Existing and future challenges: The concept of successful fish passage in South America

Abstract

Most of the large rivers of South America are impounded mainly for hydropower production. The construction of fish passes has been one of the strategies adopted by Brazilian authorities and the energy sector to diminish the effects of these barriers on migratory fish communities. Despite the high investments and efforts involved, most facilities have been considered ineffective for conservation purposes. Decades of poor monitoring and the lack of specific studies have limited our knowledge on the real role of fish passes. Efficiency has been frequently defined as the proportion of fish that ascend a fish passage facility, compared to the shoal size that reaches the dam. Inspired by the notion that fishes accumulated below the dam need to migrate upstream, the quantity of fish passed upstream historically indicated successful management, as generally inferred by the fish abundance inside the fish pass. We propose a new concept for estimating fish pass efficiency for South American rivers, based on the capability of the fish pass to maintain viable populations. This broader approach is necessary because knowledge of fish habitats below and above the pass, plus the feasibility of downstream movements of eggs, larvae and adults through the reservoir and past the dam, is needed for assessing whether a fish pass is working as a conservation tool. Copyright © 2011 John Wiley & Sons, Ltd.

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EXISTING AND FUTURE CHALLENGES: THE CONCEPT OF SUCCESSFUL FISH
PASSAGE IN SOUTH AMERICA
P. S. POMPEU,
a
*A. A. AGOSTINHO
b
and F. M. PELICICE
c
a
Department of Biology, Federal University of Lavras, Lavras, Minas Gerais, Brazil
b
Department of Biology/NUPELIA, Maringá State University, Maringá, Paraná, Brazil
c
NEAMB, Federal University of Tocantins, Porto Nacional, Tocantins, Brazil
ABSTRACT
Most of the large rivers of South America are impounded mainly for hydropower production. The construction of fish passes has been one
of the strategies adopted by Brazilian authorities and the energy sector to diminish the effects of these barriers on migratory fish commu-
nities. Despite the high investments and efforts involved, most facilities have been considered ineffective for conservation
purposes. Decades of poor monitoring and the lack of specific studies have limited our knowledge on the real role of fish passes. Effi-
ciency has been frequently defined as the proportion of fish that ascend a fish passage facility, compared to the shoal size that reaches
the dam. Inspired by the notion that fishes accumulated below the dam need to migrate upstream, the quantity of fish passed upstream
historically indicated successful management, as generally inferred by the fish abundance inside the fish pass. We propose a new concept
for estimating fish pass efficiency for South American rivers, based on the capability of the fish pass to maintain viable populations. This
broader approach is necessary because knowledge of fish habitats below and above the pass, plus the feasibility of downstream movements
of eggs, larvae and adults through the reservoir and past the dam, is needed for assessing whether a fish pass is working as a conservation
tool. Copyright #2011 John Wiley & Sons, Ltd.
key words: conservation tool; fish migration; fishway; neotropical fishes
Received 14 September 2010; Revised 1 April 2011; Accepted 2 June 2011
INTRODUCTION
Fisheries management in South American reservoirs has
been based on stocking, fisheries' harvest limits and con-
struction of fish passes. The experience of Brazil, where
most of this information originated, illustrates this scenario.
Until the 1950s, the main purpose of Brazilian management
programmes was to ensure the movement of migratory fish
through dams by constructing fish ladders, mostly in small
rivers. Since the 1960s, the companies have been obliged
to protect threatened fish species either by incorporating fish
ladders in the dam design for facilitating migration or by
creating breeding facilities to produce fingerlings of the
affected native species to be stocked in the reservoir (Sugunan,
1997; Agostinho et al., 2010).
Although these actions have been performed throughout
the country for years, we lack rigorous studies that evaluate
their efficiency in recuperating species and threatened popu-
lations, sustaining fish populations and cost–benefit analyses
(Vieira and Pompeu, 2001). The low fishery yield, the pre-
carious conservation status of native population in southern
and south-eastern Brazil reservoirs and the significant reduc-
tion of migratory species (Agostinho et al., 1994; Cesp,
1996) clearly indicate that this strategy is not satisfactory
(Agostinho et al., 2002, 2004).
