Fish assemblages respond to altered ﬂow regimes via ecological...

Fish assemblages respond to altered flow regimes viaecological filtering of life history strategies

MERYL C. MIMS AND JULIAN D. OLDEN

School of Aquatic and Fishery Sciences, University of Washington, Seattle, WA, U.S.A.

SUMMARY

1. In riverine ecosystems, streamflow determines the physical template upon which the life history

strategies of biota are forged. Human freshwater needs and activities have resulted in widespread

alteration of the variability, predictability and timing of streamflow, and anticipating the biotic

consequences of anthropogenic streamflow alteration is critical for successful environmental flow

management. In this study, we examined relationships between dam characteristics, metrics of flow

alteration and fish functional community composition according to life history strategies by coupling

stream flow records and fish survey data in paired flow-regulated and free-flowing rivers across the

conterminous United States.

2. Dam operations have generally reduced flow variability and increased flow constancy based on a

comparison of pre- and post-dam flow records (respective mean record lengths 26.2 and 43.1 years). In

agreement with ecological theory, fish assemblages downstream of dams were characterised by a lower

proportion of opportunistic species (a strategy favoured in environmental settings dominated by

unpredictable environmental change) and a higher proportion of equilibrium species (a strategy

favoured in more stable, predictable environments) compared to free-flowing, neighbouring locations.

3. Multiple linear regression models provided modest support for links between alteration of specific

flow attributes and differential life history representation below dams, and they provided strong

support for life history associations with dam attributes (age and release type). We also found support

for a relationship of both reduced flow variability and dam age with higher representation of non-native

species below dams.

4. Our study demonstrated that river regulation by large dams has significant hydrological and

biological consequences across the United States. We showed that on ecological time scales (i.e. the

order of years to decades), dams are effectively changing the functional composition of communities

that have established over millennia. Furthermore, the changes are directional and indicate a filtering by

dams for some life histories (equilibrium strategists) and against other life histories (opportunists).

Finally, our study highlights that dependence upon long-term flow records and availability of biotic

surveys extracted from national survey efforts limit our ability to guide environmental flow standards

particularly in data-poor regions.

Keywords: dams, flow regulation, freshwater fishes, life histories, United States

Introduction

Streamflow defines the physical template of riverine eco-

systems (Poff et al., 1997), provides longitudinal and lateral

access to foraging, spawning and recruitment habitat (Junk,

Bayley & Sparks, 1989), and acts as an evolutionary

selective force and an ecological filter of various survival

strategies employed by aquatic and riparian organisms

(Townsend & Hildrew, 1994; Lytle & Poff, 2004). At the

same time, human society requires water for life. Over the

millennia, humans have altered streamflow in riverine

systems for a myriad of reasons, including harnessing

water for drinking, irrigation and recreation and providing

flood control and hydropower (Gleick, 2003). The human

freshwater footprint encircles the globe with nearly half of

major river systems affected by dams (Vorosmarty et al.,

Correspondence: Meryl C. Mims, School of Aquatic and Fishery Sciences, University of Washington, 1122 NE Boat Street, Seattle, WA 98105,

U.S.A. E-mail: [email protected]

Freshwater Biology (2013) 58, 50–62 doi:10.1111/fwb.12037

50 � 2012 Blackwell Publishing Ltd

2010; Lehner et al., 2011). Future construction of dams,

particularly in economically developing nations, is an

inevitable consequence of human population growth and

the increasing freshwater and electricity needs in a chang-

ing climate (Palmer et al., 2008).

Despite providing many societal benefits, river regula-

tion by dams has also caused considerable ecological

damage and the loss of important ecosystem services

valued by society. Dams fragment rivers creating strings

of artificial lakes punctuated by often impassable barriers

(Reidy Liermann et al., 2012), and they alter physical

riverine habitat (Vorosmarty et al., 2003) and water quality

in both up- and downstream directions (Olden & Naiman,

2010). In particular, dams have been shown to be the

primary driver of altered streamflows throughout the

United States (Carlisle, Wolock & Meador, 2011), resulting

in reduced flow seasonality and variability (Poff et al.,

2007) and generally increasing short-term minimum flows

while decreasing short-term maximum flows (Magilligan

& Nislow, 2005). These changes alter the historical

disturbance regime, rendering some biotic adaptations

to these regimes obsolete while potentially favouring

others. For example, reduced flow variability by dams has

been associated with significant losses of native fish

species (Meador & Carlisle, in press) while concurrently

creating new niche opportunities that are often occupied

by non-native species with life histories novel to the

system or basin (Bunn & Arthington, 2002; Olden, Poff &

Bestgen, 2006).

Social, political and scientific pressure to ‘legitimise’

rivers as water users has resulted in a move to find

compromises in dam management to meet both human

and ecosystem water needs (Arthington et al., 2010)

through the provisioning of ‘environmental flows’

(Arthington et al., 2006) and large-scale flood events

(Konrad et al., 2011). However, despite the growing

recognition of ecologically sustainable water manage-

ment, natural resource agencies cite a lack of scientific

information required to support the application of this

approach. Poff & Zimmerman (2010) conducted an

extensive review of ecological responses to flow regula-

tion and reported that the biotic integrity of fish assem-

blages generally decreased with increased flow alteration.

Dam-induced changes to the flow regime can vary by

region and modes of dam operation (Magilligan &

Nislow, 2005; McManamay, Orth & Dolloff, 2012); how-

ever, due to overall sample size and disparate reporting

practices across studies, Poff & Zimmerman (2010) were

unable to distinguish the impacts of different components

of the flow regime and were limited to reporting the

effects of altered flow magnitudes.

Broader applicability of realised relationships between

flow alteration and biotic responses is important for

understanding the ecological impacts of dams and for

guiding environmental flow practices for ecologically

sustainable river management (Poff et al., 2010; Konrad

et al., 2011). General frameworks for prediction of biotic

responses to flow regulation, such as vegetation-flow

response guilds built upon traits rather than taxonomy

(Merritt et al., 2010), are more widely applicable than river

or reach-specific models that often do not translate

beyond their intended locality. Increasingly, freshwater

ecologists have turned to the use of species traits as a

universal currency for studying flow:ecology relation-

ships across diverse taxonomies and geographies

(reviewed in Poff et al., 2006; Frimpong & Angermeier,

2010). Life history traits of freshwater fishes are well

studied and well suited as a platform to test general

relationships between the flow regime and biological

communities; they also have direct implications for

ecosystem functioning (Winemiller, 2005). Winemiller &

Rose (1992) identified three life history strategies in both

freshwater and marine fishes that represent the essential

trade-offs among the basic demographic parameters of

survival, fecundity and onset and duration of reproduc-

tion. Opportunistic strategists are small-bodied species

with early maturation and low juvenile survivorship and

are predicted to be associated with habitats defined by

frequent and intense disturbance. Periodic strategists are

characterised by large body size, late maturation, high

fecundity and low juvenile survivorship and are likely to

be favoured in highly periodic (seasonal) environments.

