The Role of Discharge and Channel Complexity on the Downstream Transport of Semibuoyant Fish Eggs

River fragmentation and associated flow alteration and habitat degradation are significant factors in the decline and extinction of freshwater biota. One group of imperilled fishes, pelagic-broadcast spawning cyprinids release semi-buoyant eggs that require sufficient velocities and river-fragment length during early development. However, it is unknown how discharge and habitat features interact to alter the transport dynamics of fish eggs. The objective of this study was to assess the relationship among drift of semi-buoyant egg surrogates (gellan beads), flow patterns and habitat complexity. Downstream-transport patterns were assessed using a combination of reach scale field trials and fine-scale laboratory experiments. Video monitoring was used to track gellan bead trajectories across artificial habitats within an experimental flume. These trajectories were then related to observed patterns of velocity and turbulence. In the field, reach complexity was assessed by calculating several habitat metrics determined using analyses of aerial photographs. Results highlight the role of discharge in driving downstream transport, but also indicate that greater habitat complexity increases retention of beads. These results could be used to target restoration activities or prioritize water use to create and maintain appropriate discharge regimes and habitat complexity within large, fragmented rivers.