The construction of fish passes was the oldest manage-
ment strategy adopted by the Brazilian energy sector to
diminish the effects of barriers on fish communities, espe-
cially migratory species. The first fish ladder in Brazil was
constructed in 1911, at the Itaipava Dam, in the Pardo River,
upper Paraná Basin (Godoy, 1985). In 1927, the construc-
tion of fish ladders became a legal requirement in São Paulo
State (Agostinho et al., 2002). With the increasing number
of hydropower dams in the 1960s, fish passes became
required by the legislation of other states. As a result, dozens
of Brazilian dams have been equipped with ladders and
other such facilities (Agostinho et al., 2008).
Despite the substantial fiscal investments and engineering
efforts, the ecological effectiveness of most fish passes in
South America was never assessed (Agostinho et al.,
2002). Some fish passes have been considered ineffective
to conserve migratory species (Godinho et al., 1991; Oldani
et al., 2007), and others are reported to be promoting re-
gional fishery collapses (Lopes et al., 2007; Pelicice and
Agostinho, 2008). Decades of poor monitoring and the lack
of specific studies have limited our knowledge on the real
*Correspondence to: P. S. Pompeu, Department of Biology, Federal University
of Lavras, 37200-000, Lavras, Minas Gerais, Brazil.
E-mail: pompeu@ufla.br
RIVER RESEARCH AND APPLICATIONS
River Res. Applic. 28: 504–512 (2012)
Published online 18 July 2011 in Wiley Online Library
(wileyonlinelibrary.com) DOI: 10.1002/rra.1557
Copyright #2011 John Wiley & Sons, Ltd.

role of fish passes. The studies performed in South America
mainly concentrated on the selectivity of individual fish
ladders on the ascending fish (Fontenele, 1961; Godoy,
1987; Godinho et al., 1991; Agostinho et al., 2002, 2007b,
2007c; Fernandez et al., 2004; Alves, 2007; Makrakis
et al., 2007b; Bizzotto et al., 2009), focusing on their selec-
tivity concerning the ascending fish. In addition, efficiency
has been frequently defined as the proportion of fish that
ascend a fish passage facility compared to the school
size immediately below the dam (Novak et al., 2003;
Agostinho et al., 2007c) and was evaluated for at least two
fish lifts (Oldani and Baigun, 2002; Pompeu and Martinez,
2007). Inspired by the notion that fishes need to migrate
upstream, the quantity of fish passed upstream, whatever
the species, historically indicated successful management.
As a consequence, the role of passes in aiding population
recruitment and completion of life cycles has been largely
neglected.
In this paper, we propose a new concept for evaluating
fish pass efficiency in South America, based on the potential
of the fish pass to maintain viable populations regionally,
meant by a positive trend in abundance. This broader ap-
proach is necessary, because the current criterion (successful
upstream passage) does not assure fish conservation
(Kraabøl et al., 2009). Expanding on Fausch et al. (2002),
we emphasize that fish habitat components below and above
the pass, as well as the feasibility of downstream movements
of eggs, larvae and adults, must be included in evaluating
fish pass effectiveness as conservation tools. Although this
manuscript mainly considers fish passage in South America,
it could also have applications worldwide, because many
biases in fish passage research that occur globally have been
highlighted (Roscoe and Hinch, 2010).
THE LIFE CYCLE OF SOUTH AMERICAN
MIGRATORY SPECIES
Although few South American species migrate long distances
(Petrere, 1985; Godinho and Godinho, 1994; Carolsfeld
et al., 2003), they are the most important species for com-
mercial (Goulding, 1979; Bittencourt and Cox-Fernandes,
1990; Godinho, 1993) and artisanal fisheries because of
their larger size, abundance and market value (Northcote,
1978; Hoeinghaus et al., 2009). Migration occurs in a
wide range of South American taxa, although the most
conspicuous migrations are principally associated with
Characiformes and Siluriformes. Nonetheless, short-distance
migrations and fish movements over ecological time are
critical components of meta-population dynamics, evolution
and speciation (Fausch et al., 2002).
