2008 NORTHEASTERN NATURALIST 15(4):557–576
Biogeography of Stream Fishes in Connecticut:
Defining Faunal Regions and Assemblage Types
Yoichiro Kanno1 and Jason C. Vokoun1,*
Abstract - Stream fish survey data were analyzed to describe patterns of fish distributions
in wadeable streams (primarily 1st–4th order) in Connecticut. Species occurrence
within the United States Geological Survey 8-digit hydrologic unit code watersheds
were used to aggregate similar watersheds into stream fish faunal regions.
Within each identified region, multivariate analyses were used to identify major
fish assemblage types and associate stream habitat with assemblage types. The
analyses revealed an eastern and western faunal region defined primarily by distribution
of a few native species. Native species associated with the western watersheds
were: Semotilus atromaculatus (Creek Chub), Exoglossum maxillingua (Cutlips
Minnow), and Cottus cognatus (Slimy Sculpin). Native fishes associated with
the eastern watersheds were: Erimyzon oblongus (Creek Chubsucker), Esox niger
(Chain Pickerel), and Esox americanus (Redfin Pickerel). Inclusion of non-indigenous
species in the analyses resulted in a similar east–west grouping of watersheds.
Five and four assemblage types were identified in the eastern and western faunal
regions, respectively. Both regions harbored 3 fluvial assemblages defined longitudinally
from headwater streams to larger wadeable streams and a macro-habitat
generalist assemblage inhabiting streams with proportionately more pool habitat,
but taxonomic membership and indicator species rankings among assemblages
were not necessarily identical between the regions. A distinct assemblage dominated
by Redfin Pickerel was recognized only in the eastern region. Streams in the
western region were generally higher in elevation and colder in water temperature.
The discovery and description of eastern and western fish faunal regions and their
fish assemblage types will be useful in stratifying the biological monitoring of
streams and other aquatic resource management actions in Connecticut.
Introduction
The presence of a fish species or an assemblage of fish species within a
particular stream reach is a result of past and current influences operating
across hierarchical spatial scales (Frissell et al. 1986, Poff 1997). Physiography,
climate, basin geomorphometry, and human introductions influence
the composition of regional-scale species pools, but stream fish distribution
also is influenced by many local factors including stream size (Lyons 1996,
Maret et al. 1997, Newall and Magnuson 1999, Zorn et al. 2002), elevation
(Fausch et al. 1994), stream gradient (Lyons 1996, Maret et al. 1997, Waite
and Carpenter 2000), water temperature (Lyons 1996, Waite and Carpenter
2000), and hydrological variability (Taylor and Warren 2001, Zorn et al.
2002). Typically, stream habitat and the organisms it supports change predictably
as streams increase in size (Goldstein and Meador 2004, Vannote
1Department of Natural Resources Management and Engineering, College of Agriculture
and Natural Resources, University of Connecticut, 1376 Storrs Road, Storrs,
CT 06269-4087. *Corresponding author - jason.vokoun@uconn.edu.
558 Northeastern Naturalist Vol. 15, No. 4
et al. 1980), and in some systems, stream fishes may show a longitudinal
pattern of coldwater–coolwater–warmwater transitions from headwater to
downstream segments (Herlihy et al. 2006, Rahel and Hubert 1991).
Documenting and inventorying stream fish distributions and their habitat
hierarchically are useful for aquatic conservation planning. Herlihy et al.
(2006) hierarchically classified fish assemblages across the conterminous
United States and related the assemblage types to environmental variables
such as stream size, nutrient level, and water temperature. At a smaller geographic
scale, Pflieger (1989) divided Missouri into 3 physiographic regions
(Lowland, Ozark, and Prairie), subdivided each physiographic region into a
cluster of similar drainages based on stream fish species and assemblages,
and further described faunal changes from headwater creeks to large streams
in each drainage cluster. Sowa et al. (2005) recently expanded Pflieger’s
faunal classification into even finer spatially nested hierarchical levels
including segment- and reach-scale information. Zorn et al. (2002) used a
hierarchical classification of stream assemblages that describes the stream
size-hydrology gradient in Michigan’s lower peninsula, and their work led to
a valley-segment classification which incorporated landscape variables and
assemblage distributions (Seelbach et al. 2006).
We believe that an aquatic classification system in Connecticut will be useful
and necessary for aquatic resource management. The state harbors a dense
network of wadeable streams, and spatially-extensive sampling has occurred in
these streams, but knowledge of stream fish distributions remains largely qualitative.
Our objectives were twofold. First, we aggregated watersheds based on
species occurrence to define stream fish faunal regions. Second, in each faunal
region, we quantitatively defined the major fish assemblage types and described
differences in stream habitat among assemblage types.
Methods
Defining fish faunal regions
Fish data. Stream fish survey data on wadeable streams were assembled
to examine species occurrence within major watersheds. Given connectivity
of watersheds beyond political boundaries, analysis extended beyond Connecticut
to include adjoining portions of New York, Massachusetts, and Rhode
Island (Fig. 1). The data were obtained from the Connecticut Department of
Environmental Protection, New York State Department of Environmental
Conservation, Massachusetts Department of Fish and Game, and Rhode Island
Department of Environmental Management. Most of the stream surveys
were conducted between 1990 and the present, but some samples were taken
in the 1980s. Sampling periods primarily ranged from June to September, and
stream lengths sampled varied by state agency and stream size.
