Lake Sturgeon (Acipenser fulvescens) Population Attributes, Reproductive Structure, and Distribution in Namakan Reservoir, Minnesota and Ontario

Population declines of lake sturgeon (Acipenser fulvescens) have resulted in their protected status as a species of special concern in the state of Minnesota and threatened in the province of Ontario. Water bodies that lie along the Minnesota-Ontario border are home to historic populations of lake sturgeon. Research has been conducted on the neighboring lake sturgeon populations (i.e., Rainy Lake, the lower Seine River in Ontario, and Rainy River/Lake of the Woods). However, little research has been conducted on the lake sturgeon population of the Namakan Reservoir. The objectives of this project were to 1) assess the lake sturgeon in Namakan Reservoir by quantifying age, growth, mortality, and reproductive structure, and 2) to examine the influence of gender and reproductive condition on seasonal distribution and movement patterns of lake sturgeon in the Namakan Reservoir. I evaluated year class strength and reproductive structure of lake sturgeon collected in the U.S. and Canadian waters of Namakan Reservoir. Reproductive structure of lake sturgeon was assessed during spring 2008 and 2009 using plasma testosterone and estradiol-17β concentrations. Age of lake sturgeon > 75 cm ranged from 9 to 86 years of age (n=533, mean=36 years). Using logistic regression analysis, I found that total annual precipitation was positively associated with lake sturgeon year class strength in Namakan Reservoir. Plasma steroid analysis revealed a sex ratio of 2.4 females:1 male and, on average, 10% of female and 30% of male lake sturgeon were reproductively mature each year (i.e., potential spawners). Moreover, I found evidence based on re-captured male fish of both periodic and annual spawning, as well as the ability of males to rapidly undergo gonadal maturation prior to spawning. Knowledge of lake sturgeon reproductive structure and factors influencing recruitment success contribute to the widespread conservation efforts for this threatened species. I evaluated the influence of gender and reproductive condition on seasonal distribution and movement patterns of lake sturgeon in Namakan Reservoir. Sixty adult lake sturgeon were implanted with acoustic transmitters prior to spawning in spring 2007 and 2008. Fish movement was monitored using an array of 15 stationary receivers covering both U.S. and Canadian waters of Namakan Reservoir and its major tributaries. Blood samples were collected from 133 lake sturgeon prior to spawning and plasma concentrations of testosterone and estradiol-17ß were analyzed using radioimmunoassay. The greatest recorded distance traveled by a lake sturgeon was 130 km. In general, females traveled greater distances (mean=38.8 km) than males (mean=29.9 km). Lake sturgeon of both sexes traveled greater distances during the spawning and post-spawning periods compared to all other seasons. Distance traveled was lowest (mean=5.3 km) and site fidelity was highest (mean=17.8 days) during winter months. Five females were characterized as potentially reproductive and 14 as non-reproductive based on plasma steroid concentrations. Potentially reproductive females had lower site fidelity and traveled greater distances across all seasons than non-reproductive females. Distance traveled by reproductive females was highest in the fall (mean=38.8 km) compared to other seasons (means ranged from 3.2 to 24.0 km) and may be linked to gonadogenesis (e.g., increased energy acquisition) prior to spring spawning. A link between migration distance and female reproductive condition may have implications for lake sturgeon conservation particularly in water bodies that have experienced loss of preferred lake sturgeon habitat, are heavily impounded or facing future impoundment.

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