Effect of ice formation on selection of habitats and winter distribution of post-young-of-the-year Atlantic salmon parr

We determined how ice affects selection of habitats and distribution of pos t-young-of-the-year Atlantic salmon (Salmo salar) parr during winter. Night snorkeling surveys were completed between November and April to evaluate p arr habitat use and movements. Systematic measurements of water depth and v elocity were recorded during ice-free and less than or equal to 55% iced co nditions to quantify habitat availability. Ice formation altered the distri bution and reduced the abundance of habitats commonly used by parr; differe nces between parr habitat use and habitat availability were greatest when i ce was present. Edge ice formation resulted in the concentration of flows, and areas of high flow were formed in midchannel; few parr were observed in midchannel after ice had formed. Through the winter, most parr were found lateral to high flows on the ice edge boundary or in the post-ice period la teral to the stream midchannel. The correspondence of parr movements during winter to changes in the physical habitat associated with ice formation in dicates that movements and redistributions may be important for survival in streams affected by ice.