Seasonal Comparison of Catch Rates and Size Structure Using Three Gear Types to Sample Sturgeon in the Middle Mississippi River

We evaluated the efficacy of three gears commonly used to sample shovelnose sturgeon Scaphirhynchus platorynchus, pallid sturgeon S. albus, and lake sturgeon Acipenser fulvescens in large rivers. We stratified habitats and randomly sampled sites with trawls, gill nets, and trotlines, in the middle Mississippi River from June 2003 through May 2005 (N = 3,476 samples). A total of 3,523 shovelnose sturgeon, 31 pallid sturgeon, and 13 lake sturgeon were captured. When sample sizes were adequate (based on power analysis), we used a mixed analysis of variance (ANOVA), procedure to determine the relative impact of season and gear type on sturgeon catch per unit effort (CPUE). Kolmogorov-Smirnov tests were used to determine whether the length frequency distributions for each species differed among gears. Overall, the power analyses indicated that there were adequate sample sizes for comparing standardized CPUE for shovelnose sturgeon among seasons; however, too few pallid or lake sturgeon were collected to make statistical comparisons. Shovelnose sturgeon catch rates varied among gears and seasons: season and gear type interacted in the ANOVA model. Among gear types, 5.08-cm gill nets produced the highest standardized CPUE, and catch rates were highest during spring. The length frequency distributions of shovelnose sturgeon depended on gear type. Shovelnose sturgeon can be sampled in large rivers using stratified random sampling with this combination of gear. However, pallid sturgeon and lake sturgeon may be too rare to sample with this approach.