Ichthyoplankton in a regulated river: community structure and distribution patterns in a run-of-river dam cascade system in the Neotropics

Run-of-river (ROR) hydroelectric systems drastically reduce discharge in the reduced flow stretch (RFS). We evaluated the influence of RFS formation and environmental variables on the species composition and spatio-temporal distribution of fish eggs and larvae in a ROR system. Samples were collected fortnightly (Oct 2016 to Jan 2017) at the Rio das Antas Energy Complex. A total of 5,681 eggs and 2,124 larvae were collected. The study confirmed ichthyoplankton structure is shaped by strong spatio-temporal variations; species composition and egg and larval densities differed significantly among sites and months, primarily influenced by flow, temperature, dissolved oxygen, and transparency. However, the expected longitudinal progression of larval stages from upstream spawning grounds was absent, indicating the natural drift process is disrupted by the dam cascade. This disruption is confirmed by distinct ichthyoplankton densities and compositions between the upstream (S1) and downstream (S4) sections of the Complex. Furthermore, no reproductive evidence was found for migratory species historically present in the basin, corroborating our initial predictions. These findings demonstrate that cascaded ROR systems modify natural drift dynamics and species distribution, challenging their sustainable image and highlighting the importance of preserving free-flowing river stretches to maintain ichthyofauna reproductive processes.

Keywords:
Fish eggs and larvae; Fragmentation; Freshwater fish; Hydropower; Reduced flow stretch