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Diurnal variation in zooplankton communities and its relationship to whale shark (Rhincodon typus) movements at Ningaloo Marine Park, Western Australia

West, Lisa (2013) Diurnal variation in zooplankton communities and its relationship to whale shark (Rhincodon typus) movements at Ningaloo Marine Park, Western Australia. Honours thesis, Murdoch University.

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The seasonal appearance of the whale shark (Rhincodon typus) at Ningaloo Marine Park (NMP) is generally understood to occur primarily for feeding purposes. Despite this, knowledge of their foraging ecology at Ningaloo Marine Park remains scarce. Passive feeding, whereby R. typus swims with a partly or fully open mouth, is commonly observed during the day at Ningaloo Marine Park. Active feeding results in much higher prey consumption with R. typus individuals observed breaking the surface and exhibiting rapid accelerations in their efforts to catch prey. While there are only limited observations of R. typus actively feeding at Ningaloo Marine Park the majority of observations have been around sunset and during the night. It has previously been hypothesised that this diurnal variation in foraging effort is related to the diurnal vertical migration of their zooplanktonic prey.

This study aimed to test this hypothesis by examining the variation in zooplankton abundance and composition at a study site within Ningaloo Marine Park over a 24 hour period. Plankton samples were also collected near passively feeding R. typus individuals to examine the requirements of passive feeding and how it may be influenced by zooplankton diurnal vertical migration at Ningaloo Marine Park. Finally, aerial surveys were undertaken to study the surface abundance of R. typus, comparing the number of surface sightings along a set transect between day and dusk.

A significant increase in zooplankton surface concentration was recorded at night compared to the day, which is consistent with the classical zooplankton diurnal vertical migration. All diurnal migrators recognised to contribute to this trend were classified as Crustacea, an energy dense prey source for R. typus. These results support the theory that the increase in R. typus actively feeding following sunset at Ningaloo Marine Park coincides with an increase in prey concentration. Passive feeding activity did not appear to relate to prey abundance, suggesting that it is unlikely to be affected by the diurnal variation in potential prey distribution. Although results from aerial surveys confirmed that a greater number of surface sightings of R. typus occurred at dusk compared to day, the validity of these results were reduced by the high variability in R. typus sightings between days. Electronic depth data logging tags are recommended for future studies to further explain the vertical distribution and movements of R. typus.

Publication Type: Thesis (Honours)
Murdoch Affiliation: School of Veterinary and Life Sciences
Supervisor: Morgan, David and Norman, Brad
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