A general pattern for reproductive migration includes
breeder displacement to the upper areas of the basin
(>1000km, Godoy, 1975) during the beginning of the wet
season, a period of high temperatures and turbid waters (that
reduce predation on eggs and larvae by visual predators).
Spawning occurs in flowing waters (e.g. the main stem
and upper tributaries), where eggs drift downstream for
kilometres while they develop and hatch (Nakatani et al.,
2001; Agostinho et al., 2002). Floods take larvae to nursery
grounds in floodplains (Agostinho et al., 2003) or tributary
mouths flooded by high waters in the main river channel
(Zaniboni Filho and Schultz, 2003). After spawning, spent
adults migrate downstream, followed by the downstream
drift of larvae and fry (Petrere, 1985). Adults also enter
flooded areas, where they feed and improve their condition
(Gomes and Agostinho, 1997). Some of these species re-
main in floodplain lakes for 1 or 2years (Agostinho et al.,
1993), returning to the river in subsequent flood seasons.
Although this pattern may be dominant, there are several
variations, including basins where migratory fish complete
their life cycles using in-river habitats, where floodplain is
absent (Godinho and Kynard, 2009), such as the Uruguay
River (Zaniboni Filho and Schultz, 2003). The most
complex movements are observed in the Amazon region,
involving adult migration upstream and downstream for
spawning or feeding, in tributaries and in the main channel
(Carolsfeld et al., 2003).
Migratory dynamics also include estuarine and marine
environments. Some large Amazonian catfishes run more
than 3000km between estuaries in Belém-Pará and head-
waters in upper tributaries (Barthem and Goulding, 1997).
The dourada Brachyplatystoma rousseuaxii, for example,
reaches the upper courses of white water rivers to spawn,
showing a remarkable spatial variation in population struc-
ture in the Amazon basin (Barletta et al., 2010). Juveniles
(6cm TL) drift to the Amazon estuary and flooded
forest lakes (nursery areas), located in downstream reaches.
Juveniles dominate the population up to 1200km from the
estuary, whereas adults compose at least 70% of the popula-
tion above 1800km. Only adults can be found 3000 km from
the estuary (Alonso and Picker, 2005). Diadromous species
(mainly galaxiids) are confined to the more austral part of
South America (Moyle and Cech, 1988), where they have
an amphidromous behaviour (i.e. reproduction in fresh
water, larvae migrate to the sea and juveniles return to fresh
water) (stricto sensu, McDowall, 2007). However, some
species, found mainly in the Brazilian coastal basins, can
also be considered amphidromous (lato sensu), migrating
from the sea to the rivers for feeding purposes (Pompeu
and Martinez, 2006).
In short, migration involves complex behaviours that
are still not fully understood for many species, but re-
search has shown that specific habitats for spawning, nur-
sery and feeding are a common need among migratory fish
species. Migration is perhaps the evolutionary solution
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that maximized population persistence, by allowing the
use of appropriate habitats according to the demands of
different stages of species life histories. If populations
are impeded access to these habitats, individual fitness
and reproduction success are predicted to decrease. In
the context of dams and reservoirs that fragment habitats
along rivers, a successful fish pass should restore access
to such critical habitats (Pelicice and Agostinho, 2008)
that are biologically meaningful and enable the population
to maintain itself.
EVALUATION OF SOUTH AMERICAN FISH PASSES
There is little information concerning the efficiency of South
American fish passes. The vast majority of fish passes were
never monitored, or if they were monitored, the monitoring
was not based on valid protocols, or the study designs and
results were unavailable.
