DOI: 10.5061/dryad.v15dv41st
title: Estimating densities of larval Salmonflies (Pteronarcys californica) through multiple pass removal of post-emergent exuvia in Colorado rivers

description: 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 (R2 = 0.90) and total
 larval density (R2 = 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.
description: Three benthic subsamples were taken at each site between 15-18
 April 2010 from the Colorado and Fraser Rivers and 10 May 2010 from the
 Gunnison River, approximately one month prior to the typical adult
 emergence of <em>P. californica</em>. All sites were located
 in riffle areas dominated by cobble substrates interspersed with boulders
 and gravel. A modified Surber sampler with a 0.25 m² sampling frame (55.0
 cm x 45.5 cm) and 150 μm mesh net was used. Cobbles larger than 10 cm in
 diameter were individually scrubbed with a brush, invertebrates washed
 into the net, and then the cobbles removed from the sampling frame.
 Remaining substrate within the frame was disturbed to a depth of 10 cm to
 dislodge invertebrates into the net. Contents were preserved with 80%
 ethanol in 2 L plastic jars. In the lab, all <em>P.
 californica</em> larvae were sorted, sexed [21, 38], and measured
 for total length (TL) from the anterior tip of head to the posterior tip
 of the epiproct to the nearest millimeter under a dissecting microscope
 with a calibrated ocular micrometer. Length frequency histograms for male
 and female larvae were separately constructed based on TL to separate
 annual year classes. Body size measurements can be used to separate
 cohorts of this merovoltine species due to the long lifespan, fast growth,
 and large size. We based our approach on separating annual cohorts on
 Townsend and Pritchard except we constructed length frequency histograms
 from total length measurements rather than head-capsule width and wing-pad
 length. The largest size cohort of both males and females was considered
 to be mature larvae, the final instar larvae expected to emerge in that
 year. Densities of mature larvae and densities of all larvae were
 calculated and used in separate analyses for correlation with exuvia
 densities. Exuvia sampling began with the onset of P. californica adult
 emergence on the Colorado River at site nine on 2 June 2010 and proceeded
 upstream to end at site one on the Fraser River on 21 June 2010; sampling
 at site 10 on the Gunnison River lasted from 16-23 June 2010. Each site
 was sampled beginning on the day when the first exuvia was found or winged
 adults were observed and continued daily until exuvia were no longer
 found. Data collection was performed by searching for exuvia within a
 maximum width of 10 m of the bank along two 30.5 m transects on one side
 of the river. Exuvia sampling sites were on the stream banks directly
 adjacent to benthic sampling sites. Searching at each individual site
 extended from the water’s edge laterally until no more exuvia were found,
 the search distance varying by site, generally depending on the complexity
 of the shoreline and thickness of riparian vegetation. The average width
 searched was measured at each site. Collections at a site were conducted
 by two to four people completing two or three removal passes with
 identical effort and personnel on each pass. Specimens were taken only
 when attached to dry riparian and emergent substrates; none were taken
 from the water to avoid counting ones that possibly drifted into the site.
 Exuvia were enumerated using hand held counters, stored in sealable bags,
 and removed from the search area.