Environ Biol Fish (2011) 91:51–61
DOI 10.1007/s10641-010-9759-x
Distribution and abundance of fish eggs and larvae in three
tributaries of the Upper Uruguay River (Brazil)
Rogério Nappi Corrêa & Samara Hermes-Silva &
David Reynalte-Tataje & Evoy Zaniboni-Filho
Received: 28 July 2009 /Accepted: 24 November 2010 /Published online: 8 December 2010
# Springer Science+Business Media B.V. 2010
Abstract The present study aimed to investigate the
distribution and abundance of fish eggs and larvae in
three important tributaries (Chapecó, Peixe, and Ligeiro
rivers) of the Upper Uruguay River. The spatial and
temporal distribution of fish eggs and larvae were
studied as well as the correlation between
environmental parameters and the abundance of
ichthyoplankton species. The study was conducted
between October 2005 and September 2006. Ichthyoplankton samples were collected at night with
cylindroconical 0.5-mm mesh plankton nets every 5
days. Of the 591 samples collected, 170 contained
ichthyoplankton organisms, resulting in the capture of
12,847 fish eggs and 962 fish larvae. Twenty-seven fish
species were observed, of which 69% were
Characiforms and 27% were Siluriforms. Among the
fish species captured, the representatives were predominantly young forms of small and medium size
fishes, with rheophilic species occurring infrequently.
Eggs occurred exclusively between October and
:
:
:
R. N. Corrêa S. Hermes-Silva D. Reynalte-Tataje E.
Zaniboni-Filho (*)
Laboratório de Biologia e Cultivo de Peixes de Água Doce
(LAPAD), Aquaculture Department, Agrarian Sciences
Center, Federal University of Santa Catarina,
Florianópolis Santa Catarina, Brazil
e-mail: [email protected]
Present Address:
E. Zaniboni-Filho
Rodovia SC 406, 3532,
Florianópolis, Santa Catarina CEP 88066-000, Brazil
January, while the highest larvae occurrence was
observed between November and December. Fish
larvae assemblage structure was shown to be related
to some environmental variables. There was a
tendency of higher values of water temperature and
velocity at the lower sampling sites than at the upper
ones, as well an increase number of eggs and larvae.
The study tributaries serve as reproduction sites and
nursery areas for several fish species of the Upper
Uruguay River; thus, the maintenance of their
integrity is important for the preservation of diversity
and enhancement of fisheries in the region.
.
Keywords Ichthyoplankton Spatiotemporal distribution
.
.
.
Spawning sites Nursery areas Abiotic factor
Introduction
Passive migration of fish eggs and larvae through rivers
is one of the most important events of many fish
species’ life cycles because it is related to increased
chances for survival (Oliveira and Araujo-Lima 1998).
Ichthyoplankton studies are an important tool for
providing information for ichthyology, environmental
inventory, stock monitoring, and fisheries management. The precise identification of fish spawning sites
and natural nursery areas is also fundamental for the
implementation of guidelines to protect such areas
(Nakatani et al. 2001). Additionally, information
Environ Biol Fish (2011) 91:51–61
52
regarding the location, dimension and characteristics
of reproduction sites is the basis for the management
and enhancement of fisheries resources and the
preserva-tion of fish species (Bialetzki et al. 2005).
To maintain fish stock equilibrium, it is important
to maintain the integrity of the spawning sites and to
allow the dispersion of fish eggs and larvae. To do
so, it is necessary to identify and map such areas and
to determine their predominant environmental
character-istics (Nakatani 1994).
Considering that river damming may interrupt the
migration routes of many fish species, tributaries serve
as alternative migration routes and have an important
role in the maintenance of fish diversity and the
diversity of habitats (Baumgartner et al. 2008). In the
Upper Paraná River, a substantial portion of the fish
community uses the tributaries in the last free running
portion of the Paraná River to spawn (Baumgartner et
al. 2008). Some authors have also observed the
existence of differences in the distribu-tion of eggs and
larvae along the tributaries, with higher densities of
eggs at the upper portions and of larvae at the lower
portion of some rivers (Agostinho et al.
1993;
Baumgartner et al. 2004).
Recently, the Upper Uruguay River has gone through
an intense damming process, which modified the
environment and may have changed the structure of the
fish community, possibly affecting the rheo-philic
species. Damming of the Upper Uruguay River by
hydroelectric power plants constructed in a cascade
system increases the importance of tributaries as
alternative routes for migratory fish species.
