Living in the tropics : the foraging ecology of the masked booby in the Pacific Ocean
Tropical regions represent half of the oceans on earth, yet our understanding of the ecological interactions in these areas lags far behind that for temperate or polar regions. By studying the foraging ecology of seabirds, ecological information about remote tropical regions can be obtained. However, in order to interpret the foraging ecology of seabirds, it is necessary to take account of local oceanography, inter-annual variations in environmental conditions at the colonies, and the sex and breeding stage of the birds. In this study, I used the masked booby Sula dactylatra as a model species to analyze the effects of the aforementioned factors and their interactions on the foraging ecology of a pantropical distributed seabird. Fieldwork was conducted at two remote islands in the Pacific, Motu Nui and Clarion Island, during consecutive years (2016 and 2017 at Motu Nui and 2016, 2017 and 2018 at Clarion Island), using GPS, time-depth recorders, diet samples, and satellite data, and taking account of the sex and breeding stage of the individuals. Masked boobies did not utilize the specific hydrographic features at Motu Nui and Clarion Island while foraging. Nor did they change their foraging behavior between years, likely because of relatively stable environmental conditions across years at both study sites. Notably, the El Niño event 2016 did not have a major effect on the environmental conditions at Clarion Island. While foraging trip parameters did not differ between sexes, isotopic signatures of females differed from those of males at Motu Nui. This suggests that, even when the sexes use the same areas, females may be feeding at a different trophic level than males. Foraging trip duration of birds at Motu Nui was affected by breeding stage: they foraged almost twice as long during incubation as during chick rearing. Masked boobies at Motu Nui foraged closer to their colony than those at Clarion Island, which was related to colony size but not to local environmental predictors such as chlorophyll-a concentrations. This study provides the first description of the foraging ecology of a seabird species at Motu Nui and Clarion Island, and demonstrates the value of local studies for providing information on underexplored pelagic areas. Similar to the findings of studies on the foraging ecology of tropical seabirds elsewhere, Motu Nui and Clarion Island seascape did not provide any specific areas where prey species accumulated. The unusual environmental stability around Motu Nui and Clarion Island, may maintain local availability of the main prey species of masked boobies even between El Niño events, which may explain the observed consistency in foraging parameters across years. Consistent with previous studies, masked boobies showed few sex-specific differences in their foraging ecology. However, the difference in isotopic signatures between sexes is in contrast to previous reports that found no differences between male and female masked boobies, and therefore merits further investigations. Given that breeding stage affects foraging behavior, future studies should aim to use data from birds at the same breeding stage when comparing foraging parameters between sites or years. The results from this study also confirm that seabirds from larger colonies are forced to travel further during foraging trips as a result of prey depletion closer to the colony due to continuous exploitation by a larger number of individuals. In summary, this study corroborates that local oceanography, inter-annual variations in environmental conditions at the colonies, and the sex and breeding stage of the individual need to be considered when interpreting the foraging ecology of seabirds. The findings presented in this thesis are important for interpreting the foraging ecology of masked boobies and other tropical seabird species breeding on remote islands elsewhere. They contribute to our understanding of complex food-webs in marine ecosystems, and thus are vital information for decision makers in marine spatial planning, management and conservation.