Equilibrium strategists are typically small to medium in

body size with intermediate times to maturity, low

fecundity per spawning event and high juvenile survi-

vorship largely due to high parental care and small clutch

size. Equilibrium strategists are predicted to be favoured

in more stable habitats with low environmental variation.

The three life history strategies of the continuum are

hypothesised to be adaptive with respect to variability,

predictability and seasonality of environmental regimes

(Winemiller, 2005). Convergence of trait composition

along hydrologic gradients has been demonstrated for

freshwater fishes (e.g. Lamouroux, Poff & Angermeier,

2002; Logez, Pont & Ferreira, 2010), and empirical inves-

tigations that test predictions from life history theory

support these relationships (e.g. Tedesco et al., 2008;

Olden & Kennard, 2010; Carlisle et al., 2011). Recently,

we reported that life history composition of fish assem-

blages in free-flowing rivers of the United States are

associated with critical flow dimensions of variability,

predictability and seasonality and that these associations

Fish assemblages respond to altered flow regimes 51

� 2012 Blackwell Publishing Ltd, Freshwater Biology, 58, 50–62

are largely predicted by Winemiller and Rose’s trilateral

continuum model (Mims & Olden, 2012). Taken together,

a traits-based approach to establishing flow–ecology

relationships provides a promising yet largely untested

framework to evaluate and generalise the ecological

effects of flow regulation by dams.

In this study, we quantified the relationships between

major facets of the flow regime and life history compo-

sition of fish assemblages downstream from large dams

across the United States and tested whether these asso-

ciations are predicted by life history theory. We posited

and tested three main hypotheses. (i) We expected dams

to reduce variability and increase predictability of flows,

and we also expected to see changes in timing and

duration of seasonal flows. (ii) We predicted fish assem-

blages at flow-regulated sites to show proportionally

lower number of species exhibiting opportunistic and

periodic life histories and higher richness of equilibrium

life history strategies relative to fish assemblages at free-

flowing sites. (iii) We predicted that observed changes in

three primary components of the flow regime (variability,

predictability and seasonality) caused by dam operations

will predictably shape native and non-native fish assem-

blage structure via the ecological filtering of life history

strategies.

Methods

Study sites

Study sites consisted of ‘triplet’ sets of a dam, flow gauge

and fish survey that met strict search criteria to ensure

that the hydrologic conditions measured at the gauge

station reflect the conditions where fish assemblages were

surveyed (following Larned et al., 2010). Dams were

identified through the Army Corps National Inventory

of Dams (NID: U.S. Army Corps of Engineers, 2000), a

data set of c. 79 000 large dams (>2 m) throughout the

United States. Pre- and post-impoundment daily flow

records were obtained from the U.S. Geological Survey’s

(USGS) gauge records, reflecting over 25 000 gauging

stations. Fish occurrence data for sites were synthesised

from a number of regional and national survey efforts,

including the Environmental Protection Agency’s (EPA)

Environmental Monitoring and Assessment Program

(EMAP) and the regional counterpart (Regional EMAP

or REMAP) (Hughes, Paulsen & Stoddard, 2000), the

USGS National Water Quality Assessment Program

(NAWQA) (Gilliom, Alley & Gurtz, 1995) and from

various state agencies. This resulted in a national database

of 5951 unique sampling sites (data collected between

1989 and 2002) with 530 native and non-native fishes in

North America represented (Herlihy, Hughes & Sifneos,

2006). Because the data available to us represented one-

time surveys spanning a 14-year period from potentially

disparate (but effective and efficient) survey techniques,

we examined species presence ⁄absence rather than abun-

dance.

Identification of candidate study sites was performed

by overlaying (in a Geographic Information System)

locations of dams, gauges and fish surveys onto a national

hydrography of streams and rivers (NHD Plus: Simley &

Carswell, 2009). Dams with both a fish survey and a gauge

within a radius of 20 km were identified. Candidate

triplets were then manually evaluated to ensure (i) the

gauge and survey were downstream of the dam; (ii) the

components of the triplets were within 20 river km of each

other and (iii) maximum difference in mean annual flow

(modelled by NHD Plus) between the components of the

triplets was <20% in order to ensure that tributary input

did not disproportionately influence flow conditions at

the fish survey location. Additionally, we screened for any

upstream dams (within 50 river km on the mainstem or

any major tributaries with >10% of mean annual flow at

the dam) built prior to the pre-impoundment flow record.

Once candidate sites were identified, gauge records

were evaluated for completeness. We required at least

15 years of data (<10 days of missing data per year) prior

to the fish survey to ensure accurate and precise estimates

of hydrologic metrics following Kennard et al. (2010). Our

only exception to this rule was the Ridgeway Dam study

site for which only 12 years of flow data are available

post-impoundment and pre-fish survey due to the rela-

tively recent construction of the dam. The McCloud and

Trenton Dam study sites required a second gauge to

provide complete pre-impoundment flow records. In both

these cases, the total maximum distance of the triplet was

minimally relaxed to incorporate the second gauge. The

maximum difference in mean annual flow for these

additional gauges adhered to our search criteria (13.3%

for McCloud and 6.1% for Trenton).

Finally, each study site was assigned a reference fish

survey from a nearby free-flowing stream because pre-

impoundment fish surveys were not available. Search

criteria were designed to identify the best available

reference survey and were as follows: (i) candidate

surveys were geographically close (within a 200-km

search radius) and located in the same major river

drainage; (ii) reference site had no dams within 50-km

upstream along the mainstem or any major (>10%)

tributaries and (iii) if multiple candidate sites were

identified, the reference site selected was that most similar

52 M. C. Mims and J. D. Olden


in cumulative drainage area to triplet fish survey (mean %

difference in cumulative drainage area = 33.8, SD = 26.0).

