Detectability of Pacific Lamprey Occupancy in Western Drainages: Implications for Distribution Surveys

The historical North American range of Pacific Lamprey Entosphenus tridentatus extends from the northern Baja California of Mexico to Alaska and inland in suitable habitat as far as there is access to the sea. The population of Pacific Lamprey has declined recently, yet we know little about its actual distribution, and that distribution is changing as the species declines in abundance. We evaluated detection and occupancy probabilities for Pacific Lamprey over a broad region encompassing a wide range of landscapes and drainage sizes. We analyze 110 sites, each representing individual drainages or major subdrainages from the southern border of British Columbia to San Francisco, including over 1,900 km of coastline and the Columbia River basin. Using lamprey-specific electrofishers in suitable habitat low in a drainage we found detection probability of ammocoetes at single sites in occupied drainages was generally high at >90%. If not initially detected, then two additional sites provided >95% confidence of absence. The four geographic regions (Washington, Oregon coast, Columbia River basin, and northern California coast) had similar detection and occupancy probabilities, making these results broadly applicable and comparable. Results are incorporated into guidance for development of effective surveys over a broad scale and will be crucial to establishing occupancy of Pacific Lamprey for purposes of management, planning and monitoring of its changing distribution.