We reviewed the literature and located information con-
cerning the ability of facilities at 16 locations for providing
fish passage, including ladders, lifts, locks and trap and truck
systems (Table I; Figure 1). Selectiveness and dominance in
ascension were the most common aspects evaluated. Most
studies reported a negative selectivity <40%, meaning that
>60% of the species found downstream of the dam were
observed using the pass at some time. On the other hand, a high
dominance was observed in all passes, because few species
(3–5) usually represent >80% of the passed individuals.
The negative selectiveness of six fish passes was evalu-
ated. This selectiveness corresponded to the per cent of
species entering the passes but not reaching the reservoir.
In the lifts and trap and truck systems, we may consider that
all the species entering the system are successfully passed.
However, the pass may cause mortality from contact with
the moving structures and confinement in the fish chamber.
In the Santa Clara case, the percentage of dead or injured
fish was estimated as 0.5% and 0.8% for the total fauna
and for the migratory group, respectively (Pompeu and
Martinez, 2007). There is variation between fish ladders,
but in general, approximately half the fauna that enters the
mechanism does not reach the exit or even the reservoir
(Table I). The selectiveness tends to be slightly lower for
highly migratory species, probably because of their greater
swimming capabilities.
In South America, quantitative estimates of the upstream
efficiency of fish passes are available only for the Santa
Clara trap and truck system and the Yaceretá fish lift. Both
indicate that a small proportion of the downstream fish are
passed. Quantitative estimates require tagging and recapture
programs or detection at another fish pass downstream,
where the number of passed individuals are estimated. Such
estimates can be used as target population size information.
Even in North America, where monitoring is more frequent,
such studies are scarce. For a database of 213 projects with
at least one published paper concerning fish passage, infor-
mation on the number of fish using the mechanism was
available for only eight, and only three provided sufficient
data for quantitative efficiency estimates (Novak et al.,
2003). In South America, indirect inferences of the effi-
ciency of upstream passage were obtained for the fish ladder
at Igarapé, where a minimum of 14% of the individuals
marked downstream were recaptured upstream, and for the
ladders at Canoas I and II, where a high upstream passage
is presumed due to the decline of downstream populations.
Upstream fish pass selectivity has a complex origin. It
depends largely on the behaviour and ability of each species
in locating and accessing the facility, together with hy-
draulic aspects, structural design and operation (Larinier,
2002a; Knaepkens et al., 2006; Lundqvist et al., 2008;
Godinho and Kynard, 2009; Roscoe and Hinch, 2010). To
improve fish ascension, different behaviours, needs and
local dynamics must be considered, which is a great chal-
lenge in neotropical rivers, given the rich diversity of fish
species. Engineers and biologists, however, have achieved
considerable progress in improving upstream passage
(Larinier, 2002b; Mallen-Cooper and Brand, 2007; Mallen-
Cooper and Stuart, 2007; Stuart et al., 2008a, 2008b), and
the knowledge of the swimming performance of neotropical
fishes has markedly increased (Santos et al., 2007, 2008,
2009; Castro et al., 2010), aiding further improvements.
Evaluations of downstream passage in South America are
scarce. Adult downstream passage was evaluated in only
four reservoirs (see Table I), indicating that such an event
is null or extremely reduced. There are several reasons that
could explain this pattern. Lack of attractiveness and problems
with site location probably occurs, but the rheophilic be-
haviour of South American fish likely plays a major role.
Attraction to lotic habitats induces fish to avoid areas
with lentic characteristics, especially large reservoirs.
Mark–recapture and tagging studies indicate the following
behaviours. Fish released in areas influenced by large
dams tended to avoid the lentic environment, migrating
to upper lotic reaches or tributaries (Antonio et al.,
2007; Makrakis et al., 2007a). Fish passed upstream are able
to migrate through the impounded area (Antonio et al.,
2007), but once in lotic upper sites, it is likely that there is
little incentive to return downstream. Even if fish sought to
migrate downriver, they would have to travel across huge
lentic environments, where flows are low or nonexistent.
