Transactions of the American Fisheries Society 141:305–312, 2012 American Fisheries Society 2012 ISSN: 0002-8487 print / 1548-8659 online DOI: 10.1080/00028487.2012.662201 NOTE Occupancy and Detection of Larval Pacific Lampreys and Lampetra spp. in a Large River: the Lower Willamette River Jeffrey C. Jolley,* Gregory S. Silver, and Timothy A. Whitesel U.S. Fish and Wildlife Service, Columbia River Fisheries Program Office, 1211 Southeast Cardinal Court, Suite 100, Vancouver, Washington 98683, USA function (Close et al. 2002). Information on basic biology, Abstract ecology, and population dynamics required for effective Pacific lampreys Entosphenus tridentatus (formerly Lampetra conservation and management of Pacific lampreys is lacking. tridentata) are declining in the Columbia River basin, and the use of Pacific lampreys have a complex life history that includes large, main-stem river habitats by larvae of this species is unknown. We used a deepwater electrofisher to explore occupancy, detection, multiple-year larval (ammocoete), migratory juvenile, and adult and habitat use of larval Pacific lampreys and larval Lampetra spp. marine phases (Scott and Crossman 1973). Larvae and juve- in the lower Willamette River, Oregon. We used a generalized ran- niles are strongly associated with stream and river sediments. dom tessellation stratified approach to select sampling quadrats For multiple years after hatching, larvae live burrowed in stream × (30 30 m) in a random, spatially balanced order. Pacific lam- and river sediments, where they consume detritus and organic preys, Lampetra spp., and unidentified lampreys were found in the Willamette River; larvae were detected in all areas except the material by filter feeding (Sutton and Bowen 1994). Larvae Multnomah Channel. We calculated reach- and quadrat-specific metamorphose into juveniles from July to December (McGree detection probabilities and the amount of sampling effort required et al. 2008), and major migrations are made downstream to the for 80% confidence that larval lampreys were in fact absent when Pacific Ocean in the spring and fall (Beamish and Levings 1991). they were not detected. Lampreys were detected in a variety of The sympatric western brook lamprey Lampetra richardsoni areas (although relatively low numbers were collected), including shallow, nearshore areas; midchannel areas (depth up to 16 m); does not have a major migratory or marine life stage, although and anthropogenically affected areas. Detection probabilities (i.e., adults may locally migrate upstream before spawning (Renaud in occupied areas) were 0.07 (reach) and 0.23 (quadrat). The sam- 1997). For both species, the majority of the information on pling effort required for 80% confidence that lampreys were absent habitat preferences of larvae comes from Columbia River basin when undetected was 20 quadrats (in the lower Willamette River tributary systems (Moser and Close 2003; Torgersen and Close reach) and 6 subquadrats (within a quadrat). Differences in lam- prey detection by depth were not observed. A variety of sizes was 2004; Stone and Barndt 2005; Stone 2006) and coastal sys- collected (20–144 mm total length), indicating the likely occurrence tems (Farlinger and Beamish 1984; Russell et al. 1987; Gunckel of multiple ages of larvae. Our study identifies how the occurrence et al. 2009). of larval Pacific lampreys can be quantified with statistical rigor in Larval lampreys are known to occur in sediments of low- a large river (i.e., larger than fourth order [1:100,000 scale]). The gradient streams (smaller than fifth order [1:100,000 scale]; effect of channel management activities on larval lampreys should be considered in efforts to conserve these important species. Torgersen and Close 2004), but their use of habitats in relatively deep areas within large rivers is less known. Larval downstream movement, whether passive or active, occurs year-round (Nur- Pacific lampreys Entosphenus tridentatus (formerly Lam- sall and Buchwald 1972; Gadomski and Barfoot 1998; White petra tridentata) in the Columbia River basin and other areas and Harvey 2003). Anecdotal observations exist regarding have experienced a great decline in abundance, are culturally larval lamprey occurrence in large-river habitats, mainly at important to Native American tribes, and are ecologically hydropower facilities (Moursund et al. 2003; CRITFC 2008), important within the food web; the decline of this species where the larvae become impinged on downstream screens, provides insight into the effect of human actions on ecological are observed in juvenile bypass facilities, or are observed *Corresponding author: jeffrey [email protected] Received March 9, 2011; accepted August 19, 2011 Published online March 12, 2012 305 306 JOLLEY ET AL. during dewatering events. These occurrences are thought to be METHODS associated with downstream migration, and specific collections We estimated occupancy of larval lampreys at several explicit of supposedly migrating ammocoetes have been made in spatial scales within the lower Willamette River by adapting an large-river