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Identification of Important Habitats in the Lower Casco Bay (Maine) Watershed

Chapter 11. Seabird Habitats

GENERAL: In addition to wading birds and the roseate tern (analyzed separately), seven seabird species were common to the Gulf of Maine Council's list of species for designating Regionally Significant Habitats, and the MDIF&W Seabird Nesting Island Database (the primary data source for this theme). However, only two of the seven (common eiders, Somateria mollissima, and common terns, Sterna hirundo) nest on Casco Bay islands. MDIF&W uses seabird nesting as a basis for designation of "Significant Wildlife Habitats;" islands proposed for such designation (draft NRPA islands) were added to the coverage even if none of the constituent species were on the Gulf of Maine Council's list.

In addition to nesting habitat, this analysis identifies aquatic habitats (foraging and probably resting) for eiders and terns, and sensitivity zones in which development can be expected to degrade the adjacent habitats. These coverages are not intended to depict the limits of areas being managed or under regulatory control.

SOURCES OF BIOLOGICAL AND SPATIAL DATA : Data for all islands within or bordering Casco Bay were obtained from the MDIF&W Seabird Nesting Island Database. Foraging and resting areas were determined from MDIF&W's CWCA coverage. Additional spatial information included the coastal shoreline (OGIS), bathymetry (MGS), and shellfish and eelgrass beds (DMR).

HABITAT CONSIDERATIONS

Eider: The common eider is a large, social sea duck with circumpolar distribution (Blumton et al. 1988). It breeds on small coastal islands in the Gulf of Maine. Guillemette et al (1993) noted that common eiders feed along rocky shores, diving for blue mussels, urchins, and crabs. They prefer shallow waters over kelp beds where their prey is most abundant; in the Gulf of St. Lawrence these areas occur at depths of 0 -6 m. Goudie and Ankney (1988), however, note that apparent interspecific competition between 4 species of wintering sea ducks displaced eiders from some shallow foraging areas. They characterize eider foraging depths in the presence of other sea ducks as -3 m down to -10 m.

Tern: Common terns also nest on small coastal islands, and also on islands in some large lakes (Veit and Petersen 1993). Nesting terns often are displaced by gulls from the most favorable insular sites. Terns feed on small fishes, often over tide rips (typically where current flows over shallow bars), in relatively sheltered areas. Prey items include young herring (Clupea), mackerel, bluefish, sand lance, and anchovy (Heinemann 1992).

MANAGEMENT CONCERNS

Human disturbance can be a problem for either species, reducing availability of foraging areas or driving birds from nests and thus increasing exposure of young and eggs to predators. Therefore, we mapped zones within which development would likely degrade value of the adjacent habitat.

Nesting: We used the literature and available data sets to assign sensitivity distances for individually for terns and eider nesting islands; we did not assign a sensitivity distance specifically for draft NRPA islands. Erwin (1989) found that nesting common terns and black skimmers flushed when approached within 200 - 400 m. He recommended that a 200 m buffer be established for human intrusion near colonies. While a 200 m zone may be sufficient as a human approach- distance for common terns, development is likely to prolong and increase the types of disturbance. We used GIS to examine distances between developed lands and seabird colonies along the New Hampshire, Massachusetts, and Maine coast. We found the closest that common terns nested to an industrial site was 260 m, while the minimum distance between residential land uses and nesting birds was 480 m (45 colonies examined). We selected a 300 m sensitivity zone for sites on which common terns nested. None of the common tern nesting sites in Casco Bay occur this close to development.

Blumton et al., (1988) developed a nesting habitat model for the eider which included sensitivity to human disturbance as a factor. Their optimal distance from permanent human settlement was 2 km or more. The minimum distances observed between eider colonies and development in Casco Bay are 580 m (Crow Island-Great Diamond), 410 m (Pinkham Island-South Harpswell), and 490 m (Seal Island-Small Point). Because the minimum distances still may be relatively stressful, we rounded the minimum up to 500 m for sensitivity zones around nesting islands for eiders.

Feeding: available data did not allow us to examine proximity of foraging sites and development. One can argue that birds can more readily abandon foraging areas than nest sites, since the former involve less of an "investment". We generally accepted the MDIF&W disturbance distances (Jones et al. 1988), using a 30 m sensitivity zone for relatively low value foraging habitats, and a 90 m zone for moderate or high value foraging habitats.

MAPPING OF HABITATS

The steps in mapping habitats were:

EIDER FORAGING:

1) Select polygons from the MDIF&W CWCA's hosting 1% or more of the total eider count for the study area, for any of the survey intervals. The 1% criterion reduces the scope of the analysis to habitats likely to be significant from a population standpoint.

2) Select areas within these polygons that are <= 10 m deep. Assign these a relative score of 4.

3) Select areas having mussel beds, eelgrass beds, or other submerged vegetation within the polygons from step (2). Assign these a relative score of 8.

4) Select areas outside the CWCA's that are <= 10 m and have mussel beds, eelgrass beds, or other submerged vegetation. Assign these a relative score of 4.

EIDER NESTING:

1) Identify islands with 1% or more of the Casco Bay total eider nest count. Assign these a relative score of 8.

COMMON TERN FORAGING:

1) Select polygons from the CWCA coverage hosting 1% or more of the study area total tern count, for any of the 3 seasons during which they occurred there. It was assumed that the unidentified terns counted in this coverage were predominantly common terns. Assign these polygons a relative score of 4.

COMMON TERN NESTING:

1) Identify islands with 1% or more of the Casco Bay total tern nest count. Assign these a relative score of 8.

OTHER SEABIRD NESTING ISLANDS:

Assign draft NRPA islands a score of 8.

ADJUST FOR IMPACTS FROM EXISTING DEVELOPMENT:

Reduce the foraging habitat values by half if within impact zone around existing development: zones are 30 m for habitats scored 4, 90 m for areas scored 8. Areas which are currently developed were scored 0.

The combined habitats for eiders and terns are shown in Figure 11.

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