Effects of currents and tides on fine-scale use of marine bird habitats in a Southeast Alaska hotspot

Areas with high species richness have become focal points in the establishment of marine protected areas, but an understanding of the factors that support this diversity is still incomplete. In coastal areas, tidal currents—modulated by bathymetry and manifested in variable speeds—are a dominant physical feature of the environment. However, difficulties resolving tidally affected currents and depths at fine spatial-temporal scales have limited our ability to understand their influence on the distribution of marine birds. We used a hydrographic model of the water mass in Glacier Bay, Alaska, USA, to link depths and current velocities with the loca- tions of 15 common marine bird species observed during fine-scale boat-based surveys of the bay conducted during June of 4 consecutive years (2000 to 2003). Marine birds that forage on the bot- tom tended to occupy shallow habitats with slow-moving currents; mid-water foragers used habi- tats with intermediate depths and current speeds; and surface-foraging species tended to use habitats with fast-moving, deep waters. Within foraging groups there was variability among spe- cies in their use of habitats. While species obligated to foraging near bottom were constrained to use similar types of habitat, species in the mid-water foraging group were associated with a wider range of marine habitat characteristics. Species also showed varying levels of site use depending on tide stage. The dramatic variability in bottom topography—especially the presence of numer- ous sills, islands, headlands and channels—and large tidal ranges in Glacier Bay create a wide range of current-affected, fine-scale foraging habitats that may contribute to the high diversity of marine bird species found there.