Double-crested Cormorants and Fisheries: An Issue Spanning both Wildlife and Fish

Double-crested Cormorants have made a remarkable recovery in North America over the last four decades, a testimony to the success of conservation efforts. However, their increased abundance and piscivorous food habits have brought them into conflict with a variety of fish management priorities. Throughout their range, cormorant predation on fish presents a variety of biological as well as management and policy challenges. Impacts, whether real or perceived, influence management of free-swimming fishes in recreational and commercial fisheries, aquaculture, and even threatened and endangered salmonid stocks. These cormorant depredation issues have resulted in considerable controversy, and even litigation over the past several decades resulting in recent changes in cormorant management policies in the U.S. and Canada. This symposium invites presentations on the science behind cormorant and free-swimming fish interactions, as well as their management and policy implications. We will present case studies emphasizing actual evaluations of cormorant predation influences on fish populations and cormorant and fish population responses to management. We will present and discuss how this information can inform management decision frameworks and policy moving forward. Desired are presentations by both wildlife and fisheries professionals. This joint AFS/TWS Conference provides the perfect opportunity to bring both professions together to share research findings and help explore the implications of these wildlife and fisheries interactions.

8:00AM History of Policy and Management of Double-Crested Cormorants in the United States Under the Public Resource Depredation Order
  Rachael Pierce
Double-crested cormorant (Phalacrocorax auritus) populations in the eastern United States increased rapidly in the mid-1970’s. By the early 1990s, negative economic (aquaculture; sport and commercial fisheries), and ecological (vegetation; co-nesting species) impacts, both real and perceived, were on the rise. The U.S. Fish and Wildlife Service (USFWS) has statutory authority under the Migratory Bird Treaty Act to manage migratory bird populations, including cormorants. Historically, tools for managing cormorant conflicts were restricted to non-lethal harassment, exclusion, and limited lethal take under depredation permits for private property and human health and safety issues. These tools were inadequate to alleviate the persistent cormorant conflicts. Widespread public outcry and the desire to develop a more consistent and effective management strategy led the USFWS to adopt the Public Resources Depredation Order (PRDO) in 2003 to facilitate the control of depredating cormorants. These orders provided a framework for managing cormorant conflicts in a consistent and scientifically defensible manner. They increased information on cormorant abundance, distribution, trends, and ecological impacts through improved monitoring programs and a surge in research. However, some stakeholders had concerns about a lack of criteria for documenting adverse effects of cormorants on public resources, the way some entities were enacting cormorant management programs, and whether fish populations were actually responding to cormorant management actions. These concerns ultimately resulted in the Public Employees for Environmental Responsibility (PEER) to file a lawsuit against the USFWS. In May 2016, a federal judge vacated the PRDO citing that the USFWS was arbitrary, capricious, and contrary to law in its decision-making, thus violating NEPA and the Administrative Procedures Act. We will present this information to set the stage for the symposium and provide historical context for the problem of managing the fish-cormorant conflict.
8:20AM Policy and Management of Double-Crested Cormorants with Respect to Free Swimming Fish Issues
  Tom Cooper
In May 2016, the U.S. District Court for the District of Columbia vacated the Aquaculture and Public Resource Depredation Orders for double-crested cormorant (cormorant) management. The Court ruled that the U.S. Fish and Wildlife Service (FWS) did not take a “hard look” at the effect of the depredation orders on cormorants and other affected resources (i.e., fish populations). Since this ruling, the FWS, working in collaboration with the Animal and Plant Health Inspection Service, Wildlife Services (APHIS WS), completed an Environmental Assessment (EA) in November 2017 covering the issuance of depredation permits for managing cormorant damage. The scope of the EA covered issuing depredation permits for the following circumstances: 1) Health/Human Safety; 2) Aquaculture; 3) Property Damage; and 4) Concern for co-nesting species. The EA did not address potential damage to free-swimming fish due to a lack of readily available scientific information on the impacts to fish populations and the complexity of these interactions; however, the FWS made the commitment to engage state fish and wildlife agencies, tribes and other stakeholders in addressing the cormorant-fish conflict. During August 2018, the FWS, in cooperation with APHIS WS, hosted a series of regional meetings with federal, state, and tribal partners to discuss management issues and concerns over interactions between cormorants and free-swimming fish. These meetings were the first step in our efforts to collect information concerning the conflict and to chart an appropriate path forward in the management of the conflict. Based on information shared at these meetings and a follow-up meeting in March 2019 with the Association of Fish and Wildlife Agencies’ Bird-Fish Conflict Working Group, the FWS developed a suite of alternatives to explore moving forward. Public engagement and stakeholder input in this process is a priority for the FWS in meeting its responsibilities under the Migratory Bird Treaty Act.
