This paper introduces the second Special Publication of the Waterbird Society to address the biology and management of the Double-crested Cormorant (Phalacrocorax auritius) in North America. Since the late 1960s and early 1970s, when the species was at very low population levels, the Double-crested Cormorant has rebounded to its greatest population level in over 100 years. Such a significant increase has resulted in changes in community structure, and new stressors, in many aquatic ecosystems. Both Special Publications (1995 and 2013) have been focused on the biology and management of the species. The first volume dealt mainly with population growth and the resulting, immediate management issues. In the current volume, studies address the longer term situation, the implementation of two U.S. depredation orders and new research directions identified in the first Special Publication and in subsequent smaller cormorant symposia. Seventeen papers which comprise this volume are presented under six headings: introduction, impacts to natural resources, population dynamics, evaluation of control efforts, assessing fish consumption and bioenergetics, migration ecology and local and seasonal movements, and summary overview and future information needs. A second Special Publication on Double-crested Cormorants gives us an opportunity to assess how well cormorant biologists have addressed and answered questions we posed to ourselves 15 years earlier; it also provides us with a vision for the next 18 years.

Colonially nesting waterbirds transfer large quantities of aquatically derived nutrients into terrestrial systems, potentially altering community and ecosystem structure. Over the past three decades, the Double-crested Cormorant (Phalacrocorax auritus) has undergone rapid population expansion throughout much of its historic range in North America, recolonizing habitats that had not supported colonial waterbirds for decades. Mounting evidence suggests that these populations are degrading the habitats they colonize primarily through the destruction of vegetation and the alteration of soil conditions. The study examined the effects of cormorants and cooccurring long-legged wading bird species including Black-crowned Night-Herons (Nycticorax nycticorax). Great Egrets (Ardea alba) and Snowy Egrets (Egretta thula) on their nesting habitats by observing plant and arthropod community structure as well as soil and leaf litter characteristics at colonized and non-colonized sites on two islands in New York Harbor. Understory plant species richness and total plant cover were reduced, and the arthropod community shifted from primarily plant feeders to primarily carrion and dung feeders beneath cormorant nests in comparison to adjacent non-colonized habitats. On the island where cormorants have been established longer, the colony tended to be denser and larger and was associated with larger ecological impacts on plants, arthropods and soils. Long-legged wading bird colonies and more recently established cormorant colonies were smaller, less dense, and were generally associated with fewer ecological impacts.

Carolinian habitats on Middle Island in the western basin of Lake Erie have recently experienced a dramatic rise in nesting Double-crested Cormorants (Phalacrocorax auritus). Nesting cormorants on the 0.3 × 1.1 km island increased from three pairs in 1987 to 4,690 pairs in 2006. The physical attributes of individual trees and poles were assessed using common indices of tree health to determine whether forest damage increased with cormorant nesting densities. Crown density, branch damage, foliage transparency and decay were measured at 54 sampling stations along twelve transects in June of 2004, 2005 and 2006. All damage indices except for tree crown density and pole decay class increased over time, with trees more damaged than poles. Nests were more than four times more likely to be found in superstory trees than overstory trees and were virtually absent from understory trees, suggesting that Double-crested Cormorants prefer larger trees as nesting sites. However, despite greater cormorant preference for large trees, understory and open canopy trees had significantly greater levels of foliage transparency compared to overstory trees. The spatial distribution of damage varied across the island. Stem damage was lower in the center of the island compared to the western and eastern sections. Branch damage and foliage transparency were also greater on the eastern edge of the island than in the center suggesting that impacts were not yet concentrated within the forest interior. Densities of Double-crested Cormorant nests per station were significantly related to all damage indices except decay class. Stations with high numbers of cormorant nests were more likely to have lower crown densities, more transparent foliage and greater branch damage than stations with fewer cormorant nests. The data suggested that the distributional variation in damage could be used to better target specific areas of the island for management such as deployment of sonic deterrent systems, egg oiling or culling.

