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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.

DOI

http://doi.org/10.5038/2074-1235.54.1.1688

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