Temporal dynamics of size spectrum of a fish population in neotropical reservoirs

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Introduction
River impoundments are important agents in reorganizing aquatic communities, with different influences on population structure and causing several direct impacts on the environment. Aside from changes in physical and chemical properties of water (Agostinho et al., 1999;Jinpeng Li et al., 2013;James et al., 2015;Cooper et al., 2016;Fearnside, 2016), disturbances caused by dams lead to changes in the species habitat, altering community composition with negative effects on flora and fauna, including the decline in species richness (Richter et al., 2010;Tonella et al., 2022) and prevent mass migration for reproduction (Fearnside, 2014).
While large dams generate electricity, they bring serious consequences for populations located downstream of dams for hundreds of kilometers (Richter et al., 2010). These environmental and social impacts should be considered during planning and decision making (Fearnside, 2016). There are several ways of measuring the size of individuals; the most accepted measure is biomass because it efficiently presents the range of weight classes in logarithmic scale, allowing the observation of the integrations between physiological characteristics of organisms and their relationship to ecosystem functioning (Han & Straskraba, 1998;Dickie et al., 1987;Boudreau et al., 1991). Studies of body size allow us to predict predator-prey interactions, reflecting biomass of the trophic structure (Thiebaux & Dickie, 1993).
Also, to predict the patterns of energy transfer, since the biological properties of individuals change with size, which is often a good indicator of trophic level of the species (Jennings & Brander, 2010;Jennings et al., 2001). Understanding the processes that act upon individual success and survival is crucial to understand the population dynamics (Poulos & McCormick, 2015). The variation in the biomass spectrum is an efficient method because it indicates human interference and its effects on the energy flow of the ecosystem (Rice & Gislason, 1996). Organisms of the same size are treated

Temporal dynamics of size spectrum of a fish population in neotropical reservoirs
International Journal for Innovation Education and Research Vol. 10 No. 11 (2022), pg. 109 as energetic equivalents regardless of the taxa, attesting the anthropogenic impacts on biological production (Kerr & Dickie, 2001), since it reduces the complexity of the food web into a single quantitative variable.
Piscivorous species are essential for the maintenance and balance of ecosystems, by regulating the abundance of species at the top of the food chain and their prey (Novakowski et al., 2007), controlling the natural fish stocks and increasing the mortality rate among individuals (Link & Garisson, 2002).
The present study sought to identify the effect of the cascade of reservoirs in a neotropical basin (Iguaçu River basin) on the stock and length distribution of the population of the piscivorous Oligosarcus longirostris Menezes & Gèry (Osteichthyes, Acestrorhynchinae). The central hypothesis is that the of the stock size is reduced along the reservoirs in the cascade (downwards) due to the retention of nutrients and its effects on the dynamics of the size spectrum of this population.

Study area
The Iguaçu River basin has an area of 72,000 km2 approximately, of which 79% belongs to Parana State, 19% to the Santa Catarina State and 2% to Argentina. It flows 1,060 km, with an east-west direction from its headwaters on the western slope of the Serra do Mar, near Curitiba, to its mouth on the Parana River (Eletrosul, 1978).
Parana river is located on the third plateau and exhibits a high slope in its watershed, which gives it an enormous potential for hydroelectric power generation. Currently, there are five major power plants in its bed, turning the stretch into a succession of large lakes, which led to a series of changes in the hydrological regime  (Table 1).

Data collection
Fish were sampled through quarterly collections conducted in the reservoirs, from March 2004 to December 2008. Simple gill nets were used to capture the specimens (meshes from 2.4 cm to 16 cm) and trammel nets (6 cm to 8 cm between opposite knots), which were assembled in three layers (margin, surface, and bottom) and remained exposed for 24 hours with inspection every 8 hours.
After sampling, fish were anesthetized with benzocaine hydrochloride (250 mg/L), as required by the

Temporal dynamics of size spectrum of a fish population in neotropical reservoirs
International Journal for Innovation Education and Research Vol. 10 No. 11 (2022), pg. 110

Data analysis
The stock size was estimated by catch per unit effort (CPUE), in number of individuals per 1000m2. The net was exposed for 24 hours for each collection month and assessed by univariate analysis of variance followed by Tukey's test for comparison of means to check for possible differences (P <0.05) between the reservoirs of the cascade.

Results and Discussion
The catch per unit effort (CPUE -ind.1000m2net-1.day-1) was different among the reservoirs. The reservoir upstream in the cascade (FA) showed the highest CPUE, while the following reservoirs showed increasingly lower values, according to the succession of reservoirs in the cascade (Fig. 1).

Temporal dynamics of size spectrum of a fish population in neotropical reservoirs
International Journal for Innovation Education and Research Vol.    (Fig. 3).

Temporal dynamics of size spectrum of a fish population in neotropical reservoirs
International Journal for Innovation Education and Research Vol. 10   The combination of temporal and spatial variables along the river revealed marked changes in the dynamic of the size spectrum of fish population in the five reservoirs. The expected value for the constant c for the assemblage was close to -1, for a population in which a higher value was expected. This was possibly due to high positive asymmetry in fish populations. Most species show a positive symmetry in body size distribution when analyzed on a regional scale, regardless of the ecological or taxonomic group (May, 1986;Brown & Maurer, 1989;Lawton 1991;Knouft, 2004). The proliferation of small fish suggests the presence of favorable conditions of food and shelter. This observation is explained by the Cascading Reservoir Continuum Concept (CRCC), which states reservoirs located at the beginning of a series present higher nutrient availability due to changes in abiotic factors, providing high production at all trophic levels (Barbosa et al., 1999). Additionally, the reservoir receives all the nutrients from a metropolitan urban area generating a high aquatic production (Gubiani et al., 2008).
Along the succession of reservoirs, the proportion of smaller individuals in a large number was replaced gradually for smaller amounts of larger fish. This can provide information on size-based predation dynamics as from predator-prey models (Thiebeaux & Dickie,1993) and may represent trophic positions (Goyke, 1995).

