Lake Sturgeon Population Status and Demography in the Kettle River, Minnesota, 1992–2007

Abstract

We assessed population status and estimated selected demographic parameters for lake sturgeon Acipenser fulvescens in the Kettle River, Minnesota, using capture–recapture tagging data collected from 1992 to 2007. The lake sturgeon is an imperiled species of special concern in Minnesota, with population declines attributed to overfishing, pollution, and establishment of dams. In 1995, the recreational fishery was closed and the only dam on the Kettle River (Sandstone Dam) was removed, in part to promote lake sturgeon recovery. Recent incidental capture of lake sturgeon by anglers is prompting interest in re-opening the recreational fishery. We gathered information to determine (1) status and demography of the Kettle River population, (2) the effect of dam removal, and (3) the feasibility of re-opening the recreational fishery. Open-population capture–recapture models were used to examine population status and estimate survival, seniority, and the relative importance of survival and recruitment for influencing population trends. The rate of population change (λ) indicated that the population was barely maintaining itself (average λ across years = 1.02; 95% confidence interval [CI] = 0.86–1.16). Annual population estimates ranged from about 130 fish to almost 300 fish but had wide CIs. Similarly, estimates of new lake sturgeon entering the population each year (either from within-population recruitment or immigration) were variable and ranged from 0 to over 100. Annual survival was nearly constant at about 80%. Growth was slow relative to other populations but averaged about 28 mm/year for ages 6–21. A seniority parameter indicated that 49–87% of individuals in the population in a given year consisted of population members that survived from the previous year. This suggested that the population was essentially maintaining itself through a combination of episodic recruitment and relatively constant survival. Recent low recruitment may be due to a short-term disturbance from sediment released when the dam was removed in 1995. Maintaining closure of the recreational fishery still appears necessary. Bolstering of recruitment may be needed to increase the population. Full recovery will require patience for this long-lived, late-maturing, infrequently reproducing species.