A Stepwise Approach to Assess the Occupancy State of Larval Lampreys in Streams

Abstract

Pacific Lamprey Entosphenus tridentatus is an ecologically and culturally important anadromous species of conservation concern for which fisheries managers use information on occupancy state in streams to assess species status and inform stream management decisions. Here we developed a stepwise approach that incorporates the potential for nondetection and a preselected expected maximum probability of stream occupancy if field crews do not document larval Pacific Lamprey during sampling. Our approach includes seven steps: define the occupancy question; select the maximum acceptable probability of occupancy, if the species is not documented during sampling; define an assumed detection probability for the target organism; calculate required sampling effort; select sampling units; conduct sampling; and interpret sampling results into probabilistic occupancy conclusions. We examined detection probability of our approach for larval lamprey using data from multiple occupied streams in the Pacific Northwest. We illustrated our approach by evaluating Balm Grove Dam as a barrier to Pacific Lamprey migration in Gales Creek, Oregon. Bayesian estimates of detection probability in occupied streams ranged from 0.15 to 0.94, with an overall median of 0.70 (95% credible interval: 0.60–0.79). Assuming detection probability is at least 0.15 (i.e., lowest estimate), 19 reaches are required for the expected maximum probability of occupancy to be not more than 0.05, if the species is not documented through our sampling approach. Although detected downstream, we detected no larvae upstream of Balm Grove Dam; thus, we conclude that the maximum probability of occupancy upstream of Balm Grove Dam was not more than 0.05 at an assumed detection probability of 0.4, suggesting the dam as a barrier to adult migration. We provide an occupancy assessment tool with standardized sampling requirements that incorporates the potential for nondetection and the flexibility to select an expected maximum probability of occupancy if researchers document no larvae, to aid management and restoration in a single stream.

Sampling Methods and Survey Designs for Larval Lampreys

“…Studies have determined the baseline occupancy and detection probabilities of larval lampreys (Harris et al 2020), evaluated larval use of pools and riffles in small streams (Starcevich and Clements 2013), and determined larval distribution in deepwater habitats (Jolley et al 2012; Harris and Jolley 2017). In addition, Goertler et al (2019) described larval distribution in estuaries, although it is unclear whether their specimens were in fact larvae or juveniles (see Clemens 2019 for life stage terminology).…”

“…With this approach, selection of sampling units is randomized with spatial balance and the assessment of occupancy status is probabilistic. As a result, estimates of detection probability are applicable across the spatial area where surveys were conducted (USFWS 2014; Harris et al 2020). Stratification can improve precision and increase efficiency, and one has the potential to examine differences among strata to increase the scope of inference.…”

“…The desired scope of inference can guide selection of an appropriate study design, the type of units within each site to be sampled, and a method for selecting each of these (Table 4). Whereas some lamprey researchers have used simple random selection of sampling units, many more have used stratified random or clustered random selection (Table 4; Moser et al 2007; LATF 2019; Harris et al 2020) or targeted selection of sampling units (Reid and Goodman 2015). Larval lamprey surveys typically focus on assessing distribution and status at one point in time (e.g., Reid and Goodman 2015; Harris et al 2020) or in relation to habitat (Starcevich and Clements 2013; Schultz et al 2014).…”

“…Whereas some lamprey researchers have used simple random selection of sampling units, many more have used stratified random or clustered random selection (Table 4; Moser et al 2007; LATF 2019; Harris et al 2020) or targeted selection of sampling units (Reid and Goodman 2015). Larval lamprey surveys typically focus on assessing distribution and status at one point in time (e.g., Reid and Goodman 2015; Harris et al 2020) or in relation to habitat (Starcevich and Clements 2013; Schultz et al 2014). Use of well‐established survey protocols can facilitate comparisons of biological questions across larval lamprey studies.…”

Broad‐scale eDNA sampling for describing aquatic species distributions in running waters: Pacific lamprey Entosphenus tridentatus in the upper Snake River, USA

“…These have been supplemented by seasonal counts at index sites originally chosen for monitoring other species [ e.g ., screw traps for capturing outmigrating salmon smolts in road‐accessible streams (Hayes et al ., 2013) or redd counts in reaches chosen for monitoring steelhead O. mykiss spawning (Clemens et al ., 2021)]. Although these techniques can be effective for determining the presence of Pacific lamprey (Mayfield et al ., 2014; Reid & Goodman, 2015, 2016; Harris et al ., 2016), index sites or reaches are uninformative for determining the upstream extent of occupied habitat, whereas the labour‐intensive sampling required to describe larval habitat occupancy (Harris et al ., 2020) precludes its broad‐scale application. However, precise yet broad‐scale monitoring of Pacific lamprey is critical to assessing whether declines are continuing in some headwater tributaries (IDFG, 2011) and to gauge the success of adult translocations in others (Ward et al ., 2012).…”