Using mark‐recapture data in an individual‐based model to evaluate length‐at‐age differences between two S nake R iver white sturgeon ( A cipenser transmontanus R ichardson, 1836) populations in I daho, USA

Summary Despite showing similar abundance estimates, population surveys of wild white sturgeon ( Acipenser transmontanus Richardson, 1836) in the Bliss and Hells Canyon reaches of the Snake River display very different stock structures. To better understand these differences, an individual based model was developed using empirical mark‐recapture metrics from 343 white sturgeon recaptured in the Bliss reach [at‐large 1.0–19.9 years], and 580 white sturgeon recaptured in the Hells Canyon reach [at‐large 1.0–21.2 years]. After calculating annual growth increments from these recaptures, individuals from each reach were assigned to size bins then randomly chosen from within each bin to ‘grow’ theoretical fish. This process was bootstrapped 1000 times, producing a mean length‐at‐age trajectory for each reach. From these data, mark‐recapture samples were assigned estimated ages and fitted to a three‐parameter logistic growth curve. Results indicate a wide disparity in length‐at‐age of white sturgeon between reaches, with the Bliss population displaying an intrinsic rate of increase [K = 0.118] twice that of the Hells Canyon population [K = 0.059]. This is due primarily to slow juvenile growth in the Hells Canyon population relative to juvenile growth in the Bliss population. The much slower growth rate of juveniles in the Hells Canyon reach has consequences to population demographics, such as age at first maturity. Although growth differences are large, both populations appear to be stable or increasing in abundance despite being regulated by very different mechanisms.