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Comparative diet analysis of three larid species nesting at Isla Rasa, Gulf of California, Mexico.


Authors

ENRIQUETA VELARDE1 & LLOYD T. FINDLEY2
1Instituto de Ciencias Marinas y Pesquerías, Universidad Veracruzana, Boca del Río, Veracruz, Mexico (enriqueta_velarde@yahoo.com.mx)
2Centro de Investigación en Alimentación y Desarrollo, A. C.—Unidad Guaymas, Guaymas, Sonora, Mexico

Citation

Velarde, E., & Findley, L. T. (2026). Comparative diet analysis of three larid species nesting at Isla Rasa, Gulf of California, Mexico. Marine Ornithology, 54(1), 87-97.
http://doi.org/10.5038/2074-1235.54.1.1688

Received 09 June 2025, accepted 03 September 2025

Date Published: 2026/04/15
Date Online: 2026/04/15
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Key words: comparative seabird diets, fishery impacts, Gulf of California, Isla Rasa, Larus heermanni, Thalasseus elegans, Thalasseus maximus

Abstract

Isla Rasa, situated in highly productive waters of the Gulf of California, has the largest nesting colony of Royal Terns Thalasseus maximus (as many as 17,000 individuals) in the Mexican Pacific, sharing this site with 300,000 Elegant Terns T. elegans and 260,000 Heermann's Gulls Larus heermanni. During eight breeding seasons between 1996 and 2011, we found that the Royal Tern diet contained over twice the number of prey species as the other two seabirds' diets: the Royal Tern's diet included 36 species of fishes, in 20 families, and pelagic squat lobster Grimothea planipes, whereas the Elegant Tern and Heermann's Gull preyed upon 17 taxa (15 fishes and two pelagic crustaceans), in nine families. Most fishes taken by Elegant Terns and Heermann's Gulls (93.8%) represented only three pelagic species (Pacific Sardine Sardinops sagax, Northern Anchovy Engraulis mordax, and Pacific Chub Mackerel Scomber japonicus), whereas these three species made up only 73.3% of the Royal Tern diet—a high proportion, nonetheless. The Royal Tern diet also reflected more diverse prey habitats. Only ~50% of Royal Tern prey species were pelagic, far fewer than for the other two seabirds (99%). The dietary differences among these three species appear to be key to their sharing of a crowded nesting site. The continuing regional growth of the commercial fishery for pelagic forage fish represents a threat to these seabirds and other components of the Gulf of California food web.

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