The Influence of Discharge on Duration,Ascent Distance,and Fidelity of the Spawning Migration for Paddlefish of the Yellowstone-Sakakawea Stock,Montana and North Dakota,USA

Paddlefish, Polyodon spathula, of the Yellowstone-Sakakawea stock, Missouri and Yellowstone Rivers, Montana and North Dakota, were radio-tagged to assess the influence of spring discharge on duration of river residency, ascent distance, and site-fidelity during spawning migrations of 1999–2002. Contrary to expectations and reported results from other paddlefish populations, fish remained in the river for similar periods of time and ascended to similar reaches in years of higher, more sustained discharge and in years of lower, more fluctuating discharge. In all years, 65 of the 74 migrants (88%) restricted their ascent to reaches below Yellowstone River kilometer (YRkm) 55; only six migrants were found to further ascend to upriver reaches within 20 river kilometers (rkm) of the Intake Diversion Dam (YRkm 114). The lack of detectable annual differences in ascent distance over the study period despite annual differences in Yellowstone River spring flow regimes may have been partially attributed to the apparent site-fidelity demonstrated by the tagged fish over the study period. Ten of the 22 paddlefish contacted in more than one spring migration repeatedly limited their upriver movement to sites that were within 10 rkm of each other. In addition, similar to the reproductive homing tendencies documented in other large-river migratory fishes, site-fidelity occurred in different reaches of the river system. Results from this study suggest that, in years of moderate discharge, site-fidelity may be as influential as the spring flow regime in determining the reaches to which migratory paddlefish ascend. Further research is needed to investigate potential differential spawning success in fish that return to different reaches of the lower Yellowstone River.