Population Dynamics of Double-Crested Cormorants in Two Interior Breeding Areas

Because of rapid population expansion, conflicts between double-crested cormorants (Phalacrocorax auritus) and humans have increased, particularly at aquaculture facilities in the Lower Mississippi River Valley (LMRV), United States. Although key demographic information on the Interior population is sparse, management decisions for population reduction are already in place across their breeding range. New evidence suggests the Appalachian Mountains may act as a migration divide for Interior cormorants, causing some birds to migrate between LMRV and the western Great Lakes, whereas others migrate between Florida and the eastern Great Lakes. Most of the banded cormorants recovered near LMRV aquaculture facilities were banded as nestlings at colonies from the central Great Lakes west to the northern Great Plains. We selected 2 geographically distinct Interior cormorant breeding areas: west of the Great Lakes in Lake of the Woods (LOW) and eastern Lake Ontario (ELO), located on either side of the migration divide in the province of Ontario, Canada, to study population dynamics. We used age-specific classes for our mark-recapture analysis (Program MARK) on data collected from 9,498 color-banded cormorants during the breeding seasons of 2000–2008. Adult survival was the same for both areas (LOW¼0.840.09 SE, ELO¼0.830.05 SE). Finite rates of population growth (l) were also similar (LOW¼1.01, ELO¼0.97) and sensitivity analyses indicated that adult survival was the most influential vital rate contributing to l for both breeding areas. However, young-of-the-year survival estimates were distinctly less in ELO (0.190.02 SE) than in LOW (0.450.10 SE) and sensitivity analysis revealed that young-of-the-year survival played a key role in ELO population growth. Moreover, ELO breeding area fidelity (F) was much greater (0.940.05 SE) than the F estimates for all age classes in LOW (between 0.680.19 SE and 0.800.13 SE). Considering these regional demographic differences, we recommend that future management efforts be based on migratory flyways within the Interior population