A Field Study to Investigate Repeat Homing in Pacific Lampreys

—In-season homing of Pacific lampreys Lampetra tridentata was investigated using radiotelemetry in the lower Columbia River from 1998 through 1999. A total of 50 Pacific lampreys were captured: 25 at Willamette Falls (river kilometer [rkm] 45 on the Willamette River, a tributary to the Columbia River, with its confluence at rkm 163) and 25 at Bonneville Dam (rkm 238 on the Columbia River). Each fish was fitted with a radio transmitter, transported, and released in the Columbia River approximately 26 km downstream from the confluence of the Willamette River. Movement of the radio-tagged Pacific lampreys was monitored for several months using mobile and fixed receiver stations to observe rates of homing towards the site of original capture. Results indicated that the lampreys exhibited nonsignificant in-season homing fidelity (p = 0.622) based on the null expectation that onehalf of the total recoveries would home and the other half would stray. Final location classifications were 17 homed, 20 strayed, and 13 undetermined. The undetermined classification included individuals that were not detected upstream of the confluence of the Willamette River or in other Columbia River tributaries. Final location classifications were not influenced by fish length (p = 0.594). Although considered weak swimmers, Pacific lampreys were capable of traveling at velocities near 2.5 km/h and sustaining that activity for at least 24 h. * Corresponding author: hatd@critfc.org Introduction Pacific lamprey Lampetra tridentata is endemic to the Columbia River basin and plays an important role in the region’s river and marine ecosystems, along with having cultural, medicinal, and religious significance to Native American Indians of the Pacific Northwest. Counts of Pacific lampreys at fishway counting stations in the Columbia River basin are currently a fraction of the numbers that were reported 40–50 years ago, indicating a dramatic decline in lamprey abundance (Close et al. 2002). Pacific lampreys have a unique anadromous life history. Mating individuals build nests and spawn in headwater streams; larvae live as filter feeders in freshwater for several years prior to their metamorphosis and then migrate to the ocean. Lampreys spend several years feeding in the ocean prior to returning to freshwater to spawn (Beamish 1980). Besides perpetuating the species, these spawning migrations provide a vehicle for deposition of marine nutrients to the streams, as well as a buffer for avian (Merrell 1959), marine mammal (Jameson and Kenyon 1977; Roffe and Mate 1984), and piscivorous (Beamish 1980; Poe et al. 1991) predation on Endangered Species Act-listed Pacific salmon Oncorhynchus spp. Since natal homing is common in other anadromous species, it is often assumed that Pacific lampreys also exhibit this life history characteristic. Natal homing has been described for a 192 hatch and whiteaker broad range of taxa, including the highly studied salmonids. Natal homing was demonstrated in Gulf of Mexico sturgeon Acipenser oxrhynchus desotoi (Stabile et al. 1996), American shad Alosa sapidissima (Carscadden and Leggett 1975), and rainbow trout O. mykiss (Lindsey et al. 1959), and repeat homing occurs in American shad (Melvin et al. 1986), Windermere char Salvelinus willughbii Günther (Frost 1963), brook trout S. fontinalis (O’Connor and Power 1973), and brown trout Salmo trutta (Tilzey 1977). In-season homing was demonstrated in Colorado pikeminnow Ptychocheilus lucius (Tyus 1985), lake trout Salvelinus namaycush (Martin 1960), flathead catfish Pylodictis olivaris (Hart 1971; Hart and Summerfelt 1974), muskellunge Esox masquinongy (Margenau 1994), black-and-yellow rockfish Sebastes chrysomelas (Hallacher 1984), mosshead sculpin Clinocottus globiceps (Green 1973), Windermere char (Frost 1963), Florida largemouth bass Micropterus salmoides floridanus (Mesing and Wicker 1986), cutthroat trout O. clarkii (Platts 1959; McCleave 1967; McCleave and Horrall 1970), rainbow trout (Lindsey et al. 1959), brook trout (O’Connor and Power 1973), brown trout (Harcup et al. 1984; Halvorsen and Stabell 1990), largemouth bass Micropterus salmoides (Quinn et al. 1978), and American shad (Dodson and Leggett 1973). Prior to this work, homing has not been studied in Pacific lampreys; however, Beamish (1980) speculated that at least some Pacific lampreys homed as evidenced by differences in adult length compositions among rivers that he investigated. Bergstedt and Seelye (1995) used coded wire tag and recovery methods to reject the hypothesis that Lake Huron sea lampreys Petromyzon marinus home to natal streams. However, Wright et al. (1985) used allozyme data to identify statistically significant differences among five sample sites within Lake Huron, which they interpreted to indicate stock structure. For stock structure to exist, Lake Huron sea lampreys must exhibit natal homing. Gerking (1959) used the term “homing” in a general sense to mean the return of fish after migratory, accidental, or experimental displacement “to a place formerly occupied instead of going to other equally probable places. ” McCleave (1967) described equally probable places as areas “occupied by other individuals of the same species. ” In spawning migrations of fishes, three types of homing are recognized (McCleave 1967): (1) natal homing: the return of adults to spawn in the same location in which they were hatched, termed “reproductive, parent stream, or natal homing” by Lindsey et al. (1959); (2) repeat homing: the return of adults to spawn in subsequent breeding seasons at the location of initial spawning; and (3) in-season homing: the return of adults within the same breeding season to the location of initial choice after displacement. This study uses the third definition of homing. Pacific lamprey homing fidelity has many implications for restoration and management of the species. If lampreys demonstrate high homing fidelity, then substantial population substructuring can be inferred and restoration efforts will need to be focused on multiple distinct populations, as is the case for Pacific salmon. On the other hand, if homing fidelity is low, then a large panmictic metapopulation can be inferred and recovery efforts can be focused at a basin or multibasin scale. The objective of this study was to investigate in-season homing of Pacific lampreys using radiotelemetry.