Susceptibility of juvenile salmonids to avian predation : Are Caspian terns and double-crested cormorants only taking the sick and injured? Public Deposited

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  • The ability to non-destructively assess fish condition and subsequently track fish behavior and survival can be vital in understanding natural and anthropogenic stressors and sources of mortality, especially in populations of fish listed as threatened or endangered. I investigated the use of a quick, non-lethal, external examination technique to assess general health status of run-of-the-river juvenile steelhead (Oncorhynchus mykiss) migrating from the Snake River basin. The relationship between external symptoms of captured fish (including body injuries, de-scaling, external symptoms of disease, fin damage, and ectoparasite infestations) and their health status was investigated through pathology analysis of a euthanized subsample of fish. I assessed the relative fitness of steelhead smolts assigned to various categories of health status by measuring the subsequent survival and susceptibility to avian predation of fish that were tagged with passive integrated transponder (PIT) tags and released unharmed. Histological analysis was conducted on 222 steelhead sacrificed at the juvenile collection facility of Lower Monumental Dam (LMN) on the lower Snake River during steelhead out-migration in 2008; results indicated that external symptoms of disease, body injuries, de-scaling, and fin damage were all associated with increased prevalence of infectious pathogens. Capture-recapture models for 14,177 steelhead PIT-tagged and released at LMN in 2007 and 2008 indicated that the presence of external symptoms of disease, body injuries, or de-scaling, but not fin damage, were associated with significantly reduced survival probabilities during either short-distance (ca. 119 Rkm) or long-distance (ca. 354 Rkm) migration. PIT tags from 3.0% and 1.9% of all released steelhead (n = 16,258) were recovered on a Caspian tern (Hydroprogne caspia) colony and a double-crested cormorant (Phalacrocorax auritus) colony, respectively, located < 79 Rkm downstream of release locations. A consistent factor relating susceptibility of steelhead to predation by both bird species was compromised health, particularly steelhead with external symptoms of disease and body injuries. River conditions, including decreased discharge and elevated water temperatures at the time of release, were also associated with increased susceptibility of steelhead to avian predation, but the strength of these explanatory factors differed between species of avian predator. These results support the conclusion that external condition, measured non-destructively on individual fish, is associated with quantifiable differences in general health status and fitness of fish. Results also indicate that some reduction in avian predation on juvenile salmonids (Oncorhynchus spp.) may be accomplished by modifying hydrosystem operations. The higher susceptibility of unhealthy steelhead to avian predation suggests that a portion of any reduction in mortality due to avian predation will be compensated for by other sources of smolt mortality. The portion of juvenile steelhead that are at greater risk of avian predation is small, however, based on the small proportion of steelhead that exhibited visible symptoms of compromised health.
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