The tributaries of the Upper Uruguay River are short
and characterized by many rapids making it difficult for
fishes to migrate between the main river and its
affluents (Zaniboni-Filho and Schulz 2003). The first
dam was constructed in 1999, and as of yet no studies
have been completed to assess the role of tributaries in
the fish life cycle after river damming. Several authors
have emphasized the need for such studies to identify
management practices and strate-gies for the
conservation of genetic diversity (Reynalte-Tataje et al.
2008; Zaniboni-Filho et al. 2008; Hermes-Silva et al.
2009). Thus, this study aimed to assess the presence of
fish eggs and larvae in the tributaries of the Upper
Uruguay River. Our hypothesis is that there is a
spatiotemporal variation of eggs and larvae distribution
in the main tributaries of the Upper Uruguay River. To
test this hypothesis
we will do the following: 1) assess the spatial and
temporal abundance of the ichthyoplankton in different portions of three tributaries of the Upper Uruguay
River and 2) correlate environmental variables of
these tributaries with the density of eggs and larvae.
By doing this, we hope to be able to determine the
importance of the tributaries for the conservation of
fish species.
Materials and methods
Three important tributaries of the Upper Uruguay
River were studied: the Chapecó, Peixe, and Ligeiro
rivers (Figure captions Fig. 1). Samples from nine
sites were analyzed, of which two were in the Ligeiro
River, three in the Chapecó River, and four in the
Peixe River. The number of sampling points per
tributary was proportional to the river stretch free
from any geographical barrier that could stop fish
coming from the Uruguay River. Therefore, sampling
sites were distributed throughout the area between
the tributary mouth and the first natural obstacle to
fish migration.
Chapecó River flows into a stretch of the Uruguay
River away from the influence of dams, 140 km
downstream of the Itá hydroelectric power plant (27°
05′37.24″, S 53°00′59.79″ W). It is a long and
sinuous river with a waterfall located 150 km upstream of its confluence with the Uruguay River.
Saudade Fall is about 30 m high and creates a
impassable barrier to fish movement. Three sampling
sites were chosen in this river: C1–22 km
downstream of the Saudade Fall and 128 km
upstream of its confluence with the Uruguay River;
C2–65 km upstream of its confluence with the
Uruguay River; and C3–8 km upstream of its
confluence with the Uruguay River.
Peixe River is very steep and flows into the
Uruguay River in a lotic/lentic transition area of the
Itá reservoir (27°27′50.35″ S, 51°53′54.81″ W).
Sampling sites were chosen downstream of the
Tedesco Fall (which cannot be climbed by fish),
which is located 208 km upstream of the confluence
with the Uruguay River (Caçador, Santa Catarina
state). Four sampling sites were established: P1– 34
km downstream of the Tedesco Fall; P2–146 km
upstream of the confluence with the Uruguay River;
P3–61 km upstream of the confluence with the
Environ Biol Fish (2011) 91:51–61
53
Fig. 1 Location of the sampling sites. Arrows are showing direction of rivers flow
Uruguay River; and P4–22 km upstream of the
confluence with the Uruguay River.
Ligeiro River is the only tributary in the short
running stretch of the Uruguay River (6 km extension) between the Itá reservoir and the Machadinho
dam. Rio Ligeiro encounters the Uruguay River 5 km
downstream of the Machadinho dam and 130 km
upstream of the Itá dam (27°31′24.34″ S, 51°50′
09.82″ W). The river has an obstacle 13 km upstream
of its confluence with the Uruguay River, and it also
cannot be climbed by fish. In very rare hydrological
conditions, the natural fall may allow the upstream
migration of a few fish species. Two sampling sites
were chosen: L1–3 km downstream of the natural
obstacle and L2–50 m upstream of the confluence
with the Uruguay River.
Samplings were done at night (21:00–22:00)
between October 2005 and September 2006 at 5 day
intervals. Cylindroconical plankton nets of 0.5 mm
2
mesh size and 0.11 m mouth area were used.
Plankton nets were fixed near the margin about 10
cm from the water surface and left for 30 min.
Surface water velocity was estimated by measuring
the time required for a buoy to run 5 m. During
sampling, water temperature was measured with a
mercury thermometer and pH with a colorimetric kit.
