Abstract
AbstractWe evaluated the impact of predation on juvenile steelhead Oncorhynchus mykiss and yearling and subyearling Chinook Salmon O. tshawytscha by piscivorous waterbirds from 11 different breeding colonies in the Columbia River basin during 2012 and 2014. Fish were tagged with both acoustic tags and PIT tags and were tracked via a network of hydrophone arrays to estimate total smolt mortality (1 – survival) at various spatial and temporal scales during out‐migration. Recoveries of PIT tags on bird colonies, coupled with the last known detections of live fish passing hydrophone arrays, were used to estimate the impact of avian predation relative to total smolt mortality. Results indicated that avian predation was a substantial source of steelhead mortality, with predation probability (proportion of available fish consumed by birds) ranging from 0.06 to 0.28 for fish traveling through the lower Snake River and the lower and middle Columbia River. Predation probability estimates ranged from 0.03 to 0.09 for available tagged yearling Chinook Salmon and from 0.01 to 0.05 for subyearlings. Smolt predation by gulls Larus spp. was concentrated near hydroelectric dams, while predation by Caspian terns Hydroprogne caspia was concentrated within reservoirs. No concentrated areas of predation were identified for double‐crested cormorants Phalacrocorax auritus or American white pelicans Pelecanus erythrorhynchos. Comparisons of total smolt mortality relative to mortality from colonial waterbirds indicated that avian predation was one of the greatest sources of mortality for steelhead and yearling Chinook Salmon during out‐migration. In contrast, avian predation on subyearling Chinook Salmon was generally low and constituted a minor component of total mortality. Our results demonstrate that acoustic and PIT tag technologies can be combined to quantify where and when smolt mortality occurs and the fraction of mortality that is due to colonial waterbird predation relative to non‐avian mortality sources.Received November 4, 2015; accepted February 1, 2016 Published online June 27, 2016
Highlights
Tagging studies are commonly used to quantify survival rates in fish species of conservation concern
As part of Juvenile Salmonid Acoustic Telemetry System (JSATS) survival studies conducted in the Columbia River basin during 2012 and 2014, researchers tagged smolts with both Acoustic telemetry (AT) tags and PIT tags, thereby providing an opportunity to determine the proportion of total fish mortality (1 – survival) that could be attributed to predation by colonial waterbirds via tag recoveries from bird colonies
Bird Colony Location and Size The number and locations of avian breeding colonies included in the study varied by year, with a total of six colonies scanned for tags after the 2012 nesting season and 11 colonies scanned for tags after the 2014 nesting season (Figure 1; Table 2)
Summary
Tagging studies are commonly used to quantify survival rates in fish species of conservation concern. Previous studies of avian predation on smolts have relied on PIT tag recoveries (detections) at bird colonies to estimate impacts on the survival of juvenile salmonids from the Columbia River basin (Collis et al 2001; Ryan et al 2003; Antolos et al 2005; Evans et al 2012; Sebring et al 2013; Hostetter et al 2015). As part of JSATS survival studies conducted in the Columbia River basin during 2012 and 2014, researchers tagged smolts with both AT tags and PIT tags (double-tagging), thereby providing an opportunity to determine the proportion of total fish mortality (1 – survival) that could be attributed to predation by colonial waterbirds via tag recoveries from bird colonies. Results were used to identify where and when smolt losses occurred during the out-migration period and the cause of smolt mortality (colonial waterbird predation or unaccounted-for mortality)
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