Species life history strategies

Fish life history strategies were evaluated according to the

model of Winemiller & Rose (1992) that positions species

along three primary life history axes defining the oppor-

tunistic, periodic and equilibrium endpoints. The three

axes of the model are (i) ln(fecundity) defined as the

number of eggs or offspring per female per spawning

season; (ii) ln(length at maturation) defined as mean

female total length at maturation (mm) and (iii) ln(egg

size + 1) + ln(parental care + 1) (termed juvenile invest-

ment), defined as the mean diameter of mature, fully

yolked ovarian oocytes (mm) and an index of parental

care following Winemiller (1989). Fish life history traits

were obtained from a comprehensive database for fresh-

water fishes of the United States and Canada synthesised

from various literature, agency and expert accounts

(reported in Mims et al., 2010). Because many species

occupy intermediate positions in life history space, we

followed Olden & Kennard (2010) by conducting a ‘soft’

classification according to a species’ relative affinity with

each strategy. This was accomplished by calculating the

Euclidean distance in trivariate life history space between

the species’ position and the strategy endpoints, normal-

ising the distances between 0 and 1, and then computing

the inverse of these values to provide a ‘strategy weight’

for each species. This resulted in each fish species having

a proportional weight or affinity to the opportunistic,

periodic or equilibrium strategies. For each site, we

summed the strategy weights of species present and

computed the proportional value for each strategy to

represent relative life history composition.

Hydrologic metrics

We examined six key hydrologic metrics: annual coeffi-

cient of variation (AnnCV), high pulse count (HPC), base

flow index (BFI), flow predictability (FPred), flow con-

stancy ⁄flow predictability (Const ⁄Pred) and high pulse

duration (HPD). These metrics were chosen because they

address fundamental dimensions of the flow regime

(variability, predictability and seasonality) that drive

patterns of fish occurrence via the selection of life history

strategies (Bunn & Arthington, 2002; Lytle & Poff, 2004;

Mims & Olden, 2012) and represent the broader suite of

hydrologic metrics that are available from the literature

(Olden & Poff, 2003). The metrics were calculated for pre-

and post-impoundment records using the software Indi-

cators of Hydrologic Alteration (Mathews & Richter,

2007).

We also calculated an overall flow alteration index by

performing a principal component analysis (PCA) of the

six hydrologic metrics for each pre- and post-alteration

record to extract synthetic axes representing the dominant

gradients of variability in flow conditions. All hydrologic

metrics were first ln(x + 1) transformed to help achieve

normality, and we performed a PCA by computing a

singular value decomposition of a correlation matrix to

account for data measurement along disparate scales. The

index of flow alteration was then computed for each site

as Euclidean distance between the pre- and post-alteration

scores from the first three PCs (all statistically significant

according to a Monte Carlo randomisation test of the

eigenvalues). All statistical analyses were performed in

R version 2.14.1 (R Development Core Team, 2011) using

the vegan 2.0-3 package (Oksanen et al., 2012).

Statistical analyses

A paired Student’s t-test was used to evaluate directional

differences in flow metrics (variability, predictability and

seasonality) and proportional life histories (opportunistic,

periodic and equilibrium) between free-flowing and flow-

regulated river sites. A Wilcoxon signed rank test was

performed for those comparisons where the assumption

of normality was not met.

Next, we examined whether dam-induced changes in

flow variability, predictability and seasonality were

related to differences in proportional life history compo-

sition at flow-regulated versus free-flowing sites. This

involved modelling a series of response variables as a

function of the hydrologic metrics and dam attributes

using multiple linear regression. Comparisons of fish

assemblages in flow-regulated versus free-flowing rivers

resulted in four response variables representing per cent

change in proportional life history composition of the

opportunistic, periodic and equilibrium strategies and

proportional species composition of fishes non-native to

the particular locality at the scale of the 6-digit hydrologic

unit code (HUC) as described by Lawrence et al. (2011). A

total of 11 predictor variables was examined: the %

difference of the six previously described hydrologic

metrics, number of years since dam construction refer-

enced to the year of fish survey (YearsAlt), location of

water release from dam (hypolimnetic or surface) and the

overall index of flow alteration. We also considered %

difference in cumulative drainage area (%DCDA) and

linear distance between the flow-regulated versus free-

flowing sites to account for the possible effects of



reference river selection. Similarly, we also tested for

relationships between the response variables and the

straight-line distance between surveys to check for spatial

autocorrelation that may confound analyses.

An iterative but strategic approach was used to evaluate

candidate models. Each response variable was modelled

independently as a function of both dam descriptors and

hydrologic alteration. Hydrologic alteration was mea-

sured either by the Flow Alteration Index or by a subset of

the six major hydrologic variables. Rather than modelling

all six major hydrologic variables, we modelled a maxi-

mum of three hydrologic metrics at one time. This was

done both to avoid over-parameterisation of the model

(given our limited sample size) and to minimise correla-

tion between hydrologic metrics. The six major hydrologic

variables were divided into two groups of three variables

each (first group: HPC, HPD and BFI; second group:

AnnCV, constancy ⁄predictability and flow predictability).

The fundamental characteristics of the flow regime (var-

iability, predictability and seasonality) were represented

in each set. We then evaluated models using backward

stepwise model selection. Goodness of fit of models was

evaluated by the Akaike information criterion, corrected

for low sample size (AICc) (Burnham & Anderson, 2002).

DAICc, likelihood and the evidence ratios were calculated

for the top four candidate models for each response

variable.

Results

We identified 12 sites that met our strict criteria for

inclusion (Table 1, Fig. 1). The length of pre-impound-

ment discharge records in our study ranged from 15 to

52 years (mean = 26.2 years), and the length of post-

impoundment discharge records ranged from 12 to

95 years (mean = 43.1 years). Distance between flow-reg-

ulated and free-flowing survey pairs was not correlated

with any measure of per cent difference in fish assem-

blages, and although free-flowing sites were generally

located higher in watersheds than flow-regulated surveys,

per cent difference in cumulative drainage was not

correlated with any observed difference in fish assem-

blages.

Dams significantly reduced downstream flow variabil-

ity with lower AnnCV (Wilcoxon signed rank test, n = 12,

z = )2.12, P = 0.034) and fewer high flow pulses (HPC,

Wilcoxon signed rank test, n = 12, z = )2.11, P = 0.037)

(Fig. 2a). Dam operations also increased flow con-

stancy ⁄predictability ratios (ConstPred, paired t-test,

t11 = 2.26, P = 0.045) but decreased flow predictability

(FlowP, paired t-test, t11 = 2.39, P = 0.036) (Fig. 2a). Dams

did not cause a significant overall reduction or increase

BFI or HPD (Fig. 2a). Fish assemblages downstream of

dams had proportionally lower representation of the

opportunistic life history strategy than their free-flowing

counterparts (paired t-test, t11 = 3.43, P = 0.006), propor-

tionally higher representation of the equilibrium life

history strategy (paired t-test, t11 = )3.575, P = 0.004),

and no significant difference in the proportion of species

displaying periodic strategies (Fig. 2b).

We identified a consistent assemblage shift towards

dominance by equilibrium strategists and away from

opportunistic strategists downstream of dams (Fig. 3a).

