Double‐Crested Cormorants: Too Much of a Good Thing?

Dorr, Brian S.; Fielder, David G.

doi:10.1080/03632415.2017.1356121

Cited by 15 publications

(19 citation statements)

References 32 publications

(51 reference statements)

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“…The resurgence of these fish‐eating birds has resulted in increasing resource conflicts, especially with the aquaculture industry and agencies that are responsible for managing sport fisheries (e.g., Meyer et al. ; Dorr and Fielder ).…”

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confidence: 99%

Colonial Waterbird Predation and Angler Catch of Hatchery Rainbow Trout Stocked in Southern Idaho Fisheries

2019

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The abundance of piscivorous colonial waterbirds such as double‐crested cormorants Phalacrocorax auritus and American white pelicans Pelecanus erythrorhynchos has increased dramatically in recent decades in North America, resulting in increased conflict between these birds and fisheries resources, which raises the need to quantify the effects of waterbird predation on fisheries. We estimated avian predation and angler use of two sizes of hatchery‐reared Rainbow Trout Oncorhynchus mykiss (250 mm and 300 mm on average; termed standard catchables and magnum catchables) at 15 Idaho waters with known cormorant and pelican presence. Fish were tagged with passive integrated transponder (PIT), radio, and T‐bar anchor tags prior to release; the tags were subsequently recovered from bird nesting, roosting, and loafing sites. Estimated angler use, which included fish harvested as well as those that were catch‐and‐release, averaged 17%, while avian predation averaged 35%. Angler catch at study waters declined exponentially as avian predation increased. Predation specifically attributed to double‐crested cormorants and American white pelicans was related to their relative abundance at the waterbodies where the fish were stocked. Avian predation rates were similar for standard and magnum catchables, suggesting that larger fish were not able to escape predation any better than smaller ones. Our results add to existing evidence indicating that in some southern Idaho fisheries, piscivorous birds, rather than anglers, are the dominant consumers of hatchery trout. This study further contributes to the larger issue of competition between humans, wildlife, and fisheries populations.

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Colonial Waterbird Predation and Angler Catch of Hatchery Rainbow Trout Stocked in Southern Idaho Fisheries

2019

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“…Allele frequency shifts can accompany population expansions (Kimmel et al 1998), which may have occurred in cormorants as they recovered from a demographic bottleneck (Dorr and Fielder 2017). To test whether allele frequencies had shifted over the time frame of our study, we selected 2 banding sites (Spider Island, WI and St. Martin's Shoal, Lake Huron, MI) where sufficient numbers of chicks were banded from the same location across the study period and performed STRUCTURE analyses as described above.…”

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confidence: 99%

“…Cormorants have undergone an astonishing population recovery since the 1970s. A combination of adaptation to anthropogenic change (aquaculture, construction of reservoirs), pollution reduction, and regulatory protection facilitated a rebound from around 200 nesting pairs in the 1970s to approximately 115,000 nesting pairs in 2000 in the Great Lakes region alone (Weseloh et al 2002, Dorr and Fielder 2017). Because of its contemporary abundance and widespread distribution, the double‐crested cormorant is now often associated with human‐wildlife conflicts (Dorr et al 2014 b ).…”

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confidence: 99%

“…Because of its contemporary abundance and widespread distribution, the double‐crested cormorant is now often associated with human‐wildlife conflicts (Dorr et al 2014 b ). These conflicts, particularly with respect to commercial and natural resources such as aquaculture and sport fisheries, have resulted in often‐controversial management of cormorants as a nuisance wildlife species (Dorr and Fielder 2017). One question central to this contentious issue is whether cormorants should be classified into subspecies, and, if so, how those subspecies should be regulated and managed (Mercer et al 2013, Sheehan et al 2017).…”

