THE DOUBLE-CRESTED CORMORANT IN EASTERN AND CENTRAL NORTH AMERICA: A SUMMARY OF STATUS AND SELECTED RESEARCH

L.R. Wires¹, F.J. Cuthbert¹, D.V. Weseloh² and D.R. Trexel¹

1 University of Minnesota, Dept. Fisheries and Wildlife,
200 Hodson Hall, 1980 Folwell Ave., St. Paul, MN 55108-6124, USA

2 Canadian Wildlife Service-Ontario Region,
4905 Dufferin St., Downsview, Ontario M3H 5T4, Canada

Introduction: Since the late-1970s, numbers of Double-crested Cormorants, Phalacrocorax auritus (DCCO), have increased significantly in many regions of North America. A variety of problems, both real and perceived, are associated with these increases (e.g., impacts to aquaculture, sport and commercial fisheries, biodiversity). The U.S. Fish and Wildlife Service (USFWS) is preparing an Environmental Impact Statement (EIS), and in conjunction with the U.S. Department of Agriculture/Wildlife Services (USDA/WS) and state resource management agencies, will develop a national management plan for the DCCO. A status assessment of the DCCO in eastern and central North America (Wires et al. 2000) is being finalized, and an assessment of the species in western North America is currently underway.

Populations and trends: In the eastern and central portions of the continent, 3 breeding populations have been defined (Canadian and U.S. interior, Northeast Atlantic Coast and Southern U.S.) (Hatch and Weseloh 1999; Wires et al. 2000). Numbers of DCCOs have increased in all 3 breeding populations since at least 1980. Much of the growth occurred between the late 1970s-early 1990s; from the early 1990s-2000 growth rates have slowed or appeared to stabilize in many states and provinces. Comprehensive regional population data are available for the Great Lakes and the New England states, and are presented in Figure 1. On the wintering grounds, large increases have also occurred, especially in the Mississippi River Delta region. Review of historical records (Wires et al. 2000) suggests DCCOs may have been more abundant and widespread than currently presumed; very large numbers of cormorants were present in certain regions during European settlement (1600-1800s). In some areas where the DCCO has been documented as a recent breeder, the species may actually be re-colonizing after an absence of 50-300 years.

Reasons for population increases: There appear to be 5 major factors that led to dramatic increases in DCCOs in North America since about 1970: the ban of DDT and the regulation of other pesticides in the 1970s; addition of the DCCO to the Migratory Bird Treaty Act in 1972; human induced changes in fish communities in the breeding range; development of aquaculture in the southern U.S. (especially catfish farms in the Mississippi Delta region) that provided a new food source; and creation of additional breeding habitat (Hatch and Weseloh 1999).

 Diet and native fish populations: DCCO diet is characterized by great temporal and spatial variation. Trapp et al. (2000) and Wires et al. (2000) reviewed >40 DCCO diet studies. These reviews indicated most sport and commercially valuable fish species do not contribute substantially to DCCO diet and/or are not negatively impacted by DCCOs, though there are exceptions. Most studies concluded DCCOs have minor effects on fish populations relative to sport and commercial fishing and other mortality factors. With the exception of one ongoing study in aquaculture ponds (see next section), no studies have been undertaken to explore whether fish mortality from DCCOs is compensatory or additive. While DCCOs may cause fish populations to decline, no study reviewed convincingly demonstrated DCCOs negatively impact sport or commercial fisheries.

Diet and aquaculture facilities: Studies show DCCOs may eat large numbers of catfish locally and temporally. J. Glahn, USDA/WS, has initiated studies to quantify economic impacts on net harvest and is examining the issue of additive and compensatory mortality at aquaculture ponds (J. Glahn, unpubl. data).

Impacts to biodiversity: At roosts and breeding colonies, cormorants may impact vegetation through deposition of guano and stripping of leaves and small branches while building nests. These changes are of greatest concern when they affect rare plant communities or private property. However, impacts to vegetation have not been systematically investigated. Impacts to other colonial waterbirds may occur through direct competition for nests and nest sites, but such impacts have not been documented through careful study. Most impacts to other avian species appear to occur indirectly through habitat degradation by DCCOs. While there is some evidence that DCCOs may displace other species (e.g. Moore et al 1995; Jarvie et al. 2000), no studies have clearly established DCCO impact on other birds at even a colony level scale.

Management options: Currently in the U.S. all lethal take requires permission from the USFWS. Depredation permits can be obtained by individuals to prevent economic impacts or impacts to endangered, threatened or species of conservation concern. In 13 states where DCCO predation at aquaculture facilities is perceived to be the highest, the USFWS issued a Depredation Order in 1998 that negates the need for obtaining individual permits. No permits are required for non-lethal harassment of birds depredating or about to depredate. To reduce cormorant impacts primarily to fisheries, aquaculture, vegetation and other colonial waterbirds, a large number of techniques has been developed or proposed. These techniques utilize both lethal and non-lethal measures and may be used at local, regional or population levels. Effectiveness of these measures is difficult to assess because in many cases impacts have been poorly quantified.