Additionally, water turbidity (clean or muddy water),
fluviometric level (low, normal or high) and rain
(raining or not) were registered every sampling day.
Samples were preserved in 4% buffered formalin
solution and transported monthly to the laboratory
for analysis. Fish eggs and larvae were separated
from debris under a stereoscopic microscope (10x)
using a Bogorov counting chamber. Following
ichthyoplank-ton separation and quantification,
larvae were identi-fied to the lowest taxonomic level
following Nakatani et al. ( 2001) and Reynalte-Tataje
and Zaniboni-Filho ( 2008). The abundance of eggs
3
and larvae was standardized to a volume of 10 m of
filtered water (Nakatani et al. 2001).
To assess the spatiotemporal variation of fish eggs
and larvae distribution (i.e., the variation among
sampling sites and months), a bifactorial analysis of
variance (ANOVA) was applied at a 5% significance
level.
The relation between fish larvae assemblage
structure and all the environmental variables was
analyzed by Canonical Correspondence Analyses
54
(CCA; ter Braak 1986). The environmental variables
were previously log-transformed (log x+1) and to
reduce the effect of rare species in the ordering; only
species with frequency of occurrence greater than 1%
were chosen. The selection of the environmental
variables was based on a forward selection procedure.
The statistical significance of the correlation speciesenvironment was tested through the Monte Carlo test
(999 randomizations). To determine the fraction of
variation of larval assemblage structure explained by
environmental variables, the Partialling Out Method
proposed by Boccard et al. ( 1992) was used. To
confirm the spatiotemporal segregation of data observed at the CCA, the unifactorial ANOVA was used
with the selected axis (α=0.05) and then, when
necessary, the Tukey test. All the ordinations were
performed by the program PCord 5.0 and a cut-off point
of 0.05 was used as pattern. Finally, larval densities
previously log-transformed (log x+1) were correlated to
the environmental variables selected by CCA through
Pearson’s correlation.
Results
During the study period, 591 samples were collected, of
which 170 were positive for fish eggs or larvae, resulting in
a total of 12,847 eggs (93% of all of the ichthyoplankton
organisms collected) and 962 larvae collected. Larvae
belonging to 5 orders, 17 families, 26 genera and 27
species were collected (Table 1). Of the total larvae
collected, 69% were Characiforms, 27% were Siluriforms,
and 4% were Gymnotiforms, Perci-forms and
Atheriniforms. Among the identified fami-lies, Characidae
had the greatest number of taxa (seven). Some fish species
had a distribution restricted to determined sampling sites.
Auchenipterus
sp.,
Pachyurus
bonariensis,
Parapimelodus valenciennis and Trychomycterus sp.
were present only in the Ligeiro River. Acestrorhynchus
pantaneiro,
Apareio-don
affinis,
Leporinus
obtusidens, Loricariichthys spp., Cetopsis gobioides,
Rineloricaria sp. and Tatia spp. appeared exclusively in
the Chapecó River (Table 1).
The abundance of ichthyoplankton varied among
the sampling sites and months of the study period.
Eggs were all captured between October and January,
and 65.3% of the larvae were captured between
November and December. Between May and June,
no larvae were captured. At the Ligeiro River, the
Environ Biol Fish (2011) 91:51–61
bifactorial ANOVA showed a higher abundance of eggs
at site L1 during November, December and January and
at site L2 during November; the higher abundance of
larvae at this river was also observed at site L1 during
November, December and January (Table 2 and Fig.
2). At the Peixe River, this analysis showed higher
densities of eggs during October and November; while
for larvae, higher densities were observed at site P3
during November and December. At the Chapecó River,
the bifactorial analysis showed a higher concentration of
eggs at site C3 during November, December and
January; while for larvae, this analysis showed a higher
concentration on November and December, and at site
C1 and C2.
Fish larvae assemblage structure showed to be
related to environmental variables (p<0.01), explaining
4.0% of total variability of the taxa abundance. Only
water temperature and velocity were selected (forward
selection) for incorporation in the final model (Table
3). Overall, the correlation of larvae abundance and
these environmental variables at the three tributaries
studied showed the pattern observed in Fig. 3a and b.