Fish, therefore, remain in upper lotic stretches of the im-
poundment (Agostinho et al., 2007b) and do not reach the
dam, where they could access the fish pass, assuming that
it could effectively pass fish downriver. In other words,
rheophilic behaviour likely precludes downstream migration
through large dams and their reservoirs.
P. S. POMPEU ET AL.506
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DOI: 10.1002/rra

Table I. Some functioning aspects evaluated in fish passes installed in South America, such as selectivity (% of species not registered in the fishway, based on fish surveys in the region),
selectivity along the passage (% of species passed to the reservoir or that reached upper parts of the pass, based on the number of species registered in the fish way), upstream passage
efficiency (% of fish passed upstream, based on the number of fish recorded below the dam), and downstream passage (qualitative/quantitative indications of downstream passage)
Hydropower
dam (height)
Fish
passage
type
(length) Basin
Reservoir
area, km
2
Local
situation
(Figure )
Selectivity
(dominance?)
Selectivity
along the passage
Upstream passage
efficiency
Downstream
passage
References
Total
fauna Migratory
Total
fauna Migratory Adults Eggs/larvae
Canoa
Quebrada (25 m)
Ladder
(400 m)
Amazon 11.7 ? –(yes) ––––– –Junho et al. (2007)
Peixe Angical
(39 m)
Ladder
(575 m)
Tocantins 294 A 59% (yes) 53% 38% –––Unlikely Freitas et al. (2009)
Lageado (37 m) Ladder
(874 m)
Tocantins 630 A 37% (yes) 48% 53% ––Unlikely Unlikely Agostinho et al.
(2007b, 2007c)
Igarapé (6 m) Ladder (?) São
Francisco
<1 A ? (?) –––14% ––Alves (2007)
Risoleta Neves
(49 m)
Trap and
Truck
Doce 3.5 A 71% (yes) ––––– –Braga et al. (2007)
Santa Clara
(60m)
Trap and
Truck
Mucuri 7.5 D 3% (yes) 0.5% 0.8% 3.1% 7.0% Very
reduced
Possible Pompeu (2005), Pompeu
and Martinez (2007)
Salto Moraes
(11 m)
Ladder
(78 m)
Paraná –? 17% (yes) 98% –––– –Godinho et al. (1991)
Funil (45 m) Fish lift Paraná 33.5 E 8% (yes) –––––Unlikely Pereira and
Pompeu (2010),
Suzuki (2009)
Igarapava
(18m)
Ladder
(282 m)
Paraná 36 B 44% (yes) ––––– –Bizzotto et al. (2009),
Casali et al. (2010)
Canoas I (29 m) Ladder
(210 m)
Paraná 30 B 21% (yes) ––Probably
high
Probably
high
No Reproduction
areas absent
Britto and Sirol (2005),
Hoffmann et al. (2005),
Lopes et al. (2007)
Canoas II (25 m) Ladder
(228 m)
Paraná 22 F 28% (yes) ––Probably
high
Probably
high
No Reproduction
areas absent
Britto and Sirol (2005),
Hoffmann et al. (2005),
Lopes et al. (2007)
Porto Primavera
(22 m)
Ladder
(520 m)
Paraná 2250 B 54% (yes) –22% –––Reproduction
areas absent
Makrakis et al. (2007a)
Itaipú (196m) Ladder
(155 m)
Paraná 1350 A 59% (yes) 15% 11% ––– –Fernandez et al. (2004)
Itaipú (196 m) Channel
(10 km)
Paraná 1350 A 10% (yes) 58% 41% ––– –Makrakis et al. (2007b),
Okada et al. (2005)
Yaceretá (21m) Fish lift Paraná 1600 ? 32% (yes) ––1.88% 0.68% ––Oldani and Baigun
(2002),
Oldani et al. (2007)
Salto Grande
(30 m)
Fish
locks
Uruguay 800 ? 25% (yes) ––––– –Oldani et al. (2007)
Aspects that were not investigated are marked with ‘–’.