Stream survey datasets were screened by several criteria. Wadeable stream
samples were limited to those collected with backpack or tote-barge electrofishers and generally corresponded to 1st–4th order streams at the 1:24,000
scale. Records from larger streams collected with boat electrofishing were
not available in all watersheds and were excluded. Hybrids, unidentified
2008 Y. Kanno and J.C. Vokoun 559
individuals, and samples with ≤10 total individuals were deleted from analysis.
When repeat visits were made to the same sampling location, all visits
were used to score whether a species was present or absent at the site. Species
present in fewer than 5% of the sites were excluded to reduce potential effects
of rare species on multivariate analysis (McCune and Grace 2002), but
Erimyzon oblongus (Creek Chubsucker) was retained because it is known to
occur in eastern Connecticut (Whitworth 1996)—its presence is a zoogeographical
signal, rather than noise. Diadromous species, Anguilla rostrata Lesueur
(American Eel) and Salmo salar Linnaeus (Atlantic Salmon), also were
excluded from analyses. The resultant dataset contained 2218 stream sites with
25 native and non-indigenous species (n = 1184 for Connecticut, n = 391 for
New York, n = 481 for Massachusetts, and n = 162 for Rhode Island; Table 1).
Statistical analysis. Species occurrence was evaluated to group similar
watersheds in Connecticut and adjoining areas. Given the small size of the
study area, the landscape was assumed to be relatively homogeneous terrestrially,
being defined by regionally operating factors such as physiography
and climate (Omernik 1987). Our intent in the first series of analyses was to
Figure 1. A map of the USGS 8-digit hydrologic units (HUC) watersheds and two
faunal regions recognized in this study for Connecticut and its adjacent watersheds.
The HUC watersheds are encircled by solid lines with watershed names. The western
and eastern faunal regions are indicated by open and stippled polygons, respectively.
Dashed lines are the state boundaries of Connecticut.
560 Northeastern Naturalist Vol. 15, No. 4
divide the study region into clusters of similar watersheds based on stream
fish occurrence, corresponding approximately to the drainage-level classifi-
cation of Pflieger (1989).
Sixteen watersheds were delimited using the United States Geological Survey’s
National hydrography dataset 8-digit hydrologic unit code (HUC). The
study area encompassed 17 HUC watersheds (Fig. 1), but because of its small
size, the Bronx watershed in New York was joined with the adjacent Saugatuck
watershed. Relative occurrence of each fish species in each watershed was calculated
as the total number of sites where the species was present divided by
the total number of sites in the watershed, resulting in a species occurrence by
HUC watershed matrix. Occurrence values were arcsine square-root transformed
for use in multivariate analyses and Mantel tests.
Cluster and ordination analyses were conducted separately for native
species only and native plus non-indigenous species using PC-ORD version
5 (MJM Software, Gleneden Beach, OR). The distributions of native
species may represent historical biogeographic patterns of occurrence. A
Table 1. Twenty-five native and non-indigenous fish species used to define stream fish faunal regions
in Connecticut and adjoining watersheds. Species are listed in descending order of occurrence
among 2218 stream sites across the study region. Ecological characteristics of fish species
are from regional references (Armstrong et al. 2001, Halliwell et al. 1999, Whitworth 1996).
Abbreviations are: % = percent occurence in Connecticut and others; Temp (temperature): C =
cold water, C-W = cool water, W = warm water; SF (stream flow): FS = fluvial specialist, FD =
fluvial dependent, MG = macro-habitat generalist; To (tolerance): I = intolerant, M = intermediate,
T = tolerant; Fe (feeding): GF = generalist feeder (e.g., omnivorous fishes), BI = benthic
insectivore, TC = trophic carnivore.
Species
Common name Scientific name coding % Temp SF To Fe
Blacknose Dace Rhinichthys atratulus Hermann BL 68.3 C-W FS T GF
White Sucker Catostomus commersoni Lacepède WS 64.2 C-W FD T GF
Brook Trout Salvelinus fontinalis Mitchill BK 51.8 C FS I TC
Brown Trout* Salmo trutta Linnaeus BN 42.2 C FD I TC
Pumpkinseed Lepomis gibbosus Linnaeus PS 39.0 W MG M GF
Tessellated Darter Etheostoma olmstedi Storer TD 37.9 C-W FS M BI
Longnose Dace Rhinichthys cataractae Valenciennes LD 36.9 C-W FS M BI
Bluegill* Lepomis macrochirus Rafinesque BG 33.1 W MG T GF
Largemouth Bass* Micropterus salmoides Lacepède LM 31.3 W MG M TC
Common Shiner Luxilus cornutus Mitchill CS 31.0 C-W FD M GF
Creek Chub Semotilus atromaculatus Mitchill CR 28.2 C-W MG T GF
Fallfish Semotilus corporalis Mitchill FF 27.1 C-W FS M GF
Golden Shiner Notemigonus crysoleucas Mitchill GS 20.1 W MG T GF
Brown Bullhead Ameiurus nebulosus Lesueur BB 19.7 W MG T GF
Chain Pickerel Esox niger Lesueur CP 17.1 W MG M TC
Redbreast Sunfish Lepomis auritus Linnaeus RS 14.7 W MG M GF
Redfin Pickerel Esox americanus Gmelin RF 14.3 W MG M TC
Yellow Perch Perca flavescens Mitchill YP 12.7 C-W MG M TC
Cutlips Minnow Exoglossum maxillingua Lesueur CM 11.1 W FS I BI
Rock Bass* Ambloplites rupestris Rafinesque RB 9.6 C-W MG M TC
Smallmouth Bass* Micropterus dolomieu Lacepède SM 9.5 C-W MG M TC
Rainbow Trout* Oncorhynchus mykiss Walbaum RW 9.2 C FD I TC
Slimy Sculpin Cottus cognatus Richardson SC 9.0 C FS I BI
Yellow Bullhead* Ameiurus natalis Lesueur YB 5.4 W MG T GF
Creek Chubsucker Erimyzon oblongus Mitchill CH 2.8 W FS I GF
*Non-Indigenous to the study region.