8:40AM Double-Crested Cormorant Impacts to Yellow Perch in the Les Cheneaux Islands and Management Outcomes
  David Fielder
The Les Cheneaux Islands is a 36 island, 11,650ha archipelago in northern Lake Huron and an annual nesting colony for Double-Crested Cormorants (Phalacrocorax auratus) since 1980. Nesting peaked in 2002 at 5,516 or more than 11,000 mature birds estimated to consume nearly 1.5 million kilograms of fish that year. The region is home to an important recreational Yellow Perch (Perca flavescens) fishery. Cormorants are estimated to have consumed as many as 470,000 Yellow Perch per year as early as 1995. The fishery collapsed by 2000 and the population was greatly reduced. Subsequent regression analysis determined that Cormorant nest numbers best explained trends in the Yellow Perch fishery and population. Most sensitive Yellow Perch population metrics were total mortality rate, growth rate (as a density-dependent response), and mean age. Yellow Perch recruitment remained strong during this time and other forces were ruled out or deemed minor. Management in the form of culling and egg oiling was implemented in 2004 and nest numbers were reduced 90% by 2012. Yellow Perch population metrics reversed their decline in response to declining Cormorant numbers and have largely returned to precollapse levels. Other fish species responded favorably as well such as Northern Pike (Esox Lucius) and Brown Bullheads (Ameiurus nebulosus). Collectively, Yellow Perch exhibited classic over harvest responses and fishery managers came to view Cormorant predation as a competitive allocation to the recreational fishery. Even at 90% reduction in nesting, Cormorants are still estimated to consume more yellow perch than harvested by anglers, and the recreational fishery remains reduced in scope. Since vacating of Cormorant control authorities in 2016, nest numbers have risen by 97% (beyond control target levels). Bird/fish models predict an eventual return to peak abundance of Cormorants and collapse of the Yellow Perch population/fishery if left unmanaged.
9:00AM Evaluation of Harassment of Migrating Double-Crested Cormorants to Limit Depredation on Selected Sport Fisheries in Michigan
  Brian Dorr, Ashley H. Moerke, Tony Aderman
Diverse management techniques have been used to mitigate conflicts between humans and double-crested cormorants (Phalacrocorax auritus) including harassment methods supplemented by lethal take. In this study we evaluated impacts of programs to harass spring migrating cormorants on the walleye (Sander vitreus) fishery in Brevoort Lake and the yellow perch (Perca flavescens) and walleye fisheries at Drummond Island. Cormorant foraging declined significantly (pb0.05) at both locations subsequent to initiation of harassment programs. Overall harassment deterred 90% of cormorant foraging attempts while taking less than 6% lethally on average at each site. Yellow perch were a predominate prey item in number and biomass at both locations. Walleye made up a small proportion of the diet at both locations. However, both walleye and yellow perch abundance increased significantly (pb0.05) at Drummond Island. Walleye abundance at age 3 increased to record levels in 2008 following 3 years of cormorant management at Brevoort Lake. The estimated cormorant consumption of age-1 walleye in the absence of management at Brevoort Lake during 2005 would account for 55% of the record 2006 age-1 walleye population. These results support the hypothesis that cormorant predation on spawning aggregations of sportfish was a significant mortality factor and cormorant management reduced sportfish mortality and increased abundance at both locations. Continuation of harassment programs and fishery assessments will determine whether improvement of targeted sport fisheries through control of spring migrating cormorants is sustainable.
