Imitating the Physical Properties of Drifting Semibuoyant Fish (Cyprinidae) Eggs with Artificial Eggs

ABSTRACT

Six Rio Grande basin cyprinid taxa compose a reproductive guild that spawns during increases in stream flow and broadcasts semibuoyant (drifting) eggs in the current. Spawning by guild members is naturally initiated by elevated flows during spring snow-melt or summer rainstorms, but reservoir releases now also result in spawning. Location or development of a material that mimics the physical properties of semibuoyant eggs of guild members would enable quantification of the displacement of drifting eggs and assessment of the additive impacts of stream flow, instream barriers, and habitat modifications on populations of these fishes. To initiate this search, the size and terminal velocity of guild members' eggs was determined and their specific gravity (SG=1.00589 ± 0.00011) calculated. Products with specific gravities similar to semibuoyant eggs were obtained, and tests were conducted on their rates of travel and settling times when exposed to current. Materials with specific gravities slightly greater than semibuoyant cyprinid eggs (SG>1.01) sank too rapidly, while materials less dense than water (SG<1.00) traveled downstream more rapidly than guild members' eggs. Particle size, not shape, noticeably affected rates of travel and settling times. Laboratory and field tests indicated that a modified nylon 12 particle (SG=1.00457 ± 0.00017) most closely mimicked the properties of semibuoyant cyprinid eggs.