*Dirección para correspondencia:
]]> Abstract

New impoundments provide opportunities to check whether species that present enough feeding flexibility in natural conditions may take advantage of this situation and, without reproductive restriction, can occupy the most conspicuous habitat in a large reservoir (open areas) and present higher success in ]]> A. osteomystax in two environments, one natural (Sinhá Mariana floodplain lake) and one dammed (Manso Reservoir), during two periods: the first year after the filling phase and three years later. Our goal was to evaluate the occupation of the new hábitat (Manso Reservoir), by this species, as well as to test the hypothesis that in the reservoir, unlike the natural environment, there are remarkable changes in diet between the periods. Fish were sampled monthly in the floodplain lake and in the reservoir ]]> A. osteomystax we employed the Kruskal -Wallis test, and the diet analysis was carried out using frequency of occurrence and volumetric methods. Temporal differences in the diet were tested by Kruskal-Wallis test using the scores from a detrended correspondence analysis. A. osteomystax was significantly more abundant in the floodplain lake, where the captures were higher than in the reservoir in almost all months analyzed, and significant variations in abundance ]]> A. osteomystax are more relevant in an artificial environment than in a natural one. Rev. Biol. Trop. 60 (2): 699-708. Epub 2012 June 01.
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Key words: pantanal, floodplain lake, feeding changes, insectivores.

Resumen
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Los embalses nuevos ofrecen la oportunidad de comprobar si especies que presentan suficiente flexibilidad en la alimentacion en condiciones naturales pueden aprovechar esta situacion y, sin restricciones de reproduccion, ocupar la mayor parte del habitat visible en un gran embalse (espacios abiertos), ademas, presentar un alto exito en la colonizacion del nuevo entorno. Asimismo, examinamos variaciones en la abundancia y alimentacion de A. osteomystax, ]]> A. osteomystax empleamos la prueba de Kruskal-Wallis, y el analisis de la dieta se llevo a cabo con el uso de la frecuencia de ocurrencia y metodos volumetricos. Las diferencias temporales en la dieta fueron probadas con Kruskal-Wallis, se usaron los resultados a partir de un análisis de correspondencia sin tendencia. A. osteomystax fue significativamente mas abundante en el lago de la llanura de inundacion, donde las capturas fueron mas altas, que en el embalse en casi todos los meses analizados, y no se registraron variaciones significativas en la ]]> A. osteomystax son mas relevantes en un ambiente artificial que en uno natural.

Palabras clave: pantanal, llanura de inundacion, cambios en la alimentacion, insectivoros.

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Success in the colonization of new reservoirs by fish has been explained by morphological and behavioral ]]> et al. 2008). The scarcity of pre-adapted species occupying the open areas in Neotropical reservoirs, which is certainly related to the absence of large lakes in the main river basins of South America, can explain the distribution of fish in such an environment, as they are concentrated essentially in marginal areas (floodplain lakes species), and fluvial zones (rheophilic and migratory species). In general, this success is assessed by comparisons between pre- and ]]> et al. 1997, Marques et al. 2009).

The catfish Auchenipterus osteomystax (Miranda Ribeiro, 1918) belongs to the restricted group of Neotropical species with morphological pre-adaptations that enable them to occupy the pelagic areas of reservoirs (body shape, orientation of ]]> et al. 1999). Abundance of the genus Auchenipterus in reservoirs during the first years has been reported (Ferreira 1984a, Benedito-Cecilio et al. 1997, Marques et al. 2009). Internal fertilization is a characteristic associated with its success in the occupation of impounded environments (Agostinho et al. 2008), ]]> et al. 2007). However, its abundance can be severely reduced after the heterotrophic period (Mol et al. 2007), suggesting that food availability is a key factor in this success.

