Abstract
Fish passages are constructed with the aim of reestablishing connectivity between critical habitats, in order to sustain those species that move over a large area during their lives. The upstream movements of most of the known migratory species are made for purposes of reproduction (adults) or dispersal (juveniles). The present study was conducted at the ladder constructed at Lajeado Dam (Luis Eduardo Magalhães Hydroelectric Power), on the Tocantins River. The objective was to assess whether there is a temporal pattern in the abundance of fish below the dam and in the ladder, that correlates with their breeding seasons. Additionally, it was examined whether reproduction is the predominant motivation of the shoals of fish that ascend the ladder. Samples were taken monthly from November, 2002 through October, 2003, downriver from the dam with gillnets, and in the resting pools of the fish ladder with cast nets. The analysis of seasonality in the aggregation of the shoals was based on the temporal variations in abundance and species richness. The possibility of a reproductive motivation for ascending the ladder was inferred from the frequency of the stages of gonadal development. However, during the entire study period we observed a high frequency of individuals with gonads in the pre-vitellogenic stage for all analyzed species, both downriver from the dam (97%) and in the fish ladder (98%). These findings suggest that there was no reproductive motivation for the aggregation of the fish downstream and for their entry into the ladder. On the other hand, the dominance of these stages downriver suggests that the spawning habitats are distant, and that the upstream movements occur for other reasons such as dispersal and search for more appropriate habitats for feeding and growth. The entrance into the ladder resulted more from rheophilic behavior, which is more pronounced in migratory species, than from an endogenous motivation linked to reproductive migration.
Highlights
The Neotropical fish fauna includes a staggering variety of migratory species, and their life histories are incredibly diverse (Carolsfeld et al, 2003)
These findings suggest that there was no reproductive motivation for the aggregation of the fish downstream and for their entry into the ladder
Species richness in the ladder showed a lesser degree of seasonality, which resulted mainly from the low variability in the number of migratory species (Fig. 1)
Summary
The Neotropical fish fauna includes a staggering variety of migratory species, and their life histories are incredibly diverse (Carolsfeld et al, 2003). In spite of the profusion of life forms, and their broad distributions, all of them are confronting a common dilemma: the increasing number of impoundments, which obstruct their access routes to areas crucial to their life cycle, which are located in different parts of their home range and which are often separated by hundreds or even thousands of kilometers (Barthem et al, 1991; Barthem & Goulding, 1997; Agostinho et al, 2004a) These interruptions of migratory routes are probably the main factor that affects the abundance of migratory fishes (Bayley & Petrere Jr., 1989; Ribeiro et al, 1995; Northcote, 1998; Agostinho et al, 2005). The construction of these facilities is generally decided upon for diffuse objectives, it is expected that they would serve primarily to allow these species to reestablish access to the habitats where they spawn, undergo the first stages of their development, and feed
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