Estimating densities of larval Salmonflies (Pteronarcys californica) through multiple pass removal of post-emergent exuvia in Colorado rivers

Abstract

Traditional methods of collecting, sorting, and identifying benthic macroinvertebrate samples are useful for stream biomonitoring and ecological studies, however, these methods are time consuming, expensive, and require taxonomic expertise. Estimating larval densities through collection of post-emergent exuvia can be a practical and time efficient alternative. We evaluated the use of multiple pass depletion techniques of the post-emergent exuvia of Pteronarcys californica to estimate larval densities at ten sites in three Colorado rivers. Exuvia density was highly correlated with both final-instar larval density (R 2 = 0.90) and total larval density (R 2 = 0.88) and the multiple pass removal technique performed well. Exuvia surveys found P. californica at three low density sites where benthic sampling failed to detect it. At moderate and high density sites the exuvia surveys always produced lower density estimates than benthic surveys. Multiple pass depletion estimates of exuvia proved to be an accurate and efficient technique at estimating larval densities and provided an effective alternative for traditional benthic sampling when objectives are detecting and monitoring P. californica, especially at low density sites.

Quantifying the habitat preferences of the stoneflyPteronarcys californicain Colorado

“…We estimated the density of salmonflies at our sites using the method described in Nehring et al (2011) and evaluated by Heinold et al (2020). This technique is an improvement on the exuvia collection methods of Richards, Rolston, and Dunkle (2000) by applying a multiple-pass removal technique to account for imperfect detection probability.…”

“…There is a high correlation (R 2 = 0.90) between postemergence exuvia density estimates and more traditional preemergent quantitative benthic sampling and this method is more effective than traditional benthic surveys at detecting P. californica at low density sites. (Heinold et al, 2020).…”

“…Nehring, Heinold, and Pomeranz (2011) used multiple-pass removal density estimates of the shed exuvia as an index of salmonfly density in rivers in Colorado. This technique was validated and applied to other studies as a cost-and time-efficient index of salmonfly density (Heinold, Kowalski, & Nehring, 2020;Walters et al, 2018). Therefore, we applied this technique to index salmonfly density and explore its relationships with stream habitat variables.…”