183
13 Estimating Fish Production in the Itaipu Reservoir (Brazil): The Relationship Between Fish Trophic Guilds, Limnology …
Predation has an important effect as it controls the excess
of prey available, allowing better survival of the remaining
fishes on the limited food supply, especially when the reproduction rate of the prey is very high, such as tilapia (Odum
1983).
In systems where there are high fish densities, there may
be loss of weight and animal death (Mcqueen 1990). The
presence of a predator provides stability to the system, since,
with predation, there is a reduction of the stock of prey, thus
avoiding the lack and competition for food by the forage species. Paiva et al. (1994) conducted a study where the number
of predator species interfered with the trophic dynamics in
reservoirs of northeastern Brazil. They noted that the reservoirs in which there were two species of predators fish catching was high, while in reservoirs with less than two species
of predators, competition among prey probably contributed
to the decrease in catch.
Several authors suggest that the community structure and
biomass of fresh water are regulated by predators (Hrbáček
et al. 1961; Shapiro et al. 1975; Shapiro and Wright 1984;
Carpenter et al. 1985; Persson et al. 1988). It can be said that
an increase in piscivore biomass is associated to reduction
of planktivore biomass, increase of large zooplankton biomass and reduction of phytoplankton (Carpenter et al. 1985;
Mcqueen 1990; Meijer 1999; Gulati 2001). The interactions
established by predation can promote the reduction of the
planktivore biomass and these, associated with increased
biomass of large zooplankton and the increase of algae consumers (grazing), reduce the phytoplankton biomass. Several authors (Shapiro and Wright, 1984; Vanni and Findlay
1990) have noted that predators are important to zooplankton
alterations.
According to the trophic cascade hypothesis, the increase
in piscivore (carnivore) biomass in a lake or reservoir causes
the planktivorous fish biomass to decrease, increasing the
zooplankton biomass and reducing phytoplanktonic biomass
(Shapiro et al. 1975; Carpenter et al. 1985).
Lazzaro et al. (2003) point out that top-down effects in
tropical environments are more complex than in temperate
environments, as there are no species that are essentially
zooplanktivorous but omnivores (such as tilapia), and the
piscivores give way to general macro carnivores. The chlorophyll-a concentration increases with the omnivorous fish
biomass and decreases with macro carnivore biomass. However, in this work, the chlorophyll-a concentration was not
related to total fish biomass and to the macrozooplankton
biomass. Interactions between fishes and chlorophyll concentration and between fish and zooplankton biomass were
inversely related to the trophic state. The fish-phytoplankton regulation caused by complex omnivore interactions is
present in the feeding behavior of various populations of
consumers.
The results show that the Itaipu Reservoir presented topdown and bottom-up relationships, supporting the findings
by Rejas et al. (2005) in experiments in a lake in Bolivia,
with direct and indirect effects on the trophic cascade. The
top-down effect was found only in the first trophic level and
the biomass of piscivores exercised a control over omnivores
and detritivores, and had no predatory effect on the benthivore and zooplanktivore biomasses, whose biomasses actually increased. This suggests that the piscivores may have
an effect on the water quality of the reservoir, not positively,
but negatively, since the indirect relations between the guilds
consumed by predators have shown to have a controlling effect on both chlorophyll-a and cyanobacteria. The control effect of piscivores was also reported by Pelicici et al. (2005)
on 31 reservoirs of the state of Paraná.
The effect of omnivores on the other trophic groups of
fishes was also evaluated, since this group is composed
largely of big fish ( Pimelodus sp and Pterodoras granulosus, for example). This analysis indicates that the increase
in omnivore biomass caused depletion of benthivore and detritivore biomass, contrary to what happened with the zooplanktivore, which had their biomass increased.
An important relationship studied was the one between
detritivores and primary production in the reservoir. Although the results indicate that this group suffers a strong
predation pressure by piscivores and omnivores, detritivores
promote control over chlorophyll-a and cyanobacteria.
When the omnivore filter-feeders dominate the planktivorous fishes, the hypothesis “fishes versus trophic state of the
lake’’ can no longer be valid because the growth stimulation
by omnivores is intensified by the increased load of nutrients (Drenner et al. 1996). The systems dominated by omnivores show weak trophic links, owing to a combination
of weakened mechanisms. Consequently, they are difficult
to predict with the trophic cascades hypothesis. Among the
mechanisms involved, Lazzaro et al. (2003) comment on the
‘‘intraguild predation’’ (IGP), which applies to omnivores
that feed on herbivorous zooplankton and on their phytoplanktonic food and also ontogenetic omnivory (the same
mechanism that occurs during the fish ontogeny from the
juvenile phase to the adult).
The lack of zooplanktivore predation effect demonstrated in the statistical analysis also indicates damage to water
quality, with positive effect related to the cyanobacteria.
Studies by Abujanrra and Agostinho (2002) emphasize the
great adaptability that zooplanktivorous species have owing
to their gill system, as their gills are large, long, thin, and numerous. For instance, the filtration mechanism of Hypophthalmus marginatus is of the passive filter kind that consists
in swimming with an open mouth and with extended operculum, and their diet is composed primarily of zooplankton.
