Latitudinal Variation in Lake Sturgeon Size as Related to the Thermal Opportunity for Growth

The relationships between age-specific total lengths, weights, condition factors, and a measure of the thermal opportunity for growth (TOG; calculated as the sum of degree-days greater than 5°C) were estimated for lake sturgeon Acipenser fulvescens from data available for 16 populations spread throughout much of the speciesˈ natural range. The TOG explained a significant proportion of the observed variation in length at age irrespective of age. Correlations between weight at age and condition factor improved significantly as lake sturgeon aged, although TOG remained a poor indicator of the differences in condition at age among sturgeon populations. As elsewhere in the literature, declines in length and weight at age occurred at higher latitudes. However, growth rates (mm/degree-day), when adjusted for TOG, showed a significant inverse relationship between growth rates and latitude. The latitudinal countergradient in growth rates partially compensates for reduced growth opportunities in northern populations of lake sturgeon.