Relative Vulnerability to Avian Predation of Juvenile Salmonids Tagged with Passive Integrated Transponders in the Columbia River Estuary, 1998–2000

Abstract

Caspian terns Sterna caspia and double-crested cormorants Phalacrocorax auritus that colonize dredge-spoil islands in the Columbia River estuary prey upon millions of juvenile Pacific salmonids annually. We estimated the relative vulnerability of various salmonid stocks to these predators by using data from passive integrated transponder (PIT) tags detected on these colonies; 96,382 tags were detected from the 1998-2000 migration years. On tern colonies, detection rates were highest for tags from steelhead Oncorhynchus mykiss and lowest for tags from yearling chinook salmon O. tshawytscha. However, detection rates on cormorant colonies were similar for tags from steelhead and coho salmon O. kisutch but lower for tags from yearling chinook salmon. Analyses based on migration history showed tags of transported fish were frequently detected in lower proportions than those of their counterparts that migrated in-river, the pattern being most pronounced in steelhead. Analyses based on origin (hatchery versus wild) showed similar detection proportions for the tags of wild versus hatchery steelhead on both tern and cormorant colonies. In contrast, 3.1% of hatchery versus 1.1% of wild chinook salmon tags previously detected at Bonneville Dam were detected on a colony, the greater vulnerability of hatchery fish being more pronounced on tern colonies. These tags accounted for 11.5% of steelhead, 4.6% of coho salmon, and 2.6% of yearling chinook salmon detected at Bonneville Dam, the last downstream impoundment encountered by seaward migrants. These estimates of predation are minimal because detection efficiency was not 100% and tags from many salmonid prey were not deposited on a nesting colony.

Survival Estimates for the Passage of Spring-Migrating Juvenile Salmonids through Snake and Columbia River Dams and Reservoirs, 2008.

“…From 1993 through 2007, the National Marine Fisheries Service (NMFS) estimated survival for these stocks using detections of PIT-tagged (Prentice et al 1990a) juvenile salmonids passing through Snake River dams and reservoirs Muir et al , 1996Muir et al , 2001aMuir et al ,b, 2003Smith et al , 2000aSmith et al ,b, 2003Smith et al , 2006Hockersmith et al 1999;Zabel et al , 2002Faulkner et al , 2008. In 2008, NMFS completed the sixteenth year of the study.…”

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

“…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). Similar advantages and disadvantages also likely occur for variation in fish size (e.g., Willette et al .…”

Research, Monitoring, and Evaluation of Avian Predation on Salmonid Smolts in the Lower and Mid-Columbia River, 2008 Draft Season Summary.

“…Each year, thousands of these PIT-tagged fish are consumed by colonial waterbirds and many of the ingested tags are subsequently deposited on piscivorous waterbird colonies throughout the Columbia River basin. The recovery of PIT tags on bird colonies can be used as a measure of predation rates on salmonid ESUs that are listed under the Endangered Species Act (ESA), and these data can be used to assess the relative vulnerability of various salmonid species, stocks, and rearing types to avian predators , Ryan et al 2003. Furthermore, PIT tag recovery data can be used to test hypotheses on the effects of smolt morphology, condition, abundance, and origin on vulnerability to avian predation (see Section 4).…”

“…Data collected as part this research will help regional fishery managers identify and potentially address those intrinsic and extrinsic factors that influence smolt vulnerability to avian predators. Steelhead were selected as the model species for this study because prior research has shown that they are the most vulnerable to predation by birds nesting on the Columbia River Ryan et al 2003;. The benefits of using steelhead for this study are three-fold: (1) we were likely to recover a sufficient number of PIT tags from steelhead on bird colonies along the Columbia River to address a multitude of predation-related questions (more so than any other salmonid species or run), (2) the incidence of morphological abnormalities (e.g., fungal infections, de-scaling, parasites, body injuries, etc.)…”