08/08/22 – DAR completes marine survey of Lehua Island
In early August, the Division of Aquatic Resources (DAR) conducted the first marine surveys by the Division around Lehua Island. Lehua is a small, uninhabited island that lies just off the northern shore of Niʻihau. The remains of a cinder cone several hundred thousand years old, the weathered slopes of volcanic tuff provide habitat for thousands of native seabirds and plants. The surrounding reefs are some of the most pristine in the Main Hawaiian Islands, and are home to many amazing animals such as monk seals, whales, dolphins, and manta rays. In April of 2021, Lehua Island was finally declared rat free after a 5-year project by the Division of Forestry and Wildlife (DOFAW) to exterminate the pests. The rats preyed on the eggs and chicks of sea birds and destroyed fragile native plants. Already, DOFAW is seeing increasing numbers of ground nesting seabirds and the sprouting of native plants that were heavily impacted by the rats.
The rat eradication may also confer benefits to the near shore reefs. With the increase in seabird nests, the amount of nitrogen-rich guano has also increased. Nitrogen is a “goldilocks” nutrient, too little and the growth of corals and important phytoplankton is limited. But too much, and algae and bacteria take over the reefs and smother out corals and other life. For much of Hawaii, the issue is too much nitrogen in the water caused by land-based sources of pollution. But around uninhabited Lehua, the small bump in nitrogen from bird guano has the potential to fertilize the corals and increase productivity on the nearshore reefs.
Lehua also serves as an important reference point for our reefs. Apart from the occasional tour boat or fishing vessel, there is very little human activity around Lehua. Studying this pristine ecosystem provides a contrast to the rest of the Islands, where human impacts are numerous. DAR uses the data gathered from our marine surveys to aid in making management decisions and conservation efforts.
DAR’s marine surveys
Led by DAR biologist Dr. Heather Ylitalo-Ward, two teams of divers conducted a total of 19 surveys over 3 days. To conduct the surveys, the team of divers descend to a depth of 60-80 feet and locate a suitable place to begin the survey. The first diver lays out a 25-meter line along the bottom to mark the survey transect. As they lay out the line, they identify, count, and estimate the size every single fish within 5 meters of the line. In a place like Lehua, that can mean a lot of fish!
The second diver follows behind recording the dominant coral species, habitat types, and major invertebrates such as sea cucumbers and urchins that are present. They then take a series of photos of the reef bottom along the transect line, which are used to analyze the communities of coral and algae that make up the reef.
Across all the surveys, a total of 5498 fish of 109 different species were recorded. Plus, numerous “off-transect” sightings of curious monk seals, manta rays, several species of sharks, turtles, and a school of playful spinner dolphins. Schools of predatory fish such as mu, omilu, and kamanu were also seen, with some of the schools measuring in the hundreds. Gardens of rare leather corals covered large areas of the northern tip of the island, and many reef fish that are not commonly seen around Kauai such as the pyramid butterflyfish and pennant butterflyfish were found to be abundant at Lehua.
The Kauai DAR team plans to re-survey Lehua bi-annually to examine any ecosystem changes following the rat eradication, and to further our understanding of this incredible place.