Colonial waterbird predation on Lost River and shortnose suckers based on recoveries of passive integrated transponder tags

We evaluated predation on Lost River suckers (Deltistes luxatus) and shortnose suckers (Chasmistes brevirostris), both listed under the Endangered Species Act (ESA), from American white pelicans (Pelecanus erythrorhynchos) and double-crested cormorants (Phalacrocorax auritus) nesting at mixed species colonies on Clear Lake Reservoir, CA and Upper Klamath Lake, OR during 2009-2014. Predation was evaluated by recovering passive integrated transponder (PIT) tags that were implanted in suckers, subsequently consumed by pelicans or cormorants, and deposited on the birds’ nesting colonies. Data from PIT tag recoveries were used to estimate predation rates (proportion of available tagged suckers consumed) by birds to evaluate the relative susceptibility of suckers to avian predation in Upper Klamath Basin. Data on the size of pelican and cormorant colonies (number of breeding adults) at Clear Lake and Upper Klamath Lake were also collected and reported in the context of predation on suckers.

Results indicate that predation rates varied by sucker species (Lost River, shortnose), sucker age-class (adult, juvenile), bird colony location (Upper Klamath Lake, Clear Lake), and year (2009-2014), demonstrating that predator-prey interactions in the system were dynamic during the study period. Tagged suckers ranging from 72 mm to 730 mm were susceptible to cormorant or pelican predation; all but the largest of the tagged Lost River suckers were susceptible to avian predation. Estimates of minimum, annual predation rates ranged from