Only the first axis of the CCA showed statistical
significance for the species-environment correlation
(p<0.01) (Table 3) and revealed a conspicuous
spatiotemporal gradient (Fig. 3a). It was possible to
observe during the study period, at all of the
tributaries studied, a tendency of higher values of
water temperature and velocity at the lower sampling
sites than at the upper ones (Table 4). This tendency
was registered by the first axis of CCA (Fig. 4a).
Similarly, these environmental variables showed a
temporal segregation presenting the highest values
between November and February (Fig. 4b). The
ANOVA applied to the first axis of CCA showed that
both sampling sites at Ligeiro River positively
influenced the formation of this axis (Tukey; p<0.05)
and that sites C1, P1, and P2 influenced negatively
the formation of this axis (Tukey; p<0.05), showing
an inverse correlation with water temperature and
velocity (Fig. 4a, b).
Water temperature showed a direct correlation to the
increase of eggs (r=0.357; p<0.05) and larvae (r=0.470;
p<0.05) abundances. Species such as Schizodon nasutus
(r=0.450; p<0.05), Pimelodus atrobrunneus (r=0.387;
p<0.05) and Leporinus amae (r=0.401; p<0.05) were
positively correlated to water temperature; while
Auchenipterus sp. was positively (r=0.501; p<0.05) and
Bryconamericus stramineus (r=-0.235; p<0.05)
Environ Biol Fish (2011) 91:51–61
55
3
Table 1 Taxonomic composition and mean density (individuals/10 m ) of fish larvae collected between October 2005 and September
2006 in the Ligeiro, Peixe and Chapecó rivers
Taxononomic Groups
FO (%) Ligeiro River Peixe River
L1
Atheriniforms
Atherinidae
Odonthestes aff. perugiae
a
Characiforms
L2
P1
P2
Chapecó River
P3
P4
C1
0.028
C2
1.01
0.056
1.34
0.042 0.014 0.014 0.028 0.014 0.014 0.014 0.056
2.86
0.17
0.681 0.014
5.55
0.569 0.278
C3
0.014
Anostomidae
Leporinus amae
Leporinus obtusidens
Schizodon aff. nasutus
Acestrorhynchidae
Acestrorhynchus pantaneiro
0.014
0.014 0.014
0.014
0.056 0.069 0.042 0.806 0.028
0.50
0.042
Characidae
Astyanax bimaculatus
Astyanax fasciatus
2.02
4.20
0.028
0.236 0.014
Astyanax gr. scabripinnis
Bryconamericus iheringii
3.87
4.54
0.153
0.097
Bryconamericus stramineus
Oligosarcus cf. jenynsii
6.89
0.014 0.014 0.028 0.014 0.097 0.014
0.028 0.125 0.056 0.083 0.083 0.014
0.028 0.014 0.250
0.125 0.583 0.014
0.097 0.847 0.014 0.500 0.597 0.042
0.111 0.014
0.042 0.236 0.014 0.472 0.083
1.01
Serrasalmus maculatus mamaculamaculatusmaculates 3.36
0.389 0.014
0.569 0.014 0.056
0.042 0.097 0.083 0.014
Curimatidae
Steindachnerina brevipinna
1.18
0.083
1.51
0.056 0.014
0.042
Erythrinidae
Hoplias spp.
Paradontidae
Apareiodon affinis
b
Lebiasinidae
0.208 0.042 0.028
0.50
0.014 0.042
0.17
0.014
Gymnotiformes
Gymnotidae
Gymnotus carapo
Sternopygidae
Eigenmannia virescens
Perciformes
0.34
0.014 0.014
0.67
0.014
0.34
0.014 0.014
0.014
0.056
Scianidae
Pachyurus bonariensis
a
Siluriforms
0.17
Auchenipteridae
Auchenipterus sp.
1.01
Tatia spp.
0.17
0.014
0.014
0.028 0.056
0.014
Cetopsidae
Cetopsis gobioides
b
Heptapteridae
1.18
0.181
Cetopsorhamdia aff. iheringii
0.67
1.01
0.014
0.042
Pimelodella sp.
1.34
0.069 0.028
0.028
0.014
0.028
0.056 0.028 0.014
0.042
56
Environ Biol Fish (2011) 91:51–61
Table 1 (continued)
Taxononomic Groups
FO (%) Ligeiro River Peixe River
L1
L2
P1
P2
Rhamdia quelen
Loricariidae
Hypostomus spp.