2008 Y. Kanno and J.C. Vokoun 561
parallel analysis, which might help in understanding patterns in the current
distribution of stream fishes, was performed with inclusion of non-indigenous
species, many of which are naturalized and widespread in the study watersheds
that . The Sorensen distance measure with the flexible beta linkage
method (beta = -0.25) was used in the cluster analysis (McCune and Grace
2002). Nonmetric multidimensional scaling (NMS), an ordination technique
unencumbered by assumptions of multivariate linearity, was used with Sorensen
distance to simultaneously ordinate watersheds and fish species in
fish-occurrence space. NMS extracts major gradients in the data structure by
iteratively searching for positions that minimize departures of an ordination
structure from the original data. It graphically presents (dis)similarities of
sampling entities, such that watersheds plotted close together in the ordination
space are more similar in their fish occurrence than those far apart. The
general guidelines provided by McCune and Grace (2002) were followed to
execute NMS. Specifically, a preliminary run was executed to identify the best
dimensionality of the data. A scree plot also was examined to help determine
dimensionality. Then, a final run was executed with the number of dimensions
set and the starting configuration that worked best in the preliminary run.
The influence of non-indigenous species on watershed groupings was tested
by a Mantel test, which evaluates the congruence between two distance matrices
(Sorensen distance, Monte Carlo randomization test, 10,000 permutations).
If the addition of non-indigenous species exerts a major influence on watershed
aggregation, distance matrices derived from native species only and the native
species plus non-indigenous species will not be significantly associated.
Characterization of fish assemblage types and stream habitat
Fish data. Characterization of fish assemblage types was limited to a
subset of stream fish data in Connecticut, which was collected by the Connecticut
Department of Environmental Protection between 1988 and 1994
and incorporated a number of stream habitat variables including stream
width, depth, gradient, elevation, water temperature, and pool/riffle ratio
(Hagstrom et al. 1989, 1990, 1991, 1992, 1995, 1996). Sampling occurred
from June to October, and all fish samples were taken using 3-pass electrofishing depletions of block-netted sections. Fish catch was calculated
as the proportional abundance of each species at each stream site. In each
faunal region delineated as previously described, rare species (<5% occurrence
in each region) were excluded from analyses. In addition, 2 diadromous
species, the American Eel and Petromyzon marinus Linnaeus (Sea
Lamprey), and 2 non-indigenous trout species, Salmo trutta (Brown Trout)
and Oncorhynchus mykiss (Rainbow Trout), were excluded from analyses.
Migrations of diadromous fishes are often obstructed by numerous dams
in Connecticut, and the presence of a diadromous species in a stream may
primarily indicate current access to the ocean. The 2 trout are stocked widely
across Connecticut; each spring hundreds of thousands of individuals of
these species are released in publicly accessible streams. Their presence
in a stream primarily reflects current stocking practices. The final dataset
contained 24 fish species and 891 sites (Table 2).
562 Northeastern Naturalist Vol. 15, No. 4
Table 2. Results of indicator species analysis for 4 fish assemblage types identified in the western faunal region and 5 fish assemblage types identified in the
eastern faunal region in Connecticut. Assemblage type was used as the grouping variable, and in each faunal region the largest indicator value for each species
is listed with its associated P-value. Indicator values > 10 with P-value < 0.05 are marked with an “*”.
Western faunal region in Connecticut Eastern faunal region in Connecticut
Assemblage type Species Indicator values (%) Assemblage type Species Indicator values (%)
Assemblage A Brook Trout 72.9 (P = 0.0001)* Assemblage A Brook Trout 62.8 (P = 0.0001)*
(n = 76) (n = 103)
Assemblage BW Blacknose Dace 53.3 (P = 0.0001)* Assemblage BE Blacknose Dace 66.1 (P = 0.0001)*
(n = 150) Creek Chub 38.5 (P = 0.0001)* (n = 135)
Assemblage CW White Sucker 47.0 (P = 0.0001)* Assemblage CE Fallfish 54.7 (P = 0.0001)*
(n = 163) Longnose Dace 38.3 (P = 0.0001)* (n = 168) Redbreast Sunfish 42.1 (P = 0.0001)*
Common Shiner 34.5 (P = 0.0001)* Tessellated Darter 39.1 (P = 0.0001)*
Tessellated Darter 29.5 (P = 0.0003)* White Sucker 35.0 (P = 0.0002)*
Redbreast Sunfish 20.8 (P = 0.0017)* Common Shiner 34.4 (P = 0.0003)*
Fallfish 19.7 (P = 0.0036)* Longnose Dace 30.3 (P = 0.0003)*
Cutlips Minnow 19.5 (P = 0.0004)* Largemouth Bass 23.5 (P = 0.0122)*
Rock Bass 11.3 (P = 0.0140)* Bluegill 22.5 (P = 0.0149)*
Smallmouth Bass 8.9 (P = 0.0267) Spottail Shiner 20.2 (P = 0.0018)*
Fathead Minnow 5.3 (P = 0.1165) Smallmouth Bass 18.5 (P = 0.0025)*
Green Sunfish 4.4 (P = 0.3727)
Assemblage DW Pumpkinseed 70.8 (P = 0.0001)* Assemblage DE Brown Bullhead 38.5 (P = 0.0001)*
(n = 26) Brown Bullhead 40.5 (P = 0.0001)* (n = 52) Chain Pickerel 33.1 (P = 0.0010)*
Golden Shiner 36.1 (P = 0.0001)* Pumpkinseed 21.6 (P = 0.0483)*
Bluegill 35.2 (P = 0.0001)* Golden Shiner 17.5 (P = 0.0551)
Chain Pickerel 34.0 (P = 0.0001)* Yellow Perch 14.5 (P = 0.0485)*
Largemouth Bass 33.3 (P = 0.0001)* Creek Chubsucker 7.0 (P = 0.0811)
Yellow Perch 23.2 (P = 0.0001)*
Redfin Pickerel 22.9 (P = 0.0001)*
Assemblage E Redfin Pickerel 87.9 (P = 0.0001)*
(n = 13)
2008 Y. Kanno and J.C. Vokoun 563
Statistical analysis. Within faunal regions, analyses were conducted to
identify fish assemblage types and species that typified each assemblage.