Population Models: Models have identified data gaps critical for understanding population dynamics and predicting control effectiveness. Prediction of future DCCO population trends and analysis of control methods is hampered by lack of age-specific data for this species. Until better data are available, modeling efforts should include rigorous sensitivity analyses to investigate uncertainties in parameters used and assumptions made in the model.

Current research and monitoring efforts: In eastern and central North America, research is currently being conducted on cormorant diet, bioenergetics, impacts to aquaculture, sport and commercial fisheries, foraging range and foraging behavior. Additional studies are attempting to determine effectiveness of harassment at day and night roosts, and nutrient enrichment in aquatic and terrestrial habitats. A satellite telemetry study is underway to determine migration patterns, breeding locations and winter movements of cormorants at catfish farms.

In the U.S. and Canadian Great Lakes, where large numbers occur, much significant research has recently been initiated:

1) The USDA-National Wildlife Research Center, with field support from the New York State Department of Environmental Conservation (NYSDEC), the Canadian Wildlife Service (CWS) and the Ontario Ministry of Natural Resources (OMNR) put 25 satellite and 50 VHF transmitters on a total of 75 adult breeding cormorants at two Lake Ontario sites.

2) The CWS color-banded 125+ young cormorants on two colonies and plan to follow them for the next 2-5 years to obtain data on age specific return to natal colonies, breeding, mortality, etc. Additionally, approximately 3,000 fledglings were banded on the Canadian Great Lakes for studies planned by J. Glahn and co-workers on aging cormorants from bone sections.

3) OMNR launched a 5 year study to determine impacts of cormorants on Great

Lakes fisheries and to assess whether experimental control procedures result in improved fishery characteristics at selected areas in the Great Lakes.

4) NYSDEC continued with year 2 of their cormorant egg oiling program on

Little Galloo Island, a 5,000+ nest colony in eastern Lake Ontario.

5) The United States Geological Survey / Biological Resources Division concluded a radio transmitter study of cormorant movements in western Lake Erie the summer of 2000.

6) The University of Minnesota began work on two new DCCO projects in 2000:

a) Breeding pair estimates were obtained in June-July 2000 in 4 study areas in Lakes Michigan and Huron. These data will be combined with previous Great Lakes-wide census estimates from 1977, 1989 and 1997 to investigate local population changes and patterns of colonization in areas with high numbers of nesting cormorants and public concern regarding DCCO impacts. Study approach is similar to work done on Great Cormorants (e.g., Van Eerden and Gregersen 1995; Bregnballe and Gregersen 1997).

b) A second study will examine the impact of DCCOs on island biodiversity (e.g., vegetation, other colonial waterbirds).

Figure 1. Changes in numbers of breeding pairs of Double-crested Cormorants on the Great Lakes and in New England, 1970-1997 (from Wires et al. 2000).

References

Bregnballe T. and J. Gregersen. 1997. Changes in growth of the breeding population of cormorants Phalacrocorax carbo sinensis in Denmark. Supplemento alle Ricerche di Biologia della Selvaggina 26(1):31-46.

Hatch, J.J. and D.V. Weseloh. 1999. Double-crested Cormorant (Phalacrocorax auritus). In The Birds of North America, No. 441 (A. Poole and F. Gill, eds.). The Birds of North America, Inc., Philadelphia, PA.

Jarvie, S., H. Blokpoel and T. Chipperfield. 2000. A geographic information system to monitor nest distributions of Double-crested Cormorants and Black-crowned Night Herons at shared colony sites near Toronto, Canada. Symposium on Double-crested Cormorants: population status and management issues in the Midwest. USDA Technical Bulletin 1879. p.121-129.

Moore, D.J., H. Blokpoel, K.P. Lampman and D.V. Weseloh. 1995. Status, ecology and management of colonial waterbirds nesting in Hamilton Harbour, Lake Ontario, 1988-1994. Technical Report Series No. 213. Canadian Wildlife Service, Ontario Region.

Trapp, J.L., Lewis, S.J. and Pence, D.M. 2000. Double-crested Cormorant impacts on sport fish: literature review, agency survey, and strategies. Symposium on Double-crested Cormorants: population status and management issues in the Midwest. USDA Technical Bulletin 1879. p.87-96.

Van Eerden, M.R. and J. Gregersen. 1995. Long-term changes in the Northwest European population of cormorants Phalacrocorax carbo sinensis. Ardea 83(1):61-78.

Wires, L.R., F.J. Cuthbert, D.R. Trexel, and A.R. Joshi. 2000. Status of the Double-crested Cormorant (Phalacrocorax auritus): eastern and central North America. Draft report to USFWS. February 2000.

L.R. Wires¹, F.J. Cuthbert¹, D.V. Weseloh² and D.R. Trexel¹, ¹ University of Minnesota, Dept. Fisheries and Wildlife, 200 Hodson Hall, 1980 Folwell Ave., St. Paul, MN 55108-6124, USA. ² Canadian Wildlife Service-Ontario Region, 4905 Dufferin St., Downsview, Ontario M3H 5T4, Canada