Dazed and confused: Behavioural constraints impose major challenges to fish passage in the neotropics

Abstract

The role of fish translocation upstream of dams, for the conservation of migratory species, is controversial. An important but still unaddressed question is whether migratory patterns are affected, after the translocation events, by a hydropower dam. In existing dams, a useful means to test this might be to compare the behaviour of translocated fish with those in undammed river segments in the same region. The objective of this study was to evaluate the effectiveness of fish translocation in promoting access to spawning areas through a behavioural approach using Prochilodus costatus as a study model in the São Francisco River, south-east Brazil. The reservoir acted as a barrier to the dispersal of translocated individuals with only a small fraction of the population able to locate lotic stretches in the tributaries. Individuals translocated to undammed stretches of river upstream of the reservoir exhibited movement behaviour that was markedly different from that of individuals native to this region. Translocated individuals mainly displayed an erratic pattern of movement between upstream and downstream stretches of river with no specific route. In contrast, local individuals made long-distance upstream spawning migrations and downstream return migrations to feeding sites during specific periods of the year. The results indicate that translocating individuals upstream of Três Marias dam tends to be ineffective for the conservation of P. costatus in the region. The study showed that the behaviour of translocated individuals, a largely neglected factor for the tropical region, may be an additional barrier to the success of conservation programmes based on fish passage systems. For the population studied, the maintenance of a free-flowing stretch where fish are able to complete their life cycle is a more viable and effective alternative for conservation.