Similar stream sites were grouped using arcsine square-root transformed
proportional abundance of fish species in a cluster analysis (PC-ORD, Sorensen
distance, flexible beta linkage, beta = -0.25). Several criteria were
used to define the number of fish assemblage types in the resultant cluster
dendrogram (i.e., “pruning” the dendrogram into meaningful groups). First,
indicator species analysis was used as a quantitative criterion (PC-ORD;
McCune and Grace 2002). Indicator species analysis combines a measure of
ubiquity of a species in a particular group and a measure of fidelity of a species
to the particular group, and calculates indicator values that range from
0% (no indication) to 100% (perfect indication). As such, a species that is
exclusively found in an assemblage type and is ubiquitous among sampling
sites of that particular type receives a high indicator value, identifying it as
indicative of that particular fish assemblage. Fish species with an indicator
value of ≥10% and an associated P-value < 0.05 were considered good indicators
of an assemblage type (Herlihy et al. 2006). P-values of indicator
values (Monte Carlo method, 10,000 permutations) were averaged across all
species when the cluster dendrogram was pruned at 2, 3, 4, 5, and 6 levels
of division, and the minimal average was used as a guide in determining the
number of meaningful fish assemblages types (McCune and Grace 2002).
Second, canonical discriminant analysis (CDA; SAS version 9.1, SAS Institute
Inc., Cary, NC) was used to visualize relations between fish assemblage
types and as a dimension-reduction technique to contrast among-group differences.
Fish assemblage type was used as an a priori categorical variable
in the CDA to construct linear combinations of the quantitative variables that
maximize among-group separation while minimizing within-group dissimilarity.
Bi-variate plots from this analysis were examined for the presence of
noticeable separation among fish assemblage types. Additionally, spatial distributions
of assemblage types were examined to see if finer pruning of the
cluster dendrogram supported recognition of additional assemblage types.
Habitat characteristics (stream width, depth, gradient, elevation, water
temperature, and pool/riffle ratio) were compared among the fish assemblage
types in each faunal region. Kruskal-Wallis tests (SAS) were used to test for
differences among habitat and fish assemblages because the raw data were nonnormal
and heteroscedastic; transformations did not improve the data structure.
When a significant difference (P < 0.05) among fish assemblages was detected
for a habitat characteristic, a post-hoc two-tailed t-test was applied for all possible
pair-wise comparisons using a contrast statement in PROC MULTTEST
(permutation adjusted P-value < 0.05 for each pair-wise comparison).
Results
Defining fish faunal regions
Both cluster and NMS analyses indicated an east–west pattern of watershed
alignment based on native fish species (Fig. 2a, b). The division in
the cluster analysis generally separated eastern watersheds (Narragansett to
564 Northeastern Naturalist Vol. 15, No. 4
Figure 2. Cluster analysis dendrogram (a) and nonmetric multidimensional scaling
(NMS) ordination (b) of 18 native stream fish species. Codes located to the left of each
watershed in the dendrogram indicate watersheds located to the east (E) or west (W)
of the Lower Connecticut watershed. The NMS graph was rotated to highlight the
east–west pattern of watershed grouping. Open and filled triangles indicate eastern and
western faunal regions, respectively (see Fig. 1), and crosses denote fish species (see
species coding in Table 1). Watershed abbreviations are: BLA = Blackstone, NAR =
Narragansett, PAW = Pawcatuck-Wood, QBA = Quinebaug, SHE = Shetucket, THA
= Thames, LOC = Lower Connecticut, QPI = Quinnipiac, CHI = Chicopee, WES =
Westfield, FAR = Farmington, HOU = Housatonic, SAU = Saugatuck, MIH = Middle
Hudson, HUW = Hudson-Wappinger, and LOH = Lower Hudson.
2008 Y. Kanno and J.C. Vokoun 565
Quinnipiac) from western watersheds (Chicopee to Lower Hudson) (Fig. 2a).
NMS yielded a 2-dimensional best-fit solution (50 iterations, final stress =
7.661, instability = 0.00048), and grouped watersheds across an east to west
gradient (Fig. 2b, Axis 1: r2 = 0.761, Axis 2: r2 = 0.185). The gradient placed
western watersheds to the left on Axis 1, which was associated with Exoglossum
maxillingua (Cutlips Minnow), Semotilus atromaculatus (Creek Chub),
and Cottus cognatus (Slimy Sculpin). Eastern watersheds were primarily
located to the right on Axis 1, which was associated with Creek Chubsucker,
Esox niger (Chain Pickerel), and Esox americanus (Redfin Pickerel). However,
as evidenced by their more central positions in the ordination space, the
associations of these species were not as strong as those of Cutlips Minnow,
Creek Chub, and Slimy Sculpin in the western region. Axis 2 appeared to represent
a north–south gradient in the western faunal region. Northern inland
watersheds (Chicopee and Westfield) were contrasted with more southern
coastal watersheds (Saugatuck, Lower Hudson, and Hudson-Wappinger).
