Fieldwork at Heʻeia and Huilua Fishponds

This past Thursday I went to the field to scout field sites on the Northeast side of Oʻahu. Here, many sites with groundwater inputs were traditionally used for aquaculture in Hawaiian fishponds. These embayments already provided a good habitat for juvenile fish; the Hawaiians, recognizing the natural nurseries of valued food species, imposed rock walls around these areas in order to cultivate these species in their natural environment. These areas provided a sheltered, steady supply of fish while ocean fishing is not always as reliable. The pond also provides a sheltered natural environment for limu to grow, thus, these herbivorous fish species do not need to be fed as their natural food source is cultivated by the pond system. The six kinds of fishponds found in Hawaii are a unique and advanced type of aquaculture found no where else in the world.

A key feature of these ponds is the mākāhā, the sluice gates that allow smaller fish to leave the pond, but prevent larger fish from escaping. Combined with the brakish water this creates an environment for species that travel between fresh and salt water during their life cycle, such as ‘ama’ama (striped mullet, Mugil cephalus), awa (Milkfish, Chanos chanos), pualu (surgeonfish), palani (Eye-stripe surgeonfish, Acanthurus dussumieri), āholehole (Hawaiian flagtail, Kuhlia xenura), moi (Sixfinger Threadfin, Polydactylus sexfilis), kākū (great barracuda, Sphyraena barracuda), and papio (young trevally).

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Heʻeia fishpond, in Kāneʻohe, Oʻahu.

Heʻeia fishpond was built 600-800 years ago, and is one of the largest, if not the largest, in the Hawaiian islands. Heʻeia comprises 88 acres of water. Heʻeia fishpond has been restored and maintained by Paepae o Heʻeia, a nonprofit organization founded in 2001, this organization is dedicated to the fishpond and works in partnership with Kamehameha schools, the land owners of Heʻeia. Most of this information was gleaned from their beautiful Pae Pae o Heʻeia website, where you can find lots more information about Heʻeia.

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Huilua fishpond, in Kahana, Oʻahu

Huilua is a smaller fishpond, comprising 7 acres of Kahana bay. Kahana is part of the Hawaiian Division of state parks, and is maintained and restored by a group of local residents called the Friends of Kahana. It is said that two moʻo, large lizard water spirits, are the protectors of Kahana from environmental and human treats (such as fish theft).

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Field Work on the South Shore of Oʻahu

Yesterday I visited three sites on the south shore here on Oʻahu in search of submarine groundwater discharge (SGD) seeps. As you can see from the video, my search was successful! These seeps occur at the interface between subterranean estuaries and coastal marine waters. The basaltic islands of Hawaiʻi have three very important traits when it comes to groundwater: 1). most basalt is porous and able to hold water, however, 2). the different lava flows are very heterogeneous in their composition and porosity (their compositions and ability to hold water differ widely), and, in most places, 3). a less permeable caprock prevents water from flowing out from the island surface. The freshwater lens that occurs in volcanic islands extends below sea level, due to the buoyancy of the fresh water. The fresh water floats on top of the sea water, while gravity pushes the fresh water downwards, causing the fresh water to push the sea water down as it extends below sea level. Where permeable layers of rock meet the shoreline we see submarine groundwater discharge pushed out by this pressure. This is especially obvious at low tide, when the weight of the overlying seawater is lessened or removed.

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This SGD seep occurs below the low tide line, but has enough pressure pushing fresh water out of it that a boil occurs on the seawater surface!

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At very low tides SGD seeps can be seen above the tide line, free flowing from the beach.

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A SGD seep above the low tide line. You often feel this fresh water by its characteristically cooler temperature signature.

Tikehau: a coral atoll and the “fishiest island” in the South Pacific

My first visit to an atoll did not disappoint. Coral atolls are formed when an extinct seamount or island erodes or subsides beneath sea level. A lagoon forms over the caldera and a coral rim remains above or just below sea level around the edge of the atoll. In the case of Tikehau, the reef around the lagoon is almost continuous, with just one pass, Tuheiava Pass, connecting the open ocean to the inner lagoon on the western side of the atoll. Tikehau is located 340 kilometers northeast of Tahiti, the capital of French Polynesia. The area of the lagoon is 461 square kilometers, while the islands that form the rim consist of two major islands and many small islets. 500 people live on the atoll of Tikehau, with most living in the main village, while 8 families live on the remote islets around the lagoon.

The main village of Tikehau is called Tuherahera, and it takes about 20 minutes to bike from one end of town (and the island) to the other. We stayed on the South end of Tuherahera, at Pension Justine.

Tikehau catches enough fish to feed themselves, tourists, and to export their catch to the capital of Papieete on the island of Tahiti, and other surrounding islands.

Most of the fish are caught in this fish trap at Tueheiava pass. We were able to freedive through the pass and experience the current that these fish feel on the incoming tide. It is STRONG! The trap guides the fish into successively smaller chambers with smaller exits and they are unable to escape. Every fish you can imagine was in these traps, including huge moray eels. A large tiger shark even came up to check us out from behind while we were surveying the traps.

 

The abandoned pearl farm in the center of the lagoon is a known manta ray cleaning station. The Mantas visit this spot in the early morning to be cleaned by the wrasses at the coral head cleaning stations. It was amazing to see the manta rays during the day, as I have mostly dove with them at night in Hawaii.

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The amount of fish at the pass was really remarkable. Schools of fish swam at different depths, so there were layers upon layers of schools with all different colors. Here, my friend Morri Sides swims down into a school of Jacks. Above, you will see a school of barracuda that circled us as we floated through the pass. White tip sharks were shy but flitted in and out of the scene at the deeper edge of the pass, while black tip reef sharks hung out near the surface at the edge of the barrier reef.

So, you may be wondering, what about the algae?! There was not much macroalgae present at the pass, as you may expect with such large herbivore numbers. What was interesting is inside the coral lagoon the sand bottom extends out into the deeper waters of the lagoon. When I swam down just off the edge of the beach within the lagoon, I began to feel a layer of warm water. It seems to me that the marshes that are common across the atoll are discharging water at depth within the lagoon, and I was able to feel the heat signature of this beneath the cooler seawater. All along the rim of the atoll where this warm water discharged was a meadow of seagrass. It seems the seagrass is taking advantage of the nutrients from the marsh ecosystem as it discharges within the lagoon.