Nevertheless, for a suitable ]]> et al. 1995, Albrecht & Caramaschi 2003, Hahn et al. 2004), and they are related to the variations in the availability of food resources (Luz-Agostinho et al. 2006). These variations in natural aquatic systems are principally ]]> et al. 1979); consequently, the diet of fish in these environments is also highly seasonal (Welcomme 1979, Hahn et al. 2004). In natural environments, these changes are more or less predictable and gradual, allowing species to make better use of the resource thanks to their adaptations (Hahn & Fugi 2008). On the other hand, the formation of a reservoir changes the physical, chemical, and biological composition of the river, with several environmental effects (Mol et al. 2007, Agostinho et al. 2008), modifying or decreasing the seasonality to which the biota is adapted. Furthermore, the food availability changes on a plurianual scale in a reservoir as a consequence of alterations in nutrient concentrations (longitudinal and transversal stratification and exportation), productivity, biotic interaction, and dam operation. Thus, the formation of an impoundment provides excellent opportunity to test whether species that present enough feeding flexibility in natural conditions may take advantage of this situation and, without ]]>

In the present study, we examined the variations in abundance and feeding of A. osteomystax in two distinct environments: natural (Sinha Mariana floodplain lake) and ]]> et al. 1999).
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Material and methods

The study area is located in the watershed of Manso/Cuiaba River, Brazil, encompassing two lentic environments, an artificial one, Manso Reservoir (located in Manso River), and a natural one, the Sinha Mariana floodplain lake, situated ]]> Fig. 1). The Manso Reservoir (14°32’-15°40’ S and 54°40’-55°55’ W) was created in November 1999 by flooding an area of 427km².  About 80km downstream from the dam, the Manso River joins Cuiabazinho River, forming the Cuiaba River, which flows to lower regions, with a large catchment draining into the Pantanal of Mato Grosso, where the Sinha Mariana floodplain lake (area=11.2km²) is located (Fig. 1).

Fish were sampled monthly in the reservoir and in the lake floodplain during two annual periods: Period I between March 2000 and February 2001 (the first year after the filling phase), and Period II between March 2003 and February 2004 (three years later). Fishes were sampled by a set of gillnets 10m long with different mesh sizes (12 nets; 2.4, 3, 4, 5, 6, 7, 8, 9, 10, 12, 14 and 16cm opposite knots). Nets were set simultaneously in open ]]>

The abundance of A. osteomystax was expressed as the number of individuals captured by 1 000m² of gillnet during 24hr (CPUE). In order to evaluate spatial (reservoir and floodplain lake) and temporal (Periods I and II) differences in the abundance, we employed the Kruskal-Wallis nonparametric test (Zar 1996), since the assumptions of normality and homogeneity were not reached. The evaluation of the A. osteomystax abundance at the surface (open areas), margin (littoral areas), and bottom was performed only ]]> a posteriori test (multiple comparisons of mean ranks for all groups) was used. The significance level adopted was p<0.05.

Stomach contents were examined, and ]]> i*V_i/Σ F_i*V_i)*100, where: i=1, 2, ..., n food items; Fi=frequency of occurrence (%) of the item i, and V i=percentage of volume of the item i.

With the aim of evaluating temporal differences in diet, we tested the scores of two detrended correspondence analyses carried out individually for the reservoir and floodplain lake (DCA-Hill & Gauch 1980). The matrix used in this ordination was based on the relative frequencies of the volume of food ]]>

Results
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Auchenipterus osteomystax was significantly more abundant (H=5.97, p=0.014) in the floodplain lake, where the captures were higher than in the reservoir in almost all analyzed months (Fig. 2). We did not record significant variations in abundance between the two periods (I and II) in either the reservoir (H=1.6147, p=0.2038) or the floodplain lake (H=1.143, p=0.285). Nevertheless, a strong seasonal trend was observed in the floodplain ]]> A. osteomystax was more abundant in September and October in the two periods.


Significant differences in the abundance of A. osteomystax were observed between the habitats within the reservoir during both Period I and Period II (H=13.73, p=0.001 and H=15.827, p=0.0004, respectively) (Fig. 3). The abundance was significantly higher at the surface in both periods (Period I: S and B p=0.0039; S and M p=0.0046; Period II: S and M p=0.0004; S and B p=0.0005), while the abundance of A. osteomystax was not different between the margin and the bottom in either period (Period I: p=0.9973; Period II: p=0.9839).


The diet of A. osteomystax was basically compounded by insects during the two study periods in both locations. Meantime, when considering inferior taxonomic levels, the similarity between the periods was observed only for the population ]]> Fig. 4).

