Modification of a Passive Gear to Sample Paddlefish Eggs in Sandbed Spawning Reaches of the Lower Yellowstone River

Abstract

A passive sampling technique was developed to collect eggs and confirm potential spawning sites for paddlefish Polyodon spathula in sandbed reaches of the lower Yellowstone River, Montana and North Dakota. In 2000, egg collectors modeled after the mats used in sturgeon research proved difficult to retrieve from the riverbed and did not collect eggs. In 2001 and 2002, tubular egg collectors designed to remain suspended off the bottom were successfully retrieved 97% of the time and collected 130 acipenseriform eggs along suspected spawning sites (99% of differentiable eggs were genetically confirmed as paddlefish). In both years, eggs were typically collected in mid-June after peak periods of Yellowstone River discharge and at river temperatures of 15–22°C. During collection periods in 2001 and 2002, 20% and 45% of retrieved tubes, respectively, had at least one egg, and 84% of all eggs were found on tubes retrieved from the channel thalweg. Although eggs were spatially distributed in a clumped manner at sample sites, the mean number of eggs per tube was low (<4), suggesting either collector inefficiency, the inability to deploy collectors in close proximity to concentrations of spawning paddlefish, or the widespread distribution of spawning effort over the lower Yellowstone River.