The effect of the lunar cycle on the abundance and food source of whale sharks (Rhincodon typus) in South Ari Atoll, Maldives

Author(s): Aimee Riley
University of Plymouth Undergraduate Dissertation
Keywords: Maldives, whale shark, plankton, lunar cycle, abundance, monsoon, food source, feeding


A promising pilot study on the effect of the lunar cycle on the abundance of whale sharks and plankton by an undergraduate student completing her dissertation with the MWSRP in July & August 2014.

Abstract; “Whale sharks are known to inhabit the waters surrounding South Ari Atoll throughout the year (Riley et al., 2010). The aim of this study was to identify whether whale shark abundance increases during the Full Moon; in response to lunar induced changes in the behaviour and distribution of their food source. Archival data from the Maldives Whale Shark Research Programme (MWSRP) was analysed using Excel and ArcMap 10.2.2. to compute values for mean sightings per day and to identify the spatial distribution of whale sharks in relation to moon phase and monsoon seasons respectively. Plankton samples were obtained between 15th July and 7th August 2014 and analysed under a microscope to obtain information regarding the common species of plankton found when whale sharks were observed to be feeding. The common types of plankton found were calanoid copepods, Mollusca, spp., Onchaea spp., euphausiids, fish eggs and fish larvae. Accurate values for concentrations of plankton per sample area could not be accurately calculated. However, the presence of fish eggs in samples from the 17th July indicate that a lunar induced fish spawning event may have occurred around the Full Moon on the12th July. No statistical differences in mean sightings per day were identified between each stage in the lunar cycle (p = 0.163) or between months (p = 0.56) from 2006 to 2015. However, whale shark distribution appeared to be more concentrated to the west side of the atoll during the Northeast Monsoon (December to March) and at the east side of the atoll during the Southwest Monsoon (May to October). Research methods unaffected by current direction and wind speed, such as tagging and observations by aircraft, should be conducted to confirm the likelihood that whale shark distribution alters seasonally in response to the different monsoon seasons.”

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