Brad Wetherbee – Department of Biological Sciences, University of Rhode Island
Chris Lowe – Department of Biological Sciences, Cal State University Long Beach
Carl Meyer – Hawaii Institute of Marine Biology, University of Hawaii

Shark predation on the endangered Hawaiian monk seal (Monachusschauinslandi) is thought to have had a large impact on the seal population in the Northwestern Hawaiian Islands (NWHI).  In recent years there has been concern that tiger (Galeocerdo cuvier) and Galapagos sharks (Carcharhinusgalapagensis) are killing greater numbers of monk seal pups at French Frigate Shoals (FFS).  Previous anecdotal observations by National Marine Fisheries Service (NMFS) and U.S. Fish and Wildlife Service (USFWS) field personnel suggest that large sharks are common in the summer months at FFS when many marine animals congregate there to mate and give birth.  In addition, anecdotal observations by NMFS personnel have led them to believe that only a few individual sharks are responsible for a large number of attacks on seal pups. If these unsubstantiated observations are accurate, then removal of a small number of sharks could significantly reduce predation pressure on young seals. However, more rigorous scientific evidence is needed to justify such extreme measures, particularly within a National Wildlife Refuge.  For example, little attention has been focused on the movement patterns and site fidelity of these large sharks, particularly near areas where seals haul out.  This type of information is essential for understanding the scale of the predation problem and for assessment of the potential for shark culling efforts to effectively reduce predation on seals.

The focus of this research program was to investigate the movement patterns and site fidelity of large sharks (tiger and Galapagos) near haul out beaches at FFS using an array of methods:  1) All large sharks were tagged with large, highly visible identification tags (HVT).  These tags were placed on both sides of the dorsal fin to facilitate identification of individual sharks by observers on the shore or in boats.  2) Several tiger sharks were manually tracked over 6-8 hr periods using standard telemetry techniques to determine short-term movement patterns.  Tracked sharks were fed acoustic transmitters to eliminate any trauma associated with capture and tagging.  3) Longer-term site fidelity and movement patterns of large sharks were determined using anchored acoustic monitors. Sharks were surgically fitted with high-powered, long-life (2 year battery life) acoustic transmitters.  These transmitters are individually coded to allow identification of individual sharks detected by acoustic monitors.  Acoustic monitors were positioned around the major islands of the atoll and archived data was downloaded from the monitors every three months.  4) To determine long-range movement, fixed-fin satellite transmitters were attached to two tiger sharks. Nearly 40,000 detections were recorded on acoustic monitors over a four year period.  These detections provided insight into the movement patterns of large sharks at FFS, particularly for tiger sharks.  Much less data was collected for Galapagos sharks.  Restrictions on fishing in prime Galapagos areas and the culling of individual sharks carrying transmitters severely hampered attempts to understand the movements and population demographics of this species.

Repeated and fairly consistent detections of multiple sharks during our study offer strong evidence that at least some tiger sharks are present at FFS throughout the year.  The more sporadic detections of some tiger sharks at FFS, as well as detections at other islands within the Hawaiian archipelago demonstrate that some individuals were less site-attached and moved long distances.  Large tiger sharks spent more time around islands in the center and southern parts of the atoll during the summer, particularly East Island.  During winter months sharks spent proportionally more time around the northern islands (Tern and Trig).  Tiger sharks obviously feed heavily on fledging albatross at East Island during late spring and early summer.  Monitor data revealed that the appearance of tiger sharks at East Island is heavily skewed towards the late spring and early summer months, as well as during early morning hours.  Peak activity of tiger sharks near East Island in terms of both time of year and the time of day correspond with peak numbers of fledging albatross.  These data suggest that the movement patterns of tiger sharks around East Island are heavily influenced by the availability of this relatively easy to capture food source.

This project was undertaken under the assumption that the information gathered would be used to evaluate the potential of a Galapagos shark-culling program at Trig for reducing seal mortality.  Shortly after we began our study on movement patterns of sharks, NMFS began killing Galapagos sharks at Trig and also initiated an “observation study” to quantify shark activity near Trig through visual observation. Removal of Galapagos sharks and the increased restrictions on our fishing protocol limited our ability to catch Galapagos sharks at Trig and to collect the type of data that would be useful in addressing our original research questions.  The shift in research emphasis of movement patterns of Galapagos sharks near Trig to culling sharks and visual observations had several immediate impacts on our study.  First, we were no longer able to directly target Galapagos sharks that were seemingly orienting to seals at Trig.  Second, the culling project would potentially remove sharks that were carrying transmitters.  Therefore, not only was our access to the sharks of highest interest (those at Trig) restricted, but if a transmitter was successfully deployed in a shark, there was a possibility that the individual would be killed (and no longer yield any data) when it came near Trig, the location of highest interest.

The conditions under which our project was operating were counterproductive to the successful collection of information valuable for evaluation of the effectiveness of shark culling on seal pup survival at Trig Island.  The initial opportunities to investigate interactions between sharks and seals based on movement patterns of sharks were squandered, and to date very little information of substance that could be used to objectively evaluate either the potential for success, or actual success achieved through the culling of Galapagos sharks at Trig has been attained.