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

Chapter 6. Common Loon

GENERAL: The common loon (Gavia immer) is a highly regarded waterbird characteristic of relatively pristine lakes and coastal waters of Casco Bay.

SOURCES OF BIOLOGICAL AND SPATIAL DATA: Data for occurrences of the common loon were obtained from the MDIF&W GIS coverage of Coastal Wildlife Concentration Areas (CWCA). The CWCA's are polygons drawn around areas in which relatively high numbers of marine birds and seals were observed during aerial survey flights made along the Maine coast from 1979 through 1982. Survey data were combined into five "seasons"; winter, spring, nesting, post-nesting, and fall. Maine Audubon Society provided a database of loon use of Maine lakes, and additional information on habitat preferences. Additional spatial information included eelgrass locations and densities (DMR), coastal shoreline (OGIS) and bathymetry (MGS).

HABITAT CONSIDERATIONS

Breeding habitats: Although there is no documentation of common loons nesting in the lower 15 towns, the loon does breed in the Casco Bay watershed (Maine Audubon Annual Loon Census 1994). Loons breed on freshwater lakes as small as two acres in open or densely forested areas. Nest sites are commonly located on the ground near the water's edge, usually on sand, rocks, or other firm substrate. Loons prefer to nest on small islands to minimize possibility of disturbance and reduce predation by mammals (Stockwell and Jacobs 1992).

Coastal habitats: Loons are found on Casco Bay primarily during the winter season with the population reaching 500 birds (Hutchinson and Ferrero 1980). Important coastal habitats include bays, coves, channels, inlets, and other shallow areas (McIntyre 1986). Shallow inshore waters are utilized more frequently than deeper offshore waters, although some loons will use continental shelf waters up to 100 m deep and 100 km from shore.

While primarily piscivorous, loons are opportunistic and will eat any suitable prey they can see and capture (McIntyre 1986). Foods include fish (staple), amphibians, insects, aquatic plants, crustaceans, mollusks, and leeches. Winter foods include flounder (Pseudopleuronectes americanus), rock cod (Gadus morhua), menhaden (Brevoortia partronus), salmonids, sculpin (Leptocottus armatus), and crabs (Schneider and Pence 1992). Feeding typically occurs in water < 5.5 m deep (McIntyre 1986, Daub 1989) with maintenance activities (preening, drifting) usually taking place in deeper water. Common prey species of loons often are concentrated in eelgrass beds, making these important foraging sites.

MANAGEMENT CONCERNS: Loon nesting may be reduced from historic levels by lakeside development in southern Maine (Stockwell and Jacobs 1992). In Ontario, Canada, hatching success decreased as the number of cottages within 150 meters of loon nests increased (Heimberger et al. 1983). Disturbance in the form of boating activity at crucial times during the breeding/nesting season can have detrimental effects on nesting success by reducing the number of territorial pairs per lake and by exposing the nest to predation and/or cooling of the eggs.

Oil spills pose a serious threat. Loons wintering in coastal waters are subject to oiling of feathers and entanglement in fishing gear (Palmer 1962, Vermeer 1973). Detailed information on the wintering distribution and ecology of common loons is lacking (Rimmer 1992).

MAPPING OF HABITATS: Waters less than 6 meters deep, particularly over eelgrass beds, were regarded as preferred coastal foraging habitat for the common loon (Figure 8a). We did not have information on the proximity of foraging sites to development. Accordingly, we accepted MDIF&W disturbance buffers as sensitivity zones in which development activities would likely affect the value of neighboring habitats (Jones et al. 1988). We used a 30 m sensitivity zone for relatively low value foraging habitats, and a 90 m zone for moderate or high value foraging habitats. These distances also were used for identification of "impact zones", disturbed areas dominated by paved surfaces or buildings. Otherwise suitable habitats within these impact zones were reduced in score by half. Existing development was not given a habitat score.

Steps involved in mapping of seasonal habitats:

1) Select polygons from Coastal Wildlife Concentration Areas (CWCA) with loon counts > 1% of the study area population for each of the 5 seasonal surveys. The 1% criterion reduces the scope of the analysis to habitats likely to be significant from a population standpoint.

2) Select from resulting CWCA polygons areas where the depth is < -6 m; assign relative score = 4.

3) Select areas meeting conditions from step 2 and where eelgrass beds are present; assign these a relative score = 8.

4) Select all other areas in Casco Bay having eelgrass beds and depths < -6 meters; assign these a relative score = 4.

5) Identify a 30 m sensitivity zone around areas scored 4, and 90 m around areas scored 8.

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

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