The differences in the diet between the periods were significant in the reservoir (H=37.48, p=0.0001). During Period I, Chaoboridae larvae was the dominant food (IAi=76.1%), ]]> Fig. 5A, B, C). In Period II, Coleoptera and Chaoboridae were consumed in small amounts, but presented relevant occurrence in the diet (Fig. 5A). In the floodplain lake, we did not register a significant difference in the diet of A. osteomystax between Periods I and II (H=0.513; p=0.4738), and Ephemeroptera was predominant in both ]]> Fig. 5E, F). Besides Ephemeroptera, the Chaoboridae larvae were an important food in Period I, totaling 34.8% of the diet, whereas Chironomidae pupae and microcrustaceans were important only in relation to the occurrence (30.3% and 27.3%, respectively) (Fig. 5D). Regarding Period II, there was a decrease in the consumption of Chaoboridae larvae (IAi=3.6%) and an increase in the consumption of microcrustaceans (IAi=17.8%) ( Fig. 5F). Chaoboridae larvae presented a lower quantitative participation in the diet during Period II; however, these organisms were an important food, occurring in 54.4% of the stomachs (Fig. 5D).

Discussion

]]> The experimental fishing carried out in Manso Reservoir reveals that the density of A. osteomystax was low compared to the natural lentic environment (floodplain lake), with secondary importance in the reservoir (1.3% and 7.2% of the captures, respectively; A.A. Agostinho, unpublished). The preference of A. osteomystax for lentic habitats and its success in the colonization of ]]> et al. 1997, Agostinho et al. 1999), exceeding its density in natural environments. This same result was observed for a congeneric species (A. nuchalis) in the reservoirs of Curua-Una (Ferreira 1984a) and ]]> et al. 2009). The colonization of a reservoir by the pre-existing ichthyofauna depends on both the pre-adaptations to the new environment (Fernando & Holcik 1991, Agostinho et al. 1999) and the number of individuals of each population in the environment that will be impounded (Agostinho et al. 2008). Thus, despite the absence of data on the abundance of A. osteomystax in the previous ]]> et al. 2009).

Many of the fish species from Neotropical reservoirs occupy the margins of these environments (Araujo-Lima et al. 1995, Agostinho et al. 1999, Mol et al. 2007), ]]> et al. 1999). Nevertheless, A. osteomystax, which has morphological adaptations to displacement and feeding in pelagic regions (Freire & Agostinho 2000), principally occupied the reservoir surface. Furthermore, A. osteomystax is not considered an obligatory zooplanktivorous species ]]> et al. 1998), but it may be considered as facultative zooplanktivorous, that is, able to use the zooplankton that suddenly becomes abundant in newly formed reservoirs (Strictar-Pereira et al. 2010). Therefore, the predominance of the species in open areas of Manso Reservoir might have been favored by the ability to consume zooplanktonic organisms. A similar indication is given by the higher abundance of this species in surface water in the reservoir transition zone (Ferreira 1984a, ]]> et al. 1997), where the primary productivity and the zooplankton density are higher due to the balance between the light penetration and nutrient concentration (Kimmel et al. 1990, Marzolf 1990).

The success of zooplanktivorous species in new impoundments has been described (Ferreira 1984b, ]]> et al. 1992, Cassemiro et al. 2003). Although a slight increase in the occurrence of microcrustaceans was observed in the diet of A. osteomystax during the second period in Manso Reservoir, this species may not be classified as typically zooplanktivorous, nor did it present the expected increase in abundance after the formation of Manso Reservoir. Mol et al. (2007) reported that the congeneric species ]]> , which was abundant soon after the formation of Brokopondo Reservoir, was not present during the samplings accomplished 40 years later.

The diet of A. osteomystax was predominantly compounded by insects independently of the location and period. Meanwhile, we verified differences in the composition and ]]> Auchenipterus species has been discussed for other reservoirs (Merona et al. 2001, 2003), rivers (Tejerina-Garro & Merona 2010), and várzea lakes of the Amazon (Merona & Rankin-de-Merona 2004). However, the predominance of aquatic forms of insects in the diets of the same ]]> Auchenipterus is reported for the Itaipu reservoir (Hahn et al. 1998) and lakes of the Upper Parana River floodplain (Hahn et al. 2004).

