Ichthyoplankton density and Shovelnose sturgeon spawning in relation to varying discharge treatments
Goodman, Benjamin Joseph
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Many lotic fish species use natural patterns of variation in discharge and water temperature as spawning cues and these natural patterns are often altered by river regulation. The effects of spring discharge and water temperature variation on the spawning of shovelnose sturgeon Scaphirhynchus platorynchus and other fishes in the upper Missouri River have not been well documented. In 2006, 2007, and 2008, I had the unique opportunity to study the effects of experimental discharge levels on ichthyoplankton density in the lower Marias River, a regulated tributary to the upper Missouri River. The objective of this study was to evaluate the effects of contrasting discharge treatments and water temperature variation on spatial and temporal variation in spawning of shovelnose sturgeon (and other species sampled as bycatch) as measured by embryonic and larval fish density in the lower Marias River. Ichthyoplankton was sampled about every four days in June and July of 2006, 2007, and 2008. Overall larval fish density was greater in 2006 than in 2007, and increased density was associated with increased discharge in 2006. In 2006, shovelnose sturgeon spawning occurred in the Marias River in conjunction with the ascending, peak (134 m³/s), and descending portions of the spring hydrograph and water temperatures from 16°C to 19°C. No evidence of sturgeon spawning was documented in the lower Marias River in 2007 when peak discharge remained low (9 m³/s to 14 m³/s) despite the occurrence of water temperatures suitable for shovelnose sturgeon embryo development. In 2008, shovelnose sturgeon spawning occurred in conjunction with the peak (118 m³/s) and descending portions of the spring hydrograph, and during a prolonged period of increased discharge (28 m³/s to 39 m³/s), coupled with water temperatures from 11°C to 23°C in the lower Marias River. These data suggest that discharge must reach a threshold level (28 m³/s), and should be coupled with water temperatures suitable (12°C to 24°C) or optimal (16°C to 20°C) for shovelnose sturgeon embryo development, to provide a spawning cue to shovelnose sturgeon in the lower Marias River.