Evidence for Parr Growth as a Factor Affecting Parr-to-Smolt Survival

Abstract

Data collected on juvenile anadromous salmonids implanted with passive integrated transponder (PIT) tags are used in mark–recapture analyses to understand the factors affecting survival of fish estimated between rearing in riverine habitat and dam passage. We estimated parr-to-smolt survival of PIT-tagged naturally produced subyearling fall Chinook salmon Oncorhynchus tshawytscha to examine the previously unexplored influences of environmental and biological conditions measured prior to reservoir entry. Mean (±SE) parr-to-smolt survival of the early migrating cohorts was 45.4 ± 6.3% (n = 13) compared with 37.4 ± 4.7% (n = 13) for later migrating cohorts. Annual mean parr-to-smolt survival differed widely across years ranging from a low of 9.6 ± 0.5% (n = 2) in 2001 to a high of 81.7 ± 4.6% (n = 2) in 1999. Parr growth prior to reservoir entry and reservoir velocity provided the most information on variability in parr-to-smolt survival (N = 26, R ² = 0.75, corrected Akaike's information criterion [AIC _c ] = −5.01). We suggest that parr growth and reservoir velocity were directly proportional to parr-to-smolt survival because fast growth and downstream movement reduces the time when fish are vulnerable to predators. The effect of reservoir velocity comports with previous published studies and supports management efforts to increase reservoir velocity. Few if any published studies explicitly relate parr growth measured on individual fish to survival estimated for their cohorts in freshwater. This study provides empirical evidence that upholds the long-held belief that any anthropogenic activity that reduces growth of juvenile salmonids during freshwater rearing has the potential to reduce their survival.

Received June 21, 2011; accepted April 5, 2012

ACKNOWLEDGMENTS

We thank our colleagues at the U.S. Fish and Wildlife Service's Idaho Fisheries Resource Office whose efforts contributed to the success of this study. This study (and many other studies we have conducted) would not have been possible without personnel of the Pacific States Marine Fisheries Commission, including D. Marvin (through 2010) and N. Tancreto (2011), who helped operate and maintain the Columbia Basin PIT-Tag Information System. The statistical review by K. Steinhorst and peer-review by G. McMichael, B. Muir, and the journal's associate editor and editor improved this manuscript. This study was funded by the Bonneville Power Administration and administered by D. Docherty and J. George under project number 199102900 to meet reasonable and prudent alternatives (2004 Biological Opinion; 2008 Biological Opinion, 55.4, 50.3, 65.2). Any use of trade, firm, or product names is for descriptive purposes only does not imply endorsement by the U.S. Government. The findings and conclusions in this article are those of the authors and do not necessarily represent the views of the U.S. Fish and Wildlife Service.