The intense proliferation of zooplankton that characterizes new reservoirs (Rocha et al. 1999, Bonecker et al. 2001) was not directly exploited by A. osteomystax. Nevertheless, the initial diet of Chaoboridae larvae is based on flagellate protozoans and, during final stages, on microcrustaceans (Arcifa 2000). These larvae may belong to plankton or benthos depending on the time of day and instar (Arcifa 2000, Bezerra-Neto & Pinto-Coelho 2002). The fact that the final instars develop nocturnal migrations to the surface, coinciding with the location and time of ]]> A. osteomystax (A.A. Agostinho, unpublished), makes the Chaoboridae a dominant prey in the diet. Research performed in the Curua-Una Reservoir, six years after its formation, indicates that where the congeneric species A. nuchalis was more abundant, the diet was clearly dominated by Chaoboridae or microcrustaceans (Ferreira 1984b).

Pupae of ]]> A. osteomystax three years after the formation of the reservoir. The Chironomidae larvae are among the principal benthic organisms (Higuti & Takeda  2002, Callisto et al. 2002) and in the present study their pupae must have been consumed at the surface during the period when these insects, whose adults are terrestrial, are emerging and may be found at the surface. Besides that, the consumption of Hymenoptera by ]]> et al. 1988); these terrestrial organisms fall into the water and are mostly consumed by fish that feed at the surface.  In this way, although the diet was changing between periods, A. osteomystax continued to consume food available at the surface of the water column, evidencing that this layer was the foraging location in the second period too.

]]> In contrast to what was observed in the reservoir, at the floodplain lake the diet of A. osteomystax did not present significant temporal differences, and Ephemeroptera was the most consumed item during both periods. The high consumption of Ephemeroptera subimagos indicates that, as occurred in the reservoir, A. osteomystax feeds at the surface, since at this stage of development these insects emerge at the surface and become available for capture. ]]> et al. 2004), as recorded for A. nuchalis in the Curua-Una River upstream of the reservoir with the same name (Ferreira 1984b). Meanwhile, in the impounded environment of Itaipu, Ephemeroptera was also the main food item of this species (Hahn et al. 1998).

]]> Therefore, we conclude that the reservoir occupation by Auchenipteridae fish presents variable chance of success, with the most successful occupation being in the superficial layer of the water column in the transition zones of these environments, where the zooplankton density is higher. The seasonality observed in the abundance and the temporal coincidence with reproductive events suggests that the species show active displacements towards waters with higher turbulence. The diet is ]]> A. osteomystax are more relevant in an artificial environment than in a natural one.

Acknowledgments
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We express our appreciation to Nupelia (Nucleo de Pesquisas em Limnologia, Ictiologia e Aquicultura) and Furnas Centrais Eletricas for their financial support and infrastructure, and to the Brazilian Council of Research (CNPq) for providing a grant. We thank Sidinei Magela Thomaz for correcting the English and for helpful comments on the manuscript.

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*Correspondencia a: Elcio Barili & Angelo Antonio Agostinho: Programa de Pos-Graduacao em Biologia Comparada, Universidade Estadual de Maringa. Av. Colombo 5790, Cep 87020-900, Maringa, PR, Brasil; elciobarili@yahoo.com.br
Rosemara Fugi, Gisele Caroline Novakowski & Angelo Antonio Agostinho: Nucleo de Pesquisas em Limnologia, Ictiologia e Aquicultura. Universidade Estadual de Maringa. Av. Colombo, 5790, Cep 87020-900, Maringa, PR, Brasil; rosemarafugi@gmail.com, gcnovakowski@yahoo.com.br, agostinhoaa@gmail.com

1. Programa de Pos-Graduacao em Biologia Comparada, Universidade Estadual de Maringa. Av. Colombo 5790, Cep 87020-900, Maringa, PR, Brasil; elciobarili@yahoo.com.br
2. Nucleo de Pesquisas em Limnologia, Ictiologia e Aqüicultura. Universidade Estadual de Maringa. Av. Colombo, 5790, Cep 87020-900, Maringa, PR, Brasil; rosemarafugi@gmail.com, gcnovakowski@yahoo.com.br, agostinhoaa@gmail.com