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

Chapter 7. Black Duck Habitats

GENERAL: Waterfowl are important in Casco Bay from recreational (hunting, viewing) and ecological perspectives. One of the species on the Gulf of Maine Council's Species List for Identifying Regionally Significant Habitats, the American black duck (Anas rubripes), is of special interest, because of an historical decline in population.

This analysis attempts to identify both marine (winter foraging) and freshwater (foraging, nesting, brood-rearing and post-fledging) habitats for black ducks, and sensitivity zones in which development may degrade the adjacent habitats. These coverages are not intended to depict the limits of areas being managed or those areas already under regulatory control.

SOURCES OF BIOLOGICAL AND SPATIAL DATA: Biological data included the CWCA's (MDIF&W), eelgrass, shellfish beds and marine worm harvest areas (DMR), and wetlands (NWI). MDIF&W identified Moderate and High Value Wetlands for waterfowl from surveys made in 1974; we digitized these wetlands by identifying the corresponding NWI polygons, transferring them to a coverage, and assigning them the appropriate MDIF&W scores. Landcover was developed as part of this study (Chapter 3). Additional spatial information included the coastal shoreline (OGIS) and bathymetry (MGS).

HABITAT CONSIDERATIONS

Wintering: Black duck wintering populations in the Atlantic Flyway are concentrated in marine, estuarine, and riverine wetlands extending from the Canadian Maritimes through South Carolina. Rocky shoreline and large tidal amplitudes are typical of marine wintering habitat northeast of Cape Elizabeth, Maine. Black ducks loaf and feed on the southeast side of islands and peninsulas, where there is maximum sunlight and protection from wind (Longcore and Gibbs 1988). Black ducks also frequent ice-free salt marshes, small tidal bays, and open waters of dynamic ecosystems such as rivers and tidal inlets. Since black ducks are "dabblers", food sources must be near the surface or just buried in the substrate (Lewis and Garrison 1984).

Black ducks wintering in coastal habitats feed mostly on invertebrates living in rockweed or in shellfish beds on tidal flats (Jorde and Owen 1989). Diets differ due to habitat diversity, nutritional value of foods, and different foraging patterns.

Other seasons: Freshwater wetlands are used during the reproductive period (courtship through post-fledging) and for general foraging or cover. Black ducks generally prefer palustrine emergent wetlands for several functions (Frazer 1988), but habitat selection also depends on season. In the fall, palustrine scrub-shrub and forested wetlands receive more use. Riverine wetlands become more important as ice forms on lentic habitats. Less used, but still having functional value, are lacustrine and estuarine wetlands.

Black duck preferences for nesting habitat vary widely. They may nest in upland areas near an ephemeral pool or other wetland, or up to 1.5 km from a water source (Jerry Longcore, FWS, pers. com.). Due to this variability no attempt was made to map nesting habitats; it is assumed that some nesting habitat will fall within areas mapped for other functions, or within the sensitivity zones for those habitats (see below).

Brood-rearing ducks consistently select habitats that will meet the energy requirements of their growing young. Appropriate wetlands include those with active beaver colonies, impoundment ponds, and open water within palustrine emergent wetlands. Black ducks apparently prefer small wetland areas for pair-bonding (USFWS 1988). They also utilize small (< 0.5 ha) ephemeral pools and small permanent ponds intensively during the nesting season, as well as for stop-over points during overland movements with broods (Ringelman and Longcore 1982).

MAPPING OF HABITATS

We used the above understanding of the requirements or preferences of black ducks to characterize the habitat values of potential wintering, foraging, brood-rearing, and post-fledging areas for black ducks. NWI wetland classes and other environmental themes were used to assign relative scores for each of these life stages. Wintering areas were further defined from CWCA polygons in which the population exceeded 1% of the seasonal total count. The 1% criterion reduced the scope of the analysis to habitats likely to be significant from a population standpoint. Inland habitats which were within MHVW polygons were regarded as more likely to be suitable, and so were given a higher score. Habitat characterization and the resulting scores or values are shown below. These scores were used when combining coverages for various species.

A protective or "sensitivity zone" was identified around black duck habitats to indicate areas in which development and associated domestic activities would be expected to degrade those designated habitats. Zone widths (30 to 90 m) were based on MDIF&W wetland buffers (Jones et al. 1988). The final black duck coverage was adjusted to account for the effects from existing development in the watershed. Habitats within the sensitivity zone distances of existing development were reduced to one-half the score of pristine habitats (Figure 8b).

The steps in mapping habitats were:

WINTERING COMPONENT:

1) select those CWCA polygons hosting => 1% of the Casco Bay total black duck count.

2) select areas within these polygons that are <= 1 m deep. Assign these a relative score of 4. This depth includes foraging areas accessible to this species.

3) select areas resulting from step (2) which also have eelgrass, shellfish, or are intertidal. Assign these a relative score of 8.

4) select areas outside the CWCA's, but which have eelgrass or shellfish and are <= 1 m deep, or are intertidal. Assign these a score of 4. While black ducks were not observed in these areas during the MDIF&W surveys, they were scored for the presence of suitable food resources.

BROODREARING AND POST-FLEDGING COMPONENTS:

1) Select NWI polygons designated palustrine emergent. Where these overlap a MHVW rated 2 or 3 assign these a score of 8, otherwise score as 6.

2) Select NWI polygons designated palustrine forested, scrub shrub, or aquatic bed. Assign these a score of 4.

INLAND FORAGING:

1) Select NWI polygons designated palustrine emergent, forested, scrub shrub, or aquatic bed. Where these overlap a MHVW rated 2 or 3 assign these a score of 8, otherwise score as 4.

2) Select NWI polygons designated riverine emergent or aquatic bed, or lacustrine emergent or aquatic bed. Assign these a score of 4.

COMBINATION OF HABITAT SCORES:

1) Identify sensitivity zones for the above; use a distance of 30 m for areas scored 4 and 90 m for areas scored 8. Reduce scores for habitat within this distance of existing development to one-half the score of pristine habitats of that type. Areas which are currently developed are scored 0.

2) Scores for the above habitat components were combined so that the output at each locality was the maximum of the scores for these functions.

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