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Physical connectivity by transport of larvae between different habitats plays a fundamental role in marine population dynamics and is often assessed using circulation models assuming that computed large-scale connectivity describes the actual connectivity. This paper presents observations of drifters released into the Philippine Sea offshore of the western lagoon of Palau that were tracked as were first carried by the Mindanao Eddy toward Mindanao and other parts of the Celebes and Sulu Seas, where they were removed from the water. While following expected transport pathways for this region, our drifters remained at least several kilometers offshore of the various islands they passed by, suggesting that larvae moving similarly would have been too far offshore to recruit to nearshore reefs. Thus, estimates of connectivity made using large-scale models must be taken as upper bounds to connectivity across ocean basins.

Plain Language Summary A major consideration in marine conservation is the connectivity between different habitats or regions of the ocean, that is, the degree to which populations in those places are linked to each through the movement of eggs, larvae, eggs, or adults between them. One area of particular interest is the Western Pacific and its connections with the Coral Triangle. In this study, we deployed a set of satellite-tracked surface drifters offshore of the west side of Palau that moved in the local ocean circulation toward and then past Mindanao, in some cases completing multiple circuits around the Philippine Sea. These drifter tracks demonstrate an important aspect of connectivity in the ocean: In the absence of strong and directed swimming, local flow processes on ocean shelves that can act to transport materials toward shore may control the real extent of connectivity across ocean basins. Thus, the degree of connectivity inferred from large-scale flows (either modeled or observed remotely) is an upper bound on the actual degree of connectivity. Importantly, these results demonstrate that marine conservation efforts for coral reefs based on ocean-basin scale connectivity need to include consideration of flow behavior at ocean boundaries where reefs are located.