Selectivity of fish ladders: a bottleneck in Neotropical fish movement

Abstract

Although dozens of fish ladders have been constructed at dams of Brazilian reservoirs, there are few studies evaluating their efficiency as a tool for the conservation of Neotropical ichthyofauna, especially for migratory species. Therefore, the present study evaluated the selectivity of the species that entered and ascended the fish ladder located next to Lajeado Dam (Luis Eduardo Magalhães Hydroelectric Power Plant) on the Tocantins River. Samples were taken monthly from November, 2002 through October, 2003, in the resting pools of the ladder, using cast nets, and in the downstream stretch, using gillnets. The selectivity of the ladder in attracting fish was evaluated by comparing the occurrence, relative abundance, dominance and the congruence of abundance ranks of migratory and non-migratory species in the ladder and in the stretch of river immediately downstream. Species richness and fish abundance in the resting pools were used to evaluate selectivity along the ladder. The effects on selectivity by temporal variations in water level downriver and maximum flow velocity in the fish ladder were also analyzed. Out of the 130 species recorded downriver, 62.3% were caught in the ladder, and migratory species were clearly favored. However, more than 2/3 of the catch belonged to only three species (Rhaphiodon vulpinus, Psectrogaster amazonica and Oxydoras niger). Although the majority of the species that entered the ladder were able to reach its top, there was a sharp reduction in abundance of individuals towards the top. Temporal variations in the water level below the dam influenced richness and abundance of fish concentrated downstream and in the ladder, with lower values during periods of low water. In the ladder, a maximum flow velocity of 2.3 m/s, although also selective, proved to be more appropriate for fish ascension than a velocity of 2.8 m/s. It was concluded that the entry and ascension of the fish in the ladder were not congruent with their proportions in the downriver stretch: fish samples in the ladder were clearly dominated by a few species, including some that do not need to be translocated. Thus, selectivity constitutes an important bottleneck to initiatives for translocating fish aimed at conserving their stocks or biodiversity. It is urgent to review the decision-making process for the construction of fish passages and to evaluate the functioning of those already operating.Dezenas de escadas de peixes foram construídas em barragens de reservatórios brasileiros, mas são raros os estudos acerca de suas eficiências como instrumentos de conservação da ictiofauna Neotropical, em especial de espécies migradoras. Neste contexto, o presente estudo teve como objetivo avaliar a seletividade específica no ingresso e ascensão de peixes na escada localizada junto à barragem de Lajeado (UHE Luis Eduardo Magalhães, rio Tocantins). Amostragens foram realizadas mensalmente de novembro de 2002 a outubro de 2003 nos tanques de descanso da escada, utilizando tarrafas, e no trecho a jusante, utilizando redes de es...

Deficient downstream passage through fish ladders: the case of Peixe Angical Dam, Tocantins River, Brazil

“…A few species represented almost all captures in the facility. This result suggested that most species do not use the ladder, a trend reported in different river basins (Godinho et al, 1991;Oldani & Baigún, 2002;Fernandez et al, 2004;Agostinho et al, 2007a;Makrakis et al, 2007;Agostinho et al, 2009). At Peixe Angical Dam, large numbers of O. niger, P. nigricans, S. rhombeus, L. affinis and a few other species are expected to ascend the ladder successfully.…”

“…It cannot be assumed that all fish would successfully pass through the ladder if it was open, but it is probable that the fish inside the ladder would use the facility for dispersal. In addition, field experience and previous monitoring (Agostinho et al, 2007a; have shown that several fish species use ladders in the Tocantins River as an ascending route. Therefore, it was assumed that the fish numbers above and below the screen indicate the number of fish attempting downstream and upstream movements, respectively.…”

“…The first evaluations originated only recently and have challenged the sanctity of fishways. In fact, studies have identified important problems during upstream passage (Oldani & Baigún, 2002;Fernandez et al, 2004;Agostinho et al, 2007a;Makrakis et al, 2007;Volpato et al, 2009). The basis of these problems is that few species are able to ascend ladders and other facilities.…”

Genetic analysis of five sedentary fish species in middle Laranjinha River (upper Paraná River basin): A case study

“…All genetic parameters evaluated here suggest that the populations of the five species showed gene flow. Although the specific geological features of the Corredeira dam facilitates, the potential movement of the fish, the majority of dams built for hydroelectric purposes have a major impact on neotropical fish populations, particularly from the genetic viewpoint (Agostinho et al, 2004(Agostinho et al, , 2007. However, until markers applicable to multi-allelic analyzes are used to confirm our speculation on the cause of gene flow, this conclusion should be treated with caution.…”

“…Additionally, the fish ladder was built relatively recently. Several studies have demonstrated the inefficiency and selectivity of fish ladders in Brazil (Agostinho et al, 2007;Agostinho et al, 2008). Considering the genetic similarity between the fish sampled upstream and downstream of the Corredeira Dam, and that only P. microstoma was captured in the fish ladder, other factors must influence the maintenance of gene flow.…”