When I first started scuba diving many years ago in Kaua‘i, I noticed large schools of kikakapu (butterfly fish) following me around while diving. I was thrilled to have this happen but very confused, as butterfly fish usually feed on sick or dying coral polyps — so why are they following me around?
Hawaiian butterfly fish, generally known as kikakapu or lauhau, are often seen in pairs taking small bites out of the live coral. They eat the damaged polyps to make room for new healthy corals to grow and this process keeps the coral reef healthy. Sometimes we see the same butterfly fish come together in large schools of 20 or more individuals. I always wondered why they schooled from time to time.
Twenty years while diving in Kaua‘i, we never saw a monk seal. We knew that they were native to Hawai‘i, but there were very few if any in Kaua‘i at the time so understanding monk seal behavior was pretty much impossible. But currently we see monk seals on many of our scuba dives especially around Ni‘ihau and Lehua Rock off of Kaua‘i.
While diving at Ni‘ihau a few years back, I saw a large school of butterfly fish following a monk seal around and that seemed very strange until I looked a bit closer. The large 300-pound monk seal was looking into the cracks in the reef for food and when it came close to the reef the small fish darted away from the seal.
One of the fish to dart away was the mamo or sergeant fish. These 4-inch long reef fish lay their purple eggs on the exposed rocks and then they defend their nests by chasing away other fish that may want to eat their eggs.
The large monk seal would scare away the mamo temporarily and then the butterfly fish would zoom in and eat the mamo’s purple eggs. The butterfly fish were following the monk seal, so when it scared away the sergeant fish they would have a free meal of fish eggs. Now it was very apparent as to why the butterfly fish followed the seal and also that butterfly fish eat more than just coral polyps.
After watching the monk seal, I decided to have a closer look at the mamo eggs myself and sure enough the butterfly fish started following me around and would zoom in to eat the eggs when I scared away the sergeant fish that were protecting their eggs. It appears that the butterfly fish may think scuba divers are just a funny looking monk seals.
On one reef in Maui, I went up close to some of the purple fish eggs and over 50 butterfly fish zoomed in to eat the eggs along with triggerfish and other species. This just showed me that many of our Hawaiian reef fish are opportunistic feeders.
You can see the butterfly fish feeding frenzy up close in my movie about Maui marine life up on my webpage at Underwater2web.com and Youtube at Underwater2web.
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Terry Lilley is a marine biologist living in Hanalei Kaua‘i and co-founder of Reef Guardians Hawai‘i, a nonprofit on a mission to provide education and resources to protect the coral reef. To donate to Reef Guardians Hawaii go to www.reefguardianshawaii.org.
