Relationship of External Fish Condition to Pathogen Prevalence and Out-Migration Survival in Juvenile Steelhead

Abstract

Understanding how the external condition of juvenile salmonids is associated with internal measures of health and subsequent out-migration survival can be valuable for population monitoring programs. This study investigated the use of a rapid, nonlethal, external examination to assess the condition of run-of-the-river juvenile steelhead Oncorhynchus mykiss migrating from the Snake River to the Pacific Ocean. We compared the external condition (e.g., body injuries, descaling, external signs of disease, fin damage, and ectoparasite infestations) with (1) the internal condition of a steelhead as measured by the presence of selected pathogens detected by histopathology and polymerase chain reaction analysis and (2) out-migration survival through the Snake and Columbia rivers as determined by passive integrated transponder (PIT) tag technology. The results from steelhead captured and euthanized (n = 222) at Lower Monumental Dam on the lower Snake River in 2008 indicated that external condition was significantly correlated with selected measures of internal condition. The odds of testing positive for a pathogen were 39.2, 24.3, and 5.6 times greater for steelhead with severe or moderate external signs of disease or more than 20% descaling, respectively. Capture–recapture models of 22,451 PIT-tagged steelhead released at Lower Monumental Dam in 2007–2009 indicated that external condition was significantly correlated with juvenile survival. The odds of out-migration survival for steelhead with moderate or severe external signs of disease, more than 20% descaling, or severe fin damage were 5.7, 4.9, 1.6, and 1.3 times lower, respectively, than those for steelhead without these external conditions. This study effectively demonstrated that specific measures of external condition were associated with both the internal condition and out-migration survival of juvenile steelhead.

Received August 18, 2010; accepted February 28, 2011

ACKNOWLEDGMENTS

The U.S. Army Corps of Engineers (USACE) Walla Walla District (Project Manager: Scott Dunmire) provided funding for this research and access to the Lower Monumental Dam juvenile collection facility. M. Carper, B. Cramer, J. Tennyson, and numerous technicians provided invaluable assistance in the field. We thank M. Plummer and B. Spurgeon (USACE) who provided access and logistical support at the juvenile fish facilities, Pacific States Marine Fisheries Commission for access to the PIT Tag Information System (PTAGIS), and NOAA fisheries for operation of the PIT tag trawl and corresponding data. J. Adkins, D. Battaglia, K. Fone, S. Lind, P. Loschl, and M. Price, provided services and logistical support for which we are grateful. D. Noakes, C. Periera, and three anonymous reviewers provided helpful comments that improved earlier drafts of the manuscript.