181
13 Estimating Fish Production in the Itaipu Reservoir (Brazil): The Relationship Between Fish Trophic Guilds, Limnology …
1984). One aspect that should be considered is the type of
data collection, with water samples taken only from the
surface, the area of greatest development of cyanobacteria
(Tundisi 1990).
The low densities of phytoplankton groups studied can
be justified by the low average concentrations of nutrients.
According to Reynolds (1987), all phytoplankton species are
likely to keep growing as long as nutrients are available, and
faster growth species are selected in environments with favorable nutritional conditions.
The results of multiple regression analyses indicate that
the independent variables water transparency, turbidity, and
TKN showed a positive effect on the development of cyanobacteria in the Itaipu Reservoir, and the variable suspended
solids influenced negatively in their development. The generated model can estimate the biomass of cyanobacteria from
the independent variables mentioned above.
When cyanobacteria were related to total phosphorus and
fishes, the results indicated that the limiting nutrient for the
development of these algae is total phosphorus, and the fishes also play an extremely important role in this environment.
The proportion between the different groups that make
up the zooplankton community(rotifers, copepods and cladocerans) is considered a way to use this community as an
indicator of the trophic conditions of the environment, and
the dominance of rotifers is often associated with increased
eutrophication (Matsmura-Tundisi et al. 1990).
In the Itaipu Reservoir the most dominant group was
Copepoda, with the highest densities in the whole system
(57.97 %). Studies in reservoirs of Paraná indicate the same
dominance of copepods (Velho et al. 2005) and in the Tucuruí Reservoir, Espíndola et al. (2000) observed the same
pattern. The highest density of Copepoda as well as of Cladocera, may be associated with a higher trophic level in the
zones where the concentration of chlorophyll and cyanobacteria were higher. According Marzolf (1990), higher densities of zooplankton in the transition zone are observed when
resource availability and hydrodynamics are interacting to
determine abundance patterns.
The greatest abundance of zooplankton was recorded
in the transition zone of the Itaipu Reservoir. Studies by
Lansac-Tôha et al. (1999) in reservoirs of the state of Paraná
showed results that corroborate the results of this work. The
highest abundance in the transition zone is explained by
higher concentrations of chlorophyll-a, owing to increased
human activity observed in the basins of the San Francisco
Verdadeiro e Falso Rivers, located in the transition zone.
The distribution of small cladocerans may be related to the
abundance of cyanobacteria, especially in eutrophic environments (Lansac-Tôha et al. 2005). The biochemical properties of cyanobacteria (and some species may be toxic) and
the shape and size of colonies prevent an overexploitation
of this resource by zooplankton, keeping the abundances of
colonies stable (De Bernadi and Giussani 1999).
The second most abundant group was the rotifers. Although eutrophic and hyper eutrophic environments favor
the dominance of rotifers, in most aquatic environments of
Brazil, rotifers are often dominant, regardless of trophic status, both in density and in number of species (Rocha et al.
1995). The nondominance of rotifers in the Itaipu Reservoir
(32 %) is consistent with the results for oligotrophic and mesotrophic lakes in the USA (Blancher 1984), in which rotifers represent 20–37 %.
Regarding the distribution of the longitudinal axis of the
reservoir, the highest density of rotifers was found in the riverine zone and in the remaining zones there was a dominance
of copepods. Studies in the reservoir of the Corumbá River
(Lansac-Tôha 1999) show that the community structure was
altered after the construction of the dam, when the densities of cladocerans and copepods showed greater development. Other authors relate the trophic status in the community structure of zooplankton (Matsmura-Tundisi et al. 1990;
Lansac-Tôha et al. 2005).
The study of the determination of the time-space variation
of fish communities and the way in which these stocks are
removed from the environment are used to rationalize fishery management (Okada 2001). According to Hodgson et al.
(1996) fish populations have effects of fundamental importance in trophic cascades, with direct influence on biomass
and productivity of other trophic levels.
The results of the variation of the trophic structure of the
fishes show that the group of omnivores was most abundant
in the reservoir, followed by piscivores. These two groups
accounted for 77.7 % of the total biomass. The riverine zone
showed higher occurrence of omnivores and detritivores.
In the transition and lacustrine zones omnivores and piscivores were more abundant. Borghetti et al. (1988) also found
eight trophic groups in the Itaipu Reservoir in the period
between 1983 and 1986. Then, the piscivores were dominant (25.6 %), followed by iliophagous (15.6 %), omnivores
(12.5 %), insectivores (9.4 %), herbivores (3.4 %), planktivores (1.6 %), lepidofages-insectivores (1.6 %), and others
(9.4 %). Agostinho et al. (1999) in a 6-year study (1983 to
1989) of the Itaipu Reservoir, found similar results, with
higher occurrences of omnivores and piscivores. Oliveira
et al. (2005) found greater biomass of hake Cynoscion spp.
(piscivores), detritivores, and omnivores in the Itaipu Reservoir. The differences between the trophic guilds of fishes in
the present study compared to those of Oliveira et al. (2005),
can be explained by differences in sampling methodology, as
these authors used only gillnets in their studies. The results
presented here are derived from landings of commercial fishing, where fish biomasses are best represented by the fact
that fishermen pursue this activity with greater frequency.