These results demonstrate the importance of omnivore and
detritivore chains in tropical and subtropical aquatic environ-
13 Estimating Fish Production in the Itaipu Reservoir (Brazil): The Relationship Between Fish Trophic Guilds, Limnology …
Predation has an important effect as it controls the excess
of prey available, allowing better survival of the remaining
fishes on the limited food supply, especially when the reproduction rate of the prey is very high, such as tilapia (Odum
1983).
In systems where there are high fish densities, there may
be loss of weight and animal death (Mcqueen 1990). The
presence of a predator provides stability to the system, since,
with predation, there is a reduction of the stock of prey, thus
avoiding the lack and competition for food by the forage species. Paiva et al. (1994) conducted a study where the number
of predator species interfered with the trophic dynamics in
reservoirs of northeastern Brazil. They noted that the reservoirs in which there were two species of predators fish catching was high, while in reservoirs with less than two species
of predators, competition among prey probably contributed
to the decrease in catch.
Several authors suggest that the community structure and
biomass of fresh water are regulated by predators (Hrbáček
et al. 1961; Shapiro et al. 1975; Shapiro and Wright 1984;
Carpenter et al. 1985; Persson et al. 1988). It can be said that
an increase in piscivore biomass is associated to reduction
of planktivore biomass, increase of large zooplankton biomass and reduction of phytoplankton (Carpenter et al. 1985;
Mcqueen 1990; Meijer 1999; Gulati 2001). The interactions
established by predation can promote the reduction of the
planktivore biomass and these, associated with increased
biomass of large zooplankton and the increase of algae consumers (grazing), reduce the phytoplankton biomass. Several authors (Shapiro and Wright, 1984; Vanni and Findlay
1990) have noted that predators are important to zooplankton
alterations.
According to the trophic cascade hypothesis, the increase
in piscivore (carnivore) biomass in a lake or reservoir causes
the planktivorous fish biomass to decrease, increasing the
zooplankton biomass and reducing phytoplanktonic biomass
(Shapiro et al. 1975; Carpenter et al. 1985).
Lazzaro et al. (2003) point out that top-down effects in
tropical environments are more complex than in temperate
environments, as there are no species that are essentially
zooplanktivorous but omnivores (such as tilapia), and the
piscivores give way to general macro carnivores. The chlorophyll-a concentration increases with the omnivorous fish
biomass and decreases with macro carnivore biomass. However, in this work, the chlorophyll-a concentration was not
related to total fish biomass and to the macrozooplankton
biomass. Interactions between fishes and chlorophyll concentration and between fish and zooplankton biomass were
inversely related to the trophic state. The fish-phytoplankton regulation caused by complex omnivore interactions is
present in the feeding behavior of various populations of
consumers.
The results show that the Itaipu Reservoir presented topdown and bottom-up relationships, supporting the findings
by Rejas et al. (2005) in experiments in a lake in Bolivia,
with direct and indirect effects on the trophic cascade. The
top-down effect was found only in the first trophic level and
the biomass of piscivores exercised a control over omnivores
and detritivores, and had no predatory effect on the benthivore and zooplanktivore biomasses, whose biomasses actually increased. This suggests that the piscivores may have
an effect on the water quality of the reservoir, not positively,
but negatively, since the indirect relations between the guilds
consumed by predators have shown to have a controlling effect on both chlorophyll-a and cyanobacteria. The control effect of piscivores was also reported by Pelicici et al. (2005)
on 31 reservoirs of the state of Paraná.
The effect of omnivores on the other trophic groups of
fishes was also evaluated, since this group is composed
largely of big fish ( Pimelodus sp and Pterodoras granulosus, for example). This analysis indicates that the increase
in omnivore biomass caused depletion of benthivore and detritivore biomass, contrary to what happened with the zooplanktivore, which had their biomass increased.
An important relationship studied was the one between
detritivores and primary production in the reservoir. Although the results indicate that this group suffers a strong
predation pressure by piscivores and omnivores, detritivores
promote control over chlorophyll-a and cyanobacteria.
When the omnivore filter-feeders dominate the planktivorous fishes, the hypothesis “fishes versus trophic state of the
lake’’ can no longer be valid because the growth stimulation
by omnivores is intensified by the increased load of nutrients (Drenner et al. 1996). The systems dominated by omnivores show weak trophic links, owing to a combination
of weakened mechanisms. Consequently, they are difficult
to predict with the trophic cascades hypothesis. Among the
mechanisms involved, Lazzaro et al. (2003) comment on the
‘‘intraguild predation’’ (IGP), which applies to omnivores
that feed on herbivorous zooplankton and on their phytoplanktonic food and also ontogenetic omnivory (the same
mechanism that occurs during the fish ontogeny from the
juvenile phase to the adult).
The lack of zooplanktivore predation effect demonstrated in the statistical analysis also indicates damage to water
quality, with positive effect related to the cyanobacteria.
Studies by Abujanrra and Agostinho (2002) emphasize the
great adaptability that zooplanktivorous species have owing
to their gill system, as their gills are large, long, thin, and numerous. For instance, the filtration mechanism of Hypophthalmus marginatus is of the passive filter kind that consists
in swimming with an open mouth and with extended operculum, and their diet is composed primarily of zooplankton.
These results demonstrate the importance of omnivore and
detritivore chains in tropical and subtropical aquatic environ-