FIRST-WINTER SURVIVAL OF RAINBOW-TROUT AND BROOK-TROUT IN THE HENRYS-FORK OF THE SNAKE RIVER, IDAHO

We used caged fish with a cobble-boulder substrate to test the effect of fish size on first-winter survival of rainbow trout, Oncorhynchus m ykiss, to compare survival of rainbow trout and brook trout, Salvelinu s fontinalis, and to test for a temperature effect in each experiment. At the warmer site, over 90% of the rainbow trout in the size experim ent survived the winter, and there was no significant difference betwe en >90 and <90 mm fish. At the colder site, survival was 60% and all m ortality occurred in fish <90 mm. Survival of brook trout (60%) was si gnificantly less than that of rainbow trout (87%) but did not differ b etween the warmer and colder sites. Brook trout that survived were sig nificantly larger than those that did not. Results suggest that size-d ependent mortality may be more likely to occur when environmental cond itions are more severe (e.g., lower temperatures, less suitable habita t). Rainbow trout experienced less mortality than brook trout when usi ng cobble-boulder substrate as cover during their first winter; this m ay be the result of different winter-habitat preferences between the t wo species.