Movement Patterns and Genetic Population Structure of the Whitetip Reef Shark, Triaenodon obesus.

Nick Whitney, Hawaii Institute of Marine Biology

Project Overview

This research project is an investigation of the movements and population structure of the whitetip reef shark. These sharks are highly dependent on coral reefs for food and shelter. They rest in caves or under coral during the day, and feed on the reef at night. Some anecdotal evidence suggests that these sharks maintain a “home cave” that they will return to for days, weeks, or even years at a time. Despite this fact, whitetips are found from East Africa to Australia, here in Hawaii, and even in Panama and Costa Rica. My research focuses on the question: How do whitetip reef shark movement patterns affect their genetic population structure?

Most reef fishes with restricted movements have a pelagic egg or larval stage which drifts through the open ocean and allows them to disperse long distances. Sharks are viviparous (give birth to live young) and thus don’t have a pelagic life-stage for dispersal. This means that at some point, a few of these sharks must have made the swim from Japan or Australia to Hawaii, and eventually to the Eastern Pacific as well.

Photo by JoFan

Movement Patterns

The field portion of my study focuses on the movements and residence time of these sharks in the Hawaiian Islands. Sharks are tagged with two types of ultrasonic transmitters. One type allows sharks to be continuously followed from a boat for periods of 24 to 48 hours at a time. The other type of transmitter communicates with bottom-monitor stations which record the presence or absence of the shark, and how long it stays in a given area (or “home cave”). The goal of this part of the study is to determine how restricted the day-to-day movements of these animals really are.

Genetic Population Structure

The population genetics component of my project will address the question of how closely related sharks are within the Hawaiian Islands (how often do they move from island to island?) as well as across the Pacific. I will also test the hypothesis that the Hawaiian Islands are a stepping stone for sharks to disperse from the Indo-Pacific to the Eastern Pacific, and may even be a central mating location for the species. This will require extensive lab work as well as tissue (DNA) collection from sites throughout the islands.

Photo by JoFan

Community-based Photo-identification

(click here for the Photo-ID page)

The tracking part of this study examines shark movements over a few days to a few months. The genetics component examines relative differences in DNA that can take thousands of years (or more) to develop. An intermediate measure of movements and residence time will come from a community-based, photo-identification study. Whitetips usually have a unique series of dark spots on the sides of their bodies that can be used to identify individuals. Because of their close ties to the reef, this species is one of the most frequently encountered sharks for divers and spearfishermen. Hundreds of island divers see these sharks every day, and many people get pictures of them with underwater cameras. I am currently cooperating with dive shops and underwater photography clubs throughout the islands to collect pictures of whitetip reef sharks. Because people tend to see the sharks in the same areas day after day, I can accumulate a photo library of known individuals and track where they have been seen and when.

This is the first shark research project in the islands to directly involve members of the dive community. The response so far has been extremely enthusiastic (several hundred photos so far), and diver-contributed photos have already revealed shark movements of over 5 miles along the Oahu coast. More importantly, identifying these fish and recognizing them as individuals seems to generate a keen interest in shark conservation and marine management issues in general.