Double-crested Cormorant (Phalacrocorax auritus) populations have rapidly increased in the Great Lakes and in wintering areas in the southeast USA since the mid-1970s, resulting in conflicts with humans. To increase understanding of their population biology, band-recovery models were used to estimate temporal trends in hatch year (HY), second year (SY), and after second year (ASY) survival of cormorants banded in the Great Lakes from 1979 to 2006. SY and ASY annual survival varied among years with no apparent trend. HY annual survival exhibited a negative log-linear trend. Lack of clear temporal patterns in SY and ASY survival suggested that increases in cormorant abundance subsequent to 1979 did not impact survival in these age-classes, whereas HY annual survival declined as abundance increased. In addition, issuance of depredation orders in 1998 and 2003 appeared to have a small negative effect on HY survival, but no clear effect on SY and ASY annual survival. The percentage of band returns reported from cormorant control operations generally increased over time, and greatly during the depredation orders. Mean ± SE annual survival from 1979 to 2006 was 0.446 ± 0.022 for HY, 0.835 ± 0.026 for SY, and 0.884 ± 0.020 for ASY individuals. Although increases in cormorant abundance did not appear to be related to increases in annual survival, the relatively high average annual survival rate of ASY cormorants may have been responsible for rapid population growth in the 1980s and 1990s in the Great Lakes.

Banding records were examined to identify changes in mortality causes and locations of Double-crested Cormorants (Phalacrocorax auritus) in Door County, Wisconsin. In 14 out of 18 years between 1988 and 2005, a total of 22,469 birds were banded (300 to 5,462; average 1,605 per active banding year). Of 649 usable band returns (36.1 ± 5.4 per year), 33% were banding-year recoveries (before 1 May of the first year of life). The yearly rate of banding-year recoveries increased from 0.7% per year before 1996 to 2.2% per year after 1999. The yearly proportion of all band recoveries attributed to animal damage control operations also increased over time. The yearly proportion of band returns from Mississippi Delta states increased over time. Mortality rates, both natural and anthropogenic, of cormorants from these colonies appear to have risen as the population has grown and control activities in southern states have increased. Apparent survival rates were estimated by mark-recapture methods during 2001 to 2006. Birds color-banded as adults had a model-averaged annual survival rate of 0.696. For birds banded as nestlings, the model-averaged survival rates were: 0.305 (first year), 0.774 (second and third year), and 0.633 (adults). Simulations of these measured survival rates combined with previously estimated reproductive rates demonstrated that emigration and immigration rates complicate interpretation of these results. Also, simulations demonstrate the potential efficacy of reproductive controls in reducing local breeding populations.

Binational colonial waterbird censuses were formally initiated in the Great Lakes in 1976, and have been repeated every eight to ten years. Although population estimates from these efforts are good trend indicators for some species, including Double-crested Cormorant (Phalacrocorax auritus), information is generally not available to examine population changes between census periods, especially at a local scale. The study examined shortterm, annual population changes and distribution patterns of cormorants in the Beaver Archipelago of northern Lake Michigan between 2000 and 2007, and compared these data to decadal trends. Although no control efforts were underway between 2000 and 2006, an egg-oiling and culling program was initiated in 2007, after the completion of the decadal census. The peak number of breeders was documented in 1997, with lower population sizes between 2000 and 2006; however, in 2007, the population again approached peak numbers, suggesting that immigration from other regions occurred. Overall, yearly surveys determined that individual colonies showed great plasticity in breeding population size, with some colonies forming and others disappearing; some changes were due to environmental factors, while others indicate interactions with other species, human disturbance, and perhaps management activities in other areas of the Great Lakes. Yearly population estimates indicate that the long-term trends are probably captured with decadal colonial waterbird surveys, but the dynamic nature of cormorant populations is not. In addition, presence of yearly variation in the breeding population size of an “unmanaged” population emphasizes the importance of closely monitoring species that are managed aggressively.

