Long-term enhancement of habitat for salmonids in acidified running waters

Application of crushed limestone on the bed of an acidic brook enhanced Atlantic salmon (Salmo salar) spawning habitat, and the mitigation strategy resulted in improvements in the survival of juvenile Atlantic salmon and brook trout (Salvelinus fontinalis). There was a small, lasting increase in pH in the limed section of the brook and downstream over 8 yr. Limestone was replenished once after 4 yr because of washouts during floods; otherwise, requirements for maintenance were low. After liming, Atlantic salmon consistently dug most redds in the limed section, but this preference was possibly related to physical enhancement of spawning areas rather than to high pH. Liming resulted in a twofold increase in the number of occasions where more than a few juvenile Atlantic salmon survived severe acidic episodes in the brook. Brook trout were tolerant of lower pH episodes than juvenile salmon but, like salmon, they also responded to changes in minimum pH as a result of liming. Densities of age-0 salmon and brook trout were related to seasonal and interannual variability in pH, but they were always greater in the limed section than in the unlimed, control section upstream, especially at times when the difference in pH between sections was the largest.