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Factors Influencing Predation on Juvenile Salmonids by Double-crested Cormorants in the Columbia River Estuary : A Retrospective Analysis
| Content Provider | Semantic Scholar |
|---|---|
| Author | Lyons, Donald E. Evans, Allen F. Hostetter, Nathan J. Piggott, A. Weitkamp, Laurie A. Good, Thomas P. Roby, Daniel D. Collis, Ken Loschl, Peter J. Cramer, Becky |
| Copyright Year | 2014 |
| Abstract | Enhancing the survival of juvenile salmonid (Oncorhynchus spp.) is a priority objective to recover populations of Columbia River salmonids listed under the U.S. Endangered Species Act (ESA). In the Columbia River estuary, a significant mortality factor for juvenile salmonids is predation by doublecrested cormorants (Phalacrocorax auritus) nesting at East Sand Island. The U.S. Army Corps of Engineers is considering management alternatives to reduce this mortality. Understanding the factors that influence cormorant predation is important to understanding the potential consequences of various management strategies. We used principal components regression (PCR) to evaluate the relationship between several annual measures of cormorant predation and a combination of colony size and environmental covariates. The environmental factors considered included large-scale climate indices (Pacific Decadal Oscillation, El Nino/Southern Oscillation Index, North Pacific Gyre Oscillation, Pacific Northwest Index), regional climate measures (sea surface temperature, upwelling strength, upwelling timing), and variables describing conditions during freshwater and estuarine outmigration (river discharge, spill at hydroelectric dams, measures of salmonid smolt survival to the estuary). These covariates potentially influenced both the susceptibility of salmonids to cormorant predation and the abundance and distribution of marine forage fish and their availability as alternative prey for cormorants nesting in the estuary. Measures of cormorant predation spanned a 15-year period (1999 – 2013) and included (1) predation probabilities for multiple steelhead (O. mykiss) and Chinook salmon (O. tshawytscha) populations derived from recoveries of salmonid passive integrated transponder (PIT) tags at the cormorant colony, (2) estimates of annual consumption of steelhead and yearling Chinook by cormorants derived using bioenergetics modelling, and (3) the observed percentage of the cormorant diet that consisted of salmonids. We also related cormorant diet composition to purse seine catches in the estuary during 2007 – 2012 to assess how predation on salmonids is related to availability of alternative, non-salmonid prey and to examine cormorant selectivity of salmonids relative to other available prey. PCR analyses indicated that environmental factors explain a substantial proportion of the annual variability seen in several measures of cormorant predation on Columbia River juvenile salmonids. Cormorant colony size was an important explanatory factor in most regressions; however, it never explained more than 17% of the variability in any annual measure of cormorant predation on salmonids. In aggregate, environmental factors explained a greater proportion of the annual variability in cormorant predation than did colony size; in particular, river discharge and the North Pacific Gyre Oscillation (NPGO) were prominent environmental explanatory factors. Based on comparisons to estuary purse seine catches, cormorants appeared to take salmonids in proportion to their relative availability in the Columbia River estuary, not their absolute abundance. Conversely, changes in absolute abundance of alternative prey, both marine and freshwater/estuarine forage fishes, did influence how much cormorants relied on salmonids as prey. While colony size is an important determinant of cormorant impacts on salmonid populations, environmental conditions that regulate the availability of alternative prey might outweigh the effects of changing colony size in any given year. Potential management efforts to reduce the size of the double-crested cormorant colony on East Sand Island to benefit ESA-listed salmonids would best be evaluated in the context of environmental conditions, particularly if evaluation occurs on an annual basis, with special attention given to river discharge and the NPGO. Multiyear data sets following any implementation of management would likely be more useful to evaluate potential benefits. |
| File Format | PDF HTM / HTML |
| Alternate Webpage(s) | http://www.birdresearchnw.org/ESI%20DCCO%20Revised%20Retrospective%20Analysis%202014%2011%2007.pdf |
| Language | English |
| Access Restriction | Open |
| Content Type | Text |
| Resource Type | Article |
