Neuston and the influence of regional and mesoscale oceanography along 110 °E in the south-eastern Indian Ocean

Luke A Matisons

The Indian Ocean remains the least studied of the major ocean basins even though it sustains much of the world’s population and has a significant role in global climate and thermohaline circulation. Likewise, despite inhabiting the easily accessible surface of the ocean, neuston has only recently started to receive attention in biological-oceanographic surveys. This study was part of the second International Indian Ocean Expedition (IIOE-2) and aimed to fill in some of the knowledge gaps surrounding neuston in the south-eastern Indian Ocean (SEIO) by sampling along the historic 110 °E meridional transect (39.5-11.5 °S). Evening assemblages of neustonic fishes and phyllosoma were linked to regional and mesoscale oceanography based on CTD data collected at 20 sampling stations and from remotely sensed sea surface imagery. In total, 271 fishes were collected spread across 21 families, with ~50% of individuals being diel migrating larvae and juveniles of mesopelagic lanternfishes (Myctophidae). The early life stages of epipelagic fishes contributed ~ 30% to the fish abundance and consisted primarily of euneustonic flying fishes (Exocoetidae) and sauries (Scomberesocidae). Fish assemblages became more diverse and abundant towards the tropics and were significantly correlated to temperature and salinity. The northward positive temperature gradient and sub-tropical salinity peak observed along the transect resulted in grouping of fish assemblages into southern, central, and northern stations, which, along with the results of concurrent zoo- and ichthyoplankton surveys, indicated the biogeography of the SEIO. Mid-stage phyllosoma of the western rock lobster (WRL; Panulirus cygnus) were found in high abundance within a cyclonic eddy at 32 °S. Using sea surface height anomaly imagery, the eddy could be traced back to where WRL larvae were likely hatched along the west Australian shelf towards the end of November 2018, and thus, suggests that larvae became entrained and subsequently transported by the eddy. Absence of short-lived neritic fish larvae in the eddy at 32 °S and the results of Lagrangian particle backtracking examples support the theory that long pelagic larval durations, such as those of WRL, increase dispersal potential and likely make organisms more predisposed to transport via mesoscale processes. This thesis provides new information on the distributions and abundances of neustonic fishes and phyllosoma in relation to oceanography, which may play a role in continued investigation and management of open-ocean environments.

Neuston and the influence of regional and mesoscale oceanography along 110 °E in the south-eastern Indian Ocean

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