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CRITTER: Unusual marine life living in the Hale‘iwa trench

Hiding submarines in WW2 from the enemy here in Hawai‘i was not an easy job but the military was successful at doing so right offshore of Ali‘i park in Hale‘iwa O‘ahu. The military dug a huge trench right out of the shallow coral reef parallel to the park about 100 yards offshore. This trench is 90 feet deep and opens up into the boat channel going into the harbor at Hale‘iwa. They used the trench to hide submarines.

This one of a kind reef engineering project has made a very unusual dive site that is now used by very unusual marine life species. When you first dive the trench you can’t believe it is actually there because you start the dive from the shore waist deep of water. You snorkel or kayak straight out from the park then all of a sudden you hit deep blue water where you cannot even see the bottom. It is a very weird experience going from a shallow snorkel spot to the deep blue sea within ten feet. Diving on ocean ledges is common but most deep water ledge dives go over a sloping cliff, not one that is perfectly straight down.

What is also very unique for a dive site is the trench is very deep but only about 50 yards wide. The far side of the trench has a cliff going straight up to the surface making the deep trench U-shape. Most normal wall dives in the sea only have one wall facing the open ocean but the Hale‘iwa trench has two walls facing each other. This makes for a one of a kind marine life habitat and the critters that live in the trench are evolving differently than their counterparts that live out in the open sea.

I did five scuba dives in the trench to try and find some unique species I have not yet been able to video document in the rest of Hawai‘i. We have a very common whitemouth moray eel (puhi ‘oni‘o) here in Hawai‘i but the ones in the trench are reported to have yellow mouths. We have lots of pufferfish species here in Hawai‘i, but the super rare Randell’s pufferfish has only been seen in the bottom of the trench. After my dives I realized that there are a whole bunch of unique critters living in this man made marine aquarium.

Because the trench is so deep and narrow the sun does not shine down to the bottom and it is very dark and eerie. It is like diving into an unknown world where you feel like you might find a living dinosaur or some kind of strange sea monster. I found out quickly who really likes the trench. I saw over 30 sea turtles (honu) on one-hour long dive and they were sleeping in caves along the sheer underwater cliff. I think the honu were there in great numbers to relax and sleep because the tiger sharks don’t like coming into the trench because it is so closed in. So what was built by the military is now a sea turtle social club.

The other thing I noticed is that the common normal marine life species all looked different. Due to the lack of sunshine in the trench the critters are evolving to have different colors then the same species living out on the open reef. I found sea cucumbers that are normally brown but in the trench they are bright pink and green! I found moray eels that are usually green but in the trench they are white and even the common stocky hawkfish (po‘opa‘a) that is normally olive green was bright yellow and red.

The Hale‘iwa trench just may be an ocean laboratory where marine life species are evolving in the darkness differently then they are up on the top of the reef in the sunshine. Even the coral species growing in the trench look different then the ones on the reef top. As a biologist the trench is a super exciting place to study because there is nowhere else in Hawai‘i like it. I will soon make a movie about my dives in the Hale‘iwa trench and post it on my YouTube channel at Underwaster2web. Please subscribe to the channel and you will get all my movies first as soon as they are done.


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 Hawai‘i go to
Source: The Garden Island

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