Effects of increasing winter rearing habitat on abundance of salmonids in two coastal Oregon streams

We used a BACI (before-after-control-impact) experimental design to examine the effects of increasing winter habitat on the abundance of downstream mi grant salmonids. Two reference streams and two treatment streams were selec ted in the Alsea and Nestucca basins of Oregon. Population parameters for j uvenile coho salmon (Oncorhynchus kisutch), age-0 trout (Oncorhynchus spp.) , steelhead (Oncorhynchus mykiss), and coastal cutthroat trout (Oncorhynchu s clarki) were estimated each year for 8 years in each stream. Stream habit at was modified to increase the quality and quantity of winter habitat duri ng the summers of 1990 (Nestucca Basin) and 1991 (Alsea Basin). Complex hab itat was constructed by adding large woody debris to newly created alcoves and dammed pools. Numbers of coho salmon summer juveniles and smolts increa sed in the treatment streams relative to the control streams during the pos ttreatment period. Overwinter survival of juvenile coho salmon also increas ed significantly in both treatment streams posttreatment. Summer trout popu lations in the treatment streams did not change, but downstream migrant num bers the following spring did increase. These increases suggest that winter habitat was limiting abundance of all three species.