The direction of change in strategy composition was

largely independent of primary dam purpose. We also

identified via PCA two major axes characterising 74% of

the overall variation observed among the six major

hydrologic metrics (Fig. 3b). The first principal compo-

nent (PC1) explained 55% of the variation along a

gradient from more variable flows (positive scores asso-

ciated with higher Annual CV) to more steady flows

(negative scores associated with higher BFI). Sites gener-

ally shifted towards steadier, less variable flows post-

impoundment with 10 of 12 sites having a lower score

along PC1 for post-impoundment flow records than pre-

impoundment records; this pattern was largely indepen-

dent of dam purpose. PC2 explained 19% of the total

variation and represented a gradient from more seasonal

flows (negative values associated with an increase in

HPD) to less seasonal flows (positive values associated

with an increase in HPC). In contrast to PC1 (x-axis),

trends were less clear, with movement following no

general uniform direction along PC2 (y-axis).

Multiple linear regression analysis identified some

potential mechanisms for observed differences in life

history composition between flow-regulated and free-

flowing river sites (Table 2). Proportional change in

opportunistic strategist composition was positively

related to BFI; this model was more than twice as likely

given the data than the next most competitive model that

included the constancy ⁄predictability ratio. The most

supportive model for proportion of periodic strategists

included a positive relationship with dam age (YearsAlt),

with younger dams being associated with reduced repre-

sentation of the periodic life history at flow-regulated

sites. Proportional change of equilibrium strategists was

associated with the positive effect of flow predictability

(FlowP) alone, with this model having 1.7 times more

support than hypolimnetic release alone (HypoRels) and

2.9 times more support than both flow predictability and

hypolimnetic release (HypoRels) together. Finally, the

most supportive model for proportion of non-native



species included a negative association with annual

variability (AnnCV) and dam age (YrsAltered); both

drivers together had 2.9 times the support of annual

variability alone.

Overall flow alteration (Flow Alteration Index) was not

identified as a significant predictor of any response

variables. This may be due to more nuanced relationships

that vary by dam operation or flow regime. Our sample

size prohibits explicit statistical testing of relationships by

dam operation purpose; however, we plotted Flow Alter-

ation Index versus proportional life history and % non-

native changes to explore possible associations (Fig. 4).

Perhaps the most suggestive pattern is that flow alteration

downstream of hydropower dams appears to have a

negative relationship with periodic life history (Fig. 4b)

(the opposite being true for flood control dams) and a

positive relationship with equilibrium life history

(Fig. 4c); no trends are readily apparent for the opportu-

nistic life history (Fig. 4a) or the % non-native species

(Fig. 4d).

Discussion

Our study demonstrates that river regulation by large

dams has significant hydrological and biological conse-

quences for riverine ecosystems across the United States.

Despite varying release schedules due to different dam

purposes, changes in flow variability and representative

fish life history composition were in most cases consistent

in direction across study sites and largely agreed with our

Table 1 Study site attributes, including dam type that indicates primary dam purpose and includes hydropower (Hydro), flood control

(FloodC) and locks. Fish survey year’ refers to the year the flow-regulated survey was conducted; ‘Triplet distance’ refers to the maximum

distance between any two components (gauge(s), fish survey and dam) of a given triplet; ‘D MAF’ indicates absolute value of maximum %

difference (estimated using NHD+) between any two components of a given triplet; ‘D Drainage area’ refers to % difference between drainage

area at flow-regulated and free-flowing fish survey pairs (calculated as [(reference ) dam) ⁄ dam] · 100) and ‘Survey distance’ is the straight-line

distance between flow-regulated and free-flowing fish survey pairs

Site Dam River

Construction

year

Dam height

(m) and type

Hypolimentic

release?

Drainage

area at dam

(km2)

A McCloud McCloud 1965 73, Hydro Yes 912

B Morrow Point Gunnison 1967 143, Hydro Yes 10276

C Ridgeway Uncompahgre 1986 101, FloodC Yes 656

D Structure 65D Kissimmee 1965 12, Lock No 7464

E Yellowtail Big Horn 1965 160, Hydro Yes 50905

F Trenton Republican 1952 44, FloodC No 17940

G Dillon Licking 1960 36, FloodC Yes 1958

H Delaware Olentangy 1948 34, FloodC No 1060

I Mohawk Walhonding 1937 28, FloodC No 3870

J Allegheny Lock

and Dam 8

Allegheny 1931 18, Lock No 22876

K Wanship Weber 1956 53, Hydro No 854

L Glen Ferris Kanawha 1901 8, Hydro No 21702

Site DMAF

Drainage area at

survey

(km2)

D Drainage

area

(km2)

Survey

distance

(km)

Flow

gauge

Fish survey

year

Triplet

distance

(km)

A 13.3 696 )24 190 11367500 (pre)

11367760 (post)

1998 20.6

B 9.7 5501 )46 56 09128000 1998 17.1

C 8.3 607 )8 97 09146200 1998 4.5

D 1.7 3614 )52 91 02273000 1998 11.3

E 0.2 40807 )20 66 06287000 2001 4.9

F 6.1 5155 )71 89 06828000 (pre)

06829500 (post)

1995 42.0

G 1.2 1200 )39 58 03147500 1994 4.5

H 3.4 1058 )0.3 27 03225500 1999 8.5

I 0.1 976 )75 22 03138500 1994 3.3

J 0.0 10866 )53 66 03036500 1997 9.5

K 18.4 863 1 62 10130500 2001 16.1

L 0.0 17343 )20 24 03193000 1997 1.8



predictions. Dams reduced flow variability across sites

and altered seasonality and predictability of flows (as

evidenced by reduced flow predictability and increased

flow constancy). Consistent with this pattern, fish

assemblages downstream of dams generally had lower

representation of opportunistic strategists and higher

McCloud RiverMcCloud Dam (A)

Big Horn RiverYellowtail Dam (E)

Weber RiverWanship Dam (K) Gunnison River

Morrow Point Dam (B)

Uncompahgre RiverRidgeway Dam (C)

Republican RiverTrenton Dam (F)

Olentangy RiverDelaware Dam (H)

Walhonding RiverMohawk Dam (I)

Licking RiverDillon Dam (G)

Kanawha RiverGlen Ferris Dam (L)

Kissimmee RiverStructure 65D (D)

Allegheny RiverAllegheny Lock and Dam 8 (J)

HydropowerFlood controlLock

Big Horn River, Yellowtail Dam (E)

U.S. Bureau of Reclamation

Uncompahgre River, Ridgeway Dam (C)

U.S. Bureau of Reclamation

Allegheny River, Allegheny Lock and Dam 8 (J)

