Does Estuary Prey Abundance and Diversity Buffer Salmo salar (Atlantic Salmon) Smolts from Predation by Nannopterum auritum (Double-crested Cormorant)?

Danielle Frechette^1,2,*, Joanie Carrier^2,3, David Leblanc⁴, Jonathan Carr⁵, Normand Bergeron^2,6, John Murvin Vicaire⁷, and Carole-Anne Gillis^2,7

¹Maine Department of Marine Resources, 32 Blossom Lane, Augusta ME 04333. ²Centre Interuniversitaire de Recherche sur le Saumon Atlantique (CIRSA), Pavillon Alphonse-Desjardins, 2325, rue de l’Université, local 3402, Québec (Québec) G1V 0A6. ³Department of Fisheries and Oceans, Mont Joli, QC G5H 34, Canada. ⁴Restigouche River Watershed Management Council, Matapedia, QC G0C 1L0, Canada.⁵Atlantic Salmon Federation, Chamcook, NB E5B 3A9, Canada. ⁶Institut National de la Recherche Scientifique, Québec, QC J3X 1P7, Canada. ⁷Gespe'gewa'gi Institute of Natural Understanding, Listuguj, Gespe’gewa’gi, Mi’gma’gi, Canada. *Corresponding author.

Northeastern Naturalist, Volume 33, Issue 2 (2026): 295–321

First published early online: 26 June 2026

Abstract
Substantial decreases in fish stocks have been attributed in part to seabird predation. In many parts of the world, cormorant populations have been increasing since the end of the 1970s, as colonies recover from the effects of human exploitation and impacts of environmental contaminants. In coastal regions of the Gulf of St. Lawrence, Nannopterum auritum (Double-crested Cormorant, hereinafter “Cormorant”) are now abundant. To reach the ocean, Salmo salar (Atlantic Salmon) smolts originating from the Restigouche River watershed must migrate past a colony of breeding Cormorants located at the mouth of the river on Wisugunetg (Bon Ami Rocks). Cormorants were absent from this rocky islet as late as the end of the 20th century. We estimated the number of breeding pairs and assessed diet composition of Cormorants using this colony during the smolt-emigration period to assess the predation risk posed to Restigouche River smolts by Cormorants. We counted Cormorant nests from aerial-imaging surveys via fixed wing aircraft and a drone (2013–2015). We identified and enumerated prey items using otoliths extracted from Cormorant pellets collected during smolt emigration in 2014 and 2015. The Wisugunetg colony increased from 554 pairs in 2013 to 1147 pairs in 2015. We identified only 3 Atlantic Salmon smolts from 874 sampled pellets. The most prevalent prey species were Osmerus mordax (Rainbow Smelt), Lumpenus lampretaeformis (Snakeblenny), Microgadus tomcod (Atlantic Tomcod), and Pseudopleuronectes americanus (Winter Flounder). Cormorant diet reflected seasonality in prey abundance, with Rainbow Smelt dominating diet in May, and Winter Flounder becoming more dominant in June. These results indicate that Restigouche smolts contribute minimally to the diet of Cormorants nesting at Wisugunetg. Rather, the abundance of diverse, alternative prey near the breeding colony appears to provide a buffer that protects Atlantic Salmon smolts from predation by Cormorants. This natural buffer highlights the role of biodiversity in predator–prey relationships and underscores the importance of multi-species management strategies in effective stewardship.

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