9:20AM Double-Crested Cormorant Predation, Management and Fisheries Response on Oneida Lake, NY
  Robin L DeBruyne, Jeremy Coleman, James R. Jackson, Lars G. Rudstam, Anthony J. VanDeValk, Travis DeVault
Oneida Lake, NY, experienced a decline in walleye (Sander vitreus) and yellow perch (Perca flavescens) adult populations in the 1990s which was attributed to decreased age-1 recruitment and increased subadult mortality from increased double-crested cormorant (Phalacrocorax auritus) predation. Non-lethal cormorant management was initiated in 1998 and intensified from 2004-2009 to reduce the percid subadult mortality and increase the adult abundances. We found cormorant consumption of percids decreased with reductions in cormorant abundance and when age-0 gizzard shad were present. Subadult mortality for both percid species decreased when cormorant abundance was reduced. The adult walleye population increased during cormorant management but this increase was likely more strongly associated with short-term, very restrictive harvest regulations (from 5 fish/day bag limit, 15” minimum to 3 fish/day bag limit, 18” minimum) than decreased subadult mortality. The adult yellow perch population did not respond as strongly, likely due to decreased age-1 recruitment caused by factors other than cormorants. This whole-lake predator removal experiment resulted in the expected decrease in subadult mortality of both percid species, and an increase in adult walleye, but not yellow perch abundance. The adult walleye abundance did not decrease when harvest regulations were relaxed (3 fish bag limit, 15” minimum), suggesting that the very restrictive harvest regulations helped to speed adult walleye abundance recovery and cormorant management is critical to maintaining high juvenile survival into the fishery. Confounding factors include the diet shifts observed for cormorants and the decreased age-1 recruitment for yellow perch likely from other ecological changes. This study demonstrates the importance of the availability of comprehensive, system-specific data to cormorant management decisions and assessment.
09:40AM Break
1:10PM Feeding Selectivity of Double-Crested Cormorants Foraging on Oneida Lake, New York
  Robin L. DeBruyne, James Jackson, Jeremy Coleman, Anthony J. VanDeValk, Lars Rudstam
Cormorants have been reported to cause declines in some fish populations, but not others. This difference may be due to prey selection that is further dependent on the composition of the fish assemblage affected. We present 15 years of diet data collected from Oneida Lake, NY, where we have previously documented negative effects of cormorants on walleye and yellow perch. The fish community changed through the study period and this change was reflected in cormorant diets. Diets varied by season and year with emerald shiner, gizzard shad, Lepomis, logperch, walleye, and yellow perch having the highest overall relative importance. When age-0 gizzard shad were abundant they dominated cormorant diets in the fall. Consumption of emerald shiner and gizzard shad was positively related to species’ abundance, but no significant correlation between availability and consumption was found for other prey species. Positive selection was found for gizzard shad, walleye, white perch, and yellow perch during the 15 year period, as well as evidence of prey switching behavior by cormorants. We conclude that variation in fish recruitment influence cormorant food selection and stress the importance of continued diet monitoring with changing fish communities to reliably assess potential impacts on a fish community.
1:30PM Double-Crested Cormorant Impacts to Sportfish in Leech Lake, MN and Evaluation of Cormorant Management.
  Doug Schultz, Steve Mortensen
Double-Crested Cormorants (hereafter cormorant) recolonized Leech Lake (45,158 ha) in north-central Minnesota in the mid-1990’s and quickly expanded to approximately 2,500 nesting pairs by 2004. Concurrent declines in Walleye and Yellow Perch recruitment led to recreational fishery declines, which prompted several management actions including lethal cormorant control. The cormorant population has been actively managed for an annual total consumption target at or below 3.85 kg/ha (approximates 500 nesting pairs) since 2005. Diet studies during 2005-2018 (n = 6 years) identified Yellow Perch (range: 45.1-76.2%) as the primary prey of Leech Lake cormorants. Walleye comprised a smaller proportion of the diet among years (1.4-11.1%), but the majority of Walleye mass consumed were juvenile fish up to age-3. During some years of the expansion, estimated annual consumption of juvenile Walleye by cormorants exceeded the corresponding annual estimates of juvenile Walleye abundance. Several fish population metrics, such as growth and maturity rates, for both species were statistically associated with changes in cormorant abundance. Sportfish populations and the fishery they support have shown positive trends since the inception of cormorant management.