Local situation = see Figure for code explanation (A–F). Categorization is based on present author's judgement.
Dominance = a few species (3–5) summing more than 70% of captures.
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Downstream passage of young fish (eggs and larvae)
must be considered also. Recent studies indicate, however,
that the spatial distribution of young is markedly affected
by dams (Agostinho et al., 2007b; Freitas et al., 2009), es-
pecially when large reservoirs are created, with evidence
that drifting eggs and larvae disappear in the lentic areas
of the impoundment, and do not reach the dam. However,
selected ichthyoplankton species are capable of crossing
small reservoirs, such as Santa Clara (7.5km
2
), contributing
to downstream recruitment (Pompeu, 2005).
There is consensus that fish passes built to promote
upstream passage cannot provide downstream passage
(Larinier and Travade, 2002). However, these movements
are necessary to avoid the depletion of downstream stocks.
Therefore, besides upstream passage concerns (e.g. selectiv-
ity along passage and quantitative efficiency), the necessity
of downstream passage must be considered, because avail-
able evidence suggests that selectiveness and mortality are
even higher than that occurring during fish ascension
(Table I).
THE OBJECTIVES OF A FISH PASS
According to Therrien and Bougeois (2000), considerations
about fish passage efficiency must include all the species of
interest, the number of obstacles in the river and their loca-
tion and the biological and conservation objectives of the
fish pass. For migratory species, depending on the dam loca-
tion, it is essential that the obstacles are passed and that the fish
reach the spawning area at the right time for successful
recruitment. For resident species with trophic migrations or
short reproductive migrations, the major biological objective
should be to avoid population fragmentation. In this case, a fish
passage mechanism is considered efficient if it is successfully
used by a substantial number of individuals but not necessarily
by the entire population (Larinier, 1998). To conserve long-
distance migratory species, it is important for the fish to be
able to locate the entrance of the mechanism and pass the
dam and to evaluate the effectiveness of the pass in main-
taining the migratory species populations, an aspect rarely
evaluated (Cada and Francfort, 1995; Agostinho et al.,
2004). Upstream passage alone is not indicative of popula-
tion recruitment and conservation of stocks, because fish
may ascend the pass but recruitment may not occur.
Therefore, the objectives of a fish pass should be directly
related to the spatial distribution of critical habitats, such as
reproduction sites and nursery areas downstream or up-
stream. The distribution of these habitats must be previ-
ously evaluated to help understand the possible role and
relevance of a pass in maintaining the recruitment of wild
stocks or if it is applied to secondary objectives (e.g. genetic
exchange, artificial stocking). Studies of selectiveness and
efficiency (upstream and downstream) must complement
the understanding of the potential of the passage as a man-
agement tool.
Considering the location of these critical habitats for
maintaining the life cycle of migratory species, at least six
situations occur (Figure 2). Most fish passes studied were
placed in dams included in group A (Table I), where large
lotic reaches occur upstream and downstream and include
Figure 1. Location in South America of the evaluated fish passes (1—Canoa Quebrada; 2—Peixe Angical; 3—Lageado; 4—Igarapé; 5—Risoleta
neves; 6—Santa Clara; 7—Salto Moraes; 8—Funil; 9—Igarapava; 10—Canoas I and Canoas II; 11—Porto Primavera; 12—Itaipú; 13—Yaceretá;
14—Salto Grande)
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floodplains. Because the populations may become self sus-
taining in the long term in both regions, these passes would
become questionable or justified only for the maintenance of
the genetic flow between the populations. Reduced success
of downstream movements of fish could result in decreased
downstream fish stocks (Lopes et al., 2007), which justifies
performing stock-strength evaluations. However, the effi-
ciency of passage upstream was not evaluated in any of
the cases and effective downstream movements of adults,
eggs and larvae seem improbable in all reservoirs where this
aspect was evaluated (Table I).