Temporal dynamics of size spectrum of a fish population in neotropical reservoirs
International Journal for Innovation Education and Research Vol. 10 No. 11 (2022), pg. 113 Body condition affects competitiveness and survival, animals with larger body sizes are more likely to rule the territory (Poulos & McCormick, 2015). Thus, body size can determine the vulnerability of individuals, populations and communities (Rice & Gislason, 1996). The high body size condition, usually acquired with a higher consumption of food, determines the level of aggression and territoriality of organisms with a lower body size condition is more difficult to start fights and have higher rates of mortality and it is the main factor affecting survival (Donelson et al., 2009;Poulos & McCormick, 2015;Nawrocki et al.,2022). Larger individuals tend to be more territorial and competitive reflecting fewer specimens, but with a high body condition, as found in this study.
The high productivity can greatly influence the fish community. In such environments, the pressure generated by competition could determine the size distribution of the stock, since there are high numbers of individuals competing for resources. These competitive interactions directly affect species abundance (Stevens & Willing, 2000). Nevertheless, the high frequency of piscivores can be justified by the proliferation of opportunistic species, which are usually small and common in reservoirs, being an important food resource in this type of environment (Agostinho et al., 2007).
Besides that, the large number of individuals with low biomass may be related to the amount of spatial niche available, which probably resulted in the observed positive skewness (Hutchinson & MacArthur, 1959).
These authors suggest that the environment, by failing to provide enough space for large animals, creates more space for the abundance of smaller species. This theory explains the high number of species of smaller size, but it does not elucidate why smaller species are numerous. It is known that body size is related to evolutionary aspects, relative size of the ancestor, and factors associated with regional distribution which must include at least some historical information (Knouft, 2004). Meanwhile, Stanley's (1973) argues that the trends in size are the result of responses of species to environmental conditions and that this response may influence the formation of a frequent distribution; hence, the influences of the particularities of each impoundment on the aquatic community have to be considered (Agostinho et al., 1999).
The reservoirs showed a pattern consistent with the Cascading Reservoir Continuum Concept (CRCC), where there is a longitudinal gradient in the cascade of four reservoirs, with a progressive increase in CPUE along the series. The patterns of size distribution exhibited an inverse pattern of CPUE, with a reduction along the cascade, being observed a relationship between the increasing number of individuals and the decrease in size and biomass of the population. In agreement with Agostinho et al., (1999), over time and depending on the reservoir area, the dammed environments typically show a decline in fish abundance. According to these authors, after the initial phases of the impoundment, when there is a large productivity, a drop in its values is observed, being more pronounced especially with reservoir aging. The significant change in the structure of size spectra of the studied population size, which shows a large increase in population biomass in the second reservoir compared with the first, may be related to the availability of prey. According to Pelicice et al., (2005), with greater biomass of prey species, it would be possible to maintain greater biomass of piscivorous species. The biomass of piscivorous species is positively correlated with the biomass of prey species .
Nevertheless, the number of species (CPUE) in the second reservoir was smaller than in the first. The

Temporal dynamics of size spectrum of a fish population in neotropical reservoirs
International Journal for Innovation Education and Research Vol. 10 No. 11 (2022), pg. 114 primary productivity in this reservoir is lower, since the organic material possibly settles down in the first reservoir of the cascade, thus reducing the amount of nutrients in the second reservoir, affecting the fertility of the other levels of the chain. Cascading reservoirs commonly have a reduction in the concentration of nutrients throughout the series, which reduces productivity downstream (Miranda et al., 2008).
Thus, the decrease in the number of prey works limiting the growth of predators over the reservoir cascade, pointing to the bottom-up mechanism, since the availability of prey has a direct and vital influence on the population dynamics of the predator (Frederiksen et al., 2006). Piana et al., (2005) investigated the mechanism regulating fish biomass in different reservoirs of Parana State and found that the phytoplankton productivity had a predictor role in fish biomass, occurring discreetly the bottom-up mechanism (abundance determined by resources).
There is a great possibility of, along the longitudinal gradient of the reservoirs, an inversion in the mechanisms regulating the trophic structure of the environment, because the fish biomass keeps decreasing (SS, SO and SC). Aspects related to changes in water level can produce environmental disturbances that alter the biotic interactions and energy dynamics (Tundisi et al., 2003).

Conclusion
The results presented herein show changes in O. longirostris population structure, regarding the size spectra along the five reservoirs studied. The main mechanism involved in these changes of biomass and number of individuals is possibly related to the effects generated by the productivity of ecosystems, causing changes in the efficiency of energy transfer between the different trophic levels.

Acknowledgement Heading Level-1
The research is financed by Group of Research in Fisheries and Limnology-Gerpel and the Graduate Program in Comparative Biology -UEM.