3.53
0.292 0.028
3.53
0.056 0.014 0.014
Loricariichthys spp.
Rineloricaria sp.
0.17
Chapecó River
P3
P4
C1
C2
C3
0.014 0.625 0.028 0.208
0.542 0.042 0.111
0.014
0.34
0.028
Pimelodidae
Parapimelodus valenciennis
Pimelodus atrobrunneus
0.34
0.34
0.028
0.236
Pimelodus maculatus
1.51
0.042 0.028
0.34
0.042
3.336 0.572 0.070 0.293 1.654 2.334 1.585 3.197 0.196
0.014
0.028
0.028 0.014 0.014
Trichomycteridae
Trichomycterus sp.
Total
a
Larvae identified at the Order level
b
larvae identified at the Family level
FO: Frequency of occurrence of the taxonomic group in the total samples
was negatively correlated to water velocity. Larval
segregation related to environmental variables was
also evident at CCA diagram (Fig. 3b).
Discussion
confirmed to the distribution of eggs and larvae along
the rivers. Some tributaries studied presented higher
densities of eggs at the lower portion and higher
larval densities at the upper portion, while others
presented higher densities of eggs and larvae at
middle portions of the river.
This study showed that several species of the Upper
Uruguay River are using the tributaries of the region
as reproduction sites and nursery areas, but different
from what is observed in other basins, no pattern was
The species that are using the Ligeiro, Peixe and
Chapecó rivers for reproduction are mainly Characiform
and Siluriform fishes, which agrees with the ichthyofau-na
composition inventory prepared by Zaniboni-Filho et al. (
2004) in the Upper Uruguay River. The 27 species
Table 2 Results of ANOVA for the spatiotemporal variation of fish eggs and larvae distribution in the three tributaries studied
Eggs
Ligeiro
Peixe
Chapecó
Larvae
SS
GL
MS
F
P
SS
GL
MS
F
P
Site
0.224
1
0.224
1.978
0.162
0.248
1
0.248
56.68
0.000*
Month
Site/Month
16.937
5.985
11
11
1.54
0.544
13.579
4.798
0.000*
0.000*
1.16
0.826
11
11
0.106
0.075
24.20
17.14
0.000*
0.000*
Site
Month
0.172
1.42
3
10
0.057
0.142
1.367
3.383
0.254
0.000*
1.1
1.789
3
10
0.363
0.179
10.31
5.07
0.000*
0.000*
Site/Month
1.200
30
0.04
0.955
0.537
3.611
30
0.120
3.41
0.000*
Site
Month
3.29
17.50
2
10
1.647
1.751
13.936
14.81
0.000*
0.000*
0.147
1.33
2
10
0.07
0.133
4.086
7.384
0.018*
0.000*
Site/Month
11.524
20
0.576
4.875
0.000*
0.465
20
0.023
1.29
0.193
*Significantly different (p<0.05)
Environ Biol Fish (2011) 91:51–61
57
3
Fig. 2 Mean abundance values (± standard error) of eggs and larvae (individuals/10 m ) for the sampling sites of each tributary between
October 2005 and September 2006. (Due to sampling problems, no data is available from September 2006 to Peixe and Chapecó rivers)
58
Environ Biol Fish (2011) 91:51–61
A
P1
P2
P3
P4
L1
L2
C1
C2
C3
3
2
Temperature
1
0
CCA 2
Table 3 Results of the Canonical Correspondence Analyses
(CCA) performed with the most frequent larvae taxa and the
environment variables observed at different samplings sites of
the Upper Uruguay River, between October 2005 and September 2006. Monte Carlo test was significant to the first three
axes of the ordination for p<0.01 (n=999 permutations)
Axis 1
Axis 2
Axis 3
Eigenvalues
0.245
0.225
0.101
-2
Cumulative percentage
of variation explained
by species-environment
correlation
Species-environment
Correlation (r)
Correlation between
environmental
variables and
canonical axes
4.0
7.7
9.3
-3
Temperature
Water velocity
0.552*
0.375*
-1
Current
-4
0.651*
0.636
-5
-2
0.510
-1
0
1
2
3
4
5
CCA 1
B
3
P.maculatus
2
-0.128
0.131
1
* Significantly different (p<0.05)
S.nasutus
Temperature
P.atrobrunneus
L.amae
S.brevipinna
S.maculatus
C.gobioides
A.g.scabripinnis
A.bimaculatus Hoplias spp.
