Overview

• The 2024 event on the Aquidauana River in Mato Grosso do Sul was flagged by Environmental Military Police and documented by scientists for about 20 hours.
• Identified as Rhyacoglanis paranensis, the fish measure under 4 centimeters and tackled rock faces 1 to 4 meters high, beginning their ascent near dusk.
• Field notes detail fin-spreading to anchor, side-to-side wriggling and tail flips, with a suction-based aid proposed but not yet confirmed.
• Dense clusters formed on flatter surfaces, and sampled individuals were mostly mature adults with empty stomachs, consistent with a spawning migration.
• The observations, published Aug. 8 in the Journal of Fish Biology, highlight risks from river damming and habitat fragmentation that could impede such movements.