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Migratory Flyways May Affect Population Structure in Double‐Crested Cormorants

et al. 2020

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Double‐crested cormorants (Phalacrocorax auritus) recovered from a demographic bottleneck so well that they are now considered a nuisance species at breeding and wintering grounds across the United States and Canada. Management of this species could be improved by refining genetic population boundaries and assigning individuals to their natal population. Further, recent radio‐telemetry data suggest the existence of Interior and Atlantic migratory flyways, which could reduce gene flow and result in substantial genetic isolation. In this study, we used 1,784 individuals collected across the eastern United States, a large panel of microsatellite markers developed for this species, and individuals banded as chicks and recaptured as adults to explore the effects of migratory flyways on population structure, quantify the genetic effects of demographic bottlenecks, and determine whether individuals could be assigned to their natal population based on genotype. We found evidence for genetic population division only along migratory flyways, no evidence of genetic bottlenecks, and mixed effectiveness of assignment tests. Our population structure findings suggest that gene flow is high across large scales; for example, individuals from New York, Minnesota, and Alabama are all in panmixia. We also found that traditional subspecies ranges may not be valid because >1 subspecies was present in single genetic populations. The lack of evidence for genetic bottlenecks also likely underscores the vagility of this species, suggesting that even during demographic bottlenecks, populations were not isolated from allelic exchange. Finally, the failure of assignment tests to consistently perform is likely due in part to imperfect a priori sampling of Atlantic and Interior chicks and the high vagility of adults. We conclude that the demographic bottleneck is not likely to have reduced genetic diversity, and that assignment tests remain unreliable for this species. We recommend double‐crested cormorants be managed by flyway. Further development of genomic resources in this species could improve population subdivision resolution, improve assignment tests, and reveal further information on demographic histories. © 2020 The Wildlife Society.

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“…Avian predation has been an issue throughout the history of catfish aquaculture in the southern United States. (Dorr and Taylor 2003; Dorr and Fielder 2017; Engle et al 2020). Catfish operations, especially those that are located in the Mississippi River delta region of the Mississippi flyway, provide a constant and readily accessible supply of forage for birds.…”

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Economic Losses of Catfish to Avian Predation: A Case Report

et al. 2021

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Avian predation in catfish aquaculture has been a persistent issue throughout the history of the industry, and as production has expanded predation from piscivorous birds has intensified. Catfish ponds in the Mississippi River delta (in the Mississippi migratory flyway) provide a constant and readily accessible supply of forage for birds. Intensive foraging by fish‐eating birds has led to a specific regulatory policy and numerous on‐farm and regional management efforts. However, in 2016, legal challenges lead to recision of some federal policies and uncertainty as to allowable management, resulting in limitations on bird depredation. Estimating the extent of fish losses to avian predators is difficult, as loss estimates from farms are often confounded with disease‐ and management‐related mortalities. This study details the reported losses to birds that were observed in commercial‐scale catfish ponds at the Thad Cochran National Warmwater Aquaculture Center, Stoneville, Mississippi, during periods of limited bird management. The observed fish losses attributed to birds ranged from 33% to 95% loss in survival and potential yield losses of 4,396 to 8,889 lb/acre, increasing production costs and decreasing net returns. Net economic losses when accounting for negative net returns and lost profits ranged from US$3,518 to $4,060/acre. Losses of this magnitude on commercial fish farms are economically detrimental, especially because catfish farms lack the compensatory economic relief programs that are available in other agriculture sectors. Roost dispersal activities that are organized by federal agencies and avoiding delays in issuing bird depredation permits are vital for mitigating this persistent and growing regulatory problem in the U.S. catfish industry.

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Double‐Crested Cormorants: Too Much of a Good Thing?

Cited by 15 publications

References 32 publications

Colonial Waterbird Predation and Angler Catch of Hatchery Rainbow Trout Stocked in Southern Idaho Fisheries

Colonial Waterbird Predation and Angler Catch of Hatchery Rainbow Trout Stocked in Southern Idaho Fisheries

Migratory Flyways May Affect Population Structure in Double‐Crested Cormorants

Economic Losses of Catfish to Avian Predation: A Case Report

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