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

Abstract

Downstream passage through fish ladders and other facilities remains largely uninvestigated in South America. In view of this concern, we monitored ascending and descending fish movements through the fish ladder at Peixe Angical Dam, upper Tocantins River, Brazil, between September 2009 and August 2010. To investigate the extent of downstream passage, the fish fauna was sampled monthly (i) at sites upstream and downstream from the dam and (ii) inside the ladder. To distinguish between ascending and descending movements, the fish ladder remained open (permanent water flow), but a screen blocked the complete passage of fish in both directions. The results of the study indicated that the ladder restricts fish movements in both directions. Few species were found to use the ladder (31 out of 119 species recorded in the area). Moreover, monitoring revealed that downstream passage is even more limited. Only 18 species were recorded moving downstream. In addition, few fish attempted to move downstream during the year; of all fish captured in the ladder (n = 17,335), only 4% were descending. Species performing exclusive downstream movements were numerically rare or infrequent. Most downstream captures involved only four species. We emphasize that several species performed one-way upstream movements or showed a high density of ascending fish. Consequently, the numerical ratio between ascending and descending fish (ind./m 2 ) showed high positive values throughout the year (644:1 on average, considering all species); migratory species showed higher ratios (1069:1). The results of the study show that the facility fails to support the downstream passage of migratory and non-migratory fishes. We conclude that the fish ladder at Peixe Angical Dam does not play a conservation role and may complicate alternative conservation efforts.A passagem descendente de peixes em mecanismos de transposição permanece pouco investigada na América do Sul. Inspirado nessa falha, o presente estudo monitorou a escada de peixes instalada na barragem da UHE Peixe Angical, alto rio Tocantins, entre setembro de 2009 e agosto de 2010. Para investigar a extensão da passagem rio abaixo, a fauna de peixes foi amostrada mensalmente em (i) locais acima e abaixo da barragem, e (ii) dentro da escada. Para identificar corretamente movimentos ascendentes e descendentes dentro da escada, o mecanismo permaneceu aberto (fluxo permanente de água), mas uma tela impediu a passagem completa dos peixes em ambas as direções. Os resultados indicam que a escada é restritiva ao movimento dos peixes em ambos os sentidos, considerando que poucas espécies tiveram acesso ao mecanismo (de um total de 119 espécies na área, apenas 31 ocorreram na escada). O monitoramento revelou, entretanto, que a passagem descendente é consideravelmente mais limitada; apenas 18 espécies foram registradas descendo o mecanismo. Além disso, poucos peixes tentaram descer a escada ao longo do ano; de todos os peixes capturados na escada (n = 17,335), apenas 4% desempenhava movimentos descendentes. ...

Large reservoirs as ecological barriers to downstream movements of Neotropical migratory fish

“…), despite recent studies have shown that fish do not use ladders as a descending route (Agostinho et al . , ; Pelicice and Agostinho , ; Brito and Carvalho ). This inefficiency may be related to the low density of fish above the dam, which may be a consequence of the barrier imposed by the reservoir.…”

“…, ; Esguicero and Arcifa ; Terra et al . ; Pelicice and Agostinho ; Petesse and Petrere ). The paucity of migratory species in reservoirs is a well‐recognized trend and has directly affected fisheries (Petrere et al .…”

“…), but a critical limitation is the virtual absence of downstream passage for adults and young fish (Agostinho et al . , ; Pelicice and Agostinho , ; Brito and Carvalho ). The lack of downstream passage, although historically neglected by managers, may compromise the use of fishways as a management tool, because fish dispersal towards lower reaches is impeded.…”

Fish assemblages in Neotropical reservoirs: Colonization patterns, impacts and management

“…Other relevant aspects that should be considered in discussions on fish passages are their high selectivity (Agostinho et al, 2007d), the difficulty in controlling which species go through the passage (Pompeu et al, 2012), and the absence of downstream movement of adults and their offspring (Agostinho et al, 2007c(Agostinho et al, , 2011Suzuki et al, 2011;Pelicice and Agostinho, 2012). Solutions to the existing bottlenecks concerning recommendations of fish passages as a tool to mitigate impacts on migratory species must address the following issues: (i) whether the passages are efficient to attract fish and to allow free movements; (ii) whether the reservoirs represent a barrier to downstream movement of adult fish or to the drift of their eggs and larvae (Agostinho et al, 2007c; in press), (iii) whether long-distance migratory species have distinct behaviors and the swimming ability to be attracted to and to overcome the water flow in a fish passage; (iv) whether the passage is safe, with low rates of injury or predation , (v) whether clear objectives exist (e.g.…”

Ecology of Mylesinus paucisquamatus Jégu & Santos, 1988, an endangered fish species from the rio Tocantins basin

“…The Paranã and Cana Brava rivers, for example, may provide essential habitats because these tributaries are free flowing. The same is valid for the 230 km stretch between Peixe Angical dam and Lajeado reservoir, which contains large tributaries and small wetlands (Pelicice & Agostinho, 2012). Conservation efforts, therefore, must give high priority to the conservation of remnant lotic sites.…”