Non-lethal management of cormorants: system-level responses to nocturnal roost deterrents

The Double-crested Cormorant ( Nannopterum auritum) has expanded across eastern North America, raising concerns about predation pressure on fish populations. Lake Saint-Pierre (Québec, Canada) supports large fall aggregations of migratory cormorants that roost on anthropogenic navigation structures and forage in shallow littoral habitats. From 2021 to 2025, a non-lethal management program led by the W8banaki First Nation installed elevated netting deterrents on these structures to reduce roosting and associated predation. We evaluated the system-scale effects of this intervention using drone surveys of nocturnal roosts, plane surveys of foraging habitats, generalized linear mixed-effects models, and bioenergetic consumption estimates. Following full deterrent implementation (2023–2025), mean roosting group size declined by 54% at the site level and 72% at the system level. Although site-level foraging group size showed no significant change, total system-level foraging abundance decreased by 58%. Estimated cormorant foraging use-days and annual fish consumption also declined markedly following implementation. These results suggest that physical roost deterrents can reduce localized cormorant abundance and associated predation pressure without lethal control. However, additional monitoring and testing are required to determine whether these reductions translate into measurable improvements in fish populations and contribute to the long-term recovery of vulnerable species.