Spatial and Temporal Variation in Length-Weight Relationships of Age-0 Scaphirhynchus Sturgeon in the Lower Missouri River

Abstract. Length-weight relationships can be useful tools for assessing fish condition. We developed these equations (W = aLb) for wild-caught age-0 (4.1–12.0 cm) Scaphirhynchus sturgeon from eight reaches spanning over 750 river km of the lower Missouri River from 2014 to 2017. We used nonlinear modeling to estimate the constant (a) and exponent (b) of the LW equation for each reach to assess potential spatial differences. We also assessed long-term temporal effects by estimating these parameters by year at Lexington reach, which is located in the middle of our sampling area and was the only reach sampled all 4 y. Constant and exponent estimates from linearized regressions varied by reach and were inversely related during the spatial analyses. Similarly, parameter estimates were also inversely related and varied among years during the temporal analysis at Lexington. To account for the relationship between constant and exponent values, we used predicted weights at 2 cm increments (4.1–12.0 cm) for the spatial analysis (by reach) and for the temporal analysis (by year). During the 2014 and 2015 spatial analyses, weights varied by size but were usually higher in Lexington and Glasgow, which were the furthest upstream reaches sampled during those years. During 2016 and 2017, Lexington was the furthest downstream reach sampled but did not consistently yield relatively high predicted weights. Temporal analysis at Lexington yielded higher predicted weights for 2014–2015 compared to 2016–2017 for higher size categories (10- and 12-cm). In general our results suggest differences in body condition among reaches and years in the lower Missouri River. Further research is needed to identify the specific mechanisms driving spatial and temporal L-W relationship differences observed and to determine if differences in predicted body conditions affect long-term survival and recruitment of age-0 Scaphirhynchus sturgeon. Currently, factors influencing age-0 Scaphirhynchus sturgeon condition and growth are unknown and this work serves to highlight knowledge gaps regarding factors influencing Scaphirhynchus sturgeon recruitment.