Use of Radiotelemetry and Direct Observations to Evaluate Sea Lion Predation on Adult Pacific Salmonids at Bonneville Dam

Abstract

Management of protected species becomes increasingly complex when one protected population negatively affects another. This occurs along coastlines and in rivers and estuaries of the U.S. Pacific Northwest, where protected marine mammals prey on threatened and endangered Pacific salmonids Oncorhynchus spp. Over 9 years, we observed a growing aggregation of California sea lions Zalophus californianus and Steller sea lions Eumetopias jubatus preying upon adult Chinook salmon O. tshawytscha and steelhead O. mykiss at Bonneville Dam on the Columbia River. Both before and concurrent with the observation study, we monitored radio‐tagged salmon at Bonneville Dam and during their upriver spawning migrations. Springtime sea lion abundance steadily increased from 2002 to 2010 and the aggregation formed earlier each winter. The principal prey species in winter were resident white sturgeon Acipenser transmontanus and migratory steelhead and then shifted to predominantly Chinook salmon when the spring run arrived. Observation‐based estimates of salmonid consumption from January to May varied 12‐fold among years (0.4–4.9%, mean = 2.6% of adult salmonids counted at the dam), and radiotelemetry results corroborated these estimates. The highest proportional impact was in winter and early spring. As salmonid abundance increased, per capita consumption by sea lions increased (Type II functional response) but individual salmonid risk decreased (due to prey swamping). Population‐specific risk analyses indicated predation was substantially higher for early‐timed than for late‐timed salmon populations. The most at‐risk group included Snake River and upper Columbia River Chinook salmon listed as threatened under the U.S. Endangered Species Act. These predation indices should help managers simultaneously tasked with salmon recovery and marine mammal management.

High predation on small populations: avian predation on imperiled salmonids

“…7). In other systems, variation in predation rates over time by visual predators has been attributed to changes in regional environmental variables such as stream flow (Antolos et al 2005, ontogenetic requirements that trigger diet switching (Annett and Pierotti 1989), or change in the abundance of prey (Type II or III functional response [Holling 1959], Keefer et al 2012). These factors may have contributed to temporally coherent predation rates at Scott and Waddell creeks.…”

Condition‐Dependent En Route Migration Mortality of Adult Chinook Salmon in the Willamette River Main Stem

“…Inriver fisheries typically account for the largest proportion of adult salmon mortality in freshwater, but a variety of other sources can be important. Episodic or chronic adult mortality has been associated with physical and environmental migration barriers (e.g., Caudill et al 2007;Hinch et al 2012;Sigourney et al 2015), physiological limitations (e.g., Cooke et al 2012;Eliason and Farrell 2016), predation (Quinn et al 2003;Keefer et al 2012), and a variety of pathogens and parasites (e.g., Bakke and Harris 1998;Wagner et al 2005;Benda et al 2015).…”

Network analyses reveal intra‐ and interspecific differences in behaviour when passing a complex migration obstacle

“…) and marine mammals is unknown and may have been responsible for non‐passage of some lamprey, most pinniped predators depart prior to the lamprey migration and application of rates observed for salmon (Keefer et al . ) cannot fully account for the number of observed non‐passing lamprey. Future biotelemetry studies (see Cooke et al .…”