Assessing methods to monitor aquatic invertebrates in large rivers: comparing rock baskets and Hess samplers in the Snake River, Wyoming

Large rivers are difficult to sample due to their size yet critical to monitor because humans heavily rely upon and alter them. Aquatic invertebrates are commonly used to assess the ecosystem quality of streams, but methods to sample these animals in large rivers are still being developed. We sampled aquatic invertebrates using two methods in the Snake River near Jackson, Wyoming. We used a Hess sampler to collect aquatic invertebrates in areas of the river that were <42 cm in depth and rock baskets in deeper areas that were near the bank. Hess samples collected more aquatic invertebrate taxa, and a higher proportion of Ephemeroptera, Plecoptera and burrowing taxa. Rock baskets collected a higher proportion of Trichoptera, filterers and clinging taxa. Bioassessment metrics differed between sampling methods; richness, diversity, evenness, Ephemeroptera, Plecoptera and Trichoptera (EPT) and Hilsenhoff’s biotic index produced higher values in Hess samples, and percent EPT was higher in rock baskets. Non-metric multidimensional scaling and analysis of similarity indicated that the samplers collected different assemblages (p < 0.001). The standard error of total invertebrate density was smaller when at least seven samples were collected and most species were collected when 6-7 replicate samples were processed within a reach. Understanding how sampling method alters the aquatic invertebrates collected will help managers develop monitoring protocols that are best suited to the river and collect the most unbiased invertebrate assemblages.