The Elusive Ocean Sunfish

Image: The SVII and a giant Ocean Sunfish



The area around the island of Nusa Lembongan, southeast of Bali, Indonesia, is popular for scuba diving, in particular for the amount of Manta Rays and the possibility of seeing Ocean Sunfish. It is the latter of these we are interested in on this expedition. These unusual and elusive giants are generally solitary and pelagic, but for around four months of the year (July – October) the reefs around Bali are visited by the Ocean Sunfish. They are the heaviest bony fish in the world, with adults able to attain body weights of over 2,000 kg.



Difficulties in taxonomy


Sunfish belong to the family Molidae. There was once thought to be over 50 different species in 18 groups, but this categorisation has now been collapsed to 4 species in 3 genera. This confusion over the taxonomy of the species is in part due to the difficulties of identifying the species of individuals – they are thought to change proportions as they grow. Luckily, improved techology and genetic analysis is being used to assist with this issue.



Ocean Sunfish research


Whilst on Lembongan we caught up with a Marine Biologist doing her PhD studying Ocean Sunfish - Marianne Nyegaard, of Murdoch Uni, Perth, West Australia. The research here is being done on 4 levels. Firstly, satellite tagging, which hasn’t begun yet. Secondly, an ‘ID photo database’, which encourages the diving community to share photos taken of sunfish from the Nusa Lembongan / Nusa Penida area. These can help to ID individual fish by their striking patterns, to see if the same individuals are sighted over time. Thirdly, tissue samples of the Sunfish for genetic analysis - both for species verification and population genetics. And lastly, overlaying large-scale oceanographic dynamics e.g. temperature oscillations, to see what effect these have on the sightings rate of sunfish around Nusa Penida.


This research is attempting to establish not only which species of Sunfish occur at Nusa Penida (Marianne thinks they are Mola ramsayi), but also whether they are a transient or resident population, and if they are transient, is it the same individuals that return each year. Marianne believes they are a transient population, which migrates up from the Indian Ocean.



A lot of unknown factors


While Sunfish can be seen anywhere around Nusa Penida and Nusa Lembongan, they tend to be sighted most frequently at Crystal Bay. It isn’t known exactly why the Sunfish predominantly choose just this location, and why they only appear at a certain time of year.


It’s thought their movements here could be for feeding; at this time of year there are huge upwellings of nutrient-rich cold water. They also could come here to get rid of their parasites – often Sunfish are sighted at cleaning stations being cleaned by Longfin Bannerfish, adult Emperor Angelfish, Klein’s Butterfly Fish, and 2 types of Cleaner Wrasse.


Another reason for the congregation could be mating patterns. However, scientists are unable to tell what sex Sunfish are either by sight or by genetic sampling. It is believed the Sunfish that come here are mature enough to mate, even though scientists have mixed opinions on how to accurately decipher their age. There are two schools of current thinking. One estimates age according to size: 2 metres is 7 years old; 3 metres is 20 years old. This data is based on growth of Sunfish in an aquarium, and while there is no doubt that sunfish have an incredible capacity for growth, aquarium kept animals may not be representative of growth in wild populations. Another method is measuring the growth lines on vertebrae: using this method, scientists have approximated that 3 metres means the fish is 100 years old! This study was done on a sister species, Masturus lanceolatus.



More research required


With all of these unknowns, it is obvious there is much to be researched about these unusual and elusive Sunfish. Marianne is keen for her research to help guide the strategies to be put in place now that the waters around this island group have just been awarded Marine Protected Area status, in June 2014. To see the progress of the research or to send in some Sunfish photos from a dive go to: