First-winter survival of rainbow trout and brook trout in the Henrys Fork of the Snake River, Idaho

Abstract

We used caged fish with a cobble – boulder substrate to test the effect of fish size on first-winter survival of rainbow trout, Oncorhynchus mykiss, to compare survival of rainbow trout and brook trout, Salvelinus fontinalis, and to test for a temperature effect in each experiment. At the warmer site, over 90% of the rainbow trout in the size experiment survived the winter, and there was no significant difference between >90 and <90 mm fish. At the colder site, survival was 60% and all mortality occurred in fish <90 mm. Survival of brook trout (60%) was significantly less than that of rainbow trout (87%) but did not differ between the warmer and colder sites. Brook trout that survived were significantly larger than those that did not. Results suggest that size-dependent mortality may be more likely to occur when environmental conditions are more severe (e.g., lower temperatures, less suitable habitat). Rainbow trout experienced less mortality than brook trout when using cobble – boulder substrate as cover during their first winter; this may be the result of different winter-habitat preferences between the two species.