·89*
17·83
19·63
13·78
19·71
19·73
43·60
26·98
17·50
13·07
7·43
39·88
36·42
Eels Creek
40·77
16·49
23·15*
18·12
19·87
13·33
20·31
20·01
43·86
27·17
17·31
13·25
7·46
40·13
36·53
Stony Lake
Eels Creek v. Stony Lake
Females
Males
12
.   . . 
(supports Prediction 3), although there were no significant differences between
the Indian River and Rice Lake vegetated females. Generally, Rice Lake fish
had a greater body depth (supports Prediction 1), longer anal fins (inconsistent
with Prediction 2) and more posteriorly placed pelvic fins (supports Prediction
4); although these trends were not significant for the comparison of Indian River
and vegetated Rice Lake females. The dorsal fin height of unvegetated Rice
Lake females was greater than that of Indian River females (inconsistent with
Prediction 2). In males, there was also tendency for dorsal fin height of Rice
Lake pumpkinseeds to be greater than that of Indian River pumpkinseeds, but
the difference was not significant in the comparison between individuals caught
in the river and those caught in vegetated sites in the lake. In the comparison
between river males and males caught in unvegetated sites in the lake, the
regression slopes of the two groups were significantly different, but there was
almost no overlap in the best-fit lines over the size range of pumpkinseeds
sampled from these sites.
Rock bass of both sexes sampled from Rice and Stony Lakes had longer
pectoral fins than the fins of stream populations within the same sub-watershed
(inconsistent with Prediction 2), although this difference was not significant for
the comparison of Rice Lake and Indian River females [Table III(b)]. Rock bass
from 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 (inconsistent
with Prediction 2). The pectoral and pelvic fins of males were more posteriorly
placed in Rice Lake (supports Prediction 4) and Rice Lake males had a greater
body depth than those of the Indian River (supports Prediction 1). There were
fewer significant differences between the rock bass populations of Eels Creek and
Stony Lake. As noted previously, the pectoral fins of the Stony Lake fish were
longer than those of the Eels Creek fish. Also, the body of stream fish at the
point of the insertion of the pectoral fins was wider than that of lake fish in all
four comparisons, but the difference was only significant between female rock
bass 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 the
females and 82% of the males were correctly classified back to their a priori
groups (Table IV).
Indian River females were well separated in multivariate space from the
females captured in unvegetated and vegetated habitats in Rice Lake (D2 =12·0
and 12·5 respectively, Fig. 3). Female pumpkinseeds from these lake habitats
were separated from one another to a lesser extent (D2 =5·8), but the
Mahalanobis distance between them was statistically significant (P=0·003).
Similar results were found with the pumpkinseed males, with the river fish well
separated in multivariate space from the lake fish (D2 =36·1 and 25·8 in
unvegetated and vegetated habitats, respectively), and the lake males from the
two 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
T IV. Percentage of pumpkinseed and rock bass correctly classified to their a priori
groups based on the discriminant function analysis
Species
Pumpkinseed
Sex
A priori groups
% correctly
classified
Sample
size
Females
Indian River
Unvegetated
Vegetated
Pooled females
Indian River
Unvegetated
Vegetated
Pooled males
92·3
81·8
92·9
88·5
100
81·6
76·3
82·2
26
33
28
87
14
49
38
101
Indian River
Rice Lake
Eels Creek
Stony Lake
Pooled females
Indian River
Rice Lake
Eels Creek
Stony Lake
Pooled males
91·4
92·7
84·6
92·1
90·2
86·1
90·0
85·4
76·9
85·3
35
41
39
38
153
36
40
41
26
143
Males
Rock Bass
Females
Males
F75,344 =5·4, P<0·001). The DFA was able to correctly classify 90% of the
females and 85% of the males in the four waterbodies (Table IV). Females and
males from all of these populations were separated by significant Mahalanobis
distances (Fig. 4), with the Indian River–Rice Lake pair more widely separated
than the Eels Creek–Stony Lake pair (females: D2 =14·2 and 5·3; males: D2 =18·7
and 4·8, respectively).