Double-crested Cormorants (Phalacrocorax auritus) have used central New York waters for breeding and stopover habitats during migration since 1984. In response to public concern over Oneida Lake, the United States Department of Agriculture Wildlife Services and the New York State Department of Environmental Conservation initiated an integrated research, management and monitoring program aimed at mitigating cormorant impacts to fisheries and other natural resources in 1998. The history of this program was reviewed and efforts to reduce negative impacts of the Double-crested Cormorant population in central New York described. Management was successful, as demonstrated by a substantial decrease in cormorant use of Oneida Lake during spring, summer and fall seasons, and the apparent recovery of certain sportfish populations. Research identified cormorant movement patterns within and among water bodies and documented cormorant responses to hazing and other management techniques. The cormorant management program in central New York was intended to keep cormorant use of Oneida Lake at a level that prevents unsustainable impacts to fisheries populations.

The New York State Department of Environmental Conservation initiated a Double-crested Cormorant (Phalacrocorax auritus) control program in the eastern basin of Lake Ontario to mitigate cormorant impacts in 1999. Key objectives included improving the quality of Smallmouth Bass (Micropterus dolomieu) and other fisheries, restoring the structure and function of the warmwater fish community and reducing cormorant impacts to nesting habitats of other colonial waterbird species. In eight years of intensive control, cormorant numbers declined, with a corresponding reduction in estimated fish consumption. Diversity and numbers of co-occurring waterbirds either increased or have not been shown to be negatively impacted by management. Woody vegetation favorable to Black-crowned Night-Herons (Nycticorax nycticorax) has been maintained. A ca. 2.5-fold increase in the abundance of Smallmouth Bass abundance in assessment nets over the last seven years is a sign of improved recruitment to the fishery. Since the target population level of 4,500 to 6,000 cormorants has essentially been achieved, the eastern Lake Ontario cormorant program is expected to shift in 2007 from a population reduction focus towards a less intensive program intended to prevent population resurgence.

Increasing populations of Double-crested Cormorants (Phalacrocorax auritus) throughout North America have had a significant impact on fish resources in areas where they breed. A simple Excel model was used to assess the effectiveness of lethal control methods in reducing fish consumed by a breeding population of cormorants. Egg oiling was found to reduce seasonal fish consumption by an average of 504 ± 75 (SD) metric tonnes (N = 3) while culling reduced consumption by 280 ± 205 (SD) tons (N = 3). Cost-effectiveness of each method was also assessed using values from a control program in Lac La Biche, Alberta, Canada. Egg oiling cost an average of CDN$5.26 ± 0.84 (SD; N = 3) for each ton of fish saved from consumption by cormorants and culling cost $36.14 ± 8.90 (SD; N = 3). While culling alone is capable of controlling consumption by adult and young-of-the-year cormorants, egg oiling provides a practical and cost-effective alternative for management of ground-nesting cormorants when used in combination or when culling is not available as a management option.

Double-crested Cormorant (Phalacrocmax auritus) numbers in the Great Lakes of North America have increased dramatically during the past three decades. Current concerns include negative effects of cormorants on aquatic food webs, destruction of nesting habitat and competition with other avian species and odors due to feces. Control of cormorant nesting and roosting has been variably successful and sometimes involves lethal force and/or loud devices. These approaches may be unsuitable in some settings, such as parks or residential areas. A large group of cormorants (up to 2,000 individuals) has used Farr Island, a small island near houses on the North Shore of Hamilton Harbour at the west end of Lake Ontario, Canada, as a post-breeding roosting area. The concentration of birds has resulted in complaints regarding odors from the island. In an attempt to alleviate the strong odor and to examine options to limit cormorant nest site competition with other species, a tethered Steppe Eagle (Aquila nipalensis) was placed on the roost island for twelve days from 9 to 21 September 2003. The eagle was effective at causing roosting cormorants to avoid the island, apparently causing many to move their roost site a short distance to another island nearby. Complete cormorant displacement lasted at least four days beyond the period of eagle presence on the island. Using a tethered large raptor appears to be an effective non-lethal tool for local displacement of roosting Double-crested Cormorants.