The monitoring of professional fisheries in the Itaipu reser-
13 Estimating Fish Production in the Itaipu Reservoir (Brazil): The Relationship Between Fish Trophic Guilds, Limnology …
1984). One aspect that should be considered is the type of
data collection, with water samples taken only from the
surface, the area of greatest development of cyanobacteria
(Tundisi 1990).
The low densities of phytoplankton groups studied can
be justified by the low average concentrations of nutrients.
According to Reynolds (1987), all phytoplankton species are
likely to keep growing as long as nutrients are available, and
faster growth species are selected in environments with favorable nutritional conditions.
The results of multiple regression analyses indicate that
the independent variables water transparency, turbidity, and
TKN showed a positive effect on the development of cyanobacteria in the Itaipu Reservoir, and the variable suspended
solids influenced negatively in their development. The generated model can estimate the biomass of cyanobacteria from
the independent variables mentioned above.
When cyanobacteria were related to total phosphorus and
fishes, the results indicated that the limiting nutrient for the
development of these algae is total phosphorus, and the fishes also play an extremely important role in this environment.
The proportion between the different groups that make
up the zooplankton community(rotifers, copepods and cladocerans) is considered a way to use this community as an
indicator of the trophic conditions of the environment, and
the dominance of rotifers is often associated with increased
eutrophication (Matsmura-Tundisi et al. 1990).
In the Itaipu Reservoir the most dominant group was
Copepoda, with the highest densities in the whole system
(57.97 %). Studies in reservoirs of Paraná indicate the same
dominance of copepods (Velho et al. 2005) and in the Tucuruí Reservoir, Espíndola et al. (2000) observed the same
pattern. The highest density of Copepoda as well as of Cladocera, may be associated with a higher trophic level in the
zones where the concentration of chlorophyll and cyanobacteria were higher. According Marzolf (1990), higher densities of zooplankton in the transition zone are observed when
resource availability and hydrodynamics are interacting to
determine abundance patterns.
The greatest abundance of zooplankton was recorded
in the transition zone of the Itaipu Reservoir. Studies by
Lansac-Tôha et al. (1999) in reservoirs of the state of Paraná
showed results that corroborate the results of this work. The
highest abundance in the transition zone is explained by
higher concentrations of chlorophyll-a, owing to increased
human activity observed in the basins of the San Francisco
Verdadeiro e Falso Rivers, located in the transition zone.
The distribution of small cladocerans may be related to the
abundance of cyanobacteria, especially in eutrophic environments (Lansac-Tôha et al. 2005). The biochemical properties of cyanobacteria (and some species may be toxic) and
the shape and size of colonies prevent an overexploitation
of this resource by zooplankton, keeping the abundances of
colonies stable (De Bernadi and Giussani 1999).
The second most abundant group was the rotifers. Although eutrophic and hyper eutrophic environments favor
the dominance of rotifers, in most aquatic environments of
Brazil, rotifers are often dominant, regardless of trophic status, both in density and in number of species (Rocha et al.
1995). The nondominance of rotifers in the Itaipu Reservoir
(32 %) is consistent with the results for oligotrophic and mesotrophic lakes in the USA (Blancher 1984), in which rotifers represent 20–37 %.
Regarding the distribution of the longitudinal axis of the
reservoir, the highest density of rotifers was found in the riverine zone and in the remaining zones there was a dominance
of copepods. Studies in the reservoir of the Corumbá River
(Lansac-Tôha 1999) show that the community structure was
altered after the construction of the dam, when the densities of cladocerans and copepods showed greater development. Other authors relate the trophic status in the community structure of zooplankton (Matsmura-Tundisi et al. 1990;
Lansac-Tôha et al. 2005).
The study of the determination of the time-space variation
of fish communities and the way in which these stocks are
removed from the environment are used to rationalize fishery management (Okada 2001). According to Hodgson et al.
(1996) fish populations have effects of fundamental importance in trophic cascades, with direct influence on biomass
and productivity of other trophic levels.
The results of the variation of the trophic structure of the
fishes show that the group of omnivores was most abundant
in the reservoir, followed by piscivores. These two groups
accounted for 77.7 % of the total biomass. The riverine zone
showed higher occurrence of omnivores and detritivores.
In the transition and lacustrine zones omnivores and piscivores were more abundant. Borghetti et al. (1988) also found
eight trophic groups in the Itaipu Reservoir in the period
between 1983 and 1986. Then, the piscivores were dominant (25.6 %), followed by iliophagous (15.6 %), omnivores
(12.5 %), insectivores (9.4 %), herbivores (3.4 %), planktivores (1.6 %), lepidofages-insectivores (1.6 %), and others
(9.4 %). Agostinho et al. (1999) in a 6-year study (1983 to
1989) of the Itaipu Reservoir, found similar results, with
higher occurrences of omnivores and piscivores. Oliveira
et al. (2005) found greater biomass of hake Cynoscion spp.
(piscivores), detritivores, and omnivores in the Itaipu Reservoir. The differences between the trophic guilds of fishes in
the present study compared to those of Oliveira et al. (2005),
can be explained by differences in sampling methodology, as
these authors used only gillnets in their studies. The results
presented here are derived from landings of commercial fishing, where fish biomasses are best represented by the fact
that fishermen pursue this activity with greater frequency.
The monitoring of professional fisheries in the Itaipu reser-