DISCUSSION
Morphological differences between stream and lake fish were evident in both
pumpkinseeds and rock bass, though they were not always consistent among
populations or species. Stream–lake differences in body form were evident from
the DFA, despite some overlap among the populations (Figs 3 and 4). The
degree of overlap on the canonical axes and the few fish at the extreme ends of
the axes suggest that pumpkinseed and rock bass do not fall into two discrete
categories (i.e. a discrete stream morph and a discrete lake morph). A lack of
discrete morphological types was also found by Robinson & Wilson (1996) in
their study of trophic dimorphism in pumpkinseeds. They found that pumpkinseeds formed a unimodal distribution of pelagic and littoral morphs, and
that the general body shape of most individual fish was located at an intermediate position on this distribution somewhere between the extreme pelagic
and the extreme littoral forms. Robinson et al. (1993, 1996) correctly assigned
.   . . 
14
5
Axis 2
(a)
0
–5
–5
Axis 2
4
0
5
0
Axis 1
7
(b)
0
–4
–7
F. 3. Distribution of (a) pumpkinseed female and (b) pumpkinseed male canonical scores from the
discriminant function analysis (DFA) with 50% ellipsoids about the centroid of each group plotted
on the first two canonical axes. The DFA was performed on the residuals after regressing each
of the morphological variables against the centroid developed from the truss network shown in
Fig. 2. Pumpkinseeds collected from the Indian River (), Rice Lake unvegetated habitat () and
Rice 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 the
same 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 in
the stream populations, but in most comparisons, there was no significant
difference between stream and lake populations in width through the body. The
stream–lake differences in body depth are consistent with the findings of other
morphological studies. Fishes inhabiting ecosystems with more arduous hydrodynamic conditions, such as streams, tend to be more slender-bodied to reduce
the drag induced by the current (Webb, 1984; McLaughlin & Grant, 1994; Ryder
& Pesendorfer, 1989). Fishes with a more gibbose body shape suffer higher drag
penalties when swimming. Bronmark & Miner (1992) found that crucian carp
Carassius carassius (L.) with deeper bodies had a 32% increase in drag at a
    
5
15
(a)
Axis 2
0
–5
–6
0
6
0
Axis 1
5
5
(b)
0
–5
–5
F. 4. Distribution of (a) rock bass female and (b) rock bass male canonical axis scores from the
discriminant function analysis (DFA) with 50% ellipsoids about the centroid of each group plotted
on the first two canonical axes. The DFA was performed on the residuals after regressing each
of the morphological variables against the centroid developed from the truss network shown in
Fig. 2. Rock bass collected from the Indian River (), Rice Lake (), Eels Creek () and Stony
Lake ().
swimming speed of 10 cm s 1 in small ponds. The added drag penalty would
decrease the swimming performance of the fish, but minimization of resistance
does not appear to be important in the type of low-speed manoeuvring
performed by fishes that forage in complex lake environments (Webb, 1998).
With the increased burden of swimming in the stream due to the hydrodynamic conditions, selection pressures should favour the development of
mechanisms that allow sustained swimming to be maintained. Fishes selected for
sustained swimming ability are generally more slender-bodied, rounder in
cross-section and have a greater proportion of red muscle tissue, whereas more
16
.   . . 
sedentary lake fishes are generally more gibbose, more laterally compressed (or
oblong 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) may
allow the fish to reduce its need for unsteady swimming and allow it to maintain
its position with steady swimming at a slower rate. In laboratory tests, steady
swimming was two to four times less energetically costly than unsteady swimming (Webb, 1991), so this type of locomotion should be favoured. Similarly, in
field situations, the use of even small-scale current refuges reduced swimming
costs in brook trout by 10% on average, while foraging ability was not affected
(McLaughlin & Noakes, 1998). The use of such habitat structure provided
individuals with an energetic advantage. Pumpkinseed and rock bass inhabiting
streams undoubtedly use such refuges and backwater areas to reduce swimming
costs. It is likely that stream centrarchids only utilize the faster flowing water
when they are feeding on invertebrates caught drifting in the current, and this
feeding would occur from sheltered locations whenever possible.
