How much river does a fish need? For pallid sturgeon on the Missouri River there is a concern that there is not enough river for downstream-dispersing larvae. Field-based and laboratory work at the Columbia Environmental Research Center are focusing on critical unknowns in the dispersal process, using comparative studies with other sturgeon species to provide key insights. Our first experiment this year is with lake sturgeon.
Eggs collected from the lake sturgeon female spawned in the laboratory on April 15th (see previous entry, “Stream studies back in action”) hatched in the early morning hours on Monday, April 20th. A team of researchers counted out a total of 4800 newly hatched (day-0) lake sturgeon free embryos; 800 free embryos for each of the six experimental stream segments (three with gravel, and three with cobble substrate, see figure 1). The stream segments were then “seeded” by releasing the free embryos into the substrates of the experimental streams and slowly increasing the water flow over the substrate to 15 centimeters per second. The streams were monitored around-the-clock to see if free embryos stayed in the stream substrate or dispersed downstream. Nets at the end of each segment were checked at six-hour intervals for free embryos that dispersed. Free embryos collected in the nets were counted and preserved for later laboratory measurements of size and development. On Friday, April 24th, after a 96-hour (4-day) monitoring period the substrate of each segment was removed and any free embryos remaining were collected in the nets downstream. On Tuesday, April 28th, the experiment was repeated with 8-day old lake sturgeon free embryos (figure 2). Researchers will monitor the streams and record free embryo dispersal over the next 4 days. The same studies will be repeated with pallid sturgeon and shovelnose sturgeon through May and into June.
The development and dispersal strategy of each sturgeon species is adapted to the conditions where it lives. Comparing the dispersal strategies of newly hatched and 8-day old free embryos of the three sturgeon species (lake sturgeon, pallid sturgeon, and shovelnose sturgeon) in experimental stream systems provides researchers with an estimate of the time each species spends dispersing downstream. Importantly, the experiments are designed to indicate the extent to which the species delay dispersal by residing for some period of time in the substrate just after hatch. When and how long free embryos disperse, in part, determines the length of free-flowing river required for the completion of the free embryo life stage and where habitats that support the transition to the exogenously feeding larval stages should be located.