1:50PM Smallmouth Bass Population Response to Double-Crested Cormorant Predation and Management in Lake Ontario’s Eastern Basin
  Jana Lantry, James Farquhar, Steve Hurst, Steven LaPan
Lake Ontario’s Eastern Basin provided anglers with a premier Smallmouth Bass fishery for generations. This changed as the Eastern Basin’s Double-crested cormorant population increased through the 1990s and bass angler catch rates declined. By 1998, concerns of increasing cormorant numbers among anglers and local communities reached a boiling point. Fisheries assessment data documented reduced Smallmouth Bass abundance and recruitment, and a compensatory increased growth rate. Assessment data, angler surveys, and predator diet analyses implicated cormorants as the primary contributor to reduced Smallmouth Bass abundance in Lake Ontario’s Eastern Basin at that time. Management of Eastern Basin cormorants reduced the population to the management target, thereby benefitting the fish community and alleviating concerns of local communities, anglers and fisheries managers. With the cormorant population maintained near target levels, the bass population improved. The cormorant population immediately and positively responded to relaxed management efforts that began in 2016. Now, as cormorant numbers and predation pressure on the fish community increase, so do concerns among fish managers, anglers and local communities.
2:10PM Relative Demand By Double-Crested Cormorants and Anglers for Fish Production from Lakes on Manitoulin Island, Lake Huron – Background and Implications.
  Mark Ridgway
A common currency is needed to fully evaluate the relative demand for fish production by anglers and cormorants. Expressing production AND demand in kg*ha-1*yr-1 allows for this comparison. A study estimating these components on Manitoulin Island, Lake Huron, was the first attempt at forming a common currency so that the effect of cormorant demand on fish populations, in addition to angler demand, could be evaluated. This presentation describes that study and the how the approach provided new insights including how the cormorant issue was framed as science advice and how it was received by resource managers. The approach has two elements – phosphorus loading to describe aquatic productivity on one side and demand by predators for that productivity on the other. One clear advantage of this approach is that it rests within aquatic food web theory. The relationship between size of fish and productivity – and trophic transfer – adopted in the Manitoulin Island study lies at the core of aquatic size spectra theory. Indeed, the Banse and Mosher (1980) paper applied in the Manitoulin Island study is arguably one of the foundational pieces in the history of size spectra theory. The relevance and limits of a size-based approach for the cormorant issue are outlined. Finally, a relative demand perspective places cormorant abundance and consumption in aquatic food webs within the limits of the size spectra they exploit.
2:30PM Relative Demand Models to Inform Decision Making Regarding Potential Cormorant Impacts to Fisheries
  Doug Schultz, David G. Fielder, Randall M. Claramunt, Brian Dorr, Thomas Cooper, Rachael Pierce, Charles Bronte, Andrew Forbes
Double-Crested Cormorant (hereafter cormorant) predation on fish functions similar to harvest in that it can be characterized as size- and age-selective removal of piscine biomass from the fishery. Thus, a cormorant colony has its own independent yield demand on a fishery which is bounded, or localized, by the foraging area of a respective colony. Excessive demand by cormorants for a particular fish population will elicit density-dependent responses traditionally associated with overharvest, such as suppressed recruitment and associated shifts in age structure, increased growth rate, and changes in maturity schedules. Since the mid-1900’s, fisheries professionals have used varying approaches to estimate safe harvest level, or yield, using basic measures of system productivity such as total phosphorous, chlorophyll-a, and other metrics. These models have also been applied across a wide range of system types with differing fish communities, including glacial lakes and southern U.S. reservoirs. We propose applying a similar approach to existing data sets where cormorant impact to a fishery and a positive fishery response to cormorant management have been observed to determine allowable yield for cormorants while maintaining and supporting a productive harvest fishery. This approach can better inform cormorant management decisions at varying scales, including local (inland systems within a State boundary), multi-jurisdictional (Great Lakes), and the flyway for both fish stocks of management importance and the cormorant populations dependent upon them. Moreover, this method has potential to be robust to the types of system changes which inherently alter fish communities and the associated production available to higher trophic levels, including humans and cormorants.