Other native fish species, such as Luxilus cornutus (Common Shiner), Catostomus
commersoni (White Sucker), and Etheostoma olmstedi (Tessellated
Darter), showed no distinctive distribution patterns, being placed near the
center of the ordination space (Fig. 2b).
The addition of non-indigenous species did not change the east–west
pattern of watershed aggregation (Fig. 3a, b). The Mantel test yielded a
strong correlation between the natives only and natives plus non-indigenous
distance matrices (Mantel r = 0.98, P = 0.0001), indicating that non-indigenous
species had little effect on the structure of watershed aggregation.
Cluster analysis produced the east–west grouping of watersheds similar
to the native-only analysis. NMS also showed the east–west gradient (66
iterations, final stress = 7.76, instability = 0.00006, 2 dimensional best-fit solution):
eastern and western watersheds were plotted at the upper and lower
ends of Axis 1 (r2 = 0.776), respectively. Axis 2 (r2 = 0.171) again appeared
to represent a north–south gradient in the western faunal region: northern
Chicopee and Westfield watersheds were contrasted with southern Saugatuck,
Lower Hudson, and Hudson-Wappinger watersheds. Non-indigenous
species generally were plotted near the center of the NMS ordination, indicating
no strong associations with particular watersheds and/or widespread
distributions across the study area.
The strong east–west patterns observed in cluster and NMS analyses support
recognition of eastern and western faunal regions. The differences in the 2
regions resulted from distributional patterns of a few species (i.e., associations
of Cutlips Minnow, Creek Chub, and Slimy Sculpin with the western region,
and associations of Creek Chubsucker, Chain Pickerel, and Redfin Pickerel
with the eastern region). In fact, the 2 regions shared most species.
Characterization of fish assemblage types
In the eastern region, analyses supported recognition of 5 assemblage
types. The average P-value of indicator values across all species from indicator
species analysis was minimal when the eastern cluster dendrogram
(chaining = 0.49%) was pruned at 4 clusters (average P-values, 0.1067,
566 Northeastern Naturalist Vol. 15, No. 4
0.0343, 0.0140, 0.0320, and 0.0442 for 2, 3, 4, 5, and 6 clusters, respectively).
The P-value at 4 clusters was less than half of those at the neighboring
3 and 5 clusters, conferring statistical support for 4 fish assemblage types.
In addition, CDA suggested the presence of a fifth distinct fish assemblage
type dominated by Redfin Pickerel (Fig. 4a). Streams belonging to this
assemblage type were typically found in the southeastern corner of Connecticut
and the Connecticut River floodplain and valley (Fig. 5). Indicator
Figure 3. Cluster analysis dendrogram (a) and nonmetric multidimensional scaling
(NMS) ordination (b) of 18 native and 7 non-indigenous stream fish species. Codes
located to the left of each watershed in cluster analysis dendrogram indicate watersheds
located to the east (E) or west (W) of Lower Connecticut watershed. The NMS
graph was rotated to highlight the east–west pattern of watershed grouping. Open and
filled triangles indicate eastern and western faunal regions, respectively (see Fig. 1),
and crosses denote fish species (see species coding in Table 1). Watershed abbreviations
follow those in Figure 2.
2008 Y. Kanno and J.C. Vokoun 567
values characterized typical taxonomic membership of each assemblage type
(Table 2). Given the species compositions and ecological characteristics (Table
1), the 5 assemblage types in the eastern faunal region were termed (1)
Salvelinus fontinalis (Brook Trout) dominated assemblage (Assemblage A),
(2) Rhinichthys atratulus (Blacknose Dace) dominated assemblage (Assemblage
BE), (3) eastern mixed fluvial assemblage (Assemblage CE), (4) eastern
mixed macro-habitat generalist assemblage (Assemblage DE), and (5) Redfin
Pickerel dominated assemblage (Assemblage E). Assemblages A, BE, and E
were taxonomically simple; Brook Trout, Blacknose Dace, and Redfin Pickerel
were the only dominant species, respectively. In contrast, Assemblages
CE and DE were more diverse. Assemblage CE, with 10 species, was the
most diverse stream fish community in terms of the number of statistically
significant indicator species. It was best characterized by fluvial specialists
or fluvial dependents, since 5 of the first 6 indicator species were fluvial
F i g u r e 4 .
D i s c r i m i -
nant canonical
analysis
of the eastern
faunal region
(a) and the
western faunal
region (b)
in Connecticut.
Letters
in the graphs
i n d i c a t e
stream fish
assemblagetype
designations
in Table
2. Taxonomic
compositions
differ between
the two faunal
regions for
Assemblages
B(E–W), C(E–W),
and D(E–W).
568 Northeastern Naturalist Vol. 15, No. 4
Figure 5. Distributions
of the fish assemblage
types in
two faunal regions
within Connecticut.