U.S. Army Corps of Engineers

Hypolim. release (black fill)Surface release (grey fill)

Fig. 1 Study sites (n = 12) coded by dam function. Flood control sites (n = 5) are shown as triangles, hydropower sites (n = 5) are shown as

circles and locks (n = 2) are shown as squares. Black symbols represent sites with hypolimentic releases. Specific sites are coded by letters, and

dam names and rivers are as follows: A: McCloud Dam, McCloud River, CA; B: Morrow Point Dam, Gunnison River, CO; C: Ridgeway Dam,

Uncompahgre River, CO (pictured lower left); D: Structure 65D, Kissimmee River, FL; E: Yellowtail Dam, Big Horn River, MT (pictured lower

middle); F: Trenton Dam, Republican River, NE; G: Dillon Dam, Licking River, OH; H: Delaware Dam, Olentangy River, OH; I: Mohawk Dam,

Walhonding River, OH; J: Allegheny Lock and Dam 8, Allegheny River, PA (picture lower right); K: Wanship Dam, Weber River, UT; L: Glen

Ferris Dam, Kanawha River, WV.



representation of equilibrium strategists than their free-

flowing counterparts. Life history theory (Winemiller &

Rose, 1992; Winemiller, 2005) and empirical evidence

(stream reaches: Mims & Olden, 2012; catchments: Tede-

sco et al., 2008) from free-flowing rivers suggest that

reduced flow variability and increased flow constancy

will favour equilibrium strategists that are well suited to

environments characterised by low disturbance.

Although the correlation between flow alteration and

life history representation below dams was predicted

robustly by life history theory, isolating and identifying

mechanistic relationships between individual flow met-

rics and individual life histories proved more difficult.

The positive relationship between equilibrium strategists

and flow predictability is in agreement with life history

theory; however, because flow predictability is largely

reduced by dams in our data set, increased flow predict-

ability is not a likely driver for elevated representation of

equilibrium strategists below dams. Life history theory

predicts that increased variability and reduced predict-

ability ⁄constancy should favour opportunistic species;

instead, we found a positive relationship between oppor-

tunistic strategists and flow predictability (BFI) and

constancy (ConstPred). Our inability to identify mecha-

nistic relationships predicted by life history theory might

be resolved with a larger data set allowing for analysis

within regions or by dam operation type, or may point to

alternative drivers of lower opportunistic and higher

equilibrium representation below dams.

The negative correlation between streamflow variabil-

ity (AnnCV) and % non-native species is consistent

with increased invasion success of non-native species as

streamflow variability is reduced. By changing the dis-

turbance regime in rivers, dams can select against

native species well adapted to a historical regime but

poorly equipped for a new one (Bunn & Arthington,

2002). Meador & Carlisle (in press) report an association

between decreased streamflow variability and a loss of

native species, particularly fluvial specialists, at roughly

half of 97 locations in the eastern United States. Similar

declines in fluvial specialist species have been reported

for the Guadalupe and San Marcos Rivers in Texas (Perkin

& Bonner, 2011) and for the White River in Arkansas

(Quinn & Kwak, 2003) following impoundment. A new

flow regime creates new niche opportunities, and

although natural flow variability can result in a range of

life history strategies represented among native taxa

(Freeman et al., 2001), not all life history niche space is

occupied in all assemblages (Olden et al., 2006). Phyloge-

netic constraints operating at regional scales and ecolog-

ical ‘filters’ acting at local scales restrict available life

histories in a given assemblage (Southwood, 1977; Jack-

son, Peres-Neto & Olden, 2001). Dominant life history

strategies of North American freshwater fishes indeed

vary regionally and are associated with historical large-

scale climatic selection pressures (Mims et al., 2010). Dams

can create different thermal and flow regimes that

suddenly provide ecological niches to which few native

species are adapted. Non-native species equipped for new

flow and thermal regimes are particularly well poised to

establish downstream of dams. In one example, species

tending towards the equilibrium life history endpoint

strategy are notably rare from native fish assemblages of

the Colorado River Basin (U.S.A.), historically character-

ised by flashy, highly variable flows. However, as flow

Opportunis�c Periodic Equilibrium

Variability Predictability Seasonality

AnnCV HPC FlowP BFI ConstPred HPD

* *

* * * *

Flow regime

Life histories

(333.3)(802.5, 560.8)(a)

(b)

Fig. 2 Box plots of per cent difference between free-flowing reference

sites and flow-regulated rivers of (a) six flow metrics and (b) three

major life history strategies. Open circles indicate outliers; values of

large outliers are noted in parentheses (a). Variables for which paired

t-test results were significantly different from 0 (P < 0.05) are marked

with an asterisk (*).



alteration has led to more predictable, less variable flows,

many established non-native fishes such as rainbow trout

(Oncorhynchus mykiss) and channel catfish (Ictalurus punct-

atus) represent historically absent equilibrium life history

strategists (Olden et al., 2006).

Although flow regime alteration is often credited as the

primary factor leading to changes in downstream fish

assemblages (Carlisle et al., 2011), other factors may play a

role in the selection of certain life histories by dams. For

example, our results identify hypolimnetic release, pre-

dominantly from hydroelectric facilities releasing cold

water, as a likely driver of increased equilibrium strategists

downstream of dams. Like flow, water temperature plays a

critical role in metabolic rates, physiology and life history

traits of freshwater species and helps determine rates of

important ecological processes such as nutrient cycling and

B

C

A

DL

HK

G

I

J

E F

B

C

A

D

L

HK

GI

JE

F

Base flow Index

Annual CV

Hig

h pu

lse

dura

tion

Hig

h pu

lse

coun

t

(a) (b)

Opportunistic strategists (%)

Equi

libri

um s

trat

egis

ts (%

)

Fig. 3 Directional differences between (a) free-flowing and flow-regulated rivers for per cent opportunistic and equilibrium life history strat-

egies and (b) a principal components ordination summarising variation among rivers according to the six major flow metrics. Dam types are

coded by line type where solid lines indicate hydropower, dashed lines indicate flood control, and dotted lines indicate locks. Sites are labelled

at the base of each arrow, and labels correspond to Fig. 1.