During the last 25 years, North America has experienced a significant increase in Double-crested Cormorants (Phalacrocorax auritus). The increase has caused concern among wildlife and fisheries managers dealing with the perceived and/or real impacts of this bird on fisheries. Cormorant foraging and breeding ecology were examined at a large (>6,000 pairs), unmanaged colony in Lake Ontario, Canada. Chick diet, feeding rates and productivity were evaluated during 2006 and 2007. In 2007, the proportion of adult cormorants making long-range foraging trips (more than ten kilometers) during different stages of the breeding season was estimated. Alewife (Alosa pseudoharengus) composed greater than 86% of chick diet by mass during both years; suggesting that cormorants did not often prey on sport fish for feeding chicks. Chicks were fed approximately 4.5 times per day in 2006, which was significantly lower than 5.9 feeds per day during 2007. Productivity was high in both years, with 2.2 and 1.9 chicks produced per nest in 2006 and 2007, respectively. The decrease in productivity corresponded with an increase in colony size, from 6,125 to 7,241 pairs. In 2007, approximately one-fifth of the nesting colony foraged at least ten kilometers throughout the breeding season. Cormorants fed their chicks very few sport fish in the Toronto area; however, more work is required to determine whether the consumption of Alewife by cormorants represents competition with sport fish for prey.

Double-crested Cormorants (Phalacrocorax auritus) began to recolonize Leech Lake, Minnesota, in the 1990s and reached 2,524 breeding pairs before control measures started in 2005. Walleye (Sander vitreus) recruitment concurrently declined, creating concern in the local community. To better understand the impacts of cormorants on fish, a cormorant diet analysis was conducted and two models were compared: Madenjian and Gabrey (1995) and Niche Mapper™ for estimating cormorant consumption of fish. Stomach content analysis revealed Yellow Perch (Perca flavescens) to be the most dominant species, providing 80% and 40% of the overall diet in 2005 and 2006, respectively. The Madenjian and Gabrey and the Niche Mapper™ models showed similar results in estimating daily food consumption of cormorants despite their different approaches and complexity levels. In both models, consumption estimates were greater in 2005 than in 2006, with 26 to 33% of their total body mass in 2005 and 18 to 22% of their total body mass in 2006. Likely the variation was caused by the variation in diet found in cormorant stomachs and the difference in the caloric densities of their diet between the two years, with Lake Herring (or Cisco, Coregonus artedi) being more prevalent in the diet in 2006. The most sensitive parameters in the Madenjian and Gabrey model were daily energy expenditure, assimilation efficiency, caloric density of fish and weight of cormorants. In the Niche Mapper™ model, core body temperature of cormorants and caloric density of fish were the most sensitive parameters. The strengths and weaknesses of both models are discussed, while providing guidance for researchers to select the method most applicable to their specific site and available data.

The Round Goby (Apollonia melanostomus) is a small benthic fish, native to the Eurasian Ponto-Caspian region, that has rapidly spread through the entire Laurentian Great Lakes system since its 1990 discovery in Lake St. Clair. Tolerant of high population densities, the exotic Round Goby competes aggressively with native fish for food and habitat, and has increasingly been exploited by endemic Great Lakes predators. A management program for the Upper Niagara River, initiated in 2004, has provided an opportunity to study the interactions between these invaders and the Double-crested Cormorant (Phalacrocoiax auritus), a native top predator. The gut contents of 1,119 cormorants nesting at two sites in the Upper Niagara River from 2004 to 2007 were examined, and the species composition of ingested prey (by number and weight) was quantified for the 600 stomachs that contained identifiable prey. Results of these analyses indicate that Round Goby can constitute up to 85% of the biomass in cormorant diet during periods of the breeding season, and that gobies are consumed by cormorants through all dates sampled (May through August). Lengths of Round Gobies recovered in the cormorant diet were skewed towards larger members of the goby population, suggesting non-random selection relative to the range of size possibilities, and displayed significant declines in length between and within seasons.