2:50PM Refreshment Break
3:20PM Policy and Assessment Methods to Inform US Fish and Wildlife Service Permitted Take of Double-Crested Cormorants
  Mark Seamans
Lethal take of nongame migratory birds requires a permit issued by the U.S. Fish and Wildlife Service (USFWS). Determining maximum allowable take becomes difficult when a species is rare (e.g., eagles) or when large amounts of take are requested for abundant species. There is guidance when take is requested for energy development and human health and safety, but permitting take to manage public resources, such as fisheries, can be contentious. In 2017 the USFWS completed an Environmental Assessment to aid in issuing depredation permits to manage double-crested cormorant (hereafter cormorant; Phalacrocorax auritus) to protect human health and safety and alleviate property damage in the eastern U.S. Potential Take Level (PTL) models were used to determine maximum allowable annual take and its impact on cormorant populations. PTL is based on harvest theory and in its simplest form uses minimum population size (Nmin), maximum population growth rate (rmax), and a policy factor (FO): PTL = Nmin1/2rmaxFO. The value of FO depends on management goals. For example, when FO=1 the maximum allowed rate of take is ½ rmax (i.e., maximum sustained yield), whereas a low FO values a larger population near its theoretical carrying capacity. FO was set at 0.5 for the small resident Florida population and 0.75 for Atlantic and Central populations. The minimum population size estimate was (in thousands) 260.3, 462.9, and 19.0 for the Atlantic, Central, and Florida populations, respectively. I used 31 years of nest count data from Lakes Erie, Huron, and Ontario in a discrete-time logistic growth model to estimate rmax (rmax=0.358, SE=0.093). PTL was (in thousands) 26.2, 46.9, and 1.3 for the Atlantic, Central, and Florida populations, respectively. Although this assessment can be used to inform cormorant take to manage fisheries, other policy decisions need to be in place to allow take for this purpose.
3:40PM Great Lakes Double-Crested Cormorant Management Affects Co-Nester Colony Growth
  Katherine Wyman, Linda Wires, Francesca Cuthbert
The dramatic growth of the North American Great Lakes double-crested cormorant (Phalacrocorax auritus; hereafter cormorant) population beginning in the 1970s led to eventual widespread management of the population through prevention of reproduction and adult removal. One of the concerns leading to management was perceived impacts of cormorants on other colonial waterbirds. Cormorants rarely nest in single-species colonies in the Great Lakes, most often nesting alongside herring gulls (Larus argentatus), ring-billed gulls (Larus delawarensis), great blue herons (Ardea herodias), and black-crown night-herons (Nycticorax nycticorax). Our objective was to estimate effects of cormorant abundance and management on colony growth indices of these four co-nesting species. We estimated co-nester and cormorant nest colony growth indices from waterbird nest abundances reported by the Great Lakes Colonial Waterbird Survey and comparable Canadian surveys between 1976 and 2010. USDA Wildlife Services collected data on cormorant management locations, number of management visits, and number of nests or adult cormorants affected, which we used to measure management presence and intensity. We then applied linear mixed models to quantify the association between co-nester colony growth indices and cormorant abundance and management, while controlling for other factors likely to influence colony growth. According to the fitted models, black-crowned night-heron colony growth in the Great Lakes was negatively related to both cormorant abundance and management, whereas great blue herons had a neutral mean response to cormorant abundance and management. The two gull species responded positively to both cormorant abundance and management. These results suggest that cormorant management does affect co-nesters, and management plans should take likely effects on co-nesters into account to achieve desired outcomes for co-nesting waterbird populations.
4:00PM Panel Discussion

Organizers: Brian Dorr, Tom Cooper, Dave Fielder, Robin DeBrunye
Supported by: Letters of support have been obtained from USDAAPHISWSNational Wildlife Research Center, USFWS Migratory Bird Division, and The Wildlife Societies Wildlife Damage Management Working Group

Location: Reno-Sparks CC Date: October 3, 2019 Time: 8:00 am - 5:00 pm