Assimilation of pelagic red crabs, Pleuroncodes planipes, by Western gulls, Larus occidentalis

Western gulls, Larus occidentalis, ate exclusively pelagic red crabs, (Pleuroncodes planipes) during a mass stranding. The gulls regurgitated and excreted the crustaceans at specific site in San Benito Islands, allowing collection of fresh samples. A total 347 live red crabs, 63 regurgitated, and 86 scats was dried, frozen, and analyzed for proximate composition. Nutritional value of crabs was measured for its assimilation. Whole crabs had 9.46% of lipids and 40.14% of crude protein, regurgitated crabs 0.29% and 16.31%, while feces contained 0.06% and 24.96%, respectively. Assimilation for lipids and protein in regurgitated crabs was 96.9% and 59.3%; feces indicated 99.9% lipid assimilation but crude protein enrichment, probably due to bacterial load. Considering lipid fraction, regurgitation seemed more efficient since most of lipids were assimilated with a partial and less costly digestion. Western gulls prey on P. planipes not only because its abundance or easier catch, but also because of its nutritional value when regurgitated.