Since 1988, the Northern Gannet Morus bassanus (hereafter ‘Gannet’) population has exhibited an extremely low and downward trending release rate when admitted to wildlife rehabilitation facilities in the United States. Across more than 4,000 Gannets admitted to rehabilitation facilities between 1988 and 2023, patients demonstrated an overall release rate of 13.7%. More concerning is that Gannet release rates have not exceeded 10% since 2018 and dropped to 5.5% in 2019. The challenge is that Gannets admitted to rehabilitation facilities often exhibit no external signs of injury making it difficult for rehabilitators to diagnose their reason for admission. However, they are frequently in such poor body condition that they require immediate euthanasia or die within 24 hours of admission. In many instances, their clinical signs are listed as exhaustion, trauma, or emaciation. Based on data collected from 39 rehabilitation facilities along the East Coast of the United States and one facility in Canada, 46.4% of patients were admitted due to exhaustion or emaciation. To determine cause of death with more accuracy, more than 90 Gannet patients (2.2%) were necropsied. The results indicated that over 60% of the necropsied patients were suffering from aspergillosis.

The Northern Gannet Morus bassanus (hereafter ‘Gannet’) is an avian marine species that only visits the shore to breed, with juveniles believed to remain entirely at sea during their first three years of life (Mowbray 2020). Of an estimated global population of 1.5 million–1.8 million mature individuals, there are roughly 116,825 breeding pairs in North America that migrate from their breeding colonies in Eastern Canada to the Gulf of Mexico typically flying around, but not over, Florida, USA (Gunter & Burke 1977; Chardine et al. 2013; Mowbray 2020).

Based on their population extent and size, Gannets are considered a species of Least Concern (BirdLife International 2018). However, the United States Department of the Interior considers them a species of conservation concern specifically within the Federal waters of the mid-Atlantic United States (Spiegel et al. 2017). Substantial research has been conducted on the species in their breeding colonies in the Eastern North Atlantic and North America, but little research has been conducted outside their colonies (Mowbray 2020).

Every year, hundreds of migrating adult and juvenile Gannets are found injured or dead along the Florida coast. Most do not survive beyond the first 24 hours. A study of over 386,000 avian patients across more than 580 species admitted to Florida wildlife rehabilitation facilities between 2009 and 2023 showed an overall average release rate of 35% (Pahl 2024). In that study, Gannets represented more than 3,000 patients with the lowest survival rate of the 92 most frequently treated species (Pahl 2024). This study includes more than 1,000 additional Gannet patients from Florida and along the East Coast with an overall release rate of 12.7%. Even more concerning than the low overall release rate!is!that over the years the release rate continued to spiral down. In 1993, 34.2% of Gannet patients were released. By 2022, the release rate had dropped to a mere 5.9%. The question is, why?

The answer may, at least in part, be due to aspergillosis. When admitted, many Gannet patients appear exhausted and emaciated but show little to no evidence of trauma or disease. Aspergillosis is one of the most frequent fungal infections in birds, both captive and wild, with Aspergillus fumigatus being the most common cause (Huang & Mayer 2018; Sabino et al. 2019).

Aspergillosis is a fast-growing fungus with infection often resulting due to exposure to an overwhelming amount of spores or the patient being immunosuppressed (Speer 2016). The birds’ high body temperatures and the humid environment of the airways create an ideal location for the fungus, allowing it to multiply rapidly (Speer 2016). Seabirds are considered extremely susceptible to aspergillosis, but few cases are reported in wild birds. Most infections are thought to be the result of being brought into captivity at rehabilitation facilities (Speer 2016). Aspergillosis lesions can present as white-yellow nodular to confluent plaques, visible green-grey mould, and sporulation occurs primarily in the air sacs and lungs (Beernaert et al. 2010; Fischer & Lierz 2015). As the illness does not include obvious external features, it can be challenging to diagnose. Gannets may have a large global population, but their low release rate along the United States East Coast warrants concern and requires further investigation.

I would like to thank the Marine Science Center, Tri-State Bird Rescue & Research, and all of the other facilities that shared their data for this project.

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