Table 2 Multiple linear regression models quantifying relationships between flow alteration and fish assemblage structure. The best models

(evidence ratios <3) for each response type (three life histories and proportional representation of non-native species) are shown. AnnCV,

annual coefficient of variation; BFI, base flow index. DAICc is the difference between the top performing model (minimum AICc) and each

individual model AICc. The Akaike weight (wi) is calculated as the ratio of each model’s likelihood (relative strength of evidence for the model)

to the sum of all the other model likelihoods for each response type. The evidence ratio is calculated by dividing the wi of each model by the wi of

the top model

Response Model K Multiple R2 AICc DAICc Likelihood wi Evidence ratio

Opportunistic BFI (+) 3 0.321 98.9 0.0 1.000 0.582 1.0

ConstPred (+) 3 0.215 100.6 1.7 0.418 0.244 2.4

Periodic YrsAltered (+) 3 0.584 104.4 0.0 1.000 0.869 1.0

Equilibrium FlowP (+) 3 0.349 97.6 0.0 1.000 0.484 1.0

HypoRels (+) 3 0.290 98.6 1.0 0.598 0.289 1.7

HypoRels (+), FlowP (+) 4 0.474 99.8 2.1 0.343 0.166 2.9

Non-native species AnnCV ()), YrsAltered ()) 4 0.662 132.9 0.0 1.000 0.642 1.0

AnnCV ()) 3 0.402 135.0 2.1 0.343 0.220 2.9



productivity (Caissie, 2006). Fluctuations in cold water

resulting from hydropeaking operations are often associ-

ated with reductions in river productivity (reviewed in

Cushman, 1985), and our study supports predictions that

equilibrium life history strategies of optimising juvenile

survivorship by investing resources into fewer offspring

are favoured in resource-limited environments (McCann,

1998). In some cases, flow and thermal events are so tightly

coupled that only a synergistic change in both is sufficient to

illicit fish responses such as spawning, migration and

foraging (reviewed by Olden & Naiman, 2010). Cooler

temperatures below dams are often desirable for establish-

ment of coldwater (often non-native) salmonid and trout

fisheries popular with recreational fishermen, but these

cooler temperatures are detrimental to native species that

require warmer temperatures to survive and ⁄or reproduce

(Olden & Naiman, 2010). With conditions favouring non-

native species and potentially disadvantaging native spe-

cies, it is not surprising that increases in equilibrium

strategists were positively correlated to sites with hypo-

limnetic releases.

We identified dam age as another non-flow driver of life

history differences between free-flowing and flow-

regulated rivers. Dam age was positively associated with

the proportional representation of periodic strategists in

flow-regulated regimes. This at first seems counterintui-

tive given that dams (i) fragment river ecosystems and

often block movement of periodic strategists (Reidy

Liermann et al., 2012) and (ii) dampen predictable, sea-

sonal fluctuations in flow that are important in the

survival and success of many periodic strategists that

often migrate at times of large flow events (Bunn &

Arthington, 2002). However, we identified three reasons

why we may see this pattern. First, we show that,

although dams do generally dampen variability and

increase constancy of flows, seasonal changes vary

between sites and do not show a clear pattern of

alteration. It is possible that these changes act upon

periodic strategists in different ways and thus illicit

varying responses across sites. Second, the ecological

effects of dams vary with time since construction. For

example, water chemistry downstream of dams will

change as organic matter collects and decomposes in the

reservoir, and geomorphological changes to the stream

channel also manifest slowly over time (Ligon, Dietrich &

Trush, 1995). The biotic response to dams may also fall

along a temporal gradient depending on attributes such as

species generation time. Quinn & Kwak (2003) found that

fish assemblages sampled immediately post-impound-

ment on the White River, Arkansas, showed only limited

change and retained many of the native species observed

prior to impoundment. However, sampling 30-year post-

impoundment revealed that many fluvial specialists

adapted to fast-flowing water were absent and that

communities downstream of the dam were dominated

by non-native salmonid species and only a few native

species. Our observation of a positive relationship

between dam age and long-lived periodic strategists

may simply be a proportional response to the decrease

in short-lived opportunistic species. Periodic strategists

Flow alteration index

Dam operation

% Non-nativesEquilibriumPeriodicOpportunistic(a) (b) (c) (d)

Fig. 4 Pairwise per cent differences of (a) opportunistic, (b) periodic and (c) equilibrium proportional life history strategies and (d) % non-

native species versus the Flow Alteration Index. Symbols indicate dam type, and symbol fill indicates release type.



may show longer persistence simply due to differential

longevity. Re-analysing species presence ⁄absence data

from Quinn & Kwak’s (2003) study and using life history

data (described in this study), we found that equilibrium

strategists proportionally increased over time (R2 = 0.97),

opportunistic strategists decreased (R2 = 0.72) and peri-

odic strategists showed no strong trend (decreasing

immediately post-impoundment and then increasing

30 years post-impoundment) (R2 = 0.33). This lends sup-

port to this mechanism.

The third possible explanation for the positive relation-

ship between dam age and periodic strategists is that this

relationship is not mechanistic but instead is explained by

regional biogeography. The oldest dams in our data set

are generally located in the eastern or midwestern United

States, and the younger dams are in the western United

States. The presence of the periodic life history in native

fish assemblages increases from east to west across the

United States (Mims et al., 2010), such that fish assem-

blages in the western United States simply have greater

potential for change than assemblages in the eastern

United States. Lastly, dam age was also identified as a

significant predictor of % non-native species with higher

% non-native species downstream of dams associated

with younger dams. This may also be a spurious regional

association as non-native fish species constitute a greater

proportion of fish assemblages in western United States

(Rahel, 2000).

Relationships between flow and life history strategies do

appear to transcend regions (Mims & Olden, 2012); how-

ever, characterisation of biotic responses by region may

provide greater utility to dam managers attempting to set

ecological flow standards (Poff et al., 2010). Our sample size

prevents a rigorous examination of biotic impacts of dams

within-region and within-operation types, and this may

prevent us from detecting biotic relationships that are

indeed present (Magilligan & Nislow, 2005). Despite the

wide availability of dam, gauge and fish survey data, we

were only able to identify 12 sites throughout the United

States that met our strict criteria. Close to 100 candidate

triplets were identified throughout the country, but closer

inspection of flow records from these gauges revealed

many years or even decades of missing flow data. Magil-

ligan & Nislow (2005) reported similar problems when

conducting an analysis of hydrologic changes by dams

throughout the United States, and this issue highlights the

need for continued efforts at hydrologic classification of

flow regimes for data-poor streams (Olden, Kennard &

Pusey, 2012) and predicting altered flow conditions at

ungauged rivers (Eng et al., in press). Furthermore, the

continuation and creation of regional and national biomon-

itoring programmes in data-poor regions as well as the

development of information systems that increase access to

local, state and federal databases are recommended. Sur-

veys and monitoring programmes specifically designed to

understand the roles of flow regime and flow alteration will

help avoid potential biases of monitoring data not designed

for that purpose.