Some fish passes were installed immediately down-
stream of other dams. In this case, the conditions for
reproduction and recruitment are found only further down-
stream (situation B). Passes operating in these conditions
may function as ecologic traps (Pelicice and Agostinho,
2008), because they remove the fish from healthy environ-
ments and transport them to sites with no critical habitats.
The Igarapava ladder has been justified by the maintenance
of fish stocks in the reservoir for fishing (artificial stocking),
functioning as a source-sink system (Godinho and Kynard,
2009). The implementation of a pass in this case would
not help the recruitment dynamics, because the fish are
transported to areas of lower environmental quality. If any
purpose different from the conservation of natural stocks
justifies the construction of a mechanism, fish passage must
be controlled and rigorously monitored.
No studied fish passes were located at dams where the
conditions for recruitment of migratory species are found
only upstream of the dam (situation C). Although a device
in this case is not necessary, it could represent a chance to
return individuals of migratory species accidentally carried
downstream, or imprisoned downstream by the damming,
to areas where they could reproduce. The populations of
migratory species would not be able to maintain self-
sustainable populations downstream of the dam, which
means that these species would be more likely to disappear
from the reach.
Among the studied fish passes, only the trap and truck
system at Santa Clara represents the condition where migra-
tory species spawn upstream and rear downstream of the
dam. (situation D). This is the only case study where main-
tenance of connectivity between areas upstream and down-
stream is crucial for maintaining migratory species
populations. The situation at the Funil fish lift is very simi-
lar, although spawning sites may also be found downstream
(situation E). In both situations, the fish pass is appropriate if
upstream migration is equivalent to downstream fish move-
ment. If the descendent migration does not happen, the pass
loses its value to recruitment conservation. In this case, al-
ternative measures are more appropriate (e.g. rehabilitation
of spawning habitats downstream and development areas
upstream). Therefore, if there is high selectiveness, man-
agement may be difficult and expensive. A dam that frag-
ments or separates spawning and development areas
causes severe impacts, especially when it creates a huge
reservoir. To avoid this situation, the distribution of crit-
ical habitats should be thoroughly evaluated during the in-
ventory of the hydroelectric potential of the reach.
There are extreme cases where critical habitats are ab-
sent downstream and upstream of the dam. This is com-
mon in rivers having a series of dams in sequence, such
as the large tributaries of the Paraná River. Even in this
case, where there are no lotic reaches downstream or
Figure 2. Six possible locations of critical habitats (reproduction sites and nursery areas) in relation to a dam with a fish pass installed. A)
Conditions for spawning (e.g. lotic reaches and tributaries) and recruitment (e.g. floodplains and lateral habitats) exist upstream and
downstream of the dam. B) Such conditions only occur downstream or upstream (C). D) Reproduction sites are located only upstream
and nurseries only downstream. E) Migratory fish spawn upstream and downstream, but the nursery areas are located only downstream.
F) Critical habitats are completely absent in the reach
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upstream (situation F), fish passage facilities have been
installed, such as the fish ladders at Canoas II dam.
Passes aiming to maintain recruitment are irrational in
this situation if they do not reconnect critical fish habitats
in the river by incorporating passes at all the dams. If
passes were built, selectiveness studies would be neces-
sary at each dam, because the fish moving upstream must
return to lower reaches at some portion of their life
cycles. If these aspects are neglected, the construction
of a fish pass may jeopardize the conservation status of
the fish.
Among all the configurations presented in Figure 2,
there are very few cases where the passes aid recruitment
dynamics, despite their objective to conserve migratory
fish. The lack of attention to the distribution of critical habi-
tats causes some inconsistencies between the potential of the
management action and its objectives, with a huge chance of
failure or even generation of further negative impacts. This
can be observed in the ladders installed in the Paranapanema
River. Information on selectiveness and efficiency is crucial
to show the functioning and limitations of the device; ignoring
such flaws leads to failure of this management strategy, even
in cases where passage may seem opportune.
