In situ orientation of fish larvae can vary among regions

Larval coral-reef fishes have good orientation abilities. Through-water orientation of larvae in some species is location-dependent at meso-scales <10s of km, whereas other species have location-independent orientation at meso-scales. In situ observation of the damselfish Chromis atripectoralis showed that settlement-stage larvae swam in a southerly direction (mean = 175 +/- 11 degrees) at 100 to 1000 m from shore, both east and west of Lizard Island, northern Great Barrier Reef (NGBR), in 10 datasets from 1998 to 2008. Wind direction did not directly influence through-water swimming direction at NGBR. During 2014, in situ diver observation tested if orientation of C. atripectoralis differed regionally in the central Great Barrier Reef (CGBR), 620 km south of NGBR, and in the New Caledonia reef lagoon (NCRL), 1950 km east of CGBR. In all 3 regions, >90% of larvae swam directionally with similar precision and speeds, and with significant among-individual orientation. Yet through-water orientation was easterly at CGBR (72 +/- 30 degrees) and NCRL (87 +/- 20 degrees), and significantly different from NGBR. Over-bottom orientation (i.e. the result of current and larval swimming), measured by GPS at start and end of observing each larva, was weak east-southeasterly at NGBR (116 +/- 40 degrees, p = 0.045), not significantly directional at CGBR, and strongly westerly at NCRL (246 +/- 28 degrees, p = 0.0006), indicating that dispersal of C. atripectoralis is both current-and behaviour-dependent. This is the first report of location-dependent larval fish orientation at a regional scale. This might be an evolutionary response to regional hydrodynamic conditions to limit downstream dispersal.