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

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 mu mol 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.