A two-year satellite telemetry study was initiated in May 2000 at a Double-crested Cormorant (Phalacrocorax auritus) breeding colony on Little Galloo Island (LGI) in eastern Lake Ontario, New York, USA, which is managed by egg-oiling. The objective was to describe cormorant (N = 26/year) movements, specifically during the period of reproductive management by egg-oiling and seasonally (breeding, migration and wintering). Egg-oiling at two-week intervals resulted in a hatch success on LGI of 5.7% for 2000 and 2001, combined. The majority (97%) of core use areas of marked cormorants contained LGI throughout three egg-oiling treatments (six weeks), and 71% still contained LGI by the end of the final (fourth) treatment (eight weeks). Of cormorants that moved during or after control activities, three remained in the vicinity of active breeding colonies for over three months. Cormorants initiated fall migration over a 16-week period ranging from 12 July to 29 October, with a mean departure date of 6 September (N = 24, SE = 8 days) over both years. Mean duration of fall migration was 34 days (N = 19, SE = 7 days, range = 108 days). Most (75%) cormorants captured at LGI migrated east of the Appalachian Mountains, and their winter range extended from southeastern Louisiana, along the coast of the Gulf of Mexico, to the southern portion of the Atlantic coast. Although three (13%) cormorants over both years relocated to other active colonies for long enough periods (over three months) to potentially raise young, this study indicates that control efforts did not result in complete abandonment of LGI. Egg-oiling was successful in reducing recruitment within breeding seasons, and within-breeding-season renesting attempts by cormorants in this study were limited and likely unsuccessful. Further evaluation and refinement of egg-oiling as a management tool will require multiyear monitoring of the LGI cormorant breeding colony.

Numbers of Double-crested Cormorants (Phalacrocorax auritus) wintering in the southeastern United States have increased dramatically during the last 30 years concomitant with the rise of the aquaculture industry in this region. These cormorants commonly foraged at commercial aquaculture facilities and thereby came into conflict with farmers. Various interest groups are seeking ecologically sound strategies for minimizing the effects of burgeoning cormorant populations. Therefore, this study was conducted to estimate winter and summer home ranges of cormorants captured in the southeastern U.S. and determine whether age class, body mass, density of aquaculture facilities and availability of roost sites influenced size of these home ranges. Mean ± SE home range size and core use area of satellite transmitter-marked cormorants wintering in the southeastern U.S. from 1999 to 2001 were 17,490 ± 1,986 km² (N = 37) and 1,550 ± 265 km² (N = 37), respectively. Winter home range size was not affected by region, age class or body mass. Summer home range and core use area of marked cormorants was 30,547 ± 6,197 km² (N = 6) and 3,124 ± 1,019 km² (N = 6), respectively.

Migration patterns of Double-crested Cormorants (Phalacrocorax auritus) wintering in the southeastern U.S. are poorly understood. Movement data were analyzed from 28 cormorants captured in Alabama, Arkansas, Louisiana and Mississippi and equipped with satellite transmitters. Four (three immature, one adult) cormorants did not migrate and stayed in the southeastern U.S. throughout the year. During spring, cormorants captured in Alabama migrated east of the Mississippi River and primarily west of the Appalachian Mountains. Cormorants from Arkansas, Louisiana and Mississippi migrated north along the Mississippi River Valley, the Missouri River Valley and/or the Ohio River Valley. The earliest departure for spring migration was 26 March, whereas the latest departure was 12 May. Adult cormorants departed for spring migration earlier than immature cormorants. The average departure date for fall migration was 1 October. Mean duration of spring migration was twelve days, and cormorants traveled an average of 70 km per day.

An overview is provided of a symposium on the direction of research and management of Double-crested Cormorants (Phalacrocorax auritus) heading into the 2000s. The current symposium built on previous efforts and described a number of focus areas of informational need, including cormorant impacts on natural resources, demographics of cormorants, assessment of control efforts, assessment of fish consumption and bioenergetics, and cormorant spatial ecology and influences on movements. The cormorant symposium highlighted a shift in research focus relative to earlier symposia, from evaluating potential impacts on commercial and natural resources to evaluating management actions in attaining desired goals. In addition, the symposium addressed the pressing need to obtain baseline information on cormorant population demographics. The shift reflects a response to increasing management efforts and intensity in North America and the need to understand the effects and effectiveness of increased cormorant control at several spatial scales. The symposium furthered communication and the sharing of information on cormorant issues in North America. However, uncertainty regarding impacts to cormorants associated with policy changes and management actions and outcomes presents significant future challenges.