There is growing concern in the Pacific Northwest over predation by migratory male California sea lions (Zalophus californianus) on threatened and endangered salmonid (Onchorynchus spp.) stocks. We compared movements of 14 male California sea lions known to have previously consumed salmonids at Bonneville Dam on the Columbia River or Willamette Falls on the Willamette River (“river”-types), with 12 animals of unknown foraging history (“unknown”-types). We captured sea lions in the Columbia River and instrumented them with satellite-linked transmitters during 2003–2004, 2004–2005, and 2006–2007. Transmitters operated for an average of 87.9 d (range 23-200 d) resulting in 14,539 location fixes. All 14 river-type animals returned to either Bonneville Dam or Willamette Falls whereas none of the 12 unknown-types exhibited this behavior. Minimum upstream and downstream transit times between the mouth of the Columbia River and Bonneville Dam (210 rkm) were 1.9 d and 1 d. Duration at the dam ranged from 2 d to 43 d. The median start dates of the southbound migration from the Columbia River to the breeding grounds for river-type and unknown-type sea lions were 20 May and 15 June, respectively. The maximum travel speed during migration was approximately 130 km d-1 (5.4 km h-1). Our results clearly show that not all California sea lions in the Columbia River prey on salmonids at Bonneville Dam or Willamette Falls. However, factors influencing recruitment into the upriver salmonid-foraging subpopulation are unknown.
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Vol. 84 • No. 1