Contrary to Prediction 1, stream and lake fishes generally did not show a
significant difference in body width. While this dimension has not been
measured 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 influence
on the body width of the fish. According to Ryder & Pesendorfer (1989), more
fusiform fishes are typically rounder in cross-section than gibbose fishes. A
rounder 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 not
supported. It was expected that stream centrarchids would have larger paired
lateral fins for holding position and for orientating themselves in the current, and
that larger dorsal and anal fins would be used for stability in flowing water.
While stream pumpkinseed pectoral fins were longer than those of lake pumpkinseed, in most other cases the length and width of the fins was greater in the
lake 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 dwelling
rock bass.
According to Webb (1984), fishes adapted to prolonged steady swimming
should have a larger fin area relative to body size. Chinook salmon
Oncorhynchus tshawytscha (Walbaum) inhabiting areas with faster current
velocities did have larger lateral fins (Beacham et al., 1989). Brook charr use
their fins to maintain an upstream orientation in the faster, often turbulent flow
(McLaughlin & Noakes, 1998). Larger fins should move a greater volume of
water and may reduce energy expenditures from additional fin beats. In
addition, larger fins may be used by stream fishes in conjunction with steady
swimming, a propulsive mechanism observed for stream fishes in the field (Webb,
1991; McLaughlin & Noakes, 1998). The dorsal and anal fins of stream-dwelling
coho salmon are larger than those of lake-dwelling coho, although this may be
due to increased territorial behaviour in the stream (Swain & Holtby, 1989).
Territorial fishes use larger dorsal and anal fin margins to create the illusion of
increased body size.
    
17
Larger fins can create a greater drag potential in flowing water. The use of
lateral fins increases the surface area of the fish when it is viewed head-on. If a
fish is oriented in an upstream direction to forage on aquatic insects drifting
in the current, the increase in surface area exposed to the current would result
in a greater drag coefficient. This would reduce the distance covered per
tail beat, or alternatively, it would mean that a fish would have to increase its
tail beat frequency to hold position in the current (Webb, 1991; McLaughlin
& Noakes, 1998). Similarly, an increase in the surface area of the dorsal and
anal fins would be sub-optimal when a fish is not oriented precisely in an
upstream 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 orientation
and maintaining position, against the extra drag that is created by these larger
fins. The optimal solution to this trade-off may vary in species that differ in
overall body shape, or which use the fast current of a stream to a different
degree.
Prediction 3, that stream fishes will have a more robust caudal peduncle than
lake fishes (reduced depth but greater width), was partly supported. In pumpkinseed, stream fish had a more shallow depth at the anterior end of the caudal
peduncle (although this difference was only significant for males) and stream fish
had a wider caudal peduncle (although this result was only significant for
females). There were few significant differences, however, in the width or depth
of the caudal peduncle in rock bass.
For prolonged, constant speed swimming, stream fishes require a caudal
peduncle that is muscular, yet capable of large amplitude beats to increase the
forward thrust power of the fishes (Webb, 1984). To allow for faster, more
powerful swimming, stream fishes also need to be able to make these large
amplitude caudal peduncle displacements at high frequencies. A shallow caudal
peduncle with a large muscle mass (i.e. increased width) allows the fish to
maximize thrust while reducing energy lost in recoil (Webb, 1984, 1998;
McLaughlin & Noakes, 1998). McLaughlin & Grant (1994) found that juvenile
brook charr collected from sites with faster current had a shallower caudal
peduncle than those collected from sites in the same watershed with a slower
current velocity. Although the width of the caudal peduncle was not measured
by McLaughlin & Grant (1994), it would probably be greater in stream fish to
contain 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 peduncle
was smaller in stream fishes, whereas the depth at the posterior of the caudal
peduncle was greater than that of lake fishes. While it was expected that the
entire caudal peduncle would be less deep in stream fishes, energy losses in recoil
would be reduced as long as the lateral surface area of the caudal peduncle is
smaller in a relative sense. The posterior depth of the caudal peduncle was
measured from the dorsal insertion of the caudal fin to the ventral insertion of
the caudal fin. If the base of the caudal fin was larger in stream fishes, then the
depth 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 faster
flowing water had larger caudal fins.
