By Aaron DeLonay, Amy George, and Kimberly Chojnacki
Rows of hatching jars lined the shelves in the laboratory at USGS this spring and summer. In each jar, pallid sturgeon and shovelnose sturgeon gently tumbled in the 18 °C water as USGS scientists closely monitored and documented fish development. The eggs came from a captive population of pallid sturgeon adults held at the Center for research purposes and from shovelnose sturgeon adults captured from the Lower Missouri River. Four pallid sturgeon and ten shovelnose sturgeon females were induced to spawn using hormone injections in the laboratory. The eggs were fertilized by multiple males of the corresponding species and each family lot was placed in separate jars to develop and hatch. Scientists monitored each group of eggs for fertilization, development, and survival to hatch. After hatch, three to six sturgeon were removed and preserved in formalin at 2-4 hour intervals around the clock to document the rapid growth and developmental changes that occur during this early life stage. Free embryos hatch without a well-developed mouth, eyes or fins, and must rely a large yolk sac to fuel their rapid development (figure 1). At 18° C, the development from newly hatched free embryo to exogenously feeding larvae takes about 14 days for both species. Although their abilities are limited, sturgeon are capable of changing their depth and orienting to water flow even before the initiation of feeding. Once the larvae began feeding, behavioral and developmental changes are not as rapid and samples were taken every 12 hours. At this stage, they have fins, eyes, a mouth bordered by four fleshy barbels, and resemble sturgeon in miniature. Scientists at USGS closely examined the timing and trajectory of developmental changes to better estimate where sturgeon free embryos collected by our crews in the Missouri River may have been spawned, how early life stages of sturgeon disperse along the river, and where habitats needed for early feeding may be located. Differences in developmental timing, behavior, and requirements between species may provide insight into why one species, the shovelnose sturgeon, is reproducing and recruiting, while the pallid sturgeon is not.