Smoltification in an Impounded, Adfluvial Redband Trout Population Upstream from an Impassable Dam: Does It Persist?

Abstract

This study evaluated whether a population of adfluvial redband trout Oncorhynchus mykiss gairdnerii (a subspecies of rainbow trout) in Mann Creek and Reservoir, southwestern Idaho, continues to undergo smoltification. This population had an anadromous component before the construction of main-stem and tributary dams in 1958 and 1967, respectively. Smoltification was assessed by quantifying the proportion of juvenile migrants that displayed high skin reflectance, comparing mean gill Na⁺,K⁺-ATPase activity among migrants, and examining migration timing compared with that of neighboring steelhead (anadromous rainbow trout) populations. Approximately one-quarter of the 1,502 juvenile migrants trapped and examined displayed the intermediate or high skin reflectance (nonbanded silvery coloration) characteristic of smolts; the other fish maintained a banded coloration more typical of resident fish. For 78 samples of gill filaments over the course of the migration season (18 March to 3 June), Na⁺,K⁺-ATPase activity varied considerably among fish (0.95–5.81 μmol Pi · h⁻¹ · mg protein⁻¹) and doubled in nonbanded fish over the course of the migration period. ATPase activity was significantly higher for nonbanded fish than for banded fish at the end of the migration period (21 May – 4 June). Juvenile adfluvial redband trout migrated from Mann Creek in approximate synchrony with neighboring steelhead populations. These results suggest the possibility that in areas in the Snake River drainage where steelhead have been extirpated as a result of artificial barriers, remnant populations retain the potential for anadromy.

Received December 28, 2010; accepted May 3, 2011

ACKNOWLEDGMENTS

We thank R. Attebery for field assistance and data collection. R. D. Ewing performed the enzyme assay analysis and assisted in interpretation of Na⁺,K⁺-ATPase activity results. C. Moffitt, D. Schill, J. Congleton, S.Narum, R. Beamish, and two anonymous reviewers provided comments and suggestions for improving the manuscript. Funding for the Mann Creek redband trout life history study was provided by Idaho Department of Fish and Game, with specific funding for this additional component of the study provided by the Boise Valley Fly Fishermen and the Idaho Chapter of the American Fisheries Society through a student grant to D.E.H.