Our findings demonstrate that life history composition

of freshwater fish assemblages that are built over millen-

nia upon particular habitat templates are significantly

altered downstream of dams in the order of a few

decades. That we detected this result using a conservative

presence ⁄absence approach applied to a relatively small

sample size holds promise for the utility of a life history,

traits-based approach to understanding how dams alter

downstream fish communities. Further elucidation of

mechanistic links may require modelling of flow regimes

and biotic communities to inform environmental flow

standards.

Acknowledgments

We thank Alan Herlihy for providing the fish occurrence

data, LeRoy Poff and Zach Shattuck for their contributions

to the trait database and Dave Lawrence for his assistance

with the fish database. Trevor Branch and two anony-

mous reviewers provided valuable comments that im-

proved the manuscript. Funding for MCM was provided

by a University of Washington Top Scholar Graduate

Fellowship, a National Science Foundation Graduate

Research Fellowship (Grant No. DGE-0718124) and by

the John N. Cobb Scholarship in Fisheries and the

H. Mason Keeler Endowment for Excellence through

University of Washington’s School of Aquatic and Fishery

Sciences. JDO acknowledges funding support from the

U.S. Environmental Protection Agency Science To

Achieve Results (STAR) Program (Grant No. 833834).

References

Arthington A.H., Bunn S.E., Poff N.L. & Naiman R.J. (2006)

The challenge of providing environmental flow rules to

sustain river ecosystems. Ecological Applications, 16, 1311–

1318.

Arthington A.H., Naiman R.J., McClain M.E. & Nilsson C.

(2010) Preserving the biodiversity and ecological services

of rivers: new challenges and research opportunities.

Freshwater Biology, 55, 1–16.

Bunn S.E. & Arthington A.H. (2002) Basic principles and

ecological consequences of altered flow regimes for aquatic

biodiversity. Environmental Management, 30, 492–507.



Burnham K.P. & Anderson D.R. (2002) Model Selection and

Multimodel Inference, a Practical Information-Theoretic

Approach, 2nd edn. Springer, New York, NY.

Caissie D. (2006) The thermal regime of rivers: a review.


Carlisle D.M., Wolock D.M. & Meador M.R. (2011) Alteration

of streamflow magnitudes and potential ecological conse-

quences: a multiregional assessment. Frontiers in Ecology

and the Environment, 9, 264–270.

Cushman R.M. (1985) Review of ecological effects of

rapidly varying flows downstream from hydroelectric

facilities. North American Journal of Fisheries Management,

5, 330–339.

Eng K., Carlisle D.M., Wolock D.M. & Falcone J.A. (in press)

Predicting the likelihood of altered streamflows at unga-

uged rivers across the conterminous United States. River

Research and Applications, DOI: 10.1002/rra.2565.

Freeman M.C., Bowen Z.H., Bovee K.D. & Irwin E.R. (2001)

Flow and habitat effects on juvenile fish abundance in

natural and altered flow regimes. Ecological Applications, 11,

179–190.

Frimpong E.A. & Angermeier P.L. (2010) Comparative Utility

of Selected Frameworks for Regionalizing Fish-Based

Bioassessments across the United States. Transactions of

the American Fisheries Society, 139, 1872–1895.

Gilliom R.J., Alley W.M. & Gurtz M.E. (1995) Design of the

National Water-Quality Assessment Program: Occurrence and

Distribution of Water-Quality Conditions. U.S. Geological

Survey, Reston, VA.

Gleick P.H. (2003) Global freshwater resources: soft-path

solutions for the 21st century. Science, 302, 1524–1528.

Herlihy A.T., Hughes R.M. & Sifneos J.C. (2006) Landscape

clusters based on fish assemblages in the conterminous

USA and their relationship to existing landscape classi-

fications. American Fisheries Society Symposium, 48, 87–

112.

Hughes R.M., Paulsen S.G. & Stoddard J.L. (2000) EMAP

surface waters: a multiassemblage, probability survey of

ecological integrity in the U.S.A. Hydrobiologia, 423, 429–

443.

Jackson D.A., Peres-Neto P.R. & Olden J.D. (2001) What

controls who is where in freshwater fish communities: the

roles of biotic, abiotic, and spatial factors. Canadian Journal

of Fisheries and Aquatic Sciences, 58, 157–170.

Junk W.J., Bayley P.B. & Sparks R.E. (1989) The flood pulse

concept in river-floodplain systems. In: Proceedings of the

International Large River Symposium (Ed. D.P. Lodge), pp.

110–127. Canadian Special Publication of Fisheries and

Aquatic Sciences, Fisheries and Oceans Canada, Ottawa,

ON.

Kennard M.J., Mackay S.J., Pusey B.J., Olden J.D. & Marsh N.

(2010) Quantifying uncertainty in estimation of hydrologic

metrics for ecohydrological studies. River Research and

Applications, 26, 137–156.

Konrad C.P., Olden J.D., Lytle D.A., Melis T.S., Schmidt J.C.,

Bray E.N. et al. (2011) Large-scale flow experiments for

managing river systems. BioScience, 61, 948–959.

Lamouroux N., Poff N.L. & Angermeier P.L. (2002) Intercon-

tinental convergence of stream fish community traits along

geomorphic and hydraulic gradients. Ecology, 83, 1792–

1807.

Larned S.T., Arscott D.B., Schmidt J. & Diettrich J.C. (2010) A

framework for analyzing longitudinal and temporal vari-

ation in river flow and developing flow-ecology relation-

ships. Journal of the American Water Resources Association, 46,

541–553.

Lawrence D.J., Larson E.R., Reidy Liermann C.A., Mims

M.C., Pool T.K. & Olden J.D. (2011) National parks as

protected areas for US freshwater fish diversity. Conserva-

tion Letters, 4, 364–371.

Lehner B., Reidy Liermann C., Revenga C., Vorosmarty C.,

Fekete B., Crouzet P. et al. (2011) High-resolution mapping

of the world’s reservoirs and dams for sustainable river-

flow management. Frontiers in Ecology and the Environment,

9, 494–502.

Ligon F.K., Dietrich W.E. & Trush W.J. (1995) Downstream

ecological effects of dams. BioScience, 45, 183–192.

Logez M., Pont D. & Ferreira M.T. (2010) Do Iberian and

European fish faunas exhibit convergent functional struc-

ture along environmental gradients? Journal of the North

American Benthological Society, 29, 1310–1323.

Lytle D.A. & Poff N.L. (2004) Adaptation to natural flow

regimes. Trends in Ecology and Evolution, 19, 94–100.

Magilligan F.J. & Nislow K.H. (2005) Changes in hydrologic

regime by dams. Geomorphology, 71, 61–78.

Mathews R. & Richter B.D. (2007) Application of the

indicators of hydrologic alteration software in environ-

mental flow setting. Journal of the American Water Resources

Association, 43, 1400–1413.

