MICRO-SPATIAL GENETIC SURVEY OF CORAL REEFS

The Research Problem

Top predators play an important role in maintaining healthy coral reef ecosystems. In the Papahānaumokuākea Marine National Monument (Northwestern Hawaiian Islands), this role is filled by sharks (primarily tiger sharks, Galapagos sharks, grey reef sharks and white tip reef sharks) and large fishes (primarily giant trevally). To effectively conserve coral reef top predators we need to know how far they range and whether their movements have predictable patterns. We are using acoustic and satellite transmitters to quantify the movements of top predators captured in Monument waters.

We are addressing three broad questions about top predator movements:

1. Which top predators move across open-ocean between atolls?
2. How extensive are their movements within atolls?
3. Do top predators exhibit predictable patterns of movement and habitat use?

Methods

To date we have captured 394 top predators in Monument waters and surgically implanted them with small ultrasonic transmitters. We are remotely monitoring the movements of these sharks and fishes by using underwater receivers stationed around every island in the Monument and around four of the Main Hawaiian Islands. Our receiver array stretches over 2,500km from Kure Atoll to Hawai‘i Island and listens continually for the presence of sharks and fishes equipped transmitters. Receivers are periodically retrieved by divers and downloaded to find out which predators have visited, when they came and how long they stayed at each location. In 2006, we also ‘triple-tagged’ eight Galapagos and tiger sharks with both acoustic and satellite transmitters. The satellite transmitters provide information on shark movements in areas outside the detection range of our underwater listening stations, and reveal how deep these sharks typically swim.

Findings

Tiger sharks are the most wide-ranging top reef predator in Monument waters, routinely swimming thousands of kilometers along the Hawaiian Archipelago and far out into open ocean. Grey reef and Galapagos sharks occasionally cross open ocean but most are surprisingly resident at their ‘home’ atolls. Ulua (giant trevally), uku (green jobfish) and hapu‘upu‘u (Hawaiian grouper) are resident at their ‘home’ atolls and have well-defined patterns of movement with distinct diel, seasonal & lunar rhythms. All top predators range widely within individual atolls, although for most fish these movements are excursions away from core areas where they spend most of their time. For example, during summer full moons ulua from all across French Frigate Shoals swim up to 30 km to a spawning aggregation site on the southwest side of the atoll. Individual ulua return to the same spawning sites in successive years.

Related Links

To learn more please visit our Reef Predator website

Partner Links

• Top Predator Movements in the Northwestern Hawaiian Islands