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The Bali Sunfish

Common name: Bumphead Sunfish, or Giant Sunfish

Scientific name: Mola alexandrini

Sunfish are famously seen in the waters off Bali, Indonesia, most notably around the 3 Balinese islands known collectively as the Nusa Penida Islands (Nusa Penida, Nusa Lembongan and Ceningan). They appear, as if out of nowhere, during the ‘sunfish season. A quick Google search will reveal the magnitude of the associated scuba diving tourism industry to see this iconic fish first hand.

The abundance and resilience of the coral reefs of the Nusa Penida islands, the biodiversity in the region and the magnificent top side views just adds to our resolve to promote the conservation of these stunning and fragile ecosystems that support life and the fascinating annual aggregation of sunfish to the area. 

Since these strange fish were first seen by divers on the Nusa Penida reefs in the 1980s, they have been known locally as ‘mola-mola’. However, these fish are not Mola mola, but belong to the close relative, Mola alexandrini. .

Sunfish Season

The mola season is generally considered to start in July or August, and last well into October, but sightings are actually reported year round. Establishing the exact ‘mola season’ is not straight forward, however, as the mola season coincides with a large influx of tourist divers to the area. With many more divers in the waer, the chances of spotting molas increase – so are we in reality dealing with a ‘human season’, creating the illusion of a ‘mola season’? Several dive operators on both Bali and Nusa Lembongan are currently participating in the Bali Sunfish Seasonality Study by recording dives and mola sightings throughout the year, to help reveal the underlying patterns in sunfish occurrence.

This study is also useful to find patterns of seasonality in relation to other oceanographic phenomena, such as El Nino events.

What are they doing on the reefs?

The molas come into the reefs to seek out cleaner fish, to help them get rid of skin parasites, and clean up skin lessions. The most common cleanerfish to see engaging in this activity are the Schooling bannerfish, and adult Emperor anglefish. Sometimes, clouds of Klein’s Butterfly fish can be seen targetting the tail end of the sunfish, and with careful observations it may be possible to spot Cleaner wrasses and Moon wrasses take part on the cleaning as well. 

The adult Emperor angelfish acting as a cleaner fish is in itself a unique phenomenon for Bali, with no other known cases where the adult Emperor angelfish exhibits this behaviour. But despite the undeniable evidence that the molas are getting cleaned on the reefs, it is not known if there is an ulterior motive to their presence in the general area, such as for example spawning or feeding.

The Bali Sunfish

Do they like cold water?

The sunfish season is also known for its impressive thermoclines, where warmer water suddenly gives way to cold water many degrees cooler in what are known as ‘thermoclines’. Warmer water overlays cold water, without mixing (much). The cold water comes from seasonal upwelling from the Indian Ocean, and affects especially the southern sides of Nusa Penida more so than the northern. The temperatures during the mola season along the south coast of Nusa Penida are frequently a brisk 16-17 degrees Celcius, with occasional dips as low as 13-14 degrees! The northern coast of Nusa Penida stays relatively warmer during the mola season, with surface tempertaures of approximatley 24-28 degrees Celcius. Icy cold dives can be had here, too, when occasional thermoclines present divers with a chilly surprise.

The Bali molas are anecdotally thought to be attracted by this seasonally cold water, coming into the local reefs because of it. There are, however, accounts of diver sightings as far as Bali’s warm north-west coast, and occasional sightings in the warmer months ‘out of season’ are also quite common. This raises questions about the relationship between the Bali molas and the seasonal cold water upwelling, and where the molas might be during ‘off season’. Do they stick around all year, staying away from the reefs during the warm ‘off season’ months, or do they, besides a few stragglers, migrate to other (colder) areas?

To better understand the association between the presence of the Bali sunfish on the reefs, and the temperature regimes inside and outside the mola season, an array of underwater loggers have been recording the temperature around the Nusa Penida islands since 2013, as part of the Bali Ocean Sunfish Research Project.


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The Ocean Sunfish Research Trust operates 100% on volunteer efforts, in-kind support and donations. Any and all contributions are hugely appreciated. 

Thank you for your kind support 🙂


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