McCann K. (1998) Density-dependent coexistence in fish

communities. Ecology, 79, 2957–2967.

McManamay R.A., Orth D.J. & Dolloff C.A. (2012) Revisiting

the homogenization of dammed rivers in the southeastern

US. Journal of Hydrology, 424–425, 217–237.

Meador M.R. & Carlisle D.M. (in press) Relations between

altered streamflow variability and fish assemblages in

Eastern USA streams. River Research and Applications, DOI:

10.1002/rra.1534.

Merritt D.M., Scott M.L., Poff L.N., Auble G.T. & Lytle D.A.

(2010) Theory, methods and tools for determining envi-

ronmental flows for riparian vegetation: riparian vegeta-

tion-flow response guilds. Freshwater Biology, 55, 206–225.

Mims M.C. & Olden J.D. (2012) Life history theory predicts

fish assemblage response to hydrologic regimes. Ecology,

93, 35–45.

Mims M.C., Olden J.D., Shattuck Z.R. & Poff N.L. (2010) Life

history trait diversity of native freshwater fishes in North

America. Ecology of Freshwater Fish, 19, 390–400.



Oksanen J., Blanchet F.J., Kindt R., Legendre P., Minchin P.R.,

O’Hara R.B. et al. (2012) vegan: Community Ecology

Package. R package version 2.0-3. http://CRAN.Rpro-

ject.org/package=vegan

Olden J.D. & Kennard M.J. (2010) Intercontinental compar-

ison of fish life history strategies along a gradient of

hydrologic variability. In: Community Ecology of Stream

Fishes: Concepts, Approaches, and Techniques (Eds K.B. Gido

& D.A. Jackson), pp. 83–107. American Fisheries Society,

Bethesda, MD.

Olden J.D., Kennard M.J. & Pusey B.J. (2012) A framework for

hydrologic classification with a review of methodologies

and applications in ecohydrology. Ecohydrology, 5, 503–518.

Olden J.D. & Naiman R.J. (2010) Incorporating thermal

regimes into environmental flows assessments: modifying

dam operations to restore freshwater ecosystem integrity.


Olden J.D. & Poff N.L. (2003) Redundancy and the choice of

hydrologic indices for characterizing streamflow regimes.

River Research and Applications, 19, 101–121.

Olden J.D., Poff N.L. & Bestgen K.R. (2006) Life-history

strategies predict fish invasions and extirpations in the

Colorado River Basin. Ecological Monographs, 76, 25–40.

Palmer M.A., Reidy C., Nilsson C., Florke M., Alcamo J., Lake

P.S. et al. (2008) Climate change and world’s river basins:

anticipating management options. Frontiers in Ecology and

the Environment, 6, 81–89.

Perkin J.S. & Bonner T.H. (2011) Long-term changes in flow

regime and fish assemblage composition in the Guadalupe

and San Marcos Rivers of Texas. River Research and

Applications, 27, 566–579.

Poff N.L., Allan J.D., Bain M.B., Karr J.R., Prestegaard K.L.,

Richter B.D. et al. (1997) The natural flow regime. BioSci-

ence, 47, 769–784.

Poff N.L., Olden J.D., Merritt D.M. & Pepin D.M. (2007)

Homogenization of regional river dynamics by dams and

global biodiversity implications. Proceedings of the National

Academy of Sciences, 104, 5732–5737.

Poff N.L., Olden J.D., Vieira N.K.M., Finn D.S., Simmons M.P.

& Kondratieff B.C. (2006) Functional trait niches of North

American lotic insects: traits-based ecological applications

in light of phylogenetic relationships. Journal of the North

American Benthological Society, 25, 730–755.

Poff N.L., Richter B.D., Arthington A.H., Bunn S.E., Naiman

R.J., Kendy E. et al. (2010) The ecological limits of hydro-

logic alteration (ELOHA): a new framework for developing

regional environmental flow standards. Freshwater Biology,

55, 147–170.

Poff N.L. & Zimmerman J.K.H. (2010) Ecological responses to

altered flow regimes: a literature review to inform the

science and management of environmental flows. Freshwa-

ter Biology, 55, 194–205.

Quinn J.W. & Kwak T.J. (2003) Fish assemblage changes in an

Ozark river after impoundment: a long-term perspective.

Transactions of the American Fisheries Society, 132, 110–119.

R Development Core Team (2011) R: A Language and

Environment for Statistical Computing. R Foundation for

Statistical Computing, Vienna, Austria, ISBN 3-900051-07-

0, URL http://www.R-project.org.

Rahel F.J. (2000) Homogenization of fish faunas across the

United States. Science, 288, 854–856.

Reidy Liermann C., Nilsson C., Robertson J. & Ng R.Y. (2012)

Implications of dam obstruction for global freshwater fish

diversity. BioScience, 62, 539–548.

Simley J.D. & Carswell W.J. (2009) The national map

– hydrography. U.S. Geological Survey Fact Sheet 2009-

3054, 1–4.

Southwood T.R.E. (1977) Habitat, the templet for ecological

strategies? Journal of Animal Ecology, 46, 337–365.

Tedesco P.A., Hugueny B., Oberdorff T., Durr H.H., Meri-

goux S. & Merona B.D. (2008) River hydrological season-

ality influences life history strategies of tropical riverine

fishes. Oceologia, 156, 691–702.

Townsend C.R. & Hildrew A.G. (1994) Species traits in

relation to a habitat templet for river systems. Freshwater

Biology, 31, 265–275.

United States Army Corps of Engineers (2000) National

Inventory of Dams. Federal Emergency Management

Agency, Washington, DC.

Vorosmarty C.J., McIntyre P.B., Gessner M.O., Dudgeon D.,

Prusevich A., Green P. et al. (2010) Global threats to

human water security and river biodiversity. Nature, 467,

555–561.

Vorosmarty C.J., Meybeck M., Fekete B., Sharma K., Green P.

& Syvitski J.P.M. (2003) Anthropogenic sediment retention:

major global impact from registered river impoundments.

Global and Planetary Change, 39, 169–190.

Winemiller K.O. (1989) Patterns of variation in life-history

among South American fishes in seasonal environments.

Oecologia, 81, 225–241.

Winemiller K.O. (2005) Life history strategies, population

regulation, and implications for fisheries management.

Canadian Journal of Fisheries and Aquatic Sciences, 52, 872–

885.

Winemiller K.O. & Rose K.A. (1992) Patterns of life-history

diversification in North American fishes: implications for

population regulation. Canadian Journal of Fisheries and

Aquatic Sciences, 49, 2196–2218.

(Manuscript accepted 15 September 2012)



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