O.aff.perugiae
A.fasciatus
Pimelodella sp. E.virescens
C.iheringii
Hypostomus spp.
B.iheringii
0
-1
CC
A
Note: Total inertia=6.161
Auchenipterus sp
B.stramineus
2
0.225
-0.463
Current
-2
R.quelen
-3
found in this study correspond to 27% of a total of 98
fish species identified in the Upper Uruguay River
region by those authors. Overall, small and medium
size fish species were the most frequent and abundant
in the ichthyoplankton of these three tributaries of the
Upper Uruguay River.
In the Ligeiro River, 22 fish species were observed,
of which 18 were common to the other tributaries.
Fig. 3 Canonical Correspondence Analyses related to ordination of
the most frequent taxa observed at the three tributaries of the
Upper Uruguay River, between October 2005 and September 2006
Auchenipterus sp., Trychomicterus sp., Pachyurus
bonariensis, and Parapimelodus valencienis occurred
exclusively in this tributary. Reynalte-Tataje et al. ( 2008)
found larvae of these fish species in other areas of the
Upper Uruguay River, but at lower densities. The Peixe
River represented the lowest number of fish species with a
total of 19 species. Nevertheless, comparatively high
abundance values for ichthyoplankton organisms were
observed at this tributary. The greatest taxa richness was
observed in the Chapecó River (25 fish species). Some taxa
were captured exclusively at Chapecó River, such as
Acetrorhynchus pantaneiro, Apareiodon affinis, L.
obtusidens, Loricariichthys spp., Cetopsis gobioides,
Rineloricaria sp., and Tatia spp. However, some of these
taxa have also been observed by other authors in other
areas of the Upper Uruguay River (Reynalte-Tataje et al.
2008). L. obtusidens larvae were only found in the
Chapecó River, confirming the results reported by
Reynalte-Tataje et al. ( 2008) and Hermes-Silva et al.
( 2009). Their restriction to this area might be due to
the fact that this is an important tributary located in a
stretch free from dams.
In previous surveys by Zaniboni-Filho et al. ( 2008),
adult individuals of migratory fish species, such as L.
obtusidens, Prochilodus lineatus, Salminus
brasilien-sis, Steindachneridiun scriptum and
Pimelodus mac-ulatus, were found in the same study
area. In this study, the capture of eggs and larvae of
rheophilic fishes was rare. P. maculatus larvae were
found in the three tributaries, but L. obtusidens larvae
were only seen in the Chapecó River.
P. maculatus is considered a long-distance migratory species (Godoy 1987; Suzuki et al 2005);
however, according to Zaniboni-Filho and Schulz (
2003), this species performs lateral reproductive
migrations as well (i.e., migration between the main
river and a tributary). The presence of P. maculatus
-4
-5
-2
O.jenynsii
-1
0
1
2
3
4
CCA 1
Environ Biol Fish (2011) 91:51–61
CCA 1
A
59
2
Table 4 Mean (± standard deviation) temperature and water
velocity at the sampling sites between October 2005 and
September 2006 (numbers in parentheses are maximum and
minimum values observed)
1
Sampling Site
Temperature (°C)
Velocity (m·s )
0
L1
21.3±3.6
0.673±0.1221
(16.5–27.0)
(0.357–1.000)
L2
21.5±6.2
(15.5–33.5)
0.718±0.1113
(0.357–1.250)
C1
20.8±4.4
(15.6–27.0)
0.278±0.0595
(0.116–0.417)
C2
22.8±4.3
(17.8–29.0)
0.420±0.1004
(0.042–0.833)
C3
21.4±5.1
0.239±0.1253
P1
(14.2–29.0)
19.3±3.1
(0.071–0.833)
0.573±0.3211
3
F(8, 118)=5.7485, p=0.00000
-1
-2
-3
P1
P2
P3
P4
L1
L2
C1
C2
C3
Sites
B
2
F(9, 119)=5.6788, p=0.00000
1
CCA 1
0
-1
-2
-1
(12.6–22.7)
(0.172–1.370)
P2
17.9±4.7
(11.6–25.0)
0.629±0.1198
(0.217–1.087)
P3
19.7±2.3
(17.0–23.8)
0.078±0.139
(0.001–0.083)
P4
21.0±5.0
0.888±0.1483
(13.8–29.3)
(0.333–1.250)
-3
Oct
Nov
Dec Jan
Feb
Mar
Apr
Jul
Aug
Sep
Time
Fig. 4 ANOVA applied to the data matrix of CCA1 showing
spatial a and temporal b variation
larvae in different developmental stages in the three
tributaries studied indicates that these species utilize
these tributaries as spawning and nursery sites.