18
.   . . 
Prediction 4, that the lateral fins of stream fishes would have a more anterior
placement than in lake fishes was partly supported. In the pumpkinseed, the
pelvic fins of both sexes were more anterior in stream populations, but the
pectoral fins were not. In the rock bass, both the pelvic and the pectoral fins
were generally more anterior in stream populations, but this result was only
significant for males in one of the two habitat comparisons.
Although published data on the placement of the lateral fins are sparse, there
is some evidence that the pectoral and pelvic fins of stream fishes are located in
a more anterior position than those of lake fishes (Swain & Holtby, 1989). The
more anterior insertion of the lateral fins allows for additional manoeuvrability
in fishes (Webb, 1984), an adaptation that is necessary for stream fishes that must
orient and maintain their position in flowing water.
It is evident from the above discussion that the observed morphological
differences in the current study were not always consistent between the two
species studied, nor were the observed results always consistent with other
studies. Although the results for many of the variables were consistent between
the two species (i.e. if for a certain trait, stream pumpkinseed were larger than
lake pumpkinseed, then stream rock bass would also be larger than lake rock
bass), there were a few exceptions. For example, the length of the pectoral fins
and the length of the dorsal fin base were greater in stream pumpkinseed and in
lake rock bass.
There are a number of examples of conflicting results in the literature. For
example, Bodaly (1979) found two morphological forms of lake whitefish
Coregonus clupeaformis (L.), a benthic morph and a pelagic morph, in five
different Yukon lakes. Although the two morphs could be consistently distinguished within a single lake, the sets of differences that distinguished them were
not consistent among the five lakes. Bodaly (1979) suggested that the among-lake
differences might have occurred because the fish were not only adapting to the
two niches available in each lake, but also to environmental differences that exist
among the lakes. Similar mechanisms may be influencing the morphology of the
fishes in the present study.
One question that cannot be answered from the study is whether the observed
morphological differences between stream and lake populations are the result of
genetic differentiation or phenotypic plasticity. Evidence from stream–lake
studies of other species and from studies of pumpkinseed differentiation in lakes
suggest that both mechanisms may contribute to stream–lake differentiation in
pumpkinseed morphology. In threespine stickleback evidence from rearing
studies indicates that the morphological traits of stream and lake morphs are
inherited (Lavin & McPhail, 1993), and subsequent mtDNA analysis of populations from a stream and an adjacent lake indicate that the gene pools of the two
are distinct (Thompson et al., 1997). Similarly, juvenile coho salmon from a lake
and an adjoining stream showed differences in body depth, fin placement and
colouration 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 slow
and fast flowing sites within a stream appears to be due mainly to phenotypic
plasticity, as the morphological differences apparent in one year did not persist
several 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 was
attributable to phenotypic plasticity, whereas genetic differences accounted for
only 14% of the variation. A combination of genetic analysis and controlled
rearing experiments would be required to identify the relative importance of
phenotypic 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 assistance
in the field. As well, thanks are extended to B. Robinson and K. Somers for their helpful
advice during this study, and to N. Mandrak and T. Whillans for their advice and
comments on an earlier version of the manuscript. Financial support for this study was
provided by a Natural Sciences and Engineering Research Council of Canada grant to
M. Fox and an Ontario Graduate Scholarship to J. Brinsmead.
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