Colonial Waterbird Predation on Juvenile Salmonids Tagged with Passive Integrated Transponders in the Columbia River Estuary: Vulnerability of Different Salmonid Species, Stocks, and Rearing Types

Abstract

Passive integrated transponder (PIT) tags implanted in Columbia River basin juvenile salmonids Oncorhyncus spp. were recovered from breeding colonies of Caspian terns Sterna caspia and double‐crested cormorants Phalacrocorax auritus on Rice Island, a dredge spoil island in the Columbia River estuary. Tags were recovered to assess the relative vulnerability of different salmonid species, stocks, and rearing types to avian predators. We detected 50,221 PIT tags at the two bird colonies, mostly from juvenile chinook salmon O. tshawytscha and steelhead O. mykiss raised in hatcheries; 72% of the total tags were from the tern colony and 28% from the cormorant colony. Tagged steelhead smolts were more vulnerable to predation by both bird species than were yearling chinook salmon. More than 15% of PIT tags from steelhead smolts that were available in the estuary in 1998 were detected at the bird colonies compared with 2% of PIT tags from yearling chinook salmon. The greater vulnerability of steelhead may reflect size‐dependent selection by avian predators. Salmonids listed under the Endangered Species Act and unlisted salmonids were equally vulnerable to predation by both terns and cormorants. Hatchery‐raised yearling chinook salmon were more vulnerable than their wild counterparts to predation by terns, a surface‐feeding species; however, hatchery‐raised and wild yearling chinook salmon were equally vulnerable to predation by cormorants, a diving species. These results suggest that hatchery‐raised yearling chinook salmon, and hatchery‐raised steelhead in some years, are more vulnerable to tern predation than wild fish because they have a greater tendency to reside near the water surface where terns forage.

Stock‐Specific Size and Timing at Ocean Entry of Columbia River Juvenile Chinook Salmon and Steelhead: Implications for Early Ocean Growth

“…We recalculated the May growth rates for the two stocks with the earliest ocean entry timing (Willamette and west Cascade spring Chinook Salmon) using ocean recovery sizes that were 10% smaller than those we observed (to mimic size-selective predation), and the resulting growth rates (0.9 mm/d, 2.2% BW/d) were still generally higher than our estimated growth rates for stocks with later timing (Figure 6). Numerous predators are known to prey on juvenile salmon in estuarine and marine habitats (Emmett 1997;Collis et al 2001;Emmett et al 2006;Zamon et al 2013), although the extent to which this predation is size selective is not known. However, it is difficult to imagine a predation scenario that would result in the patterns that we observed in marine waters in May and June: high predation (to produce high growth rates) on some stocks but not on others within the same general area.…”

“…These size and timing differences likely interact with a suite of other factors, including prey availability and predator abundances, to influence survival in estuarine and marine waters. For example, stock-specific variation in the consumption of juvenile salmon by avian predators in the Columbia River estuary has been attributed to stock differences in size, timing, and behavior (Collis et al 2001;Ryan et al 2003;Sebring et al 2013). While there may be conditions under which early timing results in beneficial growth opportunities or survival (Scheuerell et al 2009;Satterthwaite et al 2014), other conditions may select against early timing, leading to survival advantages for later timing (Ryan et al 2003;Beamish et al 2013).…”

Cormorant predation on PIT-tagged lake fish

“…An alternative approach is to tag a high number of fish and then try to recover tags from the cormorant colonies or roosting sites. This has been done using PIT (Passive Integrated Transponder) tags, coded wire tags, radio tags and carlin tags (Feltham and MacLean, 1996;Collins et al, 2001;Koed et al, 2006;Sebring et al, 2010;Jepsen et al, 2010).…”

Migratory Behavior and Survival of Juvenile Salmonids in the Lower Columbia River and Estuary in 2009

“…Collis et al (2001) estimated that nine islands in the estuary supported up to 170,000 piscivorous water birds, including the largest nesting colonies of Caspian terns Sterna caspia and double-crested cormorants Phalacrocorax auritus in North America. Two of these islands were particularly important to survival studies of fish migrating through the study area.…”

“…Avian predation has been shown to account for a substantial amount of juvenile salmonid mortality in the Columbia River estuary (Collis et al 2001;Roby et al 2003;Ryan et al 2003;Anderson et al 2005Anderson et al , 2007. During the previous four years (2006 through 2009), 2.1% to 5.5% of yearling Chinook salmon implanted with acoustic transmitters to measure survival downstream of Bonneville Dam have been detected on the Caspian tern and double-crested cormorant nesting colonies on East Sand Island.…”