habitats (Beamish and Youson 1987; Beamish and approach used by Peterson and Dunham (2003) and refined by Levings 1991). Ammocoetes of the sea lamprey Petromyzon the U.S. Fish and Wildlife Service (USFWS; USFWS 2008) marinus have been documented in deepwater habitats within to evaluate patch occupancy and d for bull trout Salvelinus Great Lakes tributaries, in proximity to river mouths (Hansen confluentus. The approach has several requirements: (1) a site- and Hayne 1962; Wagner and Stauffer 1962; Lee and Weise and gear-specific d (assumed or estimated); (2) the probability 1989; Bergstedt and Genovese 1994; Fodale et al. 2003b), and of presence at a predetermined acceptably low level (given no in the large St. Marys River, which connects Lakes Superior detection); and (3) random identification of spatially balanced and Huron (Young et al. 1996). References to other species oc- sample sites that allow estimation of presence and the refinement curring in deepwater or lacustrine habitats are scarce (American of d. brook lamprey L. appendix; Hansen and Hayne 1962). The reach-specific d (dreach) was calculated as the proportion The Willamette River, Oregon, is a large (seventh-order) trib- of 30- × 30-m sampling quadrats that were occupied by larval utary of the Columbia River that encompasses a drainage area lampreys (i.e., larvae were captured) in the lower Willamette of 29,728 km2 (Stanford et al. 2005). Pacific lampreys and west- River, an area known to be occupied. The posterior probability ern brook lampreys inhabit the lower Willamette River, and a of reach occupancy given that a larval lamprey was not detected large portion of the main stem is unimpounded. Nevertheless, was estimated as it is unknown whether the lamprey species use the main-stem C | F · F Willamette and Columbia rivers as rearing areas for larvae in P( 0 ) P( ) P(F | C0) = , addition to using these systems as migration corridors (i.e., re- [P(C0| F ) · P(F )] + [P(C0| ∼ F ) · P(∼ F )] turning adults and outbound juveniles). (1) Sampling of larvae in deepwater (i.e., >1.0 m) areas is a chal- lenge because of specialized gear requirements and presumed where P(F) = the prior probability of larval lamprey presence patchy distributions. Bergstedt and Genovese (1994) success- and C0 = no detection. Although we knew the lower Willamette fully sampled deepwater areas for larval sea lampreys in tribu- River reach was occupied by larval lampreys, a P(F) value taries to the Great Lakes by using a modified electrofisher with of 0.5 (uninformed) was used to inform future study design suction. However, problems associated with the uncertainty in (i.e., P[F|C0]) for areas where larval lamprey presence was detection probability (d) and capture efficiency when sampling unknown. The term P(∼F)(= 1 − P[F]) is the prior probability for distribution and abundance of organisms can be encountered. of species absence, and P(C0|F)(= 1 − d) is the probability In addition, the problem of false absences in estimating occu- of not detecting a species when it actually occurs (Peterson and pancy by species that are rare or patchily distributed has been Dunham 2003). identified (Bayley and Peterson 2001; Peterson and Dunham The lower Willamette River was sampled during March– 2003; MacKenzie et al. 2005). October 2009 from Willamette Falls (rkm 42) to the confluence A goal of this study was to develop a statistically rigorous with the Columbia River at Portland, Oregon (Figure 1); design to evaluate whether larval lampreys occupy main-stem the sampled area included the Multnomah Channel (a braid habitats of a representative large river. In part, rigor can be im- of the Willamette River) and the Portland Harbor Superfund proved by determining d. Knowledge of d can be used to (1) site (rkm 3.2–19.2). A sampling event consisted of using a determine the number of site visits that are required to achieve deepwater electrofisher (Bergstedt and Genovese 1994) in a a specific certainty of lamprey absence (when not detected) and 30- × 30-m quadrat. This quadrat size was selected based on (2) assist in making inferences about lamprey distribution from the previous experience of sea lamprey researchers in the Great collected data. To this end, we sampled the lower Willamette Lakes (M. Fodale, USFWS, personal communication), as their River, which as a whole is known to be occupied by larval sampling approach evolved from systematic to adaptive (Fodale lampreys. In general, we documented the presence or absence et al. 2003a). The configuration of the deepwater electrofisher of larval lampreys throughout the lower Willamette River (i.e., is described by Bergstedt and Genovese (1994); sampling was downstream of Willamette Falls, river kilometer [rkm] 42) and standardized. The bell of the deepwater electrofisher was low- determined d with deepwater electrofishing. Our specific ob- ered from a boat to the river bottom. The electrofisher delivered jectives were to (1) use a deepwater electrofisher to document DC at 3 pulses/s with a 10% duty cycle and a 2:2 pulse train (i.e., whether ammocoetes of Pacific lampreys and Lampetra spp.