RETENTION OF ADAPTIVE RHEOTACTIC BEHAVIOR BY F1 FLUVIAL ARCTIC GRAYLING

Downstream movements of age-0 Arctic grayling Thymallus arcticus from an indigenous fluvial population (Big Hole River) and two inlet-spawni ng, lacustrine populations (Upper Red Rock Lake and Ennis Reservoir) w ere compared in a natural stream. All fish were incubated and reared t ogether in a hatchery and acclimated together in the stream before bei ng released in the stream. All fluvial test fish were F-1 progeny of p arents originating as gametes from wild fish but reared in a natural l ake (1993 trials) or in a hatchery (1994 trials). The F-1 fish in four 1993 trials were progeny of parents whose rheotaxis had also been tes ted as age-0 young. In all four trials of fish with 7-14 d stream accl imation, significantly fewer fluvial than lacustrine fish (P < 0.005) were recovered within 1 d in nets at a weir 1 km downstream from the r elease site. In two trials of fish with 1 d acclimation, significantly fewer fluvial than lacustrine fish were also recovered in one trial ( P ( 0.005) but not in the other (P > 0.500). Twice each year, 10-25 d after trials were concluded, the stream reach in which the test fish h ad been released was electrofished. Significantly more fluvial than la custrine fish were captured in the stream in each of the four electrof ishing surveys (P < 0.005). The results support earlier evidence from laboratory trials that fluvial Arctic grayling have innately stronger positive rheotaxis than those from lacustrine populations. Such eviden ce of fluvial-adaptive behavior in Arctic grayling of the Big Hole Riv er reinforces the importance of conserving this small, fluvial populat ion, which is the only known one remaining for the species south of Ca nada and Alaska.