Paddlefish growth and behavior after downriver entrainment and subsequent upriver repatriation at Garrison Dam, Missouri River, North Dakota

ABSTRACT Objective The Missouri River flood of 2011 resulted in the entrainment of Paddlefish Polyodon spathula from above Garrison Dam, a main-stem dam, into the tailrace of the Garrison Reach. A large group of entrained fish (estimated at 3,083 in 2012) remained in the tailrace for several years afterward, not undertaking yearly migrations downriver to Lake Oahe, where feeding and habitat conditions are better. We assessed movements and quantified growth (or weight loss) of individual tailrace Paddlefish from the time of original postflood tagging, as early as 2012 until 2018, and then assessed and quantified upriver movement, growth, maturation, and gonadal fat (energy) reserves of adult Paddlefish following their trap-and-transport repatriation from the Garrison Dam tailrace into Lake Sakakawea and their subsequent recapture upriver in recreational fisheries from 2019 to 2023. Methods Paddlefish were collected with gill nets. Fish were measured for length and weighed at the time of original tagging, in 2018, and upon their subsequent recovery in fisheries. Fish were tagged with individually numbered Monel jaw tags (North Dakota Garrison Reach). Dentary (lower jawbone) sections from harvested fish were collected and aged by established and validated methods. Results Of 57 North Dakota Garrison Reach-tagged fish that were tagged prior to 2018 and recaptured during the 2018 repatriation sampling, 46 fish had lost weight, nine fish had gained weight, one fish showed no change in weight, and one fish lacked adequate weight data. Mean weight dropped from 18.1 kg at original tagging to 15.3 kg in 2018, a highly significant decrease. Subsequently, 12 male and 12 female fish repatriated in 2018 were recaptured in harvest fisheries over the period 2019–2023. Four of the 12 males gained weight, two showed no weight change, and six showed a weight loss. The four males that gained weight each gained more than 0.5 kg/year, whereas only one of the six males that lost weight lost as much as 1.36 kg/year. In contrast, nine females gained weight, one female had no weight change, and two females lost weight. Repatriated Paddlefish participated in spawning-associated movements at similar rates to resident fish. Overall weight gain of repatriated fish was statistically significant for females but not for males. Conclusions Our hypothesis is that imprinting, site fidelity, and evolutionary selection for upriver movement in sexually mature fish played key roles in the observed rigid behavior of the tailrace fish. The break in their familiar life cycle was manifested in the sedentary behavior of tailrace Paddlefish below the dam and their weight loss until repatriation restored their life cycle. Although site fidelity has been found in Paddlefish in several stocks, more research is needed on site fidelity and imprinting for Paddlefish and other native, migratory, nonsalmonid species with life cycles altered by river impoundment.