MEPS 362:291-302 (2008)  -  DOI: https://doi.org/10.3354/meps07424

ABSTRACT: The largest masked booby Sula dactylatra colony in the world, with >100000 individuals, is located in the eastern tropical Pacific (ETP) on Clipperton Island. We studied their foraging ecology and their relationship with the environment as well as with the tuna purse seine fishery. We examined the movements of birds at sea and studied their diet. Breeding masked boobies foraged mainly during day time, departing early in the morning and returning late in the evening, with a few birds spending the night at sea mainly in flight. The foraging range of birds rearing chicks was on average 103 km (maximum 242 km), with 73% of the time spent in flight. Foraging zones, as measured by first passage time analyses, indicated that zones of area-restricted search (ARS) were 3 km in diameter on average (average duration 1 h) and were located at the extremity of individual trips. ARS zones were dispersed around Clipperton, with no particular concentration, and individual birds never returned to the same ARS from one trip to the next. These results suggest that the foraging sectors of masked boobies were unpredictable at a small scale. The foraging area was located in a zone of high primary production that had drifted from the coasts of Central America. This large-scale feature appears to be predictable and explains the strong seasonality in the breeding season of the population. This peak of productivity occurs year after year around the island in January to March, i.e. during the time of highest energy demand for the colony, when birds are feeding large chicks. Masked boobies dove at an average depth of 2 m (maximum 6 m) and fed almost exclusively on flying fishes, with an estimated daily catch for the population of 69 t of fish. Around Clipperton, yellowfin tuna are caught by the purse seine fishery in large numbers in association with dolphins. Since boobies are observed feeding almost exclusively in association with tuna and dolphins that drive flying fish to the surface, making them accessible for the birds, their dependence on tuna suggests that overexploitation of tuna stocks in this area could have a negative impact on the masked booby population.

KEY WORDS: Clipperton Island · Sula dactylatra · GPS · Eastern Tropical Pacific · Tuna fishery