Monthly Archives: March 2014

March 2014: The Yuzen (Wrought Iron Butterfly Fish)

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Because I was unable to dive in Japan this month due to being out of the country for a week, March’s entry is a bit of information about a fish endemic to Japan called the Yuzen, or Wrought Iron Butterfly Fish, found off the waters of Hachijojima and Ogasawara. 

Entering the dive site Nazumado on Hachijojima, an island 287km south of Tokyo, is always a difficult affair as the water is choppy and conditions are challenging. After gearing up at the car park, divers walk down a slippery slope, slowly enter the water holding a rope fixed to the ground and put on their masks and fins while the waves crash into the rocks around them. On their minds are not just the varied marine life and the many sea turtles below, but also the chance to see the Yuzen or Wrought Iron Butterfly Fish.

As the dive begins, they realize that the difficult entry was worth it. Soon they are at a huge rocky ledge called Umanone with an arch in the middle. It’s possible to swim through this arch and onto deeper depths, but a left turn around to the other side reveals another scene – rocks, deep crevices, cracks and wall formations that have built up over centuries, where moray eels roam in and out of random holes, red soldier fish hide away in dark corners, turtles swim overhead and green seaweed sway in the mild current. It is a mystical sight, buzzing with activity and plenty of marine life that is waiting to be explored. Grazing in the distance are the black and white Yuzen, taking their time hovering over the rocks and enjoying their latest meal. At first glance they are unlike other butterfly fish, dark and not particularly noticeable, but knowing that they are only found in Japan makes spotting them quite special.

The Yuzen are quite interesting. Thanks to their healthy appetite, they are capable of growing up to around 15cm in length. 10 or more individuals gather together to form large schools (a “wrought iron ball”) and one of their most noticeable features is their metallic black and white colour as well as the bright yellow tinge on their dorsal, caudal and anal fins.  Up close, their white colour is actually more silver and emits a soft shimmery tone. They are usually found between 10 and 20 meters in clear outer waters and tend to graze on benthic algae and invertebrates on rocky shoreline reefs.

The International Union for the Conservation of Nature (IUCN) has listed the Yuzen as Least Concern and states that there are no apparent threats to it.  It is spotted most often in the Izu Islands, Hachijojima and the Ogasawara (Bonin) Islands. Hachijojima, a small subtropical island with black sandy beaches, warm sea temperatures and diverse marine life, has an area of 72 kilometres and is also the second largest island in the Izu Island chain. It’s a hidden gem that is popular with divers around September and October as the sea is still relatively warm and there are a couple of long public holiday weekends in both months. The famous Kuroshio Current blesses the island with tropical waters year round. The Ogasawara Islands, meanwhile, recently became a UNESCO World Heritage Site and consist of 30 or so islands spread out over different locations in the Pacific. Underneath the beautifully clear water lie abundant coral reefs and even a couple of wrecks dating back to World War II.