The proportion of eggs (93%) and larvae found in
this study is similar to that found by Hermes-Silva et al.
( 2009) in the Upper Uruguay River, in which eggs
comprised 94.7% of the organisms captured in the
mouths of some tributaries and in the Uruguay River.
The small proportion of larvae can be explained by
natural egg mortality caused by the topography of the
Upper Uruguay River (Reynalte-Tataje et al. 2008).
The embryonic development of the eggs occurs at sites
where rapids and still waters alternate, where egg
mortality is caused by mechanical shocks and sedimentation, respectively. Reynalte-Tataje et al. ( 2008)
reported a positive correlation between spoiled egg
abundance in ichthyoplankton samples from the Upper
Uruguay River and an increase in the river flow.
Although fish reproduction in that region is
markedly seasonal, occurring mainly between October and January, larvae occurrence was verified in
almost all sampled months. This result may be
explained by the presence of multiple-spawning
species, that are generally classified as sedentary or
short-migratory
species,
and
an
extended
reproductive period. According to the classification
of reproductive strategies proposed by Suzuki et al. (
2005), among the 27 species identified in this study,
25 were non-migratory or short-distance migratory
species, and 2 (L. obtusidens and P. maculates),
were long-distance migratory species.
The topography of the Upper Uruguay River may
have contributed to the absence of a pattern in the
distribution of eggs and larvae along the tributaries.
Similar to the Uruguay River, the tributaries at this
region are a sequence of rapids and pools (still waters),
but in a small scale. This particular topogra-phy seems
to be very well used by the fish fauna, where they can
spawn in the rapids and the pools can be used as nursery
areas by their larvae. The sampling sites P3 and L1 are
located very close to pool areas in
Environ Biol Fish (2011) 91:51–61
60
both rivers, so the high larval density and diversity
observed at these sampling sites corroborate this
hypothesis. The higher occurrence of eggs observed in
the lower Chapecó River (site C3) provides additional
evidence. At the Paraná River Basin, many studies have
shown a specific spatial gradient of eggs and larvae
density variation, with the first occurring mainly at the
upper portions of the basin and the latter at the lower
portions. Baumgartner et al. ( 2004) reported higher egg
abundances in the upper portions of some rivers, while
larvae were more abundant in the lower portion. Similar
results were reported by Agostinho et al. ( 1993) for the
migratory species P. lineatus.
Although no pattern was observed when the distribution of eggs and larvae abundances was analyzed, the
CCA showed a soften relationship between ichthyoplankton and environmental variables. Following this
analysis, water temperature and velocity were important
factors determining ichthyoplankton segregation. In this
way, species such as Schizodon nasutus, Pimelodus
atrobrunneus and Leporinus amae were related
mainly to habitats with higher water temperature and
the species Auchenipterus sp. was related to habitats
with higher water velocity. The high water temperature
and velocity observed mainly at the lower portion of the
tributaries may also be important factors determining
egg and larvae abundances at these regions, even though
it is not a confirmed pattern.
As opposed to Zaniboni-Filho and Schulz ( 2003),
the presence of eggs and larvae in the Upper Uruguay
River was not restricted to tributaries’ mouth, but was
present along the length of the river, where it was
related to the existence of pools (nursery areas) and
rapids (reproduction sites) along these tributaries.
This study indicates that there is a spatiotemporal
variation of egg and larvae distribution in the three
tributaries studied of the Upper Uruguay River, corroborating our hypothesis. Temporally, it was possible to
observe a reproductive pattern in these tributaries,
where higher abundances of ichthyoplankton organisms
occur mainly between October and January. Spatially,
although no general pattern was observed, it was
possible to verify some spatial differences of eggs and
larvae distribution along these rivers, related mainly
topography and hydrology characteristics. These findings strengthen the importance of the tributaries of the
Upper Uruguay River as spawning grounds and rearing
areas of several fish species in this region, emphasizing
the necessity of preserving such environments.
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