Assemblagetype
designations
follow those in
Table 2. Where
taxonomic compositions
differ
between the two
faunal regions
(Assemblages BE–
W, CE–W, and DE–W),
stream sites in the
eastern region and
the western region
are represented by
circles and triangles,
respectively.
species including Semotilus corporalis (Fallfish), Tessellated Darter, White
Sucker, Common Shiner, and Rhinichthys cataractae (Longnose Dace). An
exception was Lepomis auritus (Redbreast Sunfish), a macro-habitat generalist,
which had the second highest indicator value in Assemblage CE. Three
other macro-habitat generalists, Micropterus salmoides (Largemouth Bass),
Lepomis macrochirus (Bluegill), and Notropis hudsonius (Spottail Shiner),
also were indicator species of assemblage CE, although their indicator values
were lower. Assemblage DE was composed of 4 statistically significant
indicator species of macro-habitat generalists: Ameiurus nebulosus (Brown
Bullhead), Chain Pickerel, Lepomis gibbosus (Pumpkinseed), and Perca
flavescens (Yellow Perch). Proportionately, Assemblage CE dominated
the sites (36%), followed by Assemblage BE (29%), Assemblage A (22%),
Assemblage DE (11%), and Assemblage E (3 %) in the eastern region. Assemblage
E was constrained geographically, but other assemblages were
distributed throughout the eastern region (Fig. 5).
Analyses of the western region supported recognition of 4 assemblage
types. The average p-value of indicator values from indicator species analysis
across all species was minimal when the cluster dendrogram (chaining =
0.66%) was pruned at 5 clusters (averaged p-values were 0.0121, 0.0166,
0.0078, 0.0032, and 0.0076 for 2, 3, 4, 5, and 6 levels of divisions, respectively).
The p-value at 5 clusters was less than half that of the neighboring 4 and 6
2008 Y. Kanno and J.C. Vokoun 569
clusters. However, when pruned at 5 groups, assemblages showed substantial
overlap in the CDA, and none showed discrete spatial distributions. Recognition
of 4 groups reduced the overlap in the CDA and produced more
meaningful separation in multivariate space (Fig. 4b). Indicator values characterized
typical taxonomic membership of each of the 4 assemblage types
(Table 2). The 4 assemblage types were ecologically similar to Assemblages A,
BE, CE, and DE in the eastern faunal region: (1) Brook Trout dominated
assemblage (Assemblage A), (2) Blacknose Dace-Creek Chub dominated assemblage
(Assemblage BW), (3) western mixed fluvial assemblage
(Assemblage CW), and (4) western mixed macro-habitat generalist assemblage
(Assemblage DW). Assemblage A was taxonomically identical to that in
the eastern region, and Brook Trout was the only statistically significant indicator.
Creek Chub, in addition to Blacknose Dace, characterized Assemblage
BW in the western region. Assemblages CW and DW were best represented by
fluvial species and macro-habitat generalists, respectively, but their taxonomic
composition and indicator value rankings differed from the eastern region.
Specifically, Assemblage CW was almost exclusively dominated by fluvial species,
and Redbreast Sunfish and Ambloplites rupestris (Rock Bass), the only
macro-habitat generalists belonging to this assemblage type, had smaller indicator
values. Interestingly, the first 6 indicator species were identical between
Assemblages CW and CE, but the rankings of their indicator values showed an
inverse relationship between the assemblages, such that Fallfish shifted from
rank 1 in the east to rank 6 in the west; Redbreast Sunfish, from 2 to 5; Tessellated
Darter, from 3 to 4; White Sucker, from 4 to 1; Common Shiner, from 5 to 3;
and Longnose Dace, from 6 to 2. Assemblage DW was again exclusively dominated
by macro-habitat generalists, but more species (8 statistically significant
indicator species) represented the assemblage than its counterpart in the eastern
region (4 significant indicator species). Species rankings also differed between
Assemblages DW and DE, and Pumpkinseed had a notably high indicator
value (70.8%) in the western region. In the western region, Assemblage CW was
again the most numerically abundant (39%), followed by Assemblage BW
(36%), Assemblage A (18%), and Assemblage DW (6%). The 4 assemblage
types were distributed across all drainages in the western region (Fig. 5).
Habitat characterization
The 2 faunal regions shared similar patterns of fish assemblage-habitat
associations related primarily to longitudinal transitions in stream size and
secondarily to a subdivision between fluvial species and macro-habitat
generalists. In each region, stream width, depth, gradient, elevation, water
temperature, and pool/riffle ratio were all significantly different among the
assemblage types (Kruskal-Wallis test: P < 0.0001 for all habitat variables
in each region; Table 3). Both regions showed a longitudinal transition of
fluvial species from Assemblage A to Assemblage BE or BW, and finally to
Assemblage CE or CW, with increasing width, depth, and water temperature
and with decreasing stream gradient and elevation. Assemblages DE and
especially Dw were associated with habitat characteristics that were intermediate
with regard to stream size, and inhabited streams with high pool/riffle
570 Northeastern Naturalist Vol. 15, No. 4
Table 3. Stream habitat characteristics in eastern and western faunal regions in Connecticut. Numbers reported are median values. In both regions, all
habitat variables are significantly different among assemblage types (Kruskal-Wallist test at P < 0.05), and for each habitat variable in each region values
followed by the same alphabetical letters are not significantly different according to a post-hoc two-tailed t-test at P < 0.05 (permutation adjusted) for each
pair-wise comparison.
