Bidirectional connectivity via fish ladders in a large Neotropical river

Abstract

The conservation of migratory fish species worldwide has been threatened by the loss of longitudinal connectivity caused by dams intercepting large rivers. One environmental management strategy for reestablishing connectivity is providing passage through fish ladders. However, ladders in Neotropical rivers have been described as ascending one‐way routes. We analysed the movements of Prochilodus lineatus through a fish ladder at a large dam—Porto Primavera—in the heavily impounded Upper Paraná River, Brazil, to determine whether the ladder connected habitats downstream and upstream of the dam, in both directions. A total of 1,419 specimens of P. lineatus were PIT‐tagged above and below the dam, and continuously monitored for 4 years. We documented bidirectional movements of P. lineatus through the fish ladder. Many individuals repeated these movements annually; one individual as many as six times. It was estimated that the cumulative probability that P. lineatus would return from downstream after descending through the ladder was 0.38, 0.50, and 0.56 in 1, 2, and 3 years, respectively. Correspondingly, return probabilities from upstream were 0.15, 0.22, and 0.26 in 1–3 years, respectively. Although return probabilities from upstream were roughly half, our results suggest the Porto Primavera fish ladder contributes to habitat connectivity, bidirectional passage, and preservation of P. lineatus. These results deviate from the perception that fishways are ineffective in Neotropical rivers. We suggest that fishways can restore the bidirectional connectivity denied to some Neotropical species, and until the services of dams are no longer needed, environmental management through fish ladders could continue to be considered as an integral part of broader conservation strategy designed to preserve native fauna.

Fish conservation must go beyond the concrete: A comment on Celestino et al. (2019)

“…The persistence of Prochilodus individuals near the dam and inside ladders has been reported in other river basins (e.g., Paraná, Tocantins, and São Francisco; Fernandes, Agostinho, & Bini, 2004; Pelicice & Agostinho, 2012; Prado & Pompeu, 2014). Celestino et al (2019) do not consider these findings or alternative explanations reported in the literature. The study was designed to investigate fish movements along the ladder, and as a result, any wider conclusions regarding migratory movements beyond the fish ladder, spawning, dispersion, recruitment, restoration of connectivity and management success is speculative, given the study design and spatial scale of data collection.…”

“…Given the constraints associated with the location of the antennas and fish release sites, limited inference can be made regarding patterns of movement beyond the ladder itself. However, Celestino et al (2019) ignore these potential constraints and assert that fish that passed through the ladder continued their migration (in both directions) and completed their life cycle. By doing so, the authors ignored the possibility that fish could remain close to the ladder throughout the study.…”

“…This misunderstanding has led to unnecessary confusion and the adoption of poor management actions (e.g., Pelicice & Agostinho, 2008). The recently published study by Celestino et al (2019) illustrates the need to recognize the wider complexities and issues associated with fish passage in regulated rivers.…”

“…Celestino et al (2019) investigated the fish ladder at the Porto Primavera Dam (Paraná River, Brazil), in order to evaluate if it provided free passage for migratory fishes, that is, allowing ascending and descending movements. The study tracked the movement of Prochilodus lineatus (marked with PIT‐tags) within the ladder over a 3.5‐year period.…”

“…The authors imply and encourage the reader to accept patterns of fish movement that cannot be substantiated by data presented (e.g., bidirectional movements and efficient passage) or were not investigated (e.g., migratory dynamics, connectivity restoration, and conservation success). By concluding that fish passages can restore river connectivity, Celestino et al (2019) underestimate the variety and severity of impacts associated with large dams and reservoirs, particularly the fragmentation of critical habitats and migration routes. As these conclusions have direct implications for the licensing process of hydropower dams and the management/conservation of Neotropical migratory fishes, in this short commentary we provide suggestions to highlight additional important factors and issues that should be considered or addressed in future research.…”