THE FUTURE OF FISH PASSES IN SOUTH AMERICA
AND THE EFFICIENCY CONCEPT
The few studies evaluating fish passes in South America
have indicated problems related to their functioning, such
as selectivity in providing upstream passage, and the
virtual absence of downstream migration. In addition,
passes have been installed without considering the distri-
bution of critical habitats. As a consequence, most passes
fail to aid recruitment of migratory species. Such obser-
vations conflict with the usual approach, which considers
the number of ascending fish as a measure of manage-
ment success. This concept of successful management
demands a fundamental revision; a broader approach is
needed, and the concept of ‘maintaining viable populations’
must replace the concept of ‘successful fish ascension’.
Survival through fish pass systems needs to be included
in a discussion of success but is rarely considered. Inde-
pendent of dam location relative to critical habitats, fail-
ures in downstream passage represent the major obstacle
to the success of fish passes as management and conserva-
tion tools. The lack of downstream migration is worrisome
because it causes additional impacts on the fish fauna. If
fish passes fail to reconnect sites fragmented by the
dam, and free movements are not restored, facilities will
work as one-way migratory routes—allowing only ascen-
sion. In this case, stocks may be redistributed along the
river, causing population imbalances or the subtraction
of stocks from downstream, as observed at the Canoas
dams on Paranapanema River (Agostinho et al., 2007a;
Lopes et al., 2007).
Depending on the spatial distribution of critical spawning
and rearing habitats, severe impacts are likely in the form of
source-sink dynamics (Godinho and Kynard, 2009) or eco-
logical traps (Pelicice and Agostinho, 2008). Because most
large rivers in South America are serially impounded,
there is substantial risk of confining populations within
short reaches lacking critical fish habitats. The probability
of creating ecological traps and other impacts is much
more likely than usually expected. Managers and the gen-
eral public must understand that passes that fragment con-
nection among critical habitats will have little significance
for management and conservation and instead will be det-
rimental to fish at considerable economic expense.
However, the aspects of fragmented critical habitats and
precluded downstream movement have been frequently
neglected, even where fish passes pass tons of fish annually.
Downstream fish passage is a major knowledge gap concern-
ing effective fish passage worldwide (Pavlov et al., 2002),
and post-departure monitoring is required to assess the fit-
ness consequences of passage (Roscoe and Hinch, 2010).
However, the needs of passing larvae and eggs in South
America are an additional challenge, because depending on
the reservoir area, they cannot reach the dam region, and
downstream bypasses would be ineffective. Serious manage-
ment programs, which employ fish passes to restore natural
recruitment in basins impaired by large dams, must consider
that downstream passage is as important as upstream migra-
tion, and the desired numbers of fish passed upstream must
depend on the downstream passage feasibility.
We propose that ‘maintaining viable populations’is the
only ecologically rational concept for fish passage efficiency
appropriate for South American rivers, because passes in-
stalled in different hydropower dams will have different goals.
This concept means that populations are self-maintained in the
wild, with the aid of the passes (e.g. recruitment, genetic
exchange, demographic inputs). We repeat that most fish
passes were never monitored, most of the limited number of
studies were incomplete, primary ecological information con-
cerning fish migration and life histories is lacking, and we are
aware of no long-term studies on migratory fish populations
both upstream of a reservoir and downstream of its dam. Such
information is needed to determine if some fish passes allow
migratory fishes to complete their life cycles, thereby working
as a valuable conservation tool.
ACKNOWLEDGEMENTS
We thank Robert M. Hughes for the English review and
suggestions on the manuscript. The manuscript was also
benefited by two anonymous referees.
P. S. POMPEU ET AL.510
Copyright #2011 John Wiley & Sons, Ltd. River Res. Applic. 28: 504–512 (2012)
DOI: 10.1002/rra

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