Water Gradient Discharge Elevation Pool/riffle
Assemblage Community characteristics Width (m) Depth (cm) temp (°C) (%) (m3/s) (m) ratio
(a) Eastern faunal region in Connecticut
Assemblage A Brook Trout dominated 2.64 a 9.30 a 18 a 1.70 a 0.06 a 79 a 1.00 ab
(n = 103)
Assemblage BE Blacknose Dace dominated 3.75 b 10.25 ab 19 a 1.60 ab 0.15 b 82 a 0.85 a
(n = 135)
Assemblage CE Eastern mixed fluvial 6.96 c 20.70 c 21 b 0.40 c 0.29 c 43 b 2.21 c
(n = 168)
Assemblage DE Eastern mixed macro-habitat generalist 3.40 b 12.15 abd 21 b 0.85 d 0.09 ab 69 ab 2.06 bc
(n = 52)
Assemblage E Redfin Pickerel dominated 2.80 ab 15.65 bcd 18 a 0.50 bcd 0.11 ab 47 ab 12.51 c
(n = 13)
(b) Western faunal region in Connecticut
Assemblage A Brook Trout dominated 2.46 a 9.41 a 16 a 3.1 a 0.02 a 181 a 0.56 a
(n = 76)
Assemblage BW Blacknose Dace-Creek Chub dominated 3.23 b 9.78 a 18 b 1.9 a 0.04 b 166 ab 0.81 a
(n = 150)
Assemblage CW Western mixed fluvial 5.71 c 17.11 b 19 b 0.8 b 0.12 c 125 c 1.30 b
(n = 163)
Assemblage DW Western mixed macro-habitat generalist 4.05 b 13.69 b 19 b 0.3 b 0.04 b 119 bc 2.33 b
(n = 26)
2008 Y. Kanno and J.C. Vokoun 571
ratios (i.e., abundant pools). In the eastern region, Assemblage E inhabited
cold, small, low-gradient streams with numerous pools.
The CDA spatially reflected differences across assemblages revealed in the
habitat comparisons (Fig. 4). In the eastern region, 3 fluvial assemblages (Assemblages
A, BE, and CE) were aligned along the first canonical axis to suggest
the effect of increasing stream size (r2 = 0.775). This pattern contrasted headwater
Brook Trout streams with relatively species-diverse larger streams in the
dataset. On the second canonical axis (r2 = 0.717), riffle-associated Assemblage
BE was juxtaposed with pool-associated Assemblages DE and E. Assemblage
E formed a distinct group, concentrated in the lower left quadrant of the plot.
Generally, the western region shared similar patterns. Stream sites transitioned
from Assemblage A to BW to CW along the first axis (r2 = 0.784), corresponding
to increasing stream size. On the second axis (r2 = 0.729), Assemblage DW was
separated from the other 3 assemblages, emphasizing a shift across the macrohabitat
generalist assemblage to the fluvial assemblages.
Despite the generally shared patterns, the two regions showed some differences
in observed habitat variables. Streams in the western region were
generally higher in elevation and colder in water temperature (Table 3). Median
values of elevation among the 5 assemblage types in the eastern region ranged
from 43 m to 82 m, and those in the western region were between 119 m and 181
m. Coincidently, Assemblage A had a colder median water temperature (16 °C)
in the western region than in the eastern region (18 °C), and inhabited highergradient
streams in the western region (3.1%) than in the eastern region (1.7%).
Discussion
The east–west zoogeographical pattern resulted from characteristic
distributions of relatively few native species at the geographic scale of
the study region. Most other native stream fishes were distributed widely
across the study region. However, assemblage-level differences also were
recognized between the two faunal regions as evidenced by differences in
indicator species rankings within mixed fluvial and macro-habitat generalist
assemblages. Furthermore, a fifth assemblage type dominated by Redfin
Pickerel was recognized only in the eastern region. These assemblage-level
differences additionally support the validity of two faunal regions in our
study area. Our analyses were limited to wadeable streams, and the effect of
the inclusion of large streams on our classification is unknown. But we believe
that our classification of the two faunal regions is robust since it can be
supported by the colonization history of native fishes to the study region.
Our data support that present-day distributions of stream fishes in the study
region are generally explained by 3 possible pathways of postglacial colonization
from Pleistocene refugia (Schmidt 1986, Whitworth 1996). First, most
stream fishes are widely distributed in the study area, and they probably originated
from the Northeastern Coastal Refugium located to the south of today’s
northeastern coast (Schmidt 1986, Whitworth 1996). Second, two eastern
species (Creek Chubsucker and Redfin Pickerel) are hypothesized to have
entered the region from the Atlantic Coastal Plain Refugium and proceeded
572 Northeastern Naturalist Vol. 15, No. 4
through glacial Lake Ronkonkoma (present Long Island Sound) from the lake
outlet located on its east (Schmidt 1986). The presence of this refugium also
explains the eastern distributions of two lentic species (Etheostoma fusiforme
Girard [Swamp Darter] and Enneacanthus obesus Girard [Banded Sunfish];
Schmidt 1986, Whitworth 1996), neither of which were included in the current
study because of their rarity in streams. The third pattern is dispersal from
the south and/or west. Whitworth (1996) suggested that the Cutlips Minnow
and Creek Chub invaded from the south, a refugia located south of the current
western third of Connecticut. However, Schmidt (1986) suggested that Cutlips
Minnow dispersed from the Atlantic Coastal Uplands Refugium (i.e., entered
Connecticut from the west), and Creek Chub colonized the study area from the
Northeastern Coastal Refugium. Our data indicated that Cutlips Minnow was
most abundant in Hudson River watersheds, became less common in western
Connecticut, and was absent in eastern Connecticut; thus the dispersal route
of this species may have been from the west (Schmidt 1986). The western distribution
of Creek Chub in our study area is difficult to explain. If Creek Chub
invaded from Northeastern Coastal Refugium like many other widespread
species of this study (Schmidt 1986), then its current distribution might be
expected to be more extensive. The species might have invaded from a refugia
located south of the current western third of Connecticut (Whitworth 1996),
but its extensive distribution pattern across North America makes it difficult
to identify post-glacial dispersal pathways to a particular region. Indeed, the
complexity of colonization history in the western region may explain why Axis
2 of NMS (Figs. 2b, 3b) depicts a north–south distributional gradient within the
western faunal region but not the eastern region.
Interestingly, Schmidt (1986) reported high assemblage resemblance
between Housatonic, Connecticut, and Thames watersheds, but our analyses
separated these watersheds into two faunal regions. High resemblance
was attributed to the ancient connections of these watersheds to glacial Lake
Ronkonkoma. However, the relatively fine resolution of our analyses may
support patterns that reflect colonization from other sources, such as Cutlips
Minnow from the Atlantic Coastal Uplands Refugium. The historical connection
of the major watersheds of Connecticut to the glacial lake was not sufficient
alone to explain present-day distributions of stream fishes in our study area.
The extent and validity of the two faunal regions outside the study region
is not known. The east–west pattern also is observed among native fishes of
Massachusetts (Hartel et al. 2002). The Creek Chub is mostly restricted to
the western one third of the state, and the distributions of Redfin Pickerel and
Creek Chubsucker are skewed toward the eastern coastal watersheds. In addition,
species such as Slimy Sculpin, Longnose Dace, and Blacknose Dace, are
distributed primarily in the western half of Massachusetts, and their eastern
distribution extent appears to be delimited approximately by the drainage divide
between the Connecticut and Merrimack rivers (Hartel et al. 2002). This
pattern also may relate to the classification of the Chicopee watershed into the
western faunal region despite its geographic location in the eastern half of our
study region. Given the fish assemblage similarity between Merrimack River
watersheds and coastal watersheds in Connecticut (Schmidt 1986), the eastern
2008 Y. Kanno and J.C. Vokoun 573
faunal region might extend north along the coast into the Merrimack River
watershed, including part of New Hampshire. However, the western faunal
region might not extend as far north, since a group of species with Mississippi
Valley affinity, such as Couesius plumbeus Agassiz (Lake Chub) and Phoxinus
eos Cope (Northern Redbelly Dace) are found in the northwestern part of
Massachusetts (Schmidt 1986, Hartel et al. 2002). The western extent of the
western faunal region might be similarly limited by the presence of species
originating from the Mississippi Valley Refugium. Smith (1985) described the
occurrence of eastern and western species in New York, which corresponded
to affinity with the Atlantic Coastal Refugium and Mississippi Valley Refugium,
respectively.
Our analyses described fish distributions at two spatially hierarchical
levels (i.e., faunal regions and assemblages within them); finer levels of spatially
nested classifications warrant further research. This analysis requires
integration of geomorphic channel types, surficial materials, and groundwater
interactions (Seelbach et al. 2006, Wehrly et al. 2006). Given the habitat differences
between the two regions, particular attention may be paid towards
understanding if there are inter-regional differences in the relative importance
of major environmental variables controlling species and assemblage distributions.
The longitudinal replacement patterns among species and assemblages
typically differ regionally (Fausch et al. 1994, Torgersen et al. 2006).
Results of our analyses provide useful information for aquatic resource
management in Connecticut. The fish assemblage types identified here can
help stratify inventories of aquatic resources in the state. From the viewpoint
of biodiversity conservation, some assemblages may require more attention
than others. Assemblages BE-W and DE-W were composed mainly of tolerant
and/or macro-habitat generalists, which are not only likely secure from endangerment
but also may become more common as human perturbations continue
across the landscape. In contrast, the Brook Trout dominated assemblage (A)
and the mixed fluvial assemblages (CE–W) likely will be more prone to declines
from human disturbances and thus merit more active management. Brook
Trout are vulnerable to a multitude of human disturbances including habitat
degradation (Lyons et al. 1996, Steedman 1988), exotic species (Larson and
Moore 1985, Waters 1983), and global warming (Meisner 1990).
Identifying high-quality stream habitat inhabited by Assemblage CE–W
warrants further inventory efforts. This process could be facilitated by developing
a tool that assesses the health of stream environments such as an index
of biotic integrity (IBI) (Karr 1981). Given the number of fluvial assemblage
types identified, it might be necessary to use more than one IBI to accurately
assess stream conditions in Connecticut streams. In Vermont, for example, 3
categories of streams exist for biomonitoring: (1) Brook Trout only streams,
(2) coldwater streams containing 2–5 species, and (3) streams containing 5 or
more species (Vermont Department of Environmental Conservation 2004).
While no IBI exists for the Brook Trout only streams, different IBIs are applied
to the other 2 categories. Connecticut may benefit from a similar approach.
Our results indicated that an equivalent of Vermont’s second stream
category would include some streams in Assemblages A and BE–W, and the
574 Northeastern Naturalist Vol. 15, No. 4
third category would mostly correspond to Assemblage CE–W. As a result, in
Connecticut, a coldwater IBI (Jacobson 1994) may be applicable primarily to
smaller streams, while another IBI (mixed-water IBI) may need to be developed
for more diverse fish assemblages in relatively larger wadeable streams.
Acknowledgments
This research was made possible by the Connecticut Department of Environmental
Protection through the State Wildlife Grants Program, the Storrs Agricultural
Experiment Station, and the University of Connecticut. Stream fish survey data in
Connecticut were made possible by the Sport Fish Restoration Act. The New York
Department of Environment Conservation, the Rhode Island Department of Environmental
Management, and the Massachusetts Department of Fish and Game all provided
stream fish survey data. We thank Melvin Warren, Jr., Eric Schultz, Rick Jacobson